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A 35-year-old man presented with a 2-day history of cramps and paresthesias in the arms, predominantly involving the fingers. This presentation was preceded by a bout of viral gastroenteritis 1 week earlier. The patient reported receiving no medications and specifically reported not using thiazide diuretics. He had a blood pressure of 100/60 mm Hg, a respiratory rate of 24 breaths per minute, and carpopedal spasm, which was reproducible by inflating a blood-pressure cuff placed on the patient's arm. Chvostek's sign, the twitching of the circumoral muscles with tapping lightly over the facial nerve, was also present (Video 1). Laboratory investigation revealed a serum calcium level of 1.9 mmol per liter (7.6 mg per deciliter) (normal range, 2.2 to 2.6 [9.0 to 10.5]), a potassium level of 2.8 mmol per liter (10.9 mg per deciliter) (normal range, 3.5 to 5.0 [13.7 to 19.5]), and a magnesium level of 0.5 mmol per liter (normal range, 0.8 to 1.2). Analysis of arterial blood gas showed a pH of 7.53, a bicarbonate level of 34 mmol per liter, and a partial pressure of carbon dioxide of 30 mm Hg. A diagnosis of hypocalcemia was made, and the patient was treated with calcium gluconate. Urinary calcium excretion was subnormal, with increased urinary loss of potassium, magnesium, and chloride, which supported the diagnosis of Gitelman's syndrome, an inherited renal salt-wasting disorder. The patient had a good response to therapy with oral magnesium and potassium supplements.
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