The peripheral nerves are made up of myelinated and unmyelinated motor and sensory axons, and are essential for the normal functioning of both the voluntary and autonomic nervous systems. Some neuropathies are characterized by exclusively or primarily motor dysfunction, others by sensory dysfunction, and some by a combination of motor and sensory dysfunction. Some of the salient features of autonomic neuropathies in dogs and cats have only recently been described. Technically speaking, a mononeuropathy refers to a dysfunction of one cranial nerve (e.g. facial nerve) or one named peripheral nerve (e.g. radial nerve). A polyneuropathy refers to multiple (i.e. more than one) nerve dysfunction. In this text, multiple cranial nerve dysfunction (without dysfunction of peripheral nerves to the limbs) and multiple peripheral nerve dysfunction in the same limb (e.g. brachial plexus neuropathy/neuritis) will be referred to as multiple mononeuropathies. This distinguishes these relatively focal disorders from more generalized polyneuropathies (e.g. idiopathic polyradiculoneuritis).
In general, neuropathies reflect a failure of the lower motor neuron (LMN). Although some of the diseases discussed in this chapter have abnormal axons and/or myelin in both the central (CNS) and peripheral (PNS) nervous systems, clinical signs of peripheral disease usually predominate. In those neuropathies in which motor nerves are affected, typical clinical signs include decreased to absent reflex activity, poor muscle tone, and neurogenic muscle atrophy (Videos 13 and 33). A recurring theme with canine and feline neuropathies is that tentative diagnoses are often based upon a combination of clinical features that are characteristic of specific diseases. Electrodiagnostic and nerve/muscle biopsy evidence may confirm the presence of a neuropathy, but rarely provides a specific diagnosis by itself. Even when a specific disease entity is confirmed histopathologically (e.g. dysautonomia), the underlying etiology often remains undetermined. Indeed, the underlying cause(s) of the majority of canine and feline neuropathies is (are) unknown. For many of these disorders, therefore, there are no effective treatments. However, some of these disorders resolve spontaneously, and others may not necessarily adversely affect the quality of the patient’s life. For some of the breed-associated neuropathies, specific genetic mutations have been identified as being responsible for the neuropathy; this trend is likely to continue over time.
It is important that the clinician be able to localize disorders to the PNS. In human studies, both acquired and inherited neuropathies are categorized as primarily demyelinating, axonal degradation and secondary loss of myelin, or a combination of both based on electrodiagnostic testing. In primarily demyelinating neuropathies, there is a significantly decreased nerve conduction velocity (NCV) versus a normal or mildly decreased NCV seen in axonal degradation and secondary demyelinating neuropathies. Moderately decreased NCVs are a feature of combination neuropathies. In human studies, these classifications lead to DNA test developments based on the specific mutation present in the myelin, axons, or both. Classification of canine and feline neuropathies is not common practice currently; however, it may lead to important treatment options in the future.
The multitude of reported neuropathies may seem intimidating, but a working knowledge of all these disorders is unnecessary. Once the neuroanatomic diagnosis is made (PNS), appropriate reference sources should be consulted in an attempt to arrive at a specific diagnosis. Knowing that there is a wide spectrum of neuropathies—with different causes, severities, and prognoses—is important both to patient management and client communication. Disorders of cranial nerve (CN) VIII (hearing and balance) are discussed in Chapter 11 and are not discussed in this chapter.
Hereditary polyneuropathy of Alaskan malamutes (HPAM)/idiopathic polyneuropathy of Alaskan malamutes (IPAM) and hereditary polyneuropathy of Greyhounds36, 45, 66, 101, 231, 314
Hereditary polyneuropathy of Alaskan malamutes refers to dogs from Norway, while idiopathic polyneuropathy of Alaskan malamutes refers to dogs from the United States. Although there are some differences between the two groups of dogs, they are considered together in this text because of their many clinical similarities. Greyhound dogs are afflicted with a comparable disorder to the Malamute dogs. This disorder is inherited in an autosomal recessive manner, and the specific gene mutation (NDRG1) responsible for this neuropathy has recently been identified. This disease is characterized by the degeneration of both motor and sensory axons and myelin throughout the PNS. In general, the Alaskan Malamute, Greyhound, and several other breed-related polyneuropathies have been likened to Charcot–Marie–Tooth (CMT) disease of humans. CMT represents a group of phenotypically heterogeneous peripheral nerve disorders of a suspected or known genetic basis.
Clinical signs typically begin at 10–18 mos of age in Malamutes and 3–9 mos of age in Greyhounds and consist initially of pelvic limb paresis and ataxia and reduced exercise tolerance. Dogs with IPAM generally present with more severe clinical signs. Progression of the disease in Malamutes is variable, but worsening paraparesis or tetraparesis—as well as regurgitation, coughing, and/or dyspnea (due to megaesophagus, laryngeal paresis/paralysis, and aspiration pneumonia)—may develop. Spinal reflexes are usually depressed or absent, particularly in the pelvic limbs. Moderate to severe muscle atrophy may be appreciated in all muscles with both forms of the disease. Atrophy may be especially prominent in the shoulder and thigh areas in cases of HPAM, while dogs with IPAM primarily demonstrate distal muscle atrophy. Additionally, dogs affected with the latter form may demonstrate appendicular and paraspinal hyperesthesia. The Greyhound polyneuropathy is very similar in clinical features and progression. Greyhounds progress from having exercise intolerance, a high-stepping gait, and “bunny-hopping” in the pelvic limbs to severe muscle atrophy, tetraparesis, ataxia, and dysphonia. As the disease progresses further, proprioceptive deficits, laryngeal paresis, and loss of spinal reflexes develop.
A tentative diagnosis is based upon history, signalment, and typical clinical features, as well as abnormal results of electrodiagnostic tests, nerve and muscle biopsies, and/or upon histopathologic findings postmortem. As mentioned, a definitive diagnosis can be achieved via identification of a point mutation at the NDRG1 gene locus.
The disease is typically progressive and there is no known treatment. The prognosis for short-term survival for HPAM varies from favorable to poor, as some dogs will improve, whereas others will continue to worsen. In general, dogs that improve tend to do so transiently and often die or are euthanized due to residual deficits and/or complications secondary to these deficits (e.g. paresis, respiratory problems, regurgitation). The prognosis for IPAM is poor, as only continued progression is typically seen. Though some Alaskan Malamutes have been reported to stabilize, these dogs often die or are euthanized within 6 mos. None of the reported Greyhounds has survived longer than 10 mos from the onset of clinical signs.
