Syndrome of inappropriate secretion of antidiuretic hormone associated with exanthem subitum.

Abstract

We treated a patient with exanthem subitum caused by human herpes virus-6 (HHV-6) infection complicated with the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). Febrile convulsions are a well-described neurologic complication of exanthem subitum as is also encephalitis or encephalopathy. To the best of our knowledge, however, there has been no previous report of SIADH associated with exanthem subitum. Case report. The patient, an 8-month-old boy with no special family or perinatal medical history, had fever with vomiting for 1 day before admission. On the day of admission he had a generalized clonic convulsion for 20 min and clouding of consciousness. The axillary temperature was 38.1°C and he showed decreased muscle tone. Skin turgor and blood pressure were normal. Laboratory investigations demonstrated a serum sodium concentration of 120 meq/l, plasma osmolality of 262 mOsm/kg H2O, urine osmolality of 569 mOsm/kg H2O and plasma arginine vasopressin concentration of 135 pg/ml (normal range, 0.3 to 3.5 pg/ml). A complete blood cell count showed a leukocyte count of 19.3 × 109/l with 86% neutrophils and 5% lymphocytes. Hemoglobin was 10.6 g/dl, and platelet count was 353 × 109/l. Other pertinent normal laboratory examinations included a cerebrospinal fluid examination and culture and tests of renal function, adrenal function and thyroid function. Because of hyponatremia without dehydration and high plasma plasma arginine vasopressin, SIADH was diagnosed. Fluid intake was restricted. On the third day of hospitalization the serum sodium concentration was 130 meq/l and on the 4th day it was 140 meq/l. Plasma osmolality was 286 mOsm/kg H2O and urine osmolality improved to 347 mOsm/kg H2O. The level of consciousness also improved after normalization of the serum sodium value. High fever lasted for 3 days and a maculopapular eruption was observed on the trunk after disappearance of high fever. HHV-6 fluorescent antibody IgM and IgG were negative on serum obtained in the acute phase, and IgM was detected at 1:10 and IgG was detected at 1:40 in the convalescent serum. Primary HHV-6 infection was diagnosed. He recovered without sequelae. Discussion. SIADH is characterized by hyponatremia, plasma hypoosmolality with normal plasma volume, osmolality of the urine greater than that appropriate for the concomitant osmolality of the plasma and normal renal adrenal function.1 There are many causes of SIADH including central nervous system disorders (meningitis, subarachnoid hemorrhage, neoplasm, psychosis), drugs (iv cyclophosphamide, chlorpropamide, oxytocin, vasopressin), pulmonary disease (pneumonia, acute attack of asthma, tuberculosis), carcinoma causing ectopic production of antidiuretic hormone (oat cell carcinoma of lung) and the postoperative state (especially in premenopausal woman).2 There has been no report of SIADH associated with exanthem subitum induced by HHV-6 infection. In our case hyponatremia without dehydration, high plasma plasma arginine vasopressin concentration and rise in IgM and IgG antibody titer for HHV-6 in the patient's serum is diagnostic of SIADH and primary HHV-6 infection. Exanthem subitum is a common infectious disease of infancy caused by HHV-6.3 The most commonly reported complication in children with exanthem subitum has been febrile convulsions. Independent of exanthem subitum HHV-6 infection has been associated with a number of neurologic conditions including encephalitis, encephalopathy and meningitis.4 Our experience suggests that SIADH should be added to the list of neurologic complications associated with primary HHV-6 infection. Takao Okafuji, M.D.; Hiroshi Uchiyama, M.D. Nobuhiko Okabe, M.D. Junichi Akatsuka, M.D. Kihei Maekawa, M.D. Department of Pediatrics; Jikei University School of Medicine; Tokyo, Japan