In cardiology, urology, nephrology the word has other meanings, but in neurosurgery, a "shunt" is almost always a diversionary cerebrospinal fluid drainage pathway.
Sometimes the shunt does not work because of material (such as blood, pus, or clumps of periventricular cellular debris) inside the tubing or valve (internal) or compressing, kinking, disruption of the shunt tubing from the outside (external)
Ventriculoperitoneal shunt
The ventricles are large spinal fluid-filled chambers deep inside the brain. Excessive water can swell (dilate) the ventricle, putting pressure on the surrounding brain and potentially increasing the pressure inside the skull (intracranial). Permanent brain damage can result from elevated pressure. One solution is called a shunt: fluid enters a tube in the ventricle and flows inside the tube along its course through the brain and skull, under the scalp and skin on its way from the head down to the peritoneal cavity. Under high pressure the fluid leaves the brain and flows into the abdomen and continues flowing until the pressure intracranial decreases to the point that it is too low to push the fluid through out tube out of the ventricle.
Shunt obstruction
Shunt obstruction can be difficult to diagnose.
CT scans show the size of the ventricles -- a previous scan showing that the ventricles were smaller at some point after shunting and one know showing the ventricles enlarged suggests that the amount of fluid leaving the ventricles through the shunt over time is inadequate and that hydrocephalus is back.
Many shunts have reservoirs or valves under the scalp that can be "tapped" (fluid drawn out through a small needle inserted into the reservoir through the skin). If fluid cannot be withdrawn the shunt may be blocked at the ventricular end.
Another way to assess for shunt obstruction is to inject a radioactive dye into the ventricle (usually through the reservoir) and then make an image of where the dye goes -- normally the fluid flows through the brain, around the spinal cord and then back up and over the hemispheres of the brain, if the shunt is working some of the dye will enter the shunt and appear in the shunt tubing.
Because shunt obstruction can usually be diagnosed non-invasively it is not necessary to put a ventriculostomy in unless the neurosurgeon wants to determine the optimal amount of shunting.. By varying the amount of draining through the ventriculostomy and correlating this to the patient's symptoms and signs, the neurosurgeon can determine that the shunt should be set to maintain a very low, low, normal, etc. fluid pressure in the ventricles.
Fluid will accumulate in the ventricles more quickly the more complete the obstruction. Some patients with a sudden complete obstruction of their VP shunt have a rapid increase in the amount of fluid in the brain which can raise the pressure in the head to the point that the brain does not function normally and the patient becomes sleepy, lapses into a coma, and can even die. In these cases immediate surgical intervention can be life saving. Most shunt obstructions do not have such a catastrophic presentation and can be operated on within days or even weeks of symptom onset.
Ventriculoperitoneal (VP) shunt
Ventriculoperitoneal shunt insertion
Ventriculoperitoneal shunt revision
Risks and complications of ventriculoperitoneal shunt insertion
