The genesis of low pressure hydrocephalus.

Background: Low pressure hydrocephalus (LPH) is an uncommon entity. Recognition of this treatable condition is important when clinicians are faced with the paradox of symptomatic hydrocephalus despite low intracranial pressures (ICP). Its etiology remains enigmatic.

Methods: We identified patients with LPH from the prospective, inpatient neuro-intensive care database over a 4-year period (2006-2010).

Results: Nine patients with LPH were identified over a 4-year period. The time from diagnosis of the initial neurosurgical condition to development of LPH varied from 7 days to 5 years. The sub-zero drainage method of Pang and Altschuler was successful in all cases. LPH was accompanied by transependymal edema in five patients despite low ICP. Four patients developed LPH during their initial admission for intracranial bleeding. As patients entered the LPH state, the ICP remained in a normal range yet daily CSF output from the external ventricular drain was reduced. When LPH patients were drained at sub-zero levels, daily CSF output exceeded baseline values for several days and then receded to baseline. Long-term management was achieved with low pressure shunt systems: six programmable shunts; one valveless ventriculoperitoneal shunt; two ventriculopleural shunts. Conditions most commonly associated with LPH are: subarachnoid hemorrhage, chronic hydrocephalus, brain tumors, and chronic CNS infections.

Conclusions: Low pressure hydrocephalus is a challenging diagnosis. The genesis of LPH was associated with a drop in EVD output, symptomatic ventriculomegaly, and a remarkable absence of intracranial hypertension. When LPH was treated with the sub-zero method, a 'diuresis' of CSF ensued. These observations support a Darcy's flux of brain interstitial fluid due to altered brain poroelastance; in simpler terms, a boggy brain state.