Shunt Malfunction

Repeated shunt malfunction is the primary long-term problem with CSF shunts (Fig. 5). The most common type of failure is obstruction secondary to overgrowth from the ependyma or choroid plexus. Approximately 50% of all shunt malfunctions arise from obstruction of either the proximal catheter (70%), distal catheter (20%), or other portions of the shunt (10%). The remainder of shunt failures can be divided into shunt infection (see next section), component fracture, skin breakdown, or symptoms related to shunt overdrainage. Approximately 50% of newly inserted shunts will fail in the first 2 to 3 years after insertion. In a randomized trial of various shunt designs, the overall 1-year failure rate was 40%. Only 30% of shunts remain functioning 3 years after insertion. Children younger than 6 months of age

Figure 5. Typical causes of shunt malfunction: obstruction of the proximal catheter (left image); obstruction of the valve with blood and debris (middle image); and fracture of the valve (right image).

at time of insertion have higher shunt malfunction rates than older children. While some children will not require a shunt revision over many years, these are the exception. Many modifications and innovations in shunt design have not dramatically affected the durability of CSF shunts.

Evaluation of Suspected Shunt Malfunction

Shunt malfunction presents by and large with symptoms identical to that of the initial hydrocephalus. Most patients have a common set of symptoms that occur with shunt malfunction and are usually the same over multiple shunt malfunctions. There is a large variation, however, in the pattern of symptoms between affected individuals. For this reason, a child's parents are often best able to detect early and subtle symptoms that might not be apparent to a medical professional. Common symptoms of shunt malfunction include headache, vomiting, changes in school-work, lethargy, changes in extremity spasticity for children with spina bifida and other changes in behavior. The diagnostic evaluation for shunt malfunction should include an imaging study of the shunt, such as a plain X-ray shunt series, and an imaging study of the brain and ventricles such as a CT scan. In children who have had a shunt system for many years, plain X-ray films are important for detecting shunt fracture. Most shunt hardware is visible on plain X-ray film although some components are not and small gaps in a shunt system can be misinterpreted as a shunt fracture.

If these studies do not convincingly demonstrate a shunt malfunction, additional diagnostic steps may be necessary. A partially obstructed shunt may allow some CSF to drain without much change in ventricular size. Tapping the shunt directly through a reservoir is also a method of evaluating shunt flow and function. Usually the access reservoir is proximal to the valve; if CSF cannot be aspirated easily, this is consistent with a proximal obstruction. Nuclear medicine tests, such as shunt function studies, involve injecting a small volume of radioactive tracer into the reservoir and then following the passive flow of the tracer over time (usually 15 to 30 minutes). The majority of the tracer should clear from the shunt and disperse into the peritoneal cavity within 5 minutes (Fig. 6). These studies can be misleading in the presence of partial shunt function.

In patients who have intermittent symptoms without clear imaging evidence of shunt malfunction, other measures of ICP such as a fundoscopic examination and/ or a lumbar puncture may provide evidence of a shunt malfunction. If no convincing evidence is available, then close observation and serial imaging studies may be the most prudent course of action. In some cases, exploration of the shunt is sometimes the only method to determine whether the shunt is functioning or not.

Technique of Shunt Revision

Once the diagnosis of shunt malfunction is clearly established, a surgical exploration and revision of the shunt should be performed without delay. The initial step in revising an existing shunt is the opening the cranial incision to allow access to the junction between the ventricular catheter and valve. A few shunt systems consist of single piece units that need to be cut in order to assess their function during surgery,

Figure 6. A normal nuclear medicine shunt study is shown. An image of the injected shunt reservoir and distal tubing is on the right side of the figure. A region of interest is drawn around the reservoir and the percentage of radioactive tracer remaining within the reservoir is measured over time. These results can be plotted over time, as seen on the graph. This is a normal study with 50% of the tracer emptying within 3 minutes.

