Osmolar relationships in infantile dehydration.

We studied serum osmolality in 167 consecutive infants admitted for gastroenteritis with 5% or more dehydration. Osmolality was determined by the freezing-point method in a 0.2-mL sample of serum immediately on admission. Forty-one patients (24.6%) had hypo-osmolar dehydration, with a mean osmolality of 257 mOsm/kg (range, 234 to 270 mOsm/kg). Eleven patients (6.5%) had hyperosmolar dehydration, with a mean osmolality of 329 mOsm/kg (range, 312 to 369 mOsm/kg). Simultaneous serum and CSF osmolalities were determined in 14 patients with hypo-osmolar and eight with hyperosmolar dehydration. In patients with hyperosmolar dehydration, serum osmolality correlated well with CSF osmolality, but a poor correlation was seen between serum and CSF sodium levels. Convulsions occurred in two patients in whom the CSF osmolality was greater than the serum osmolality by more than 10 mOsm/kg. Convulsions also occurred in two patients with hypo-osmolar dehydration in whom the CSF osmolality was lower than the serum osmolality by more than 13 mOsm/kg.