Most patients with acute hydrocephalus have ventriculomegaly and high intracranial pressure (ICP). However, there is a subset of patients who are symptomatic with acute ventriculomegaly and inappropriately low ICP. Methods: Two patient groups were defined. Each patient presented with clinical deterioration that included a significant decrease in level of consciousness with new and significant ventriculomegaly. Patients in group 1 (n = 10) were managed without endoscopic third ventriculostomy (ETV). Group 2 was a series of patients (n = 10) managed with ETV. Results: Treatment for both groups involved insertion of an external ventricular drain (EVD) with ICP <5 cmH 2O. Further treatment consisted of either neck wrapping with a tensor bandage and/or lowering the EVD to negative levels to facilitate drainage of cerebrospinal fluid (CSF), which resulted in clinical improvement and resolution of ventriculomegaly. All 20 patients had anatomical obstruction to CSF flow into the subarachnoid space (SAS) confirmed by magnetic resonance imaging (MRI) with cine MRI studies. Group 1 patients were treated until shunt revision/insertion was possible (n = 7), ICP normalized, and the EVD could be removed (n = 2), or death (n = 1) occurred. Patients in group 2 all underwent ETV, and ICP patterns normalized in all. Group 2 patients were managed with an EVD until shunt revision/insertion was required (n = 2), ICP normalized and the EVD could be removed (n = 7), or death (n = 1) occurred. Discussion/Conclusions: The syndrome of inappropriately low-pressure acute hydrocephalus (SILPAH) is an important entity in both children and adults. A possible hypothesis invokes loss of an effective SAS. ETV reestablishes communication between the SAS and ventricles, producing a rapid return of normal ICP dynamics and a significant decrease in the number of shunt-dependant patients.