Background: The etiology of idiopathic normal pressure hydrocephalus (iNPH) is currently unknown. With no visible obstructions, altered cerebrospinal fluid (CSF) dynamics may explain the accumulation of ventricular fluid. We hypothesized that elevated osmolality in the CSF of iNPH patients could potentiate formation of ventricular fluid and thereby cause the disease progression and/or predict the surgical outcome. To address this hypothesis, we determined the lumbar and ventricular CSF osmolality of iNPH patients at different disease stages and compared with lumbar CSF samples obtained from control subjects.

Methods: The osmolality of CSF was determined on a total of 35 iNPH patients at diagnosis and at the subsequent treatment with shunt surgery (n = 20) and compared with the CSF osmolality from 20 control subjects. Simultaneously collected lumbar and ventricular CSF samples from experimental pigs were used to evaluate the compatibility between CSF from different compartments.

Results: We found no evidence of increased osmolality in the CSF of iNPH patients upon diagnosis or at the time of shunt treatment months after the diagnosis, compared with control individuals. CSF tapped from the lumbar space could be used as a read-out for ventricular CSF osmolality, as these were similar in both the patient group and in experimental pigs. We further observed no correlation between the CSF osmolality in iNPH patients and their responsiveness to shunt surgeries.

Conclusions: The osmolality of lumbar CSF is a reliable reflection of the ventricular CSF osmolality, and is not elevated in iNPH patients. iNPH therefore does not appear to arise as a function of osmotic imbalances in the CSF system and CSF osmolality cannot serve as a biomarker for iNPH or as a predictive tool for shunt responsiveness.