Dancing Doberman disease27, 60, 66, 314
This is an enigmatic peripheral neuromyopathy, principally affecting the gastrocnemius muscles, seen only in Doberman Pinschers. Although clinical and pathological features suggestive of both nerve and muscle disease have been described, it is unclear whether this syndrome is primarily a myopathy, a neuropathy, or a combination of the two.
Age at onset of clinical signs of dysfunction has ranged from 6 mos to 7 yrs. The first observable abnormality is flexing of one pelvic limb while standing (Fig. 17.1). Within several months, these dogs typically begin alternately flexing and extending both pelvic limbs when standing, giving the appearance of dancing. Affected dogs often prefer to sit, rather than stand, are not lame while walking, and do not appear to be in any discomfort. Over time, Dobermans with this disease can develop conscious proprioceptive deficits and paraparesis. Gastrocnemius muscle atrophy may also develop. The disease is very slowly progressive (over years) and generally does not adversely affect the life of affected dogs.
Figure 17.1 Typical posture of a dog with Dancing Doberman disease. (Courtesy of Dr. Gregg Kortz.)
Diagnosis is primarily based upon the unusual and characteristic clinical findings and ruling out other causes for the unusual pelvic limb carriage (e.g. orthopedic diseases, myelopathies, cauda equina lesions). Electrodiagnostic testing (especially electromyography [EMG] examination of the gastrocnemius muscles) and muscle/nerve biopsy results may help support the diagnosis.
Despite the fact that there is no treatment for the disease, the long-term prognosis for an acceptable quality of life is good.
Great Dane distal sensorimotor polyneuropathy4, 25, 28, 66, 154, 353
A distal, symmetric, polyneuropathy with a suspected inherited basis has been reported in Great Danes. This disease is characterized by the distal degeneration of motor axons, with less frequent involvement of sensory or autonomic nerve fibers.
The reported age of onset has ranged from 1 to 5 yrs. Clinical signs may present acutely, and initially include decreased hock flexion and a “skipping” gait. These reportedly progress over the following weeks, and may progress to include distal hyporeflexia and, eventually, recumbency.
Diagnosis is based upon history, signalment, and typical clinical features, as well as abnormal results of electrodiagnostic tests, and nerve and muscle biopsies.
A definitive treatment is not currently available for this disorder. The prognosis for affected animals is considered poor due to the progressive nature of this disease.
This is a disease of Birman kittens with suspected autosomal recessive inheritance. Loss of myelin and axons of the PNS and CNS results in clinical signs of disease. The lesions are most severe in the distal portions of axons, suggesting a dying-back process. Axonal integrity depends upon anterograde transport of substances (e.g. proteins) from the neuronal cell body throughout the length of the axon, and retrograde conveyance of cellular waste products from the axon back to the cell body to be degraded. Disease processes that adversely affect either the neuronal cell body or axonal transport mechanisms will likely have most serious consequences on the distal-most aspect of the axon. As the disease process continues, axonal degeneration will proceed proximally, “dying back” toward the neuronal cell body. The pathogenesis of Birman cat distal polyneuropathy is unknown.
Affected kittens exhibit clinical signs of neurologic dysfunction at approximately 8–10 wks of age. Slow progression is typically seen. Pelvic limb ataxia and paresis (with frequent falling episodes), subtle hypermetria of all four limbs, and plantigrade stance in the pelvic limbs are characteristic clinical features of the disorder. Kittens with this disorder also tend to adduct their hocks.
Tentative diagnosis is based upon signalment, historical and clinical findings, and results of diagnostic tests (e.g. EMG, nerve/muscle biopsy). Definitive diagnosis is based upon a histopathology of the CNS and PNS lesions.
There is no known treatment for this progressive disease and the prognosis is poor.
This is an autosomal recessively inherited trait characterized by widespread degeneration of myelin and multiple axonal swellings (spheroids) in both motor and sensory axons of the CNS and PNS. Central lesions predominate within both the spinal cord and the caudal brain stem (predominantly the spinal tract of the trigeminal nerve, the rostral olivary, cuneate and accessory cuneate nuclei, and cerebellar white matter and nuclei). The pathogenesis is unknown, but is suspected to involve defective slow axonal transport, with resultant accumulations of neurofilaments and membranous organelles (constituents of the spheroids) along the axon. It is thought that this disease is primarily an axonopathy, with secondary demyelination.
Age of onset of clinical signs is typically 2–6 mos of age (usually 2–3 mos). Pelvic limb ataxia and hypermetria are initially exhibited and this abnormal gait remains the salient clinical feature throughout the course of disease progression. Myotatic reflexes are decreased or absent and decreased muscle tone is apparent. Muscle atrophy is typically minimal to nonexistent with this disease.
Ataxia and paresis slowly progress and may also involve the thoracic limbs. Conscious proprioception (e.g. proprioceptive positioning/placing) is often normal initially, but deteriorates over time. Pain sensation (nociception) remains intact. Mild signs of cerebellar dysfunction (ocular tremors, head-bobbing) have been described in a few dogs, late in the course of the disease.
A tentative diagnosis can be made based upon signalment, historical and clinical findings, as well as abnormal results of nerve conduction studies and nerve/muscle biopsies. Spontaneous activity is not typically seen with EMG. Definitive diagnosis requires demonstration of characteristic histopathologic abnormalities (e.g. spheroids) in both the PNS and the CNS.
The prognosis is variable. There is no treatment for this disorder, but some dogs will stabilize by 12–18 mos of age and the clinical signs will remain relatively static for months or even years. Most of these dogs are eventually euthanized due to inability to ambulate.
Giant axonal neuropathy of German Shepherds25, 28, 66, 103, 106, 107, 314
This is a rare condition characterized by distal axonal swellings in both sensory and motor axons of the PNS and CNS. There is a loss of both axons and myelin. The pathogenesis is unknown, but impaired axonal transport mechanisms are suspected. This is inherited as an autosomal recessive trait.
Onset of clinical signs is typically around 15 mos of age. Pelvic limb ataxia and paresis are noted initially, along with a plantigrade stance. Signs progress to a loss of patellar reflexes and proprioceptive placing reactions, and hypotonia of pelvic limb musculature with distal (below the stifle) muscle atrophy. Tetraparesis is typically evident by 18–24 mos of age. Voice change, megaesophagus (with subsequent regurgitation and occasional aspiration pneumonia), and fecal incontinence also commonly develop within several months of disease onset. A curly hair coat is also a frequent finding in these dogs.
