Hypernatremic Myopathy in cats 

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Episodic weakness and signs of depression were reported in a 7 month old Domestic Shorthaired cat with episodic hypernatremia (serum Na concentration ranging from 182 to 215 mEq/L; normal is 148 to 161 mEq/L) secondary to hypodipsia (failure to drink water). This rare condition was accompanied by hyperosmolality (ranging from 381 to 431 mOsm/L) and evidence of hypopituitarism (adrenocorticotrophic and growth hormone deficiencies, along with blunted thyroxine response to thyroid-stimulating hormone). The most prominent clinical sign was ventroflexion of the neck. No other neurological abnormalities were detected. Electromyographic testing revealed prolonged insertional activity, fibrillation potentials, positive sharp waves, and bizarre high-frequency discharges. Nerve conduction velocities were normal. These abnormalities were more severe during episodes of hypernatremia. Serum creatine kinase activity was increased, while CSF analysis was normal. Examination of several muscle biopsies were normal. Contrast-enhanced computed tomographic studies of the brain demonstrated marked hydrocephalus, although no hypothalamic or pituitary lesions were detected. 

The episodic weakness might have been associated with muscle membrane alterations associated with displacement of intracellular potassium by high levels of extracellular sodium. Interestingly, the clinical signs, serum CK levels, electrodiagnostic data, and muscle biopsy findings were very similar to those seen in cats with hypokalemic myopathy. Forced water intake and dietary sodium restriction (using a low-salt feline diet) corrected the hypernatremia and signs of muscle dysfunction. After restoration of eunatraemia, secretion of pituitary hormones became normal. It was suggested that hypothalamic dysfunction, possibly related to hydrocephalus, induced both hypodipsia and transient hypopituitarism.