Developmental disorders

Developmental disorders of the nervous system of the horse are usually manifested by disturbances of function evident at, or shortly after, birth. Most of these are the result of genetic defects or insults which have affected the foal during gestation. These include a variety of virus or toxemic disorders affecting the mare. The clinical manifestations depend largely upon the stage of development at which the insult is applied and the extent of the consequent deficit. Thus, virus infections affecting a pregnant mare at the time of neurological development (early in gestation) might produce severe effects on the neurological development of the foal. In many cases the effects are life-threatening to the fetus and intrauterine death is common.

In some cases the clinical effects of minor neurological problems in foals may be dramatic. In others, even apparently gross abnormalities might have surprisingly little effect.

Hydrocephalus (Figs. 11.1–11.3)

Hydrocephalus is usually an easily recognized, isolated, developmental abnormality which is not consistent with life long term. Affected foals are usually born dead or die during parturition, as a result of massive increases in intracranial pressure. They are often premature, with an obvious cranial deformity. However, cranial deformity is not invariably present in hydrocephalic foals and in such cases a diagnosis may be difficult without diagnostic imaging. Conversely, some foals born with apparently domed skulls may not be suffering from hydrocephalus. A sagittal section of the head of an affected foal shows the gross distension of the lateral ventricles with generalized compression of the cerebral hemispheres.

Hydrocephalus can also be acquired following conditions such as meningitis or following hemorrhage. Hydrocephalus can be further classified as normotensive or hypertensive. Normotensive hydrocephalus usually is incidental to hypoplasia or loss of brain parenchyma after destructive prenatal or postnatal infection or injury. The CSF volume passively expands to fill the space that is normally occupied by the brain tissue. Hypertensive hydrocephalus is a result of obstruction of the CSF conduit between the sites of production, in the third and lateral ventricles, and the sites of absorption by the arachnoid villi in the subarachnoid space. Blockage may be due to hypoplasia or aplasia of a part of this system or may be acquired. The increased CSF pressure results in dilation of the third and lateral ventricles with resulting tissue damage. The clinical signs seen include abnormal behavior patterns such as lack of recognition of dam, compulsive walking, bizarre postures and apparent blindness.

Diagnosis and treatment

• Diagnostic imaging techniques such as computed tomography or magnetic resonance imaging allow for ante-mortem diagnosis.

• Post-mortem examination confirms the diagnosis. In either case the etiology usually cannot be determined.

• There is no treatment at present.

Myelodysplasias (Figs. 11.4 & 11.5)

Failure of the cranium and overlying skin to close during development, with prolapse of either the meninges (meningocele) or the meninges with parts of the brain (encephalocele) is occasionally seen in neonatal foals. The condition which is almost exclusively in the midline may be very small, or involve almost the entire cranium, and may be accompanied by other developmental disturbances including spina bifida. Affected foals are usually either aborted or are born dead or die soon after birth.

Myelodysplasias may be clinically evident at birth or manifested soon after as stable neurological abnormalities such as paraparesis. A bunny-hopping gait and bilaterally active reflexes in the limbs at the level of the defect are prominent features.

Progressive neurological defects resulting from spinal cord compression are associated with severe vertebral anomalies.

Diagnosis and treatment

• Diagnosis by visual and clinical examination with no differentials.

• Affected foals that are born alive should be euthanized as the condition is incompatible with life.

Cerebellar hypoplasia (Fig. 11.6)

Cerebellar syndromes and degenerative lesions are seen in Thoroughbred and Paso Fino foals. Unlike ruminants no toxin or virus has been linked with abnormalities of cerebellar development in foals and congenital malformation or hypoplasia of the cerebellum appears to be rare. Signs including dysmetria and ataxia are evident as soon as the foal attempts to stand and walk.

Diagnosis and treatment

• Ataxia at birth makes this distinguishable from other conditions such as cerebellar abiotrophy that develop later. Definitive diagnosis is only possible at post-mortem. The cerebellum is grossly and histopathologically abnormal.

• No treatment is possible but less severely affected animals may survive with a satisfactory quality of life.

• As it has not been shown to be an inherited condition continuing to breed from the parents is unlikely to result in the birth of another similarly affected foal.

