Neuroscience Clerkship

 

 

SURGICAL TREATMENT OF HYDROCEPHALUS

 

A variety of CSF shunt systems are currently available. No single shunt has been clearly shown to be superior to the rest, and thus the surgeon’s familiarity with the shunt equipment is an important factor in its selection. The consistent use of the same shunt system helps to optimize the surgeon’s technical proficiency and facilitates the process of shunt revisions.

The equipment for CSF diversionary shunting includes a proximal shunt catheter, valve, distal shunt catheter, and sometimes other components such as a device to prevent siphoning, an on/off switch or telemetric sensor. Shunt systems may exist as integral units (no connectors) or as separate parts requiring assembly.

The proximal catheter is the portion of a shunt system which is placed into the CSF space before the site of obstruction. In ventricular shunt systems, the proximal catheter is most commonly placed in the lateral ventricle. When the subarachnoid space is shunted, the proximal catheter is most commonly inserted into the lumbar thecal sac. Proximal shunt catheters, particularly ventricular catheters, should incorporate several features in their design to ensure optimal function. Some ventricular catheters include a subcutaneous reservoir. The catheters may be straight or right-angled. Right-angled catheters allow easier fixation to the distal shunt system. The reservoir should be palpable underneath the scalp and allow percutaneous access to the CSF for sampling, pressure measurement, and introduction of medication or contrast agents.

Shunt valves come in several varieties. All produce unidirectional flow of CSF. Most valves are pressure regulated, that is, they respond to a differential pressure gradient across the valve. A differential pressure is generated either by an increase in pressure upstream or a decrease in pressure downstream. The ideal valve would drain only the excess CSF produced by each individual patient which could not be reabsorbed. Such a valve does not yet exist.

Ventricular shunt valves may be placed proximally or distally along the shunt system. The majority of shunt valves are proximal valves which are seated just distal to the ventricular catheter. Distal valves of the slit type are effective but have two major disadvantages: 1) they are associated with a higher rate of distal shunt malfunction and 2) they are more difficult to replace at the time of shunt revision. Examples of proximal differential pressure valves include the slit valve, ball-in-cone valve, diaphragm valve, and miter valve.


Shunt Complications

In spite of their dramatic ability to control the symptoms and signs of hydrocephalus, ventricular shunts are foreign bodies associated with a myriad of complications. One must be familiar with the full spectrum of complications that can follow shunt placement because only some occur immediately and many occur over the long-term. At times the rationale for placement of a shunt presumes that because the surgery is simple, “a shunt won’t hurt and might help”. While the latter statement is often correct, the former is a dangerous misconception. Complications that accompany shunt placement are generally related to malfunction and infection. Shunt malfunction may result from either underdrainage or overdrainage of CSF.


Shunt malfunction from Underdrainage of CSF

Underdrainage of CSF occurs if the shunt system becomes obstructed or disconnected. Most commonly, this occurs as the result of an obstruction of the ventricular catheter. This obstruction may be the result of initial misplacement of the ventricular catheter or a migration of the catheter into the subependymal tissue or choroid plexus as a result of collapse of the ventricles, growth or the head, or movement of the entire shunt system. Even a catheter properly placed within the ventricles may become occluded by choroid plexus or tissue debris. Occlusion of a catheter as a result of debris from an immune response has also been described. The shunt valve may stick or become occluded and malfunction. The distal shunt is also a site which can become occluded, although this is more apt to occur if a distal slit valve apparatus is used, and less likely to occur if open ended peritoneal tubing is used.

Underdrainage of CSF may also result from a disconnection with the shunt system. Disconnections tend to occur at connector sites, particularly when connectors have been placed along the shunt tract to bridge gaps at the time of previous shunt disconnections. Disconnections may occur anywhere along the shunt system. When shunts have been in place for long periods of time the tubing may become brittle and even calcify, making it more prone to fracture. Disconnections are common during growth spurts and tend to occur at areas of movement such as the neck or at areas subject to pressure such as the region overlying the clavicle. Disconnections have become less common with newer systems that contain fewer connector sites.

Underdrainage may also be the result of loculation within the ventricular system. Loculated ventricles may occur following hemorrhage or infection. Occlusion of the foramen of Monro can create a trapped lateral ventricle. Multiple septations may occur within the ventricles, and a single shunt may be ineffective in draining these isolated fluid collections. Ventriculoscopic techniques can be used to fenestrate the septum pellucidum as well as the walls of loculated cysts. This allows simplification of shunt systems and in some cases the need for a shunt may be eliminated (e.g., after septostomy to bypass a blocked foramen of Monro, a trapped lateral ventricle may drain through normal pathways on the other side).

Ventriculoatrial shunts have a high rate of malfunction. These shunts require frequent revision because the distal end migrates out of the cardiac atrium during growth of the child. Ventriculoatrial shunts are also subject to problems of vascular or cardiac perforation, embolism, and an immune-mediated glomerulonephritis.


Shunt Malfunction from Overdrainage of CSF

All extracranial CSF shunts (ventriculo-peritoneal, -pleural and -atrial) are subject to malfunction from overdrainage. In each of these shunts there may be markedly negative pressures generated by the hydrostatic column of fluid in the tubing distal to the shunt valve when the patient assumes the upright position. The problem of overdrainage of CSF is common to the differential pressure valves, as the negative hydrostatic pressure generated in the upright position can overcome the effect of even the “high pressure” valves.

The most common symptom from overdrainage is headache. This low pressure headache must be distinguished from the high pressure headache due to CSF underdrainage. Overdrainage headache is worse in the upright position and improved when the patient is recumbent. It is often transient and will abate if the patient is allowed to adjust to the upright position slowly. Occasionally, upgrading the valve or use of a device to prevent siphoning is necessary.

When headaches are persistent one must consider the possibility of a more dangerous condition such as a subdural hematoma or hygroma. This can be a very difficulty problem to treat, particularly in patients who are shunt dependent. Burr hole drainage of the subdural collection, or subdural to peritoneal shunting with a valveless system can be utilized.

The slit-ventricle syndrome (figure to the right) is a condition characterized by intermittent headache and symptoms suggestive of shunt malfunction from underdrainage of CSF that occurs in children with small slit-like ventricles. When these children are symptomatic, the intracranial pressure is elevated. These children have usually been shunted as infants and often have small heads. The shunt valve refills slowly. There may be papilledema or cranial nerve abnormalities and, occasionally, hypertension and bradycardia. Many authors regard the symptoms and signs of the slit-ventricle syndrome to be manifestations of intermittent shunt obstruction due to small ventricular size occurring in a brain that lacks compliance, perhaps due to a diminished craniocephalic ratio. Although many children with indwelling ventricular shunts have small or slit-like ventricles, the slit-ventricle syndrome is relatively rare, occurring in 1-5% of infants shunted for hydrocephalus.


Shunt Infection

Shunt infections make up a substantial percentage of the complications that accompany shunt placement. Despite a markedly improved trend in infection rate over the past two decades, shunt infections are still reported in 2-10% of cases. The presence of ventriculitis has been shown to adversely affect intelligence. In one study, the average IQ of shunted patients having had ventriculitis was 72 as compared to 95 in shunted patients not having had ventriculitis.

Staphylococcus epidermidis is the most common organism to infect ventricular shunts. The majority of shunt infections occur within two months of shunt insertion. For this reason it is presumed that the infection is the result of intraoperative contamination. It is interesting that cultures of the skin of shunt recipients do not consistently match subsequent cultures of infected shunts. This suggests either that the source of seeding can be from an area other than the recipient’s skin, such as the nasopharynx, or from the skin of operating room personnel.