Carbamazepine treatment of myotonia congenita in a cat

Abstract

Case summary: A 2-year-old female intact domestic shorthair cat was referred to the neurology service at the Foster Hospital for Small Animals as a result of lifelong weakness, seizure-like episodes after excitement, muscle spasms, stiffness of the limbs and shortness of breath. A diagnosis of myotonia congenita (MC) was made based on compatible history, clinical signs and electromyography findings, and excluding other causes of muscle weakness using muscle biopsy, baseline bloodwork and echocardiography. Initial treatment with procainamide had overall low efficacy. The cat was then switched to carbamazepine, which provided long-term improvement in clinical signs at a dosage of 7.4 mg/kg PO q12h with no observed clinical side effects.

Relevance and novel information: To date, there is only a single case report describing the successful treatment of MC in cats, and no report describing the use of carbamazepine to treat this condition in this species. This anti-convulsant medication was shown to be anecdotally effective in the case reported. Further research is needed to categorize the metabolism, pharmacokinetics, tolerance, dose range and efficacy of carbamazepine in cats with and without MC.

Keywords: Carbamazepine; pharmacodynamics; procainamide; treatment.

Figure 1

(a) The patient’s electromyogram (EMG) showed myotonic discharges. The insertion of a monopolar needle into the cranial tibial muscle produced an EMG characterized by continuous waxing and waning action potentials. These findings strongly suggest myotonia. (b) The patient’s nerve conduction activity of the peroneal nerve showed a normal conduction velocity and a normal amplitude of the recorded compound muscle action potentials, making neuropathy very unlikely

Figure 2

Biopsy of the left cranial tibial muscle. A hematoxlyin and eosin-stained cryosection from the cranial tibial muscle showed an excessive variability in myofiber size without other specific pathological changes. Magnification = ×20

Figure 3

Procainamide blood levels. Limited information is available on the metabolic rate of procainamide in cats. To better understand treatment with procainamide, serum levels were measured after an oral dose of 35 mg/kg at multiple time points. Results showed a fast increase of the serum levels after administration to a peak within the therapeutic range reported for humans of 4–14 µg/ml. However, the short half-life of 3 h would have required multiple treatments per day, which was difficult to achieve in this cat

Figure 4

Cerebellar mass. The histopathological analysis of the cerebellar mass showed a prominent whorling of elongated spindle-like cells (arrow) and few psammoma bodies (star). These findings are most compatible with a psammomatous meningioma, which is a common histological subtype in cats