Occipitoatlantoaxial malformation (OAAM) (Fig. 11.7)

OAAM is reported in Arabians, Morgans, Standardbreds and rarely in other breeds and includes fusion of the atlanto-occipital joint, atlantalization of the axis and hypoplasia of atlantal wings. Foals can be born dead, be ataxic at birth or show progressive ataxia as yearling animals. Occasionally restricted neck movements are the only clinical signs.

In spite of severe (physical and radiographic) malformations some cases show little or no neurological deficit until a gross displacement occurs, when a severe compromise of the neurological function will be evident immediately.

Animals often have extended neck posture with reduced flexion of the atlanto-occipital junction. A malformation of C1 and C2 may be palpated. Usually there are varying degrees of cervical spinal cord signs (tetraparesis and ataxia in all four limbs).

Diagnosis and treatment

• The disease can be confirmed by plain radiography. Malformations of the occiput, C1 and C2 are present including atlanto-occipital fusion and hypoplasia of the dens. Healed atlanto-axial fractures can mimic OAAM.

• No treatment is indicated in most cases and since the disease is most likely inherited in the Arabian, the owners should be counseled not to breed close family lines.

• Laminectomy has been used to alleviate spinal cord compression and clinical signs.

Juvenile epilepsy (Fig. 11.8)

Juvenile epilepsy occurs mainly in Egyptian Arabs or Arabian crosses. The disorder results in seizures in foals that are otherwise normal from several days to several months of age and unlike true epilepsy is usually outgrown.

Clinical signs

• Single or multiple epileptiform seizures. Frequently, these seizures are not seen and all that is noticed is evidence of self-trauma. If seen these seizures usually follow a consistent pattern which is individual for each foal.

• Prodromal signs and postictic signs are also usually consistent for a given animal. Amaurosis (central blindness) is common before and after seizures and is frequently noted by owners.

Treatment

• Although this disorder is outgrown, anticonvulsant therapy (phenobarbital) should be initiated at the first signs and maintained for several weeks to months.

It is administered at a dose rate of 12 mg/kg PO/IV loading dose and then 6 mg/kg PO/IV q 12 h. Note: Phenobarbital induces microsomal enzymes, which may alter the metabolism of the drug with repeated administration. Serum phenobarbitol concentrations should be monitored and maintained between 10–30 µg/ml.

• Failure to treat these animals may result in neuronal death and further possibly permanent seizure foci.

• Underlying problems such as fever that may initiate further seizures should be corrected.

Lavender foal syndrome/coat color dilution lethal (Fig. 11.9)

This is a rare, genetic disease of Arabian foals inherited as an autosomal recessive. It is most commonly encountered in Egyptian Arabians but has also been seen in some Crabbet lines. The condition gets its name as the most striking feature of some of these foals is the dilute or ‘bleached-out’ hair coat color. In a few cases, the color is a very striking iridescent silver to pale lavender hue, hence the name ‘lavender foal syndrome’. Coat color dilution lethal is a more appropriate name, as many affected foals do not exhibit the striking lavender color. Other dilute coat colors observed are pewter (pale slate gray) and pale chestnut (pink). Foals with LFS are unable to stand, and sometimes cannot even attain sternal recumbency (to roll from their side to lie upright, resting on the sternum, a precursor position to standing). They may lay with their necks arched back, make paddling motions with their legs, and often have seizures.

Diagnosis and treatment

• The condition should be suspected in any Arabian foal showing coat color dilution with appropriate clinical signs. It may be somewhat more difficult in foals without obvious coat color dilution.

• There is genetic testing available in the US and Australia at present.

• There is no effective treatment and affected foals are usually euthanized within a few days of birth.

Deafness (Fig. 11.10)

This is poorly documented in the horse but is thought to occur in some overo-colored horses with blue eyes.

Diagnosis and treatment

• The brainstem auditory-evoked response (BAER) test measures responses in brain waves that are stimulated by clicks in the ear to check the auditory pathways of the brainstem.

• Normal values have not been established for foals, however marked delays can be predicted from normal adult horse values.

• There is no treatment available.

