Hydrodynamic principles in hydrocephalus: the engineer's perspective

There have been significant improvements in the prognosis for patients suffering from hydrocephalus stemming from the introduction of the cerebrospinal fluid (CSF) shunt some 40 years ago. Currently, one of the major obstacles to effective shunt treatment is the mismatch between the physiology of the patient and the hydraulics of the shunt system. In order to maintain the proper relationship between CSF and cerebrovascular pressures, the implanted shunt needs to establish normal CSF outflow (absorption) and storage (compliance). Many of today's shunts establish a limited range of normal CSF outflow (absorption) and storage (compliance) once implanted, but a mismatch between CSF and cerebrovascular pressures may exist when the patient changes body position during daily activities. An uncoupling of these pressures creates mechanical strains within cerebral tissues, which are implicated in pathologies related to shunt malfunction. We suggest that re-establishment of normal CSF outflow resistance, which by definition is an indicator of both absorption and compliance, is a fundamental requirement for shunt treatment under most conditions.