Ataxic Young Horse with a Neck Lesion and a Heart Murmur
A 2-year-old Thoroughbred colt is presented for a 3-week history of progressive hindlimb ataxia and a 'wobbly' gait, worse when the head is raised. He is bright and eating. Physical exam: temperature 38.0 C, heart rate 60/min, respiratory rate 16/min. There is a grade 3/6 holodiastolic decrescendo murmur at the aortic valve area, and a grade 2/6 systolic murmur at the tricuspid area. Neurologic exam: grade 3/5 ataxia and paresis in all four limbs, worse in the pelvic limbs; proprioceptive deficits; no cranial nerve deficits; no muscle atrophy. Cervical radiographs show mild narrowing of the caudal cervical vertebral canal at C5-C6 and C6-C7 with slight facet remodeling. CBC/chemistry are normal; SAA is normal.
1. How do you reconcile the cardiac and neurologic findings, and what is your prioritized differential list?
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Neurologic: The young horse with progressive symmetric ataxia/paresis worse in pelvic limbs, worsened by neck flexion (head raise), with cervical vertebral narrowing is classic for cervical vertebral stenotic myelopathy (CVSM, 'wobbler'). The mild C5-C7 narrowing supports this, though radiographic narrowing can be incidental - myelography or CT is needed to confirm dynamic compression. Differentials include equine protozoal myeloencephalitis (EPM) (often asymmetric, may have cranial nerve signs), cervical vertebral fracture/osteomyelitis (unlikely with normal SAA), equine herpesvirus-1 myeloencephalopathy (usually acute, febrile, herd outbreak), and degenerative myelopathy (rare in young horses).
Cardiac: The holodiastolic decrescendo murmur at the aortic valve is concerning for aortic regurgitation, which in a young horse raises concern for aortic root dilation, aortocardiac fistula, or endocarditis. However, a grade 3/6 holodiastolic murmur can also be a functional/physiologic flow murmur in young athletic horses. The tricuspid systolic murmur may be physiologic. Echocardiography is essential to assess aortic valve integrity, aortic root size, and ventricular dimensions. If true AR with volume overload, exercise intolerance and sudden death risk increase.
Reconciliation: The two problems may be unrelated (CVSM plus a functional murmur) or linked via a systemic process (e.g., verminous arteritis causing both aortic root damage and spinal cord ischemia - rare). Prioritize echocardiogram and myelographic/CT evaluation of the cervical spine.
2. What is your diagnostic and management plan, and how do the cardiac findings influence the decision to pursue surgery?
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Diagnostics:
- Echocardiogram - quantify AR (jet width, pressure half-time), measure aortic root and left ventricular dimensions, assess for vegetations (endocarditis) and aortocardiac fistula. If severe AR with LV dilation, the horse is a poor surgical candidate and exercise restriction is critical.
- Cervical imaging - myelography (if available) or CT to confirm dynamic cord compression; radiographs alone are insufficient because narrowing can be incidental.
- EPM testing - serum and CSF (if myelography/CSF tap performed) to rule out protozoal disease; treat if positive.
- CBC/fibrinogen/SAA - normal, so infectious causes less likely.
Management:
- CVSM: Conservative management (controlled exercise, weight management, avoid neck flexion) for mild cases; surgical ventral stabilization (e.g., Seattle Slew or basket procedure) for moderate-severe cases with confirmed compression. Surgery carries ~50-70% success for return to athletic function but is expensive and has complications.
- Cardiac: If AR is mild/functional, proceed with cervical surgery after echo clearance. If AR is moderate-severe with LV dilation, avoid general anesthesia and strenuous exercise - the horse is at risk for sudden cardiac death and should be retired to pasture. In that case, do not pursue cervical surgery; manage conservatively.
- EPM: If positive, treat with ponazuril or diclazuril and reassess in 30 days before considering surgery.
Key judgment: The cardiac diagnosis can be the rate-limiting factor. A horse with severe AR should not undergo prolonged anesthesia or be exercised; the owner should be counseled that the neurologic and cardiac prognoses are independent and both must be favorable to justify surgery.