Hypoxic ischemic encephalopathy (neonatal maladjustment syndrome) (Fig. 11.11)

Hypoxic ischemic encephalopathy (HIE) is a specific syndrome characterized by alterations in behavior, neurological signs and ischemic and/or hemorrhagic CNS lesions associated with perinatal asphyxia. Foals are often normal for the first few minutes to hours after birth and then may be unable to locate the udder or suck properly and lose affinity for the dam.

Clinical presentation is dependent on the degree of hypoxia. Signs are variable and may include somnolence, lethargy and hypotonia. There may also be a loss of suckle reflex, dysphagia, odontoprisis, central blindness, mydriasis, anisocoria, nystagmus and head tilting. Foals that have been exposed to more moderate hypoxia/ischemia are more likely to experience seizures characterized by eye blinking, eye deviation, nystagmus, paddling movements and a variety of oral-buccal-lingual movements.

Diagnosis

• HIE is principally a diagnosis of exclusion. Foals typically present within 24–36 hours of birth.

• The condition should be expected in any foal which has a history of abnormal placentation of the mare, premature placental separation, delivery by cesarian section, prolonged dystocia, premature delivery or in foals that have required resuscitation for any reason after delivery.

Treatment and prognosis

Treatment of HIE has been described extensively and the success rates have improved significantly, however many commonly used treatments are still quite controversial. The goals of treatment are listed below with many of the recommended treatments.

• Treatment of the mare. Prevention of intrauterine asphyxia by early treatment of placentitis in mares may help to decrease intrauterine asphyxia (see Chapter 12, p. 471).

• Treatment of the foal:

• Maintaining adequate ventilation

• Maintaining adequate perfusion

• Maintaining adequate glucose levels

• Controlling seizures

• Control of brain swelling (see Treatment of CNS trauma in p. 410)

• Other treatments:

1. Free radical scavengers: Allopurinol (xanthine oxidase inhibitor)

2. NMDA (N-methyl-D-asparate) receptor blockers such as magnesium

3. Ascorbic acid (vitamin C)

4. Thiamine: 1 gram IV in 1L of fluids SID. Thiamine increases the activity of the ATP-dependent sodium pump, thus regulating ion uptake and decreasing cellular water

5. Hypothermia has been shown to be neuroprotective. However the exact level of hypothermia has not been established for equine patients but currently it is advisable to stay below rather than above normal temperature. Thus avoid overheating by the judicious use of heating lamps, blankets, etc.

6. Good nursing care is vital.

• Survival rates have been reported as high as 90% but the prognosis is poorer for premature foals, foals with concurrent sepsis and foals showing clinical signs immediately after birth.

Cervical vertebral malformation (CVM) is a common cause of ataxia in horses and tends to affect young adults. Compression of the cervical spinal cord results in lesions to proprioceptive and motor tracts to the thoracic and pelvic limbs. This results in ataxia (resulting in inconsistent foot placement and excessive circumduction of the pelvic limbs when turning) and paresis (shown by weakness when pulling on the tail while the horse is walking in a straight line). Table 11.1 outlines a system for grading neurological deficits that is useful in the evaluation of horses with suspected CVM as it allows for accurate recording and monitoring of progression. There are two different types of CVM/CVSM that are recognized and are outlined in Box 11.1.

Due to the peripheral location of pelvic limb tracts in the spinal cord they are affected more severely, resulting in paresis and ataxia, which is more notable in the pelvic limbs. In mild cases the thoracic limbs will not appear to be affected even though the lesion is in the cervical cord. It can sometimes have a history of acute onset ataxia.

Traumatic injuries

Traumatic injuries to the nervous system of the horse are a frequent occurrence and affect the central nervous system as well as the peripheral nerves.

Cerebral trauma (Figs. 11.16–11.20)

Cerebral trauma does not always have to be severe enough to cause outward evidence of damage, although this is common. Fractures of the forehead are often compound (open) and involve direct cerebral laceration and hemorrhage. These injuries are most commonly the result of kicks from other horses or from impact with solid objects. In some cases rearing or pulling back from a firmly fastened head collar or halter may be responsible for significant trauma, particularly in young horses.