A tentative diagnosis is based upon signalment, historical and clinical findings, and abnormal results of electrodiagnostic tests and nerve/muscle biopsies. Lesions are reportedly seen in diffusion-weighted magnetic resonance images of human patients with giant axonal neuropathies. Definitive diagnosis hinges upon finding characteristic histopathologic changes in the PNS and CNS.
There is no treatment for this progressive disease and the prognosis is poor.
This sensorimotor neuropathy is characterized by widespread loss of peripheral axons (axonal necrosis), especially in distal segments, in young Dalmatian and Rottweiler dogs. The pathogenesis is unknown, although an autosomal recessive mode of inheritance is suspected in Dalmatians. This disease is thought to involve a dying-back process of axons, affecting distal segments of axons most severely. Sixteen Dalmatians and five Rottweilers have been reported with this disorder. Six Pyrenean mountain dogs have been reported with a similar, inherited, laryngeal paralysis-polyneuropathy complex as well. The authors have encountered a similar condition in a litter of young Argentinean dogs.
Onset of clinical signs is typically from 2 to 6 mos of age in Dalmatians and between 2 and 3 mos of age in Rottweilers. Clinical signs relating to laryngeal paralysis, including respiratory distress (e.g. inspiratory stridor, coughing, cyanosis, dyspnea) with associated exercise intolerance, and voice change (dysphonia) are commonly seen. Limb paresis (worse in the pelvic limbs in Rottweilers) with hyporeflexia is also a consistent clinical feature. Gagging, regurgitation (due to a megaesophagus), facial and lingual paralysis, hypermetric gait, and muscle atrophy are additional clinical signs associated with the Dalmatian disease. One of five Rottweiler puppies displayed regurgitation associated with megaesophagus. A peculiar finding in the affected Rottweilers was bilateral cataracts, seen in four of the five dogs; the cause of these cataracts was undetermined.
A tentative diagnosis of this disorder is based upon signalment, history, characteristic clinical findings, and abnormal results of diagnostic tests (e.g. electrodiagnostics, nerve/muscle biopsies). Definitive diagnosis of this disorder is based upon the character and distribution of axonal lesions identified post mortem.
There is no treatment for this disorder. The prognosis is poor, as most dogs die or are euthanized shortly after presentation due to respiratory dysfunction, often involving severe aspiration pneumonia.
Inherited polyneuropathy of Leonberger dogs66, 109, 280, 298
A distal polyneuropathy has been reported in Leonberger dogs, which shares many pathologic and genetic characteristics with the CMT axonal neuropathy seen in humans. Two genetically distinct (gene mutations identified) forms of this disorder, Leonberger polyneuropathy 1 (LPN1) and Leonberger polyneuropathy 2 (LPN2), have been identified. The genetic mutation found for LPN1 (ARHGEF10 deletion) is the same mutation responsible for a similar juvenile polyneuropathy of Saint Bernard dogs (one of the breeds from which the Leonberger was derived). LPN1 is inherited as an autosomal recessive trait, whereas LPN2 is inherited as an autosomal dominant trait. This disorder is characterized by chronic, progressive, distal axonal loss and demyelination. Italian Spinone dogs have reportedly presented with a similar distal polyneuropathy, and an autosomal recessive mode of inheritance is suspected in this breed.
The age of onset of clinical signs in Leonberger dogs has reportedly ranged from 1 to 9 yrs of age, although the majority have been 1 to 3 yrs of age at presentation. Dogs with LPN1 tend to have particularly severe disease with onset less than 4 yrs of age (mean of 2 yrs), whereas dogs with LPN2 have a broader range of onset of clinical signs (1 to 10 yrs; mean of 6 yrs). Italian Spinone dogs typically present between 8 and 10 yrs of age. Affected dogs show exercise intolerance, distal muscle atrophy, decreased spinal and cranial nerve reflexes, and laryngeal paralysis/paresis. Hip and stifle hyperflexion while ambulating is also classically seen, along with a tendency to “throw” the foot forward during the pelvic limb’s swing phase. Facial nerve paralysis and/or a decreased gag reflex have been infrequently reported.
A presumptive diagnosis can be made based upon signalment, historical and clinical findings, as well as abnormal results of electrodiagnostic testing and corroborating muscle/nerve biopsy results. Definitive diagnosis for the Leonberger disorder is achieved by identifying the causative gene mutation. The LPN1 mutation accounts for 20% of all Leonbergers with polyneuropathy and the LPN2 mutation accounts for 25% of these dogs. Therefore, there is a group of Leonberger dogs with polyneuropathy that will not be identified by testing for LPN1 or LPN2.
Symptomatic treatments, such as surgical lateralization of the arytenoid cartilage to improve respiratory function, may improve quality of life. However, definitive treatments are not yet available.
Although disease severity appears to vary among affected dogs, long-term prognosis is considered guarded. A subset of dogs will continuously deteriorate, becoming tetraplegic or developing complications from laryngeal dysfunction, such as aspiration pneumonia.
This refers to a motor neuropathy of the recurrent laryngeal nerve that appears to occur in two major forms. The first form is a hereditary disease of immature animals (less than 12 mos of age) and the second form is an acquired (also referred to as idiopathic laryngeal paralysis) disorder of middle-aged to older dogs and cats. It is now accepted that acquired laryngeal paralysis is a manifestation of a generalized neuromuscular disease, rather than an isolated neuropathy. Many patients affected by acquired laryngeal paralysis have concurrent esophageal dysfunction and eventually develop a generalized polyneuropathy within 1 yr. Electrodiagnostic testing abnormalities and histopathologic findings support a generalized polyneuropathy in many dogs with acquired laryngeal paralysis.
Both forms appear to be much more common in dogs than cats. In both disorders, denervation atrophy of the cricoarytenoideus dorsalis muscle and axonal and myelin loss in the recurrent laryngeal nerve(s) are characteristic features. The pathogenesis is unknown for both forms of laryngeal paralysis. Specific information pertaining to the two forms of this neuropathy is as follows:
Hereditary laryngeal paralysis—this is best described for the Bouvier des Flandres breed, in which the disease is inherited as an autosomal dominant trait. An autosomal dominant pattern of inheritance is seen in the Bouvier des Flandres and Wallerian degeneration of the recurrent laryngeal nerve and concurrent histopathologic changes in the nucleus ambiguus have been documented. An inherited basis for this disorder has also been proposed in Siberian Huskies, and in a litter of Siberian Husky/Alaskan Malamute crossbreed dogs. Recurrent laryngeal nerve degeneration is due to neuronal degeneration in the nucleus ambiguous of the brain stem. Onset of clinical signs is typically between 4 and 6 mos of age. However, age of onset of clinical signs has been reported up to 7 yrs of age in this breed. A similar disease has been described in young Siberian Husky and Husky crossbred dogs, Bull Terriers, Rottweilers, and white-coated German Shepherd dogs.
