AFIP SYSTEMIC PATHOLOGY

JPC SYSTEMIC PATHOLOGY

NERVOUS SYSTEM

February 2014

N-M02

 

Signalment (JPC #2020918):  Two-year-old thoroughbred gelding

 

HISTORY:  This horse’s clinical presentation varied from nearly normal to severe incoordination. Necropsy revealed no gross lesions in the spinal cord or brain. When the neck was flexed, the anterior end of the vertebral body of C5 protruded into the spinal canal, resulting in an approximately 50% reduction in the canal diameter. Less pronounced protrusions were noted at C3-C4 and C5-C6. Specimens submitted were from C6-C7.

 

HISTOPATHOLOGIC DESCRIPTION:  Spinal cord, cervical:  Multifocally, affecting the white matter of the lateral and ventral funiculi, most prominently around the ventromedian fissure, approximately 25% of axons exhibit evidence of axonal (Wallerian) degeneration, characterized by dilated myelin sheaths (up to 5 times normal), containing glassy, eosinophilic, swollen axons (spheroids), or necrotic debris admixed with single to multiple gitter cells with moderate amounts of foamy eosinophilic cytoplasm(ellipsoids).

 

MORPHOLOGIC DIAGNOSIS:  Spinal cord, cervical, white matter:  Axonal degeneration, multifocal, moderate, with dilated myelin sheaths and gitter cells (Wallerian degeneration), thoroughbred, equine.

 

ETIOLOGIC DIAGNOSIS:  Spinal cord compression

 

CAUSE:  Cervical vertebral stenosis

 

CONDITION:  Cervical vertebral stenotic myelopathy

 

CONDITION SYNONYMS:  Cervical compressive myelopathy, wobbler syndrome, equine incoordination, equine sensory ataxia, cervical vertebral malformation-malarticulation

 

GENERAL DISCUSSION: 

·       Cervical vertebral stenotic myelopathy is a common condition in the horse and dog in which cervical vertebral malformations result in progressive spinal cord injury, ataxia and paresis

·       Divided into two syndromes

·       Cervical vertebral instability (dynamic compression) malarticulation is the most common syndrome; it causes damage to the spinal cord only when the neck is extended or flexed; it may be missed on necropsy because it may only be evident when the neck is ventroflexed; occurs primarily in horses 8 to 18 months of age and produces lesions at C3-C4 and C4-C5

·       Cervical static stenosis - malformation (stenosis) of vertebral canal is less common; it is caused primarily by thickening of the ligamentum flavum and the dorsal lamina of the vertebral arches; the position of the neck does not matter; it occurs primarily in horses 1 to 4 years of age and produces lesions at C5-C6 and C6-C7

·       Both static and dynamic compression are generally associated with narrowing of the vertebral canal from C3-C6, regardless of site of vertebral compression

·       Nervous signs are generally symmetric (although gait asymmetry has been reported) and affect the pelvic limbs more than the thoracic limbs

·       Affects young rapidly growing animals, especially thoroughbred, warm bloods, Tennessee walking horses and quarter horses, with a higher incidence in males compared to females

·       The term “wobbler” is a generic term for ataxia and paresis of any cause and its use is discouraged by some

 

PATHOGENESIS:

·       The cause is unknown; likely multifactorial

·       Heredity and nutritional factors are postulated

·       Likely that biomechanical changes are secondary to primary developmental issues

·       Affected horses are rapidly growing and on a high nutritional plane

·       Ad libitum feeding of high-protein, high-energy rations to weanlings to produce maximum-sized yearlings seems to be the major influence

·       Osteochondrosis is diagnosed in vertebral growth plates of many affected animals and may be a factor; degenerative lesions associated with osteochondrosis are often seen on the articular facets, which may encroach on the spinal canal or result in vertebral malalignment due to asymmetric articular facets

·       Dietary copper (low), zinc (high), and carbohydrates (high) are thought to play a role

 

TYPICAL CLINICAL FINDINGS:

·       Rapidly growing males 6 months – 3 years of age; especially thoroughbreds, warm bloods and Tennessee walking horses

·       However age (>3 years) does not preclude diagnosis

·       Onset is usually gradual, although it may be acute after a period of hard exercise

