Hydrocephalus shunts: Modelling pressure and volume changes in the brain
A problem presented at the UK MMSG Nottingham 2002.
Categories
- Presented by:
- (School of Medical and Surgical Sciences, University of Nottingham)
- Participants:
Problem Description
Hydrocephalus results from accumulation of cerebrospinal fluid (CSF), a watery liquid, in the ventricles of the brain due to occlusion of drainage pathways. In the infant, whose skull bones are not yet fused, the head circumference will increase as CSF pressure and ventricular volume increase. In the older child and adult, the skull is a rigid box and the ventricular volume increases at the expense of brain substance. The brain is not homogeneous, consisting of neural tissue interspersed with blood vessels. There are also fibrous membranes inside the skull. Therefore brain compression is not simple.In addition, the two main sources of pressure inside the head are CSF pressure and blood pressure, and these interact. The main concern of the host is to supply the brain with glucose and oxygen to maintain cerebral function and viability, and cerebral blood pressure increases to offset CSF pressure in order to achieve this. Eventually a point is reached where blood pressure cannot rise further and cerebral viability declines.
In order to treat the condition, a shunt is inserted to divert CSF from the cerebral ventricles around the occlusion and into the bloodstream. Though many different designs of shunt are available, all control the rate of CSF flow or pressure to normalise it. When the shunt malfunctions, either over-draining or under-draining, symptoms signalling potentially serious intracranial pressure / volume problems appear. Sometimes these are subtle and diagnosis is a problem. Direct CSF pressure measurement is sometimes undertaken, using a surgically inserted transducer. Indirect methods are less satisfactory. As shunts have become more sophisticated, a goal of telemonitoring, ie monitoring of CSF pressure using a radio - transducer in the shunt, capable of transmission or access via the internet to doctors at distant sites, linked by a feedback mechanism to the shunt mechanism, needs to be reconsidered.
Study Group Report
We developed a model primarily aimed at detecting in vivo a blockage or other shunt malfunction using noninvasive measurements, so that shunt valves can be adjusted accordingly. This was then extended to include the possibility of flow regulation.
The simple modelling revealed that much diagnostic information on shunt performance is available through current measurement methods. The use of pressure measurements in the reservoir to detect catheter blockage provides an alternative to the invasive and damaging procedures necessary for intercranial pressure measurements. This method does, however, require dynamic measurements, since static pressure readings reveal only the pressure inside the reservoir, which may be significantly different to that inside the ventricles.
Follow-Up Activities
The following publications have been written as a result of this problem:
- A model of in-vivo hydrocephalus shunt dynamics for blockage and performance diagnostics
- (2004)
- Mathematical Medicine and Biology 21, 347–368.