Acquired (idiopathic) laryngeal paralysis—this is encountered most commonly in older large- and giant-breed dogs, such as Labrador Retrievers, Saint Bernards, Newfoundlands, Irish Setters, and Afghan hounds. Labrador Retrievers represent between 69 and 73% of late-onset laryngeal paralysis cases, suggesting a familial or genetic predisposition in this breed. A high likelihood of older dogs presenting with laryngeal paralysis having underlying generalized neuromuscular disease has been reported. There appears to be no breed or sex predilection for cats with laryngeal paralysis. The median age of cats with laryngeal paralysis was reported as 11 yrs in one study. A focal pharyngeal laryngeal blastomycosis infection has been reported in one dog, with a clinical presentation indistinguishable from idiopathic laryngeal paralysis cases. Laryngeal paralysis secondary to a pulmonary squamous cell carcinoma has also been reported in a cat. In the majority of acquired cases, the underlying cause is undetermined.
Clinical signs reflect dysfunction of the arytenoid cartilages and vocal folds and include dysphonia, inspiratory noise (stridor), and respiratory distress (especially when exercising). Retching, gagging, and coughing associated with eating and drinking may also be appreciated. These clinical signs are directly related to the arytenoid cartilages and vocal folds remaining in a paramedian position during inspiration. Essentially, this creates an upper airway obstruction. Concurrent megaesophagus has been reported with laryngeal paralysis in a small percentage of both canine and feline cases. Clinical signs tend to progress in severity over several months. Clinical signs related to generalized neuromuscular disease—such as exercise intolerance, muscle atrophy, and absent patellar reflexes—have been reported. High-impact exercises, excitement, increased environmental humidity, and ambient temperature all exacerbate clinical signs.
Both forms of this neuropathy are diagnosed by historical and clinical features and by ruling out other causes of laryngeal paresis/paralysis (e.g. neuromuscular junction disorders, hypothyroidism). Assessment of vocal fold movement during standard laryngoscopy may be used to confirm laryngeal paralysis. A light plane of anesthesia is recommended for this procedure, as anesthetic agents may impair vocal fold movement. Thiopental, given intravenously to effect, has been shown to least influence laryngoscopy results. The CNS stimulant doxapram reportedly improves visualization of laryngeal paralysis but may also temporarily worsen airway obstruction, necessitating the intubation of affected dogs. Effective vocal fold evaluation in sedated dogs has also been reported using transnasal laryngoscopy. Abnormal EMG activity in the cricoarytenoideus dorsalis muscle and neurogenic atrophy appreciated in biopsy samples of this muscle support the diagnosis. A full electrodiagnostic evaluation should be performed to rule out generalized neuromuscular disease, particularly if corresponding neurologic signs are present. Lastly, sound spectrogram analysis may provide a noninvasive method of detecting laryngeal paralysis.
Clinical signs tend to progress in both forms of this disease. Absent patellar reflexes have been associated with a worse prognosis in affected dogs, likely due to their association with generalized neuromuscular disease. Other than exercise restriction and avoiding stressful situations, there is no effective medical treatment to halt or impede disease progression. However, many patients will do well with corrective surgery (e.g. arytenoid lateralization). Corrective surgery for canine laryngeal paralysis has been associated with variable complication rates, ranging from 34 to 74% in various reports. Complications were most likely to occur in dogs requiring bilateral arytenoid lateralization. Bilateral surgery has also been associated with a higher risk of recurrence of clinical signs in affected dogs. The most common postoperative complication was aspiration pneumonia. The placement of low-tension sutures during unilateral cricoarytenoid lateralization may be associated with a lower incidence of aspiration pneumonia, while still achieving sufficient vocal fold abduction. A technique using video assistance for unilateral cricoarytenoid laryngoplasty surgery has been reported. In an effort to assess real-time arytenoid abduction during suture tensioning, the cricoarytenoid suture was tightened under video observation of the rima glottidis with a rigid endoscope. The short-term surgical outcome in 13/14 of these dogs was good and this technique is a useful way to assess the final arytenoid position intra-operatively
Minor complications of arytenoid lateralization surgery include continued coughing, gagging, or exercise intolerance. Seroma formation at the surgery site has also been reported. Minor complications did not affect owner-determined quality-of-life scores in one survey. Successful surgical treatment of this disorder with either an endoscopically inserted cricoid implant or a calcium hydroxyapatite vocal fold injection have also been reported. Similarly, preliminary evidence has shown that surgical re-innervation of the laryngeal muscles, or functional electrical stimulation of laryngeal adductor muscles, may be effective treatments for this disorder, although full clinical evaluations of these procedures are not yet available.
This polyneuropathy of young adult Rottweilers is characterized by a widespread loss of axons (axonal necrosis) and myelin in both motor and sensory nerves of the PNS, especially in the terminal axonal segments. This appears to be a dying-back neuropathy, but the specific pathogenesis is unknown. The disease appears to be reminiscent of a human neuropathy termed hereditary motor and sensory neuropathy (HMSN) type II. Rottweiler distal sensorimotor polyneuropathy is similar in many respects to a syndrome reported sporadically in large dogs of various breeds (e.g. Irish Setter crossbred dogs, Great Danes, German Shepherds) referred to as distal symmetric polyneuropathy.
The dogs reported with this disease ranged in age from 1.5 to 4 yrs at the time of clinical disease onset. The clinical course consists of paraparesis initially, which slowly progresses to tetraparesis, with hyporeflexia and hypotonia, and atrophy of distal limb muscles. The disease is typically slowly progressive (sometimes over a year) and may even wax and wane, although acute presentations have been reported. Dogs with distal symmetric polyneuropathy may exhibit decreased nociception and masticatory muscle atrophy.
Diagnosis is based upon signalment, historical and clinical findings, and abnormalities noted on electrodiagnostic testing (especially EMG of distal limb muscles) and nerve/muscle biopsies.
Some dogs seem to transiently respond to glucocorticoid therapy, but this is a progressive disease with no known treatment. The long-term prognosis is guarded to poor.
Distal denervating disease26, 28, 103, 134, 314
A motor polyneuropathy of unknown pathogenesis has been reported in dogs in the United Kingdom. Lesions are restricted to the degeneration of distal axons and myelin of motor nerves.