Figure 6. A normal nuclear medicine shunt study is shown. An image of the injected shunt reservoir and distal tubing is on the right side of the figure. A region of interest is drawn around the reservoir and the percentage of radioactive tracer remaining within the reservoir is measured over time. These results can be plotted over time, as seen on the graph. This is a normal study with 50% of the tracer emptying within 3 minutes.

but these are the minority. The ventricular catheter is disconnected from the valve. In a clear instance of proximal catheter obstruction, CSF will either not emerge from the visible end of the catheter, or do so very slowly. In this case, the ventricular catheter is changed using the same entry point. Removal of the existing ventricular catheter is sometimes complicated by intraventricular hemorrhage because the choroid plexus has a tendency to grow around and into the end of the catheter. This can be minimized by placing a thin metal stylet into the ventricular catheter and using cautery to coagulate any tissue within the lumen of the tubing. Nevertheless, if a significant ventricular hemorrhage occurs, the procedure may need to be ended and a temporary external drainage catheter placed until the CSF clears.

In all cases, the distal peritoneal tubing should be tested using a manometer to assess the runoff of a column of saline. Typically, saline will flow though the manometer and reach the set pressure of the shunt valve in a few seconds. If there is doubt regarding the function of the distal catheter, it will also require replacement. For this reason, even though proximal obstruction occurs in the vast majority of cases, the surgeon should be prepared to replace any segment or the entire shunt system.

Shunt Infection

Evaluation

Shunt infections can present in a number of ways: (a) meningitis, (b) an indolent infection with a chronic inflammatory response leading to shunt obstruction, (c) local soft tissue infection around the shunt hardware with wound breakdown and/or purulent discharge, or (d) infection within the peritoneal cavity that presents with abdominal pain, shunt obstruction and/or an accumulation of fluid within the peritoneal cavity.

Approximately two-thirds of all shunt infections are caused by staphylococcal species (S. epidermidis and S. aureus being the most common). These bacteria probably colonize a shunt at the time of insertion, and an infection usually becomes clinically apparent within the first 6 months after insertion. The variability in time of presentation depends on the degree of colonization, virulence of the organism and host factors. Only 10% to 20% of shunt infections present more than 6 months after insertion. This observation guides the management of children with a suspected shunt infection. A child with neurological signs or signs consistent with meningitis in the first six months after insertion should have a CSF sample taken before antibiotics are started. This sample should be obtained directly from the shunt, since a lumbar puncture can occasionally provide a false negative result. If nonspecific symptoms such as fever and irritability are present, the child should be examined for more common causes such as an upper respiratory infection, gastroenteritis, or otitis media. Routine shunt aspiration or lumbar puncture is not indicated if another source for the fever is clearly identified. If nonspecific signs such as fever and irritability are present without an obvious source, and the patient is within 6 months of insertion, the shunt should be aspirated to obtain a CSF sample. A negative Gram stain is not sufficient to exclude infection as some organisms require a minimum of 48 hours of culture to be identified. Blood cultures are rarely positive with shunt infections unless a VA shunt is present. The abdomen should always be examined for signs of peritoneal infection. If there is doubt, an abdominal ultrasound will usually identify a significant fluid collection. A bacterial infection within the peritoneal cavity severely impairs its absorptive capacity, and infected CSF causes the omentum to create a 'pseudocyst' around the accumulating CSF. A large locu-lated fluid collection within the abdomen is strongly suggestive of a shunt infection.

Children who are evaluated for a febrile illness more than 6 months after shunt insertion will rarely have a shunt infection. Other sources should be pursued diligently before the shunt is attributed as the cause. However, if all diagnostic tests are negative and a febrile illness persists, the shunt should not be overlooked as the cause. Antibiotics should be withheld until all cultures are taken. If a child is sufficiently ill that antibiotics are required immediately, then subsequent CSF cultures may be negative. It may be necessary to confirm CSF sterility after a course of antibiotics is completed if symptoms persist.

