Ataxia and seizures in a 5 year old female spayed Siberian Husky
Contributed by Dr. Stacey Sullivan
Animal Neurological Clinic
Portland, ME

Clinical History
The owner reported that generalized seizures had occurred once a year for the past 3 years. The dog was not on medication for this problem. Five months prior to presentation, the dog was frequently rubbing her face and this persisted until the time of presentation. Four months prior to presentation a seizure occurred that was followed by right forelimb incoordination and a pigeon-toed stance. Shortly following this, left hind limb incoordination was described.  No evidence of discomfort was reported. Although she was still active, the overall energy level was described as decreased.

Physical Examination
General physical examination was unremarkable. Neurologic examination showed the dog was bright and alert. She would stand bearing less weight on the right front leg and when walking, ataxia was observed in the right front leg and both rear legs.  Little to no weakness or muscle atrophy was evident. Hopping, wheelbarrowing, and extensor postural thrust were poor in the right front, left hind, and to a lesser extent, in the right hind leg. Cranial nerve examination was normal, including the fundus. The patellar reflex was absent in the left hind leg with the rest of the reflexes normal. Results of the neurological examination pointed to a multifocal problem. Differentials included encephalitis/myelitis, primary or metastatic neoplasia, or multiple problems including spinal cord compression and epilepsy.

Diagnostic Tests
Routine laboratory tests

CBC and Chemistry profile including CK - normal

Blood lead - negative

Lymes - Consistent with previous vaccination

Thyroid panel - TT4 0.16 ug/dl (1.5-3.0 ug/dl); TT3 0.13 ng/ml (1.0-2.0 ng/ml); FT4 0.19 ng/dl (0.8-2.0 ng/dl); Canine TSH 0.11 ng/ml (<0.50 ng/ml); Thyroglobulin autoantibody negative. According to the interpretation provided by the laboratory performing these evaluations, these values were consistent with hypothyroidism.

BRAIN CT - normal

CSF TAP - normal

MYELOGRAM (full spine) - normal

EMG/NCV - Generalized spontaneous activity was present including positive sharp waves and fibrillation potentials. Nerve conduction velocities (NCV) were markedly slowed with the ulnar NCV from the right front limb 33.6 m/sec   (reference 50-60 m/sec) and the tibial NCV from the left hind limb 29.2 m/sec (reference 55-65 m/sec). These findings were consistent with a motor unit disorder.

MUSCLE AND NERVE BIOPSIES - A fresh biopsy was evaluated by frozen section from the biceps femoris muscle (Fig. 1). With the exception of a rare singular atrophic fiber (arrow), the muscle biopsy did not reveal specific pathological abnormalities.. A fixed biopsy from the peroneal nerve was plastic embedded and evaluated in 1um sections (Fig. 2).  There was a reduction in the number of large diameter myelinated nerve fibers (right arrowhead) with axonal degeneration and collapse of the myelin sheath (left arrowhead). Small caliber nerve fibers were numerous (asterisk).

Figure 1.

Figure 2.

Following anesthesia for diagnostic evaluations, the dog had 2 generalized seizures. The next morning she was markedly worse, non-ambulatory, dull and disoriented. Routine EKG showed frequent long runs of VPC's (15-20). Pulses and mucous membrane color were good. The arrhythmia was controlled with a lidocaine drip (25 ug/kg/min gradually increased to 50 ug/kg/min over 12hrs) and Mexilitine (150 mg PO q 8 hr). Over the next few days, her neurologic status continued to deteriorate and she became increasingly disoriented, vocalized constantly, and all reflexes were lost. As results of special laboratory testing (thyroid, lead, Lymes, muscle and nerve biopsies) were not yet available, empirical therapy was initiated including Phenobarbital 60 mg BID; Prednisone 30 mg BID; Soloxine 0.6 mg BID; Mexilitine 150mg TID; Doxycycline 150 mg BID; and a single dose of B-complex vitamins (1.5 ml SQ). Improvement began almost immediately after initiation of these medications. Following receipt of all laboratory evaluations, prednisone was discontinued. The dog improved steadily over the month following presentation, and although not yet normal, was able to run, jump, and play. She is currently maintained on Soloxine 0.6 mg BID, Mexilitine 150 mg TID, and Phenobarbital 15 mg BID.

Conclusion and Comments
Neurologic complications of hypothyroidism including ataxia, epilepsy, and decreased tendon reflexes are well recognized in humans (1), and the existence of an intrinsic diffuse sensorimotor neuropathy secondary to hypothyroidism has relatively recently been accepted. A reduction in the number of myelinated fibers, particularly those of large diameter, has been a consistent finding in human hypothyroid neuropathy (2-6). Neurologic complications have also been associated with hypothyroidism in canines. Although we cannot say with certainty, we suspect that the clinical presentation of multifocal neurological disease in this dog may have been a result of hypothyroidism. Evidence for this includes: a) laboratory confirmation of hypothyrodism; b) pathological changes within the peripheral nerve biopsy consistent with those described in human cases of hypothyroid neuropathy; c) electrophysiological evidence of a peripheral neuropathy; d) considerable improvement following replacement therapy.

Selected references
1. Pollard JD. Neuropathy in diseases of the thyroid and pituitary glands. In: Dyck PJ, Thomas PK (eds). Peripheral Neuropathy. 3rd Edition, W.B. Saunders Co., Philadelphia,1993

2. Dyck PJ, Lambert EH. Polyneuropathy associated with hypothyroidism. J Neuropathol Exp Neurol 29:631, 1970.

3. Meier C, Bischoff A. Polyneuropathy in hypothyroidism. Clinical and nerve biopsy study of four cases. J Neurol 215:103,1977,

4. Nemni R, Bottacchi E, Faxio R et al. Polyneuropathy in hypothyroidism: clinical, electrophysiological and morphological findings in four cases. J Neurol Neurosurg Psychiatry 50:1454, 1987.

5. Pollard JD, McLeod JG, Honnibal TGA, Verheiden MA. Hypothyroid polyneuropathy. J Neurol Sci 53:461,1982.

6. Shirabe T, Tawara S, Terao A, Araki S. Myxoedematous polyneuropathy: a light and electron microscopic study of the peripheral nerve and muscle. J Neurol Neurosurg Psychiatry 38:241,1975.

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