Isotonic hyponatremia and cerebrospinal fluid sodium during and after transurethral resection of the prostate

We examined the effects on the central nervous system of hyponatremia during transurethral resection of the prostate (TURP). Initially, a prospective study was done on 165 consecutively treated patients undergoing TURP, to evaluate symptoms related to the serum osmolality. There were ten patients with hyponatremia below 120 mEq·L⁻¹, and in whom the serum sodium decreased to 111.9±6.4 mEq·L⁻¹ (mean±SD) postoperatively, the measured serum osmolality remained near normal. The calculated osmolality decreased to 237.4±11.9 mOsm·kg⁻¹ and the estimated osmolar gap was 33.5±10.4 mOsm·kg⁻¹ due to absorption of the irrigating sorbitol. Neurological symptoms were mild and complications such as seizures or loss of consciousness nerver occurred. There were five other patients with hyponatremia (serum sodium 118.0±6.7 mEq·L⁻¹) from whom lumbar cerebrospinal fluid (CSF) was collected before and after TURP through a single puncture. CSF sodium did not decrease throughout 1.5 h after TURP, and there was a CSF-to-serum sodium gradient. Our study shows that in cases of acute dilution hyponatremia during and after TURP, symptoms are mild because the serum osmolality remains near normal and CSF sodium does not decrease despite severe postoperative hyponatremia.