Closed head injuries (those in which there are no skull fractures) often result in brainstem damage with localized bleeding and, often quite extensive subarachnoid bleeding. The signs that are seen are the result of mechanical injury to the brain, cerebral edema, parenchymal hemorrage and ischemia produced by brain swelling and intravascular clotting. The neurological signs are very variable depending on the site of the trauma and its relationship to underlying structures, and the extent of any intracranial bleeding and/or physical disruption.

There may be an initial period of unconsciousness, which can be of variable length, followed by fluctuating neurological signs which are dependent on the degree of intracranial hypertension associated with cerebral edema and hemorrhage.

Wandering toward the side of the lesion and depression are commonly seen but ataxia is not usually seen unless there is progressive involvement of other parts of the brain.

Pupillary light responses are usually brisk in the early stages but may become more delayed as cerebral edema worsens. There may be some asymmetry of the pupils and miosis and characteristically there is central blindness and depressed menace responses.

The development of a midbrain syndrome (dilated unresponsive pupils and tetraparesis), associated with swelling of the cerebral hemispheres and herniation caudally against the midbrain, warrants a poor prognosis, whereas an uncomplicated cerebral syndrome usually has a good prognosis as response to treatment for brain swelling can be good.

Basioccipital and basisphenoid fractures (Figs. 11.21 & 11.22)

These fractures most commonly occur after rearing accidents or head collar injuries. Typically, the neurological signs associated with this type of trauma are very variable. Individual cases may be comatose but others may show subtle neurological deficits, which may be limited to the individual cranial nerves in the region of the fracture; in which case the signs may be unilateral.

Petrous temporal bone fractures (Figs. 11.23–11.30)

Bleeding from the ear following head trauma is indicative of damage to the petrous temporal bone and the vestibular nerve and/or its nucleus. Affected horses have a marked head tilt towards the affected side, and are obviously disoriented with circling and nystagmus in which the fast phase is away from the affected side. Usually there is complicating concurrent damage in the base of the brain and weakness, depression and ataxia, amongst other signs.

Fractures involving the base of the cranium

These will frequently involve one or more of the cranial nerves, and the clinical signs of this damage may be difficult to identify in conjunction with the more severe effects of cerebral, cerebellar or brainstem disruption. Commonly, however, the signs include depression and/or dementia.

Fractures which affect the optic or vestibulo-cochlear nerves

These fractures or their central nuclei will have dramatic and easily recognized signs (see Fig. 11.20). In the former, blindness and pupil-light-reflex deficiencies or discrepancies between the two eyes will be present. Concurrent retinal damage (including detachment) and/or lens dislocation may be found.

Diagnosis

• In many cases the diagnosis of trauma is an obvious diagnosis but diagnostic imaging techniques can be useful to help identify the exact site and thus allow for a more informed assessment of involved structures. Advanced imaging techniques such as CT and MRI can provide additional information such as enlargement of cerebral hemispheres.

• Basioccipital and basisphenoid fractures may be difficult to appreciate radiographically, but endoscopic examination of the roof of the medial compartment of the guttural pouches will often identify either bruising or even the presence of a hematoma over the site of the fracture.

• Fractures of the hyoid bone may possibly be detected radiographically and by endoscopic examination of the guttural pouch(es).

• Ultrasound examination of the atlanto-occipital space can be used in some cases to determine the presence of intracranial hemorrhage.

• Cisternal puncture and aspiration of blood-stained cerebrospinal fluid confirms intracranial hemorrhage but in many cases may be contraindicated due to increased risk of mid brain herniation.

Treatment

• The general principles for treatment of CNS trauma are administration of osmotic diuretics, nutritional and fluid support and protection from self-inflicted trauma and the effects of prolonged recumbency.

• Seizures or excessive, difficult-to-manage thrashing may require sedation or short-term anesthesia. Diazepam 5 mg (foal) up to 100 mg (adult) can be repeated as necessary to control seizures. Phenobarbital or pentobarbital may also be used to control seizures. The alpha 2 agonists should be avoided for the treatment of seizures in the acute stages as they can cause transient hypertension, exacerbating CNS hemorrhage and they also suppress ventilation.

