Peripheral Nerve Disorders

Chapter 129 Peripheral Nerve Disorders



Linda G. Shell*,



ANATOMY AND PHYSIOLOGY


The peripheral nervous system (PNS) is composed of 12 pairs of cranial nerves (see Chapters 125 and 126) and 36 pairs of spinal nerves that arise from the spinal cord. Spinal nerve fibers give rise to the peripheral nerves, which usually are composed of both sensory and motor fibers. Sensory nerve fibers are activated by peripheral receptors (Fig. 129-1). Impulses are transmitted up the peripheral nerve to the spinal cord. Some disorders affect only the sensory nerve fibers or ganglia, causing clinical signs such as hyperesthesia and analgesia, proprioceptive deficits, and self-mutilation (Table 129-1). In many cases sensory losses may be difficult to detect.


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Figure 129-1 Impulse pathways in peripheral and spinal nerves.



Table 129-1 CLINICAL SIGNS OF MOTOR AND SENSORY NERVE DEFICITS



Motor Nerve Deficits


Weakness/paralysis


Muscle atrophy


Reduced reflexes


Reduced muscle tone



Sensory Nerve Deficits


Hyperesthesia/anesthesia


Self-mutilation


Loss of proprioception


Dysmetria


Reduced reflexes


Motor or efferent nerve fibers arise from nerve cell bodies in the gray matter of the spinal cord. They carry information from the central nervous system (CNS) to the striated muscles (see Fig. 129-1). Motor deficits, characterized by limb weakness, muscle atrophy, and reduced spinal reflexes, occur with injury to any of the following: lower motor neuron in the gray matter of the spinal cord, ventral nerve root, spinal nerve, peripheral motor nerves, neuromuscular junction, and muscle (see Fig. 129-1). Disorders of the neuromuscular junction and muscle are discussed in Chapter 130.


Neuropathy is a general term denoting pathologic changes and/or functional disturbances in the PNS. Polyneuropathy refers to involvement of several nerves, usually resulting in bilaterally symmetrical signs.



CLINICAL SIGNS


Motor nerve disorders generally cause weakness and muscle atrophy. Sensory nerve disorders cause hyperesthesia or anesthesia, self-mutilation, and other abnormalities (see Table 129-1). The clinical signs of motor nerve disorders are similar to those of muscle disorders and can be distinguished using muscle enzyme determinations and muscle and nerve biopsies (see Chapter 130).




Key Point


Many common peripheral nerve disorders are manifested as one of the following clinical problems: acute flaccid tetraplegia, chronic progressive tetraparesis, monoparesis or monoplegia, and sensory disturbances (Table 129-2).


Table 129-2 DIFFERENTIAL DIAGNOSIS BASED ON CLINICAL SIGNS OF NEUROPATHY

















































































Problem Differential Diagnosis
Monoparesis or Monoplegia
Acute onset Trauma to nerve root or peripheral nerve
Fibrocartilaginous infarct
Intervertebral disc herniation to lateral side
Chronic onset Tumor of nerve root or peripheral nerve
Intervertebral disc herniation to lateral side
Joint disease
Acute Flaccid Tetraplegia
  Acute polyradiculoneuritis (coonhound paralysis)
Tick paralysis (see Chapter 130)
Botulism (see Chapter 130)
Myasthenia gravis crisis (see Chapter 130)
Post-vaccinal polyneuropathy
Acute idiopathic polyneuropathy
Protozoal neuritis/myositis
Chronic Progressive Tetraparesis
Immature Globoid cell leukodystrophy
Giant axonal neuropathy (German shepherds)
Progressive axonopathy (boxers)
Hypertrophic neuropathy (Tibetan mastiffs)
Motor neuron diseases (Brittany spaniels, pointers, rottweilers)
Mature Chronic relapsing polyradiculoneuritis
Distal denervating disease
Distal polyneuropathy (Doberman pinschers)
Distal symmetrical polyneuropathy
Metabolic neuropathy
Neoplastic and paraneoplastic neuropathy
Toxic neuropathy
Nutritional neuropathy
Various myopathies
Sensory Disturbances
Immature Acral mutilation in pointers
Sensory neuropathy (longhaired dachshunds)
Mature Sensory neuropathy (ganglioradiculoneuritis)


PRINCIPLES OF DIAGNOSIS



History and Physical Examination



• The history allows classification of the disease process as acute or chronic and progressive or nonprogressive.

• Perform a careful physical examination to detect signs of involvement of other systems (e.g., endocrine disorders) that may influence the peripheral nerves.

• Perform a neurologic examination (see Chapter 125) to localize the process to the PNS if reduced muscle mass and tone and reduced spinal reflexes are found on the physical examination.