There is no age, sex, or breed predisposition. Clinical signs of a LMN tetraparesis (hypotonia, decreased to absent spinal reflexes) develop over a period of 1 wk to 1 mo. Signs of cranial nerve dysfunction may also occur, including dysphonia, facial weakness, and atrophy of masticatory muscles. There is no evidence of sensory dysfunction. Atrophy of proximal limb muscles is characteristic. Respiration, swallowing, and bladder control remain unaffected. Most dogs recover fully with supportive care within 4–6 wks.
Diagnosis is based upon clinical findings and abnormal results of electrodiagnostic tests and nerve/muscle biopsies. Treatment is supportive.
The prognosis is favorable, as most dogs recover fully within 4–6 wks.
Golden Retriever hypomyelinating polyneuropathy25, 28, 34, 66, 103, 222, 314
A peripheral hypomyelination disorder has been reported in Golden Retriever littermates. Nerve biopsies from affected dogs revealed normal axons with deficient myelination. The pathogenesis is unknown, but abnormal Schwann cell function is suspected.
The affected puppies exhibited an ataxic, mildly paretic pelvic limb gait at 7 wks of age. Mild pelvic limb muscle atrophy, a crouched pelvic limb stance, and bunny-hopping while running were also observed. The puppies improved clinically over time.
Diagnosis is based upon signalment, historical and clinical findings, and results of electrodiagnostic testing (e.g. decreased motor conduction velocity) and nerve/muscle biopsy.
The prognosis appears to be favorable, as the puppies either improved clinically or remained unchanged as they grew older.
An unusual demyelinating polyneuropathy has been identified in three related (two littermates, all shared the same dam) black Miniature Schnauzers. This disorder is characterized by the histopathologic finding of focal thickenings of the myelin sheath of peripheral nerves. These thickenings, called tomacula, are a result of excessive folding and compaction of myelin. This disorder appears to be similar to demyelinating forms of CMT disease of humans. A genetic basis is suspected.
Two (intact males) of the three reported dogs developed clinical signs at about 6 mos of age and were presented for evaluation at 14 and 31 mos, respectively. The third dog (intact female) presented at 31 mos of age and the onset of clinical signs was described as from an “early age.” All dogs, despite having electrodiagnostic (decreased motor and sensory conduction velocity) and biopsy evidence (demyelination) of appendicular nerve dysfunction, were presented primarily for respiratory dysfunction, rather than generalized limb muscle weakness. Two dogs presented for repeated regurgitation episodes (one had evidence of megaesophagus) and aspiration pneumonia, and one dog for inspiratory stridor (due to bilateral laryngeal paralysis) and subtle exercise intolerance. This latter dog had evidence of megaesophagus on thoracic radiographs on a recheck examination.
Diagnosis of this disorder in the three reported dogs was based upon clinical features, electrodiagnostic testing, and results of nerve (abnormal myelination) and muscle biopsy (normal).
Although the prognosis for this disorder cannot be determined from the three reported cases, all three dogs were alive and stable at the time of submission of the manuscript, for a considerable period after the onset of clinical disease. This suggests that this is not a rapidly progressive neuropathy.
This is a demyelinating peripheral polyneuropathy that is inherited as an autosomal recessive trait in Tibetan Mastiffs. This disorder has also been described in cats. There is widespread demyelination and remyelination with minimal axonal degeneration, leading to gross hypertrophy of the affected nerves. The pathogenesis is thought to involve a defect in Schwann cell ability to produce and maintain a stable myelin sheath.
Onset of clinical signs of dysfunction is typically at 7–10 wks of age in Tibetan Mastiff dogs, and between 7 and 12 mos of age in cats. Affected puppies usually exhibit pelvic limb weakness that rapidly progresses to the thoracic limbs. A shuffling, plantigrade gait is characteristic. Decreased spinal reflexes, muscle hypotonia, and dysphonia are also common features of the disease. Most afflicted dogs are nonambulatory tetraparetic within a few weeks of the onset of clinical signs. Pain perception (nociception) is unaffected. Cats with hypertrophic neuropathy display generalized tremors that worsen with activity, hypermetric gait, plantigrade stance, depressed spinal reflexes, and decreased sensation in the facial region and extremities. Mild limb paresis and muscle atrophy may also be appreciated.
Diagnosis of this condition is based upon signalment, historical and clinical findings, and abnormal results of electrodiagnostic tests and nerve/muscle biopsies.
There is no treatment for this progressive disease and the prognosis is poor. A subset of dogs may improve in their signs, but residual weakness is frequently seen in these cases.
Megaesophagus can result from a number of diseases of nerve, muscle, neuromuscular junctions, or gastrointestinal tract. Additionally, megaesophagus may be seen secondary to vascular ring anomalies or a persistent left cranial vena cava. A predisposition to esophageal dysmotility without megaesophagus, likely representing delayed esophageal maturation, has been reported in young Terrier dogs. However, this discussion focuses on the congenital disorder of immature animals and the acquired (idiopathic) disorder of adult animals. Congenital and acquired megaesophagus are more common in dogs than cats. Both forms of megaesophagus may represent a neuropathy (motor and/or sensory) affecting the vagus nerves, but there is no compelling evidence to support this. In short, both forms of megaesophagus are idiopathic.
Clinical signs of megaesophagus usually include regurgitation, and secondary respiratory problems due to aspiration pneumonia. Congenital megaesophagus has been reported primarily in Great Danes, German Shepherd dogs, Irish Setters, Newfoundlands, Greyhounds, Shar Peis, Miniature Schnauzers, and Wire-haired Fox Terriers. In the latter two dog breeds, this has been shown to be an inherited disease. Congenital megaesophagus has also been reported in Siamese cats. Idiopathic acquired megaesophagus can occur in dogs and cats at any age. In a recent report of cats with disorders of esophageal motility, 30% were congenital and 43% were idiopathic acquired.
Diagnosis of both congenital and acquired idiopathic megaesophagus is based mainly upon signalment and historical and clinical findings (particularly radiographic evidence of megaesophagus; Fig. 17.2), and ruling out other causes of megaesophagus. Especially in adult animals, there is a multitude of potential causes of megaesophagus. These causes should be ruled out before assigning a diagnosis of idiopathic acquired megaesophagus.
Figure 17.2 Lateral thoracic radiograph of a dog with megaesophagus.