• Horses with CNS signs following cranial trauma should probably receive dexamethasone (0.1–0.25 mg/kg q 4–6 h) for 1–4 days. However, the benefits should be weighed against the possible complications of steroid administration in the horse (laminitis).

• Intravenous administration of 20% mannitol (0.25–1 g/kg) has been used for the treatment of increased intracranial pressure. Its use however is contraindicated in cases of ongoing cerebral hemorrhage. Response to mannitol is usually noted within 1 hour and if a response is noted, mannitol administration should be repeated every 4–6 hours for the first day.

• DMSO, is frequently given slowly at 1 g/kg as a 10% solution in isotonic fluids. DMSO has several beneficial pharmacological effects including diuresis, free-radical scavenging, inhibition of platelet aggregation, vasodilation and increased penetrance of steroids and antibiotics into the brain. There are many anecdotal reports of successful treatment with DMSO but clinical trials in other species have not shown a clear benefit in treating CNS trauma. Adverse effects include intravascular hemolysis which has been associated with too rapid administration or administration of a more concentrated solution. DMSO administration can be repeated every 12 hours for 3–4 days if clinical improvement is seen.

• Hypoxemia and hypercapnia should be avoided as hypoxia exacerbates brain swelling and hypercapnia increases intracranial blood volume and pressure. Recumbent horses should be rolled every 4–6 hours to minimize pulmonary arteriovenous shunting and ventilation perfusion mismatching. Ideally the head should also be maintained at heartbase level or higher to avoid hypostatic intracranial congestion. Maintenance of hydration is important but over-hydration should be avoided as it can exacerbate brain edema.

• Close monitoring and good nursing care are essential. If an improvement is noted in 6–8 hours, treatment should be repeated. If no improvement is seen more aggressive treatment may be warranted, including exploratory craniotomy.

Prognosis

• Complicated combinations of neurological deficits can arise from even relatively minor trauma and may take some hours or days to produce their full neurological effect. Many consequences of cranial trauma such as hemorrhage, laceration necrosis, secondary ischemia and midbrain injury are inaccessible to therapy and the presence of some of these lesions is difficult to diagnose with failure to improve or deterioration in neurological condition being the only clue to their existence.

• The most accurate prognosis is based on repeated detailed neurological examinations with assessment of the rate of progression or resolution and the responses to specific therapeutic measures.

• Cerebral and cerebellar lesions usually carry a better prognosis than brainstem lesions and therefore those injuries which are accompanied by bleeding from the ear but few other outward signs are possibly more serious than the more dramatic injuries to the frontal and facial bones.

• If no improvement is seen or deterioration is noted in a comatose patient 36 hours after surgery or anesthesia, euthanasia may be indicated.

Cord trauma

Suspected spinal cord trauma is one of the most common neurological disorders presented to equine practitioners. Spinal cord trauma typically occurs following a traumatic incident such as a fall and may or may not be associated with vertebral trauma. Fractures rarely occur secondary to other pathology such as neoplasia. The cervical vertebrae are a common site for vertebral fractures, especially the occipitoatlantoaxial region in foals. The lower cervical and cranial thoracic sites are the most common areas for vertebral fractures in the adult horse. Fractures of the thoracic dorsal spinous process are not usually associated with neurological signs, whereas fractures of the vertebral body, arch or articular processes are usually associated with neurological signs.

Clinical signs

The signs seen depend heavily upon the location of the injury and the extent of damage to the cord or to the meninges and/or to the peripheral nerves as they pass through the intervertebral foramina.

Gross displacement of adjacent vertebrae which results in severance of the cord is accompanied by immediate loss of all conscious proprioception and voluntary motor function distal to the site of the lesion. In some cases, however, the extent of the apparent bony damage may not necessarily correspond to the extent of the neurological deficits.

Many cases of vertebral trauma with or without neurological signs have been reported.

Cervical spine (Figs. 11.31–11.34)

A recumbent horse with a lesion at C1–C3 has difficulty raising its head off the ground; whereas, a recumbent horse with a lesion at C4–T2 should be able to lift its head and cranial neck. C1–T2 lesions may result in tetraplegia, but may also present as tetraparesis and ataxia. Spinal cord lesions above the C6 segment will result in normal to increased muscular tone and spinal reflexes (panniculus, triceps and biceps) in all limbs. If the lesion is located at the sixth to eighth cervical spinal cord segments, the forelimb reflexes are diminished or absent and those of the hindlimbs are normal or increased.