Treatment

The presence of a shunt infection proven by CSF Gram stain or culture requires treatment with appropriate antibiotics (discussed in Chapter 13) and removal of the shunt hardware. If there is an obvious wound infection with purulent drainage, the shunt hardware is removed and an external catheter is placed simultaneously to drain CSF at another site. If the soft tissues are not involved, then an external ventricular catheter is placed through the same entry point as the shunt. For distal or peritoneal infections, the initial surgical step is to remove the peritoneal catheter

from the abdomen and connect it to an external collection bag. If the proximal valve and ventricular catheter are proven to be sterile, then only a new distal peritoneal catheter is required. However, if any doubt exists regarding the sterility of existing hardware, then the entire shunt should be replaced with a new system. In some cases, children will have nonfunctional shunt hardware either within the ventricular system or peritoneal cavity. These pieces should be removed in order to eliminate all potential reservoirs for bacteria that could recolonize a newly placed shunt.

The exact duration of antibiotic therapy required to prevent re-infection remains unknown, although most surgeons require at least 5 to 10 days of treatment prior to re-internalization of the shunt. During external drainage, CSF cultures can be sent regularly to establish when CSF sterility has been achieved. Some surgeons use a standard course of treatment for most shunt infections and dispense with regular CSF cultures. This approach may need to be modified if an unusual organism is present, or if the patient has a persisting fever or new symptoms. In rare circumstances, an intracranial abscess (epidural or subdural) can be associated with a shunt infection and, if suspected, an imaging study with contrast should be obtained. Shunt infection is a major cause of cognitive morbidity in patients with hydroceph-alus and should be treated aggressively.

Other Issues in the Management of Shunted Hydrocephalus

Outcome

The presence of a shunt itself does not predispose a patient to a poor cognitive outcome. The underlying cause of hydrocephalus is a much stronger predictor regarding functional outcome. Many children who receive a shunt for a disease or disorder such as aqueductal stenosis or a benign brain tumor may be cognitively normal and can lead long and productive lives. As expected, children who have neonatal hydrocephalus because of secondary to a grade IV intraventricular hemorrhage or meningitis tend to have poorer outcomes.

Complex Hydrocephalus

Most patients with hydrocephalus will require multiple shunt revisions, but these surgical procedures are accomplished with low morbidity. There are, however, less common forms of hydrocephalus that are far more difficult to manage. These include multi-loculated ventricles, slit ventricle syndrome, overdrainage and loss of distal shunt locations. Multiloculated ventricles typically arise following a severe infection, such as with Gram-negative organisms. Inflammation of the ventricular walls leads to adhesions within the ventricular system and lack of communication between different CSF compartments. Rather than one shunt draining the ventricles, at times several separate shunt systems are required. Endoscopic fenstrations created between compartments can allow CSF to flow more normally, but often damage to the ventricular surface leads to ongoing problems.

Slit ventricle syndrome refers to a situation in which shunt malfunction and clinically apparent hydrocephalus occur in the absence of ventricular enlargement; usually the patient has very small ventricles. The cause is believed to be reduced compliance (i.e., increased 'stiffness') of the brain. There is some evidence that a

disproportion between brain and cranial volume also contributes to the pathogenesis. These patients require multiple shunt revisions and the creation of a functioning shunt system is a challenge. Alternative sites for ventricular drainage, such as the lumbar space, or foramen magnum may be required. Although a complete discussion of these conditions is beyond the scope of this chapter, in general, a logical and consistent approach eventually results in a satisfactory outcome in the vast majority of patients.

Suggested Readings

1. Drake JM, Sainte-Rose C. The Shunt Book. New York: Blackwell Science, 1995.

2. Drake JM, Kestle JR, Milner R et al. Randomized trial of cerebrospinal fluid shunt valve design in pediatric hydrocephalus. Neurosurgery1998; 43:294-303; discussion 303-305.

3. Hoppe-Hirsch E, Laroussinie F, Brunet L et al. Late outcome of the surgical treatment of hydrocephalus. Childs Nerv Syst 1998; 14:97-9.

4. Kestle JR. Pediatric hydrocephalus: current management. Neurol Clin 2003; 21:883-95.

5. Rekate HL. Hydrocephalus in children. In: HR Winn, ed. Youmans Neurological Surgery. 5 th ed. Philadelphia: WB Saunders, 2004:chapter 215.

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