There is no specific treatment for these idiopathic conditions. Patients should either be fed with their heads elevated or preferably via a gastrostomy tube. Antacids (e.g. cimetidine) and motility modifiers (e.g. metoclopramide, cisapride) may be helpful in decreasing gastric acidity (decreased lung damage with aspiration) and increasing gastroesophageal sphincter tone, respectively. These motility-modifying agents have not been shown to improve esophageal function in dogs. In dogs with megaesophagus, increasing gastroesophageal sphincter tone may be counterproductive if the patient is being fed by mouth, but may decrease gastroesophageal reflux in a patient fed with a gastrostomy tube. In contrast to dogs, there is recent evidence that the majority (78%) of cats with esophageal motility disorders exhibit clinical improvement when treated with motility modifiers. The difference between dogs and cats in regard to clinical response to motility modifiers is likely due to the large proportion of smooth muscle in the feline esophagus. Metoclopramide and cisapride are smooth muscle stimulants. The majority of the canine esophageal musculature is skeletal (striated), whereas the majority of the esophageal musculature in cats is smooth.
Prognosis is guarded for congenital megaesophagus. In some patients, the megaesophagus resolves as the animal matures, while in others there is either no change or worsening of the megaesophagus. In these latter dogs, the combination of severe malnutrition and recurrent aspiration pneumonia is often fatal. The prognosis for acquired idiopathic megaesophagus is often guarded to poor, especially in dogs. The megaesophagus rarely spontaneously resolves in these patients, and the potential for severe, recurrent aspiration pneumonia is high.
This is an acute mononeuropathy of one (usually) or both facial nerves that has been reported in dogs and cats. The pathogenesis is unknown but a similar condition occurs in humans (Bell’s palsy). There is some evidence in the human medical literature that Bell’s palsy is due to an immune-mediated response triggered by herpes simplex viral infection. Histopathologic evidence of axonal and myelin loss, without evidence of inflammation, has been described with this disorder.
Affected animals are usually middle-aged to older (e.g. over 5 yrs of age). Although this problem has been reported in a number of breeds, Cocker Spaniels appear to be consistently overrepresented. Clinical signs reflect acute dysfunction of one or both (uncommonly) facial nerves. Drooping of the ears and lips, deviation of the nasal philtrum toward the normal side (if unilateral paralysis), decreased to absent palpebral reflex and menace response, and excessive salivation on the affected side are all typical clinical findings (Fig. 17.3). Some patients will have trouble keeping food from dropping out of the lips on the affected side. Corneal ulceration may occur, both to inadequate blinking ability and interruption of the parasympathetic input to the lacrimal gland (in the facial nerve). Though uncommon, some patients may also exhibit signs of vestibular dysfunction (CN VIII; see Chapter 11).
Figure 17.3 Dog with unilateral (left-sided) facial nerve paralysis.
Diagnosis is based on characteristic historical and clinical findings and ruling out other causes of acute facial nerve dysfunction (e.g. otitis media/interna, hypothyroidism). The clinician should bear in mind that Cocker Spaniels are predisposed also both to otitis and hypothyroidism.
The prognosis is guarded for the complete return to function of the facial nerve. Full recovery may occur in weeks to months, but in many cases some degree of facial nerve paresis appears to be permanent. Treatment is symptomatic (e.g. artificial tears to prevent corneal drying). The use of corticosteroids for this disease is controversial, especially considering the apparent lack of inflammation. There is some evidence of efficacy for corticosteroid treatment of Bell’s palsy in people.
Believed to be inherited as an autosomal recessive trait, this disorder is characterized by degeneration of principally distal sensory axons in both the PNS and the CNS. The pathogenesis of this disorder is unknown. A similar disorder has been reported in a Jack Russell Terrier and four Border Collies. An experimentally induced sensory neuropathy with distal axonal degeneration, secondary to chronic dietary deficiencies of phenylalanine and tyrosine, has also been reported in cats.
Clinical signs of dysfunction may be evident by 8–12 wks of age and include mild ataxia, loss of proprioception (especially in the pelvic limbs), and widespread reduction or loss of superficial and deep pain perception. Some patients may exhibit vomiting and/or urinary incontinence, presumably due to degenerative changes in the autonomic nervous system. Self-mutilation may also be exhibited. There is no evidence of muscular atrophy in these dogs, and spinal reflexes may be normal or slightly reduced.
Diagnosis is made via history, signalment, and clinical findings, along with results of electrodiagnostic testing (decreased to absent sensory nerve potentials) and muscle/nerve biopsy.
The prognosis for this disorder is poor, due to the progressive nature of the disorder and/or the severity of clinical signs.
An autosomal recessively inherited sensory polyneuropathy has been reported in English Pointer dogs as well as Czechoslovakian Short-haired Pointer dogs. This condition has also been reported in 13 French Spaniels, with presumed autosomal recessive inheritance. The pathogenesis is unknown, but the disease is characterized by a loss of sensory neurons (as well as their axonal processes and myelin) and an associated lack of an important nociceptive neurotransmitter called substance P.
Clinical signs typically become apparent between 2 and 12 mos of age. There is loss of pain perception to the distal aspect of the paws (e.g. the toes) and a decreased pain sensation proximal to the carpus and tarsus. There is the possibility that paresthesia/dysesthesia may contribute to the clinical picture. The dogs begin to lick and then chew their digits, ultimately leading to autoamputations (Fig. 17.4). Apparently painless fractures and osteomyelitis of the paw may occur. There are no other neurologic deficits, other than altered pain perception to the distal limbs.
Figure 17.4 Automutilation of the digits in a Pointer dog with sensory neuropathy. (Courtesy of Dr. Jacques Penderis.)
A tentative diagnosis is usually based upon history, signalment, clinical findings, and, potentially, nerve biopsy results. Results of electrodiagnostic tests are normal in this disease. Definitive diagnosis is based upon histopathologic evaluation of spinal ganglia, axonal processes of the sensory nuclei of those ganglia (both in the PNS and CNS), and a lack of staining for substance P in the spinal cord.
There is no effective treatment for this disorder and the prognosis is poor.
Also known as acral lick dermatitis, this self-mutilatory behavior of “high-strung” breeds of dogs and cats (e.g. Doberman Pinschers, German Shepherd dogs, Siamese and Abyssinian cats) may be due to a mild sensory polyneuropathy. There is electrophysiologic and histopathologic evidence to support this theory. The pathogenesis of this suspected sensory polyneuropathy is unknown.
Clinical signs of this disorder are usually limited to licking, biting, or scratching an area of skin around the tarsal or carpal areas.
Diagnosis of this syndrome is typically based on typical historical and clinical findings in a “nervous” or “high-strung” pet. Other dermatologic conditions should be ruled out. Some patients will respond (decreased self-mutilatory behavior) to the tricyclic antidepressant drug clomipramine, at a dose of 1–3 mg/kg per os, per day. Secondary skin infections should be treated with appropriate antibiotic regimens.
