Myotonia congenita (MC) is a skeletal muscle channelopathy characterized by inability of the muscle to relax following voluntary contraction. Worldwide population prevalence in humans is 1∶100,000. Studies in mice, dogs, humans and goats confirmed myotonia associated with functional defects in chloride channels and mutations in a skeletal muscle chloride channel (CLCN1). CLCN1 encodes for the most abundant chloride channel in the skeletal muscle cell membrane. Five random bred cats from Winnipeg, Canada with MC were examined. All cats had a protruding tongue, limited range of jaw motion and drooling with prominent neck and proximal limb musculature. All cats had blepharospasm upon palpebral reflex testing and a short-strided gait. Electromyograms demonstrated myotonic discharges at a mean frequency of 300 Hz resembling the sound of a ‘swarm of bees’. Muscle histopathology showed hypertrophy of all fiber types. Direct sequencing of CLCN1 revealed a mutation disrupting a donor splice site downstream of exon 16 in only the affected cats. In vitro translation of the mutated protein predicted a premature truncation and partial lack of the highly conserved CBS1 (cystathionine β-synthase) domain critical for ion transport activity and one dimerization domain pivotal in channel formation. Genetic screening of the Winnipeg random bred population of the cats' origin identified carriers of the mutation. A genetic test for population screening is now available and carrier cats from the feral population can be identified.

A 7-year-old male neutered Boston Terrier, previously diagnosed with hyperadrenocorticism, was presented to the University of Missouri Veterinary Medical Teaching Hospital Neurology service for evaluation of a large pituitary mass revealed on computer tomography (CT) and progressively worsening clinical signs despite treatment. Upon arrival, the dog's vital parameters, including heart and respiratory rate and temperature, were within normal limits. The mucous membranes were dry. The neurologic examination revealed an inappropriate mentation, characterized by drowsiness, absent menace response bilaterally, postural reaction deficits limited to delayed hopping and proprioceptive positioning in the left pelvic limb, while gait was unremarkable and with no obvious ataxia, weakness or lameness, spinal reflexes were intact, and no hyperesthesia could be elicited upon paraspinal palpation. The dog's neuroanatomic diagnosis was bilateral forebrain lesion due to a pituitary macroadenoma and associated peritumoral edema. The CBC (Sysmex XT-2000iV; Sysmex America, Inc, Mundelein, IL, USA) was normal, except for a mild lymphopenia. The chemistry profile (Olympus AU400e; Beckman Coulter, Brea, CA, USA) revealed changes associated with hyperadrenocortism, including mildly increased GGT, ALT, and ALP activities. Additionally, marked hypernatremia, hyperchloridemia, and a mildly decreased corrected chloride concentration were found. Hypernatremia and hyperchloridemia were attributed to marked dehydration due to an altered mental state. The mildly decreased corrected chloride (corrected Cl = measured Cl 9 147/measured Na) suggesting a disproportionate decrease of chloride relative to sodium has been reported with hyperadrenocortism. 1 The increased endogenous ACTH concentration was supporting pituitary-dependent hyperadrenocortism. A transsphenoidal approach for hypophysectomy was taken to remove the mass. Intraoperative imprints were submitted for evaluation ( Figure 1). Figure 1. Imprints of an enlarged pituitary gland in a dog. Wright-Giemsa. 950 objective. A B

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