Laboratory Evaluation



• Initial studies usually consist of a hemogram, blood chemistry profile, and urinalysis to evaluate for metabolic, endocrine, and neoplastic disorders.

• Elevated serum concentrations of muscle enzymes (creatine kinase, aldolase, lactate dehydrogenase [LDH], and aspartate aminotransferase [AST]) suggest a muscle disorder.

• Low cholinesterase levels in whole blood or serum may indicate exposure to organophosphates.

• Blood and tissue samples can be analyzed for heavy metal levels.

• Thoracic radiographs are often indicated in neuropathic disorders to evaluate for evidence of megaesophagus. If a paraneoplastic neuropathy is suspected, thoracic radiographs and abdominal ultrasound are indicated to screen for neoplastic disease.


Electrodiagnostics



• Use needle electromyography (EMG) studies to record electrical activity in skeletal muscle (see Chapter 125).
• Needle EMG studies are used to confirm the presence and distribution of peripheral nerve and muscle disorders and can be performed on a cooperative awake patient or an anesthetized patient.

• Spontaneous abnormal activity, such as fibrillations and positive sharp waves (denervation potentials), are found in many neuropathies.

• Use nerve stimulation studies to determine the location and nature of peripheral nerve abnormalities (see Chapter 125).
• Reduction of the conduction velocity or a change in amplitude, duration, or waveform of the evoked action potential suggests pathologic conditions.

• Nerve conduction studies are recorded using specialized electrodiagnostic equipment (usually by a specialist at a referral center) under general anesthesia. Both motor and sensory nerves can be evaluated.


Muscle and Nerve Biopsy



• Muscle biopsy specimens not only distinguish different muscle disorders but can also differentiate nerve from muscle disorders if histochemical staining is used (see Chapter 130).

• Sensory and motor nerve biopsies involve removing a fascicle of the nerve several centimeters in length, leaving the rest of the nerve intact. Because special handling and processing are required, these biopsies are probably best performed by a specialist at a referral center.


Cerebrospinal Fluid Analysis



• Spinal fluid analysis occasionally is beneficial in diagnosis of disorders of the nerve roots such as acute polyradiculoneuritis and protozoal diseases.


PRINCIPLES OF TREATMENT




Key Point


The key to treatment is to find the cause. Unfortunately, a specific cause of many of the acquired peripheral neuropathies may not be obvious even after extensive diagnostic evaluation.


Treatment often must rely on supportive care. In all peripheral nerve disorders that cause decreased mobility and muscle wasting, the following measures are important:



• Use waterbeds or heavily padded surfaces for bedding, to prevent decubital ulcer formation.

• Flex and extend the joints several times a day to prevent tendon and muscle contraction (see Chapter 95 on physical therapy).

• Maintain proper nutritional intake.

• Ensure frequent and complete urinary bladder evacuation.

• Use corticosteroids only for those neuropathies associated with immune-mediated diseases such as systemic lupus erythematosus.


SPECIFIC PERIPHERAL NERVE DISORDERS


A wide range of disease processes, varying from simple trauma to more complex inherited disorders, can affect the peripheral nerves. For the purpose of this discussion, peripheral nerve disorders are categorized according to their etiology, as anomalous/inherited/congenital, metabolic, nutritional, neoplastic, inflammatory/immunologic, idiopathic, traumatic, and toxic.



Anomalous, Inherited, and Congenital Disorders


Anomalous causes of peripheral nerve disease usually are noticed before 1 year of age. For each of the following anomalies, signalment and clinical signs are keys to a presumptive diagnosis, and thus only additional procedures are described under Diagnosis.




Key Point


Many anomalous causes of neuropathies are breed-specific and are not treatable.



Globoid Cell Leukodystrophy


This storage disease is caused by an inherited (autosomal recessive) deficiency of the enzyme betagalactocerebrosidase that results in damaged oligodendrocytes and Schwann cells in the CNS and PNS, respectively.



• Signalment: West Highland white and Cairn terriers, beagles, poodles, Pomeranians, basset hounds, and cats <1 year of age.

• Clinical signs: Pelvic limb ataxia progressing to tetraparesis, hyporeflexia, hypermetria, and head tremors. Nystagmus, blindness, and anorexia may develop prior to death.

• Diagnosis: Nerve biopsy demonstrating segmental demyelination, axonal degeneration, and endoneural globoid cell accumulation; biochemical evaluation of beta-galactocerebrosidase enzyme activity in leukocytes, brain, and spinal cord.


Glycogen Storage Disease in Norwegian Forest Cats


This metabolic defect is a deficiency of a glycogen branching enzyme and is believed to be inherited as an autosomal recessive trait.



• Signalment: Norwegian Forest cats 5 months of age.

