[A peculiar mechanism of hydrocephalus: the "water-hammering" effect].
BACKGROUND AND PURPOSE: Aneurysms of the basilar artery can cause hydrocephalus due to compression of the third ventricle or the sylvian aqueduct. The observation of a particular case led to discuss another possible mechanism of hydrocephalus. CLINICAL PRESENTATION AND RADIOLOGICAL STUDIES: An aneurysm of the basilar artery was revealed by an ischemic stroke in a 65-year-old man. Hydrocephalus developed during the following months. The MRI studies showed that it could not be explained merely by a permanent compression. However, the patient improved clearly after a ventriculo-peritoneal derivation. CONCLUSION: The hydrocephalus could be explained by a "water-hammering" effect due to the pulsating blood in the ectatic vessel, which created a cerebrospinal fluid outflow impairment through the third ventricle.