Comparative Neuromuscular Laboratory

NEUROMUSCULAR CASE OF THE MONTH - MARCH 2003

Congenital Myotonia in a Chow Mix Dog
Contributed by Dr. Deena Tiches
VCA Veterinary Referral Associates
Gaithersburg, MD

Clinical History
An approximately one-year old male neutered Chow mix dog presented with a history of non-progressive difficulty standing and prehending food. The dog was adopted from a rescue league two weeks prior to presentation.

Physical and Neurological Examination
There was sustained non-painful contracture of the tongue, periocular, and appendicular muscles. Other physical abnormalities included ptyalism and increased respiratory sounds. The dog had difficulty rising, however once standing, the dog could ambulate but with a stiff gait (See Video Clip). Normal conscious proprioception and spinal stretch reflexes were present. Dimpling of the tongue and cranial tibial muscles were noted for 30 seconds after percussion. Retinal examination and mentation were normal.

Diagnostic Testing

CBC: No significant abnormalities

Serum Chemistry: Creatine kinase 2577 U/L (reference range 59-895 U/L) and aspartate aminotransferase 149 U/L (reference range 15-66 U/L)

Antibody titers for Toxoplasma gondii and Neospora caninum were negative

Electrodiagnostics: Myotonic discharges were recorded from all muscles tested including the appendicular, epaxial, and head and neck muscles (See Video Clip). These discharges waxed and waned and ceased after 45 seconds. Motor nerve conduction velocity of the tibial-sciatic nerve (75 m/sec) was within the reference range. Repetitive stimulation of the peroneal nerve showed no decremental response. F-wave latency for the sciatic-tibial nerve was 18.1 m/sec (expected 17.1 m/sec).

Muscle Biopsies: A biopsy was collected from the right cranial tibial muscle. With the exception of a few scattered necrotic fibers undergoing phagocytosis, the muscle was histologically normal (Fig.1, H&E). Myofiber size was at the upper end of the normal range. A marked type 1 fiber predominance was present (Fig. 2. Type 1 fibers light, type 2 fibers dark in an ATPase reaction performed at pH 9.8). These findings have been identified in congenital myotonic disorders and in endocrine disorders such as hypothyroidism.

Figure 1

Figure 2

Clinical Course and Outcome
Based on the clinical signs, electrodiagnostic and histologic findings, a diagnosis of myotonic myopathy was made. Due to the young age of the dog and the Chow breed, congenital myotonia was likely. Myotonia may result from defective chloride or sodium conductance across the muscle membrane. Therapy is directed at sodium channel blockade and class 1 antiarrhythmic drugs are used most often. Oral mexiletine 50 mg (2 mg/kg TID) improved the clinical signs at three hours post-pill. The dose was increased to 75 mg (3.2 mg/kg TID) and the clinical signs improved for approximately six hours post-pill. An electrocardiogram showed no evidence of arrhythmia. The dose was increased to 150 mg (6.5 mg/kg TID) with dramatic improvement of his clinical signs (See Video Clip).

References
Vite CH, Cozzi F, Rich M, Klide AK, Volk SW, Lombardo R (1998). Myotonic myopathy in a Miniature Schnauzer: Case report and data suggesting abnormal chloride conductance across the muscle membrane. J Vet Intern Med 12:394-397.

Vite CH (2002). Myotonia and disorders of muscle cell membrane excitability. In Veterinary Clinics of North America. Small Animal Practice. Neuromuscular Diseases (Shelton GD, ed). 32;169-187.