The prognosis for control of this condition is good.
Spinal muscular atrophy represents a spectrum of uncommon disease syndromes characterized by a premature degeneration of motor neurons primarily in the spinal cord and, to a variable degree, the brain stem. There are multiple forms of SMA, described in a number of dog breeds, with various clinical presentations and levels of severity. In some forms of the disease, there may also be cerebellar neuronal degeneration. It is currently not clear as to whether all of these disorders belong in the same category (e.g. SMA) or if some should be classified as multisystem neuronal degeneration.
Most of these disorders appear to be analogous to infantile spinal muscular atrophy of humans, with onsets of dysfunction occurring during the first several weeks to months of life. Adult-onset SMA, similar to amyotrophic lateral sclerosis (ALS, or Lou Gehrig’s disease) of people and equine motor neuron disease, is rarely reported in dogs and cats. The pathogenesis of this disease is unknown, but SMA is believed to represent an abiotrophy, or premature cell death. These disorders are suspected or proven, depending upon the specific disorder, to be autosomally inherited traits. In the most completely studied form of this disease syndrome, hereditary SMA of Brittany Spaniels (autosomal dominant inheritance), there is evidence that both abnormal cytoskeletal neuronal protein production (e.g. neurofilaments) and imbalances of excitatory CNS neurotransmitters (e.g. aspartate, glutamate) are linked to premature neuronal cell death.
Spinal muscular atrophy has been reported in numerous breeds, including Brittany Spaniels, English Pointer dogs, German Shepherd dogs, Rottweilers, Swedish Lapland dogs, Great Dane crossbred dogs (Stockard’s paralysis), Cairn Terriers, Briquet Griffon Vendéen dogs, a Saluki, and several cats. The typical clinical picture is that of a rapidly progressing polyneuropathy (LMN paresis) in the first 1–6 mos of life, mainly or exclusively affecting the limbs. Paresis with decreased to absent spinal reflexes and neurogenic muscle atrophy usually progresses to paralysis within weeks. Pelvic limb paresis typically occurs prior to thoracic limb paresis. Limb joints may become malpositioned and immovable, subsequent to pronounced muscle atrophy. Some dogs have a more protracted disease course, with less severe signs. For example, the German Shepherd dog disease appears to be an asymmetric, focal loss of motor neurons in the cervical intumescence, with relatively static unilateral or bilateral thoracic limb dysfunction. The accelerated (homozygous) form of the Brittany Spaniel SMA follows the typical pattern of early onset and rapid disease progression. However, the intermediate form has an onset at 6–12 mos of age and progresses slowly to tetraparesis by 2–3 yrs of age, and the chronic form has the same age at onset as the intermediate form but is nearly subclinical.
Some breeds will exhibit signs of brain-stem (e.g. dysphonia, megaesophagus, tongue fasciculations) or cerebellar (e.g. head tremor) dysfunction, in addition to the LMN signs to the limbs. Apparent cataplectic episodes have been observed in the Cairn Terrier disease. This disorder may belong in the category of multisystem neuronal degeneration (see Chapter 7) rather than SMA.
A tentative diagnosis of spinal muscular atrophy is based upon signalment, historical and clinical findings, and abnormal results of electrodiagnostic testing and nerve/muscle biopsies. A definitive diagnosis depends upon brain-stem and spinal cord histopathologic findings.
There is currently no effective treatment for this group of diseases, and the prognosis for the majority of them is poor. There is recent evidence that 4-aminopyridine, a drug that can both improve axonal conduction and increase acetylcholine release at the neuromuscular junction, may hold some promise as a potential therapy for SMA patients. Despite the poor prognosis overall, Brittany Spaniels with intermediate and chronic forms of the disease may do well for years. The German Shepherd dog disease also appears to be self-limiting.
This is a degenerative polyneuropathy of the autonomic nervous system in dogs and cats of unknown pathogenesis. Degeneration and loss of neuronal cell bodies occurs to various degrees in parasympathetic and sympathetic ganglia, intermediolateral columns of the spinal cord, and parasympathetic nuclei of the brain stem. The tendency for cases of dysautonomia to cluster in certain geographic regions (e.g. Missouri, Kansas) suggests either a toxic or an infectious etiology. A genetic influence on susceptibility to dysautonomia has also been proposed. Risk factors for the development of dysautonomia in dogs have been identified. Living in rural areas, consumption of wildlife (e.g. birds, rabbits), spending the majority of time outdoors, and access to cattle, pastureland, and farm ponds are all associated with the development of canine dysautonomia. Most cases of canine dysautonomia are identified between the months of February and April.
Although a wide age range has been reported, most dogs and cats afflicted with this disease are young adults. In one report of canine dysautonomia, the median age at onset of clinical signs was 14 mos and in another it was 18 mos. Clinical signs develop relatively quickly (usually within 48 hrs in cats, within 2 wks in dogs) and primarily reflect parasympathetic dysfunction. However, chronic progressive autonomic dysfunction has been reported in a dog, whose clinical signs progressed over a 4-yr period. Consistently reported abnormalities include mydriasis with absent pupillary light reflex (PLR), dry mucous membranes (often with associated nasal congestion), retching, vomiting/regurgitation (megaesophagus is often present), dysphagia, prolapse of the third eyelid, urinary incontinence/dysuria (often with a distended bladder), fecal incontinence with decreased anal tone, constipation (more common in cats), diarrhea (more common in dogs), and bradycardia. Some patients will also have deficient tear production (abnormal Schirmer tear test) and abdominal pain (presumably due to intestinal ileus). Nonspecific clinical signs of lethargy, weight loss, and dehydration are also commonly observed.
A tentative diagnosis is based primarily on clinical signs of autonomic dysfunction. Orthostatic hypotension has also been reported in affected dogs. Pharmacologic testing (e.g. pilocarpine ocular drops, bethanechol challenge) can also be performed to confirm abnormal autonomic receptor function (i.e. denervation hypersensitivity). Dysautonomic patients tend to demonstrate miosis shortly after ocular installation of 1–2 drops of dilute (0.05–0.1%) pilocarpine. These patients also tend to exhibit improved ability to urinate after subcutaneous administration of a low dose (0.04 mg/kg) of bethanechol. Definitive diagnosis requires demonstrating neuronal cell loss in the parasympathetic nervous system postmortem.
