Insufficient and unstable cerebrospinal fluid drainage via an Ommaya reservoir: two cases of low-pressure hydrocephalus

External ventricular drainage via an Ommaya reservoir has been adopted as an alternative in hydrocephalus patients ineligible for ventriculoperitoneal shunting, largely owing to its perceived lower infection risk. However, the risk of inadequate drainage remains under-recognized and the overall safety of this approach remains incompletely characterized. We report two cases of Ommaya reservoir-based EVD for hydrocephalus in Han Chinese patients. Case 1 was a young male with low-pressure hydrocephalus (LPH) and nearly absent spontaneous respiration. Initial direct EVD at low drainage heights (0–10 cmH₂O) failed to produce adequate cerebrospinal fluid output, but Ommaya-reservoir EVD with titrated subzero drainage improved respiratory function (drainage pressures as low as −50 cmH₂O). The treatment was ultimately discontinued due to intracranial infection caused by extensively drug-resistant Acinetobacter baumannii (XDR-Ab). Case 2 was a middle-aged male with initial high-pressure hydrocephalus and poor scalp incision healing. Ommaya-reservoir EVD resulted in unstable CSF drainage and fluctuating bone flap tension, culminating in a low-pressure state consistent with LPH, while subsequent direct EVD with standardized subzero drainage achieved clinical stabilization and radiological improvement. EVD via an Ommaya reservoir is not suitable for routine external hydrocephalus drainage. It may cause insufficient drainage, complicate LPH management, increase the risk of intracranial infection, and may be associated with progression to LPH in patients with initial high-pressure hydrocephalus.