NEUROMUSCULAR CASE OF THE MONTH - APRIL 2002

Chronic myopathy associated with nemaline rods and presumptive hypothyroidism in 8 year old Catahoula Leopard Hound
Contributed by Drs. Pierre S. Bichsel and Julie Corbett
Animal Emergency & Referral Center
Fort Pierce, FL

Clinical History
Two months prior to presentation the owner noticed excessive panting and infrequent collapsing episodes. The dog remained conscious during the episodes of muscular weakness which lasted approximately 5 minutes. A change in bark was also described. The dog was being treated for aggression with oral clomipramine. A positive serum antigen titer (1:128) for Cryptococcus neoformans was found 1 month prior to presentation and treatment with oral fluconazole was initiated.

 



Figure 1.


Physical and Neurological Examination
On presentation the dog was bright, alert and responsive (Fig. 1). A collapsing episode was observed while hospitalized. The dog crumpled to the ground and appeared to be “sleeping” with the eyes open. There was no excessive muscle rigidity, urination, or defecation. Mucous membrane color was normal with strong pulses and no pulse deficits. Pupils were large but responded well to light. Electrocardiograms obtained just prior to the collapse and during the episode were normal. Blood glucose, electrolytes and other chemistry parameters were normal during the episode. After approximately 4 minutes the dog jumped up and was normal. A laryngeal examination under light anesthesia revealed the presence of laryngeal paralysis. The remainder of the neurological examination including spinal reflexes was normal.

Diagnostic Testing
CBC – Mild non-regenerative anemia (HCT 30.7%; reference range 37.0-55.0) and leukopenia (4.4 X 109; reference range 6.0-16.9)

Plasma lactate – 2.8 mmol/l (reference <2.0)

Thyroid profile – Thyroglobulin autoantibody 451% (reference <200) measured 3 months before presentation

Serum acetylcholine receptor antibody – 0.04 nmol/l (reference <0.6)

Cryptococcus neoformans – Negative serum antigen titer after one month of treatment with fluconazole

Electromyography - Positive sharp waves and bizarre high-frequency discharges were present in the laryngeal muscles and in the masticatory muscles bilaterally.

Motor nerve conduction velocity – Left tibial nerve 41 m/s (canine reference for distal tibial nerve 60.3 +/-3.5)


Muscle and nerve biopsies:Fresh frozen and fixed biopsies were collected from the cervical epaxial muscle. Numerous myofibers contained nemaline rod-like structures that were highlighted by the trichrome stain (Fig. 2, arrows) and central and peripheral PAS positive deposits consistent with excessive glycogen storage (Fig. 3, arrows). A marked type 1 fiber predominance was also observed (not shown). No specific abnormalities of nerve fiber density, myelin, axons or supporting structures were identified biopsies from a spinal nerve.

Muscle carnitine quantification (nmol/mg protein)
Total carnitine – 5.0 (12-41)
Free carnitine – 2.90 (11-33)
Esters carnitine – 2.10 (0-11)



Figure 2.


Figure 3.

Diagnosis
Based upon the clinical finding of laryngeal paresis, the histopathological changes within the muscle biopsy specimen that have previously been identified in hypothyroid dogs (type 1 fiber predominance, nemaline rods and PAS positive deposits), and the presence of thyroglobulin autoantibodies, a presumptive diagnosis of hypothyroid myopathy was made. An atypical finding was the cataplectic episodes. The clinical significance of the cryptococcosis titer was unclear.

Treatment
The dog was supplement with thyroid hormone (20 μg/kg BID PO), L-carnitine (30 mg/kg TID PO), and coenzyme Q10 (30 mg BID PO). Clomipramine (50 mg BID PO) was continued.

Outcome
The dog had one more cataplectic episode 2 months after discharge and became more and more aggressive. The owner could not give medications and was attacked by the dog two months after discharge. The dog was euthanitized.


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