There is no specific treatment for this disease, and spontaneous clinical recoveries are uncommon. Recovery has been reported in cats. However, this recovery may not begin to be apparent for several months and may take a prolonged period. During this convalescent time, intensive nursing care, including tube-feeding, bladder expression, enemas, correction/prevention of electrolyte abnormalities and dehydration, and anti-emetic administration (e.g. metoclopramide) may be required of the owner. The survival of severely affected cats has been estimated to be between 25 and 50% with such supportive treatment. Bradycardia may be a negative prognostic indicator. Most patients with dysautonomia either die or are euthanized due to complications of the disease.
Some of the lysosomal storage diseases discussed in Chapter 7 can have peripheral polyneuropathy as part of the clinical syndrome. In some cases, a peripheral polyneuropathy can be the only clinical abnormality (e.g. Niemann–Pick disease in Siamese cats). The lysosomal storage diseases that may include signs of polyneuropathy as part of the clinical picture include the following:
Fucosidosis—a glycoproteinosis of English Springer Spaniels.
Globoid cell leukodystrophy (Krabbe’s disease)—a sphingolipidosis most common in West Highland White and Cairn Terriers.
Glycogen storage disease type IV—a glycogenosis reported in Norwegian Forest cats.
Niemann–Pick disease—a sphingolipidosis reported in Siamese and Balinese cats.
Clinical signs for most of these disorders reflect multifocal disease of the nervous system. Tentative diagnosis is based upon historical and clinical findings, abnormal electrodiagnostic test results, and lesions supportive of the suspected disorder in muscle/nerve biopsies. Diminished enzyme activity (of the suspected enzyme of interest) in leukocytes, skin biopsies, or cultured cells (e.g. skin fibroblasts, hepatocytes) may be used as a confirmatory diagnostic test in some of these disorders.
There is no treatment for these progressive diseases, and the prognosis is poor.
This idiopathic congenital condition is uncommonly reported in dogs and rare in cats. There is a lack of neurons in the ganglion layer of the retina and atrophy of the optic nerve. Other concurrent ocular abnormalities (retinal dysplasia, retinal detachment) have been reported. Optic nerve hypoplasia can occur either unilaterally or bilaterally.
This condition is believed to be a heritable trait in Miniature Poodles. It has been reported in a number of different breeds, however. Diagnosis is based upon a history of visual problems since opening of the eyelids in infancy, clinical signs of blindness, mydriasis, and absent direct PLR on the affected side(s), and ophthalmoscopic findings of a small optic disc on the affected side(s).
The visual deficits are permanent and there is no treatment for this congenital condition. These patients will, however, make acceptable pets.
A polyneuropathy associated with diabetes mellitus has been described in both dogs and cats. The prevailing thought has historically been that this neuropathy primarily reflects a distal (dying back) axonopathy with secondary demyelination/remyelination. However, more recent evidence suggests that abnormal Schwann cell/myelin function may play a pivotal role in the development of diabetic neuropathy, with axonal damage being comparatively less important. The pathogenesis of axonal and Schwann cell/myelin dysfunction is unknown, but several hypotheses exist. It is likely that certain aspects of all these hypotheses act in concert to effect peripheral nerve dysfunction. The three hypotheses include the following:
Vascular hypothesis—microvascular disease is known to occur with diabetes mellitus. The mechanisms responsible for microvascular compromise are not clearly defined, but may include: decreased vasodilatory molecules (e.g. prostacyclin, prostaglandin E1) and increased vasoconstrictive molecules (e.g. thromboxane A2, endothelin) in vascular endothelium, due to altered lipid metabolism; abnormally functioning hemoglobin and 2,3-diphosphoglycerate, due to protein glycosylation; and thrombosis subsequent to altered vessel compliance (accumulation of glycosylated molecules in and around endothelial cells) and increased red blood cell and platelet aggregation (altered blood flow dynamics and imbalance of vasodilatative/vasoconstrictive substances). Abnormal thickening of the perineurium (the sheath surrounding fascicles of myelinated axons in the peripheral nerve) of diabetic dogs has been reported. This thickened perineurium may also lead to vascular compromise of the axons and Schwann cell/myelin. Interference with the microcirculation to the peripheral nerves may result in ischemia and axonal/myelin degeneration.
Metabolic hypothesis—several cellular metabolic aberrations occur in the diabetic state, which may interfere with axonal conduction and Schwann cell/myelin function. Most of these metabolic alterations are linked to the polyol pathway, which is dependent on the enzyme aldose reductase. There is increased activity of the polyol pathway due to excess glucose substrate, with subsequent accumulation of sorbitol. Sorbitol accumulation leads to depletion of myoinositol, a molecule necessary for normal function of cellular Na1+/K1+ ATPase. Myoinositol is also an integral component of several membrane phospholipids. In addition, sorbitol is slowly metabolized to fructose. During this oxidative process, membrane-damaging free-radical species (e.g. superoxide, nitric oxide) may be produced. Finally, excessive intracellular glucose may lead to the nonenzymatic glycosylation of proteins necessary for normal cell metabolism and axonal transport mechanisms, altering the functional capabilities of those proteins. Any one or a combination of these metabolic alterations may adversely affect axonal and Schwann cell/myelin function.
Immune-mediated hypothesis—in addition to vascular and metabolic disturbances that secondarily affect axonal integrity, there may be more direct, autoimmune-related axonal/myelin disease concurrent with the diabetic condition. There are data to support immunologic mechanisms as being involved in diabetic neuropathy. There is some evidence that an immunologic attack of myelin may be subsequent to the glycosylation of myelin proteins. Some autoantibodies (e.g. against phospholipids) also tend to cause vascular thrombosis, providing another possible mechanism for microvascular nerve injury.
Clinical signs in dogs are quite variable and range from a subclinical disorder (diagnosed via electrodiagnostics and nerve/muscle biopsy) to severe LMN tetraparesis/tetraplegia with profound neurogenic muscle atrophy. The typical clinical scenario in both dogs and cats is symmetric pelvic limb LMN paresis with proprioceptive deficits, decreased reflexes, and muscle atrophy. Bilateral Horner’s syndrome has also been reported with concurrent diabetes mellitus. In this case, resolution of clinical signs was seen once glycemic control was attained. The clinical picture of cats with diabetic neuropathy is more consistent than dogs. These cats usually exhibit a plantigrade posture in the pelvic limbs with their hocks touching the ground (Fig. 17.5). Decreased patellar reflexes, proprioceptive deficits, and muscle atrophy are also characteristic findings. Cats appear less likely than dogs to develop clinical signs of weakness in the thoracic limbs, although a palmigrade stance has been reported in cats with a diabetic neuropathy.
Figure 17.5 Characteristic pelvic limb posture of a cat with diabetic neuropathy. (Courtesy of Dr. Gregg Kortz.)
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