Thoracic spine (Figs. 11.35–11.38)

Lesions of T3–T6 may cause paraplegia or paraparesis and ataxia. A paraplegic horse that ‘dog-sits’ usually has a lesion caudal to T2. Most animals with thoracic spinal cord injury have normal to exaggerated spinal reflexes and hypertonia of the rear limbs. Some degree of asymmetry may be present with spinal cord trauma, but signs are almost always bilateral. The level of hypalgesia on the neck or back indicates the cranial extent of the lesion. In the early post-trauma phase, a region of hyperesthesia may be detected just cranial to the lesion. Strip patches of sweating may occur when thoracolumbar spinal nerve roots are damaged. Whole-body sweating, seen frequently in horses with a broken neck or back, may be due to involvement of pain pathways and sympathetic spinal cord pathways.

Lumbar spine (Figs. 11.39–11.41)

Lesions at L1–L3 spinal cord segments result in normal or hypertonic and hyper-reflexic hindlimbs. Lesions at L4–S2 may result in hypotonia and hyporeflexia of the hindlimbs. The bladder is distended, but sphincter tone is normal. Tail and anal tone are normal.

Thoraco-lumbar lesions which are associated with posterior paralysis and forelimb extensor rigidity (Schiff-Sherrington reaction) and which gradually ascend are most serious.

Sacrococcygeal spine

Lesions of spinal cord segments S1–S2 result in decreased conscious proprioceptive responses of the hindlimbs and diminished flexor reflexes of those limbs. Anal tone is diminished to absent and the bladder is distended and hypotonic. Atony of the urethral sphincter results in incontinence and urine scalding. The tail is flaccid and paralyzed.

Clinical pathology and radiographic findings

• Plain radiography is the most helpful aid in confirming vertebral trauma, but does not directly evaluate the presence or extent of spinal cord damage. Abnormalities that are seen that indicate injury are displacements of vertebral components, shortened or abnormally shaped vertebrae, slipped physeal plates and fractures.

• Fracture-induced changes in the CSF may be useful for ancillary diagnosis. These changes can be classified as acute (<24 hours) or chronic (>24 hours). The acute changes include diffuse blood contamination, a high red blood cell (RBC) count, a normal to high white blood cell (WBC) count, and a high protein concentration. Chronic CSF changes include a normal to slightly increased WBC count, normal to increased RBC count, increased protein concentration and xanthochromia.

Treatment

• Pain should be managed with non-steroidal anti-inflammatories and other medication recommendations are similar to those for cerebral injury. Good nursing care is essential, especially for recumbent patients and should include bladder and rectal evacuation if necessary.

• If the spinal fracture appears stable and the animal can stand with assistance, it may be placed in a water tank and supported for long periods. Other methods of support include slings, but these should not be used for animals that cannot support themselves as severe respiratory compromise or myositis may result. Slinging of animals with mild neurological signs may help minimize secondary complications, improve extensor tone and hasten recovery.

• Horses which suffer from spinal pain and/or posterior paralysis often show extremes of panic and their management is most difficult and frequently dangerous for the handlers involved. Priority should be given to human safety at all times.

Prognosis

• The prognosis is best judged on the basis of repeated neurological examinations. The longer a patient remains recumbent and neurologically impaired the poorer the prognosis. In general the prognosis for horses suffering from spinal trauma is poor. Some may recover with time but the intensity of nursing required and the consequences of incoordinated attempts to rise are often most distressing for the horse, the owner and the attendants.

• Healing of fractures frequently results in vertebral malalignment. Delayed callus formation and degenerative changes in adjacent articulations can result in permanent spinal cord compression even after apparent healing and resolution of clinical signs.

Other cranial nerve disorders

The trigeminal nerve (CN-V) (Fig. 11.42)

This is the largest cranial nerve and has both motor and sensory functions. Motor functions include aspects of prehension, mastication and swallowing, while the sensory components control mouth and eye sensation and sensation in the skin of the head.