Evaluation of hyponatraemia in patients with tick-borne encephalitis--a preliminary study.

Hyponatraemia is one of the most frequently observed, but sometimes overlooked, electrolyte disorder. Patients with meningitis are predisposed to develop hypovolaemic hyponatraemia. However, hyponatraemia in meningitis may also be caused by the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). Proper differentiation of these pathomechanisms is crucial in patient's treatment as dehydration requires fluid supplementation, while SIADH is treated with fluid restriction. The aim of the present study was the evaluation of frequency, potential causes, and risk factors of hyponatraemia in patients with tick-borne encephalitis (TBE). A total of 61 patients (22 women, 39 men) aged 18-80 years, with a history of TBE was included in the study. Hyponatraemia was diagnosed when sodium concentration was below 135 mmol/l. Hyponatraemia was considered mild when sodium concentration was 130-134 mmol/l, moderate when 125-129 mmol/l, and severe when <125 mmol/l. Among the 61 patients, hyponatraemia was observed in 41% (25 patients). In 20 patients (33%), hyponatraemia was mild, in 3 (5%) it was moderate, and in 2 (3%) severe. Two patients with severe hyponatraemia and one with moderate hyponatraemia fulfilled the SIADH criteria. In the non-SIADH patients, sodium concentration normalized within 1-2 (1.1±0.2) days while in the SIADH group, the disturbances lasted for 4-8 (6±2) days. Sodium concentration correlated with patients age (R Spearmann - 0.27, p<0.05). There were no significant differences as far as gender or clinical form of the disease are concerned. Hyponatraemia, usually mild, is a common disorder in the course of TBE, although it is not significantly more frequent than in other hospitalized patients. Dehydration seems to be the main cause of hyponatraemia in the course of TBE. SIADH is a less common cause of hyponatraemia in the course of TBE, although it should be taken into consideration as the treatment differs significantly. Patients >60 years of age with TBE are more susceptible to hyponatraemia than younger patients.