Cerebrospinal fluid osmolal changes in bacterial meningitis.

Sixty children with acute bacterial meningitis (ABM) were prospectively studied for their serum sodium values and cerebrospinal fluid (CSF), serum and urinary osmolality. The results have been compared with 20 age and nutritionally matched controls. Even though mean serum osmolality (283.2 +/- 13.84 mOsm/kg) and serum sodium levels (130.5 +/- 8.15 mEq/L) were significantly lower in ABM in comparison to controls (p < 0.05 and < 0.001, respectively), the overall mean CSF osmolality in patients with ABM (282.5 +/- 12.3 mOsm/kg) was not significantly different as compared to controls (288.2 +/- 7.89 mOsm/kg). As expected, cases of ABM with syndrome of inappropriate secretion of antidiuretic hormone (SIADH) had significantly lower CSF osmolality (272 +/- 9.42 mOsm/kg) as compared to those without SIADH (288.5 +/- 9.34 mOsm/kg) and controls (288.2 +/- 7.89 mOsm/kg). However, our observations indicate that whereas the mean CSF osmolality was lower than the serum osmolality in the control group as well as in ABM without SIADH, it was greater than serum osmolality in ABM with SIADH (p < 0.05). Our results suggest that in the presence of SIADH, hypo-osmolality of serum may eventually result in hypo-osmolality of CSF, but the fall in CSF osmolality is not of the same degree as that of serum. Low CSF osmolality was observed to be associated with an unfavorable prognosis (p < 0.05).