A 76-year-old Chinese man presented with 2 falls in a day and nonspecific symmetrical lower-limb weakness. This was not associated with anorexia, vomiting, syncope, or seizures. Five days prior, he had consulted his general practitioner for herpes zoster and was prescribed oral acyclovir 800mg 5times a day. His background medical history included ischemic heart disease for which he had undergone coronary artery bypass surgery, diabetes mellitus, hypertension, and dyslipidemia. There had been no recent change to his medication list of aspirin, bisoprolol, amlodipine, metformin, atorvastatin, and lisinopril. He had not taken any psychotropic drugs, diuretics, or alternative medications. His last routine labs a month prior were within normal limits, with normal serum sodium at 139mmol/L (range: 135-145 mmol/L).
He was clinically euvolemic with unremarkable parameter readings. Physical examination revealed a right parietal cephalohematoma as well as crusted vesicles over C4-6 dermatomal distribution (Figure1). Neurological examination was normal.
Biochemical investigations, including complete blood count, renal, hepatic, and thyroid function tests were unremarkable except for severe hyponatremia of 118mmol/L (Table). Chest radiograph, electrocardiogram, and troponin screen were normal. Further work-up revealed low serum osmolality paired with elevated urine sodium and osmolality. An 8 AM cortisol level demonstrated robust response. This was suggestive of syndrome of inappropriate antidiuretic hormone secretion (SIADH). Computed tomography scan of the brain, thorax, abdomen, and pelvis did not identify alternative etiologies of SIADH.
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-Raymond K.H. Goh, MBBS, MMed (Int Med), MRCP, Brenda Chiang, MBBS, MMed (Int Med), MRCP, Marvin W.J. Chua, MBBS, MMed (Int Med), MRCP, Chaw Su Naing, MBBS, MRCP
This article originally appeared in the April 2019 issue of The American Journal of Medicine.