• Clinical signs: Hyperthermia, movement associated whole body tremors, muscle atrophy, and ataxia that progresses to tetraparesis with decreased spinal reflexes, cranial nerve signs, seizures, and death.

• Diagnosis: Transient elevations in creatine kinase; spontaneous needle EMG activity; decreased amplitude of evoked action potential and normal to slightly reduced motor nerve conduction velocities on nerve stimulation; PAS-positive material in central and peripheral nervous system (as well as multiple organs) that results in neuronal loss and axonal degeneration.


Other Storage Diseases Affecting Peripheral Nerves



• Niemann-Pick disease usually presents with cerebral or cerebellar signs in 2- to 5-month-old Siamese cats, but some cats have signs of tetraparesis, reduced spinal reflexes, and hypotonia. Spontaneous needle EMG activity and reduced motor and sensory nerve conduction velocities are present. Peripheral nerves show demyelination and remyelination with vacuolated macrophages surrounding affected nerve fibers.

• GM2 gangliosidosis in dogs and cats usually presents as cerebellar signs, but isolated cases of peripheral nerve signs have been described. Peripheral nerve histopathology shows Wallerian degeneration of peripheral axons and accumulation of lamellar inclusions in Schwann cells.

• Alpha-L-fucosidosis is an autosomal recessive storage disease in English springer spaniels that produces behavioral changes and ataxia beginning at 12 to 14 months of age. Signs progress until dogs become incapacitated at around 4 years of age. Spinal reflexes become depressed and peripheral nerves may be palpably thickened. Nerve enlargement is caused by infiltration of foamy macrophages and loose fibroedematous endoneurial tissue.

• Hyperchylomicronemia is an inherited disorder of lipoprotein metabolism resulting in fasting hypertriglyceridemia and hyperchylomicronemia and subsequent deposition of excessive lipids around peripheral nerves. Clinical signs are due to compression of nerves from the lipid accumulations and are reversible if plasma triglyceride concentrations can be controlled with diet and drugs. The radial and tibial nerves appear to be most commonly affected.

See Chapter 126 for a discussion of other lysosomal storage diseases affecting the nervous system.



Hyperoxaluria Type 2 in Domestic Shorthaired Cats


This metabolic defect is believed to be inherited as an autosomal recessive trait. While the predominant feature is acute renal failure due to deposition of calcium oxalate in the kidneys, some cats have lower motor neuron weakness due to Wallerian degeneration of peripheral nerves.



• Signalment: Domestic shorthaired cats <1 year of age.

• Clinical signs: Crouching, cow-hocked stance with weakness, hyporeflexia, and diminished pain perception.

• Diagnosis: Chemistry changes compatible with acute renal failure; increased concentrations of oxalate and L-glycerate in urine; axonal swellings with neurofilament accumulations in proximal axons in spinal ventral horn cells, ventral roots, and intramuscular nerves.


Giant Axonal Neuropathy in German Shepherds (Alsatians)


This is an inherited (probably autosomal recessive) neuropathy characterized primarily by distal axonal swellings filled with tightly packed, whorled neurofilaments in the PNS and CNS. Based on the spatial-temporal spread of lesions, it is considered a central peripheral distal dying back neuropathy.



• Signalment: German shepherds 1 to 2 years of age.

• Clinical signs: Progressive symmetrical paraparesis followed by loss of patellar reflexes, distal muscle atrophy, hypalgesia in the pelvic limbs, fecal incontinence, weak bark, vomiting, regurgitation from megaesophagus, and aspiration pneumonia. Some dogs have a curly coat.

• Diagnosis: Spontaneous needle EMG activity in distal muscle groups; decreased amplitude of evoked action potential and reduced motor and sensory nerve conduction velocities on nerve stimulation; giant axonal swellings found on nerve histopathology.


Hypertrophic Neuropathy in Tibetan Mastiffs


This inherited (autosomal recessive trait) chronic demyelinating disease of the PNS is most likely due to an inability of the Schwann cells to form and maintain a myelin sheath during rapid growth periods. Remyelination can occur and some dogs may regain some strength within 4 to 6 weeks but do not usually have normal strength.



• Signalment: Tibetan mastiffs 7 to 12 weeks of age.

• Clinical signs: Pelvic limb weakness that rapidly progresses to generalized weakness, hyporeflexia, hypotonia, recumbency, and dysphonia. Some dogs may regain the ability to stand but remain weak.

• Diagnosis: Occasional spontaneous needle EMG activity that may diminish as the affected dog ages; slowed motor and sensory nerve conduction velocities; Schwann cell proliferation and relatively little axonal degeneration on nerve histopathology.

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Aug 27, 2016 | Posted by in SMALL ANIMAL | Comments Off on Peripheral Nerve Disorders

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