·       Symmetric ataxia and paresis in the pelvic limbs; forelimbs less affected

·       Cervical hyperesthesia frequently reported

·       Neurologic signs may improve with rest, remain static, or worsen

·       Forced flexion of the horse’s neck may exacerbate clinical signs

 

TYPICAL GROSS FINDINGS: 

·       Vertebral lesions include malformation with stenosis of vertebral foramen, malarticulation, degenerative osteoarthropathy of intervertebral articular processes, enlargement of the dorsal vertebral lamina and ligamentum flavum, and formation of synovial cysts

·       The lesions are limited to the spinal canal and spinal cord

·       Usually no gross lesions in the cord; rare foci of hemorrhage and/or malacia

·       Dynamic lesions occur at C3-C4 and C4-C5; static lesions occur at C5-C6 and C6-C7

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

·       Cranial to the traumatic site, ascending Wallerian degeneration primarily in ascending fibers most of which are in the dorsolateral funiculi

·       In the areas of stenosis, there is usually extensive axonal degeneration and secondary demyelination with accompanying mild to moderate microglial proliferation

·       Caudal to the site of primary insult, Wallerian degeneration of the descending nerve tracts is present in the ventral and ventrolateral funiculi

·       Severe cases may have hemorrhage and necrotic foci unilaterally or bilaterally in the gray matter in either the ventral or lateral horns;  retrograde chromatolysis of neurons may be seen

 

DIFFERENTIAL DIAGNOSIS:

·       For histologic findings:

·       Occipitoatlantoaxial malformation involves inheritable malformation of the occipital bone, atlas, and axis and is most frequently seen in Arabian horses

·       Equine degenerative myeloencephalopathy: Idiopathic disease of light breeds; degeneration and demyelination of white matter tracts of the spinal cord and degenerative lesions of spinal and brain stem nuclei

·       Intervertebral disk herniation:  Herniated disk present

·       Other causes of hind limb ataxia and paresis:

·       Equine herpesvirus-1 myeloencephalitis:  Vasculitis with necrosis and thrombosis of spinal cord and brain

·       Neuritis of the cauda equina: Severe granulomatous epidural inflammation of the cauda equina; the cause is unknown

·       Rabies, caused by a Lyssavirus in the family Rhabdoviridae: Perivascular mononuclear perivascular cuffs and Negri bodies

·       Protozoal myelitis (Sarcocystis neurona): Necrotizing nonsuppurative myelitis or encephalitis with extensive mononuclear perivascular cuffs

·       Intervertebral disk herniation:  Herniated disk present

 

COMPARATIVE PATHOLOGY:

·       Cervical vertebral malformation and malarticulation occurs commonly in larger breeds of dogs (especially Great Danes and Doberman pinschers); heredity and high nutritional and growth rates have been postulated as predisposing factors; male dogs are over represented; dogs are 1 to 11 years of age with Great Danes showing signs at an earlier age than Doberman pinschers; lesions are usually in the region of C-5 to C-6 and C-6 to C-7; an exception is the basset hound in which C3 is affected

·       The disease is uncommon in other domestic animals; lesions have been reported in calves, sheep, goats, and mink

 

References:

1.      Craig LE, Dittmer KE, Thompson KG.  Bones and joints. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. Philadelphia, PA: Elsevier; 2016:136.

2.      Janes JG, Garrett KS, McQuerry KJ, et al. Cervical vertebral lesions in equine stenotic myelopathy. Vet Pathol. 2015; 52(5):919-927.

3.      Levine JM, Scrivani PV, Divers TJ, et al. Multicenter case-control study of signalment, diagnostic features, and outcome associated with cervical vertebral malformation-malarticulation in horses. J Am Vet Med Assoc. 2010; 237 (7); 812-822.

4.      Olson EJ, Carlson CS. Bones, joints, tendons, and ligaments. In: McGavin MD, Zachary JF, eds. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier Mosby; 2016:979-980.

5.      Smith BP. Large Animal Internal Medicine. 5th ed. St. Louis, MO: Mosby Press; 2015:985-988.

6.      Summers BA, Cummings JC, de Lahunta A. Veterinary Neuropathology. St. Louis, MO:   Mosby; 1995:193-202.

 


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