Mechanical shunting is the primary treatment for hydrocephalus. Placement of a CSF shunt is the most common pediatric neurosurgical procedure performed in the United States. It is also the neurosurgical procedure with the highest incidence of postoperative complications.6 Many types of CSF shunt systems exist (Figures 175–3, 175–4, and 175–5). Most systems consist of three components, beginning with a silastic tube passed into the ventricle via a burr hole. This tubing is tunneled subcutaneously to a valve chamber. The valve chamber, the second component, establishes a pressure gradient that ensures drainage of fluid away from the ventricle. The valve chamber, or in some cases a separate reservoir, allows access to the shunt system for patency testing, pressure measurement, CSF sampling, medication injection (e.g., chemotherapy, antibiotics), or contrast administration. Distal tubing, which is the third component, connects the valve chamber to a drainage point. The most common drainage site is the peritoneal cavity. Other drainage sites include the right atrium, gallbladder, pleural cavity, and ureter.
Example of a shunt kit. Circular objects on the left are a locator and a pressure/performance indicator. Circular object on the right is an adjustment tool. All three are needed to adjust the settings. Two sizes of valves are shown in the middle.
Typical ventriculoperitoneal shunt.
Ventriculoperitoneal shunt system.
Programmable shunt valves allow for easier control of flow rates, which is particularly useful in previously difficult cases that required frequent adjustments. The valve can be adjusted and tested using a locator and indicator tool to determine the pressure programmed into the valve. An adjustment tool can then be used to increase or decrease the valve's pressure or performance as needed. Typical nonadjustable pressure-type valves are available with low, medium, and high settings. These valves open at a pressure gradient of 2 to 4, 4 to 6, and 8 to 10 cm H2O for the low, medium, and high settings, respectively. The adjustable valves typically have five preset pressure settings that can be selected or adjusted as clinically warranted. The patient (or family) should have been given a card that documents initial settings and any subsequent adjustments. Pressure setting can also be confirmed by radiography using the radiopaque dials on the valve.
Exposure to strong magnetic fields and some MRI units can change the valve pressure setting, so all patients should have the setting verified after any exposure to strong magnetic fields. Controversy still exists as to whether programmable valves offer an advantage over traditional valves in reducing the number of revisions and extending shunt life.7,8,9
Shunt malfunctions are the most common complications encountered with CSF shunts. Shunt malfunction can be due to obstruction, mechanical failure, overdrainage, loculation of ventricles, or abdominal complications.
Obstruction is the most common type of shunt malfunction. The most frequent location of obstruction is the proximal tubing, followed by the distal tubing, and then the valve chamber. Proximal obstructions usually occur within the first years after shunt insertion. Table 175–3 lists the common causes of CSF shunt obstruction.
Causes of CSF Shunt Obstruction
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Causes of CSF Shunt Obstruction
Distal obstruction is the most frequently encountered obstruction in shunts in place for >2 years.6 Shunt obstruction usually manifests with signs and symptoms of increased intracranial pressure. Infants generally present with vomiting, irritability, and a bulging fontanelle.10 Older children and adults may present with cephalgia, nausea, vomiting, lethargy, ataxia, and cranial nerve palsies.10
Mechanical failure of shunts can be secondary to fracture, disconnection, migration, or misplacement. Typically, fractures appear in distal tubing many years after shunt placement; this is due to both degradation of tubing and stress from the growth of the patient.6 The most common location for a fracture is along the clavicle or lower ribs.6 Patients present with mild symptoms of increased intracranial pressure. Local symptoms of pain, mild erythema, and edema over the affected area are not uncommon. In fact, it not unusual for a fracture to be found incidentally because the shunt tract often serves as a conduit between the fractured segments.6 Disconnection often occurs shortly after insertion and manifests as increased intracranial pressure and fluid at the skin site around the disconnection. Migration occurs when a properly placed catheter migrates to a position in which drainage is compromised partially or completely. Misplacement entails the placement of the catheter into brain parenchyma, the choroid plexus, or the temporal horns; it usually manifests postoperatively with evidence of failure.
Overdrainage and the slit ventricle syndrome are seen in approximately 5% of patients with shunts. Because of overdrainage, the tissues actually occlude the orifices of the proximal shunt apparatus. As intracranial pressure increases, the same occluding tissue is disengaged, which allows drainage to resume. This phenomenon is cyclical and is responsible for the episodic or waxing and waning aspect of the presenting complaint. Patients present with episodes of elevated intracranial pressure caused by a transient obstruction of the ventricular catheter from a collapsed ventricle. Decreased cerebral compliance may prevent the ventricles from fully expanding as intracranial pressure and volume increase, which further contributes to ventricular collapse. The rate of this complication is lower for currently used shunt systems with antisiphon devices and programmable shunt valves.8,11
Separate, noncommunicating CSF accumulations may develop within a ventricle so that the shunt device is not able to drain the entire ventricular system, leaving behind enlarging pockets of fluid that may have compressive sequelae. Trapped fourth ventricle syndrome occurs when the fourth ventricle becomes loculated, presumably from closure of the sylvian aqueduct.6 Patients present with typical symptoms of increased intracranial pressure as well as symptoms of brainstem compression, including poor feeding, disconjugate gaze, and difficulty swallowing.
Several abdominal processes can secondarily result in shunt malfunction. The most commonly encountered complication is malfunction due to pseudocyst formation. Pseudocysts are localized abdominal fluid collections that form around the peritoneal catheter. Infection is the major cause, with an infection rate of 40%.12 They often are asymptomatic until they enlarge substantially enough to cause abdominal pain.
Symptoms of CSF shunt malfunction usually develop over several days, although rapid deterioration within 24 hours has been reported. Clinical features include mental status changes, headache, nausea, vomiting, abdominal pain, lethargy, decreased intellectual performance, ataxia, coma, and autonomic instability. Often, the presenting complaint is vague. No single sign or symptom is accurate in predicting shunt malfunction, although a decrease in level of consciousness may have the highest correlation with shunt malfunction.6 As intracranial pressure increases, paralysis of upward gaze, dilated pupils, and papilledema may develop. Paralysis of upward gaze (or sundowning) is caused by impingement on the brainstem by the third ventricle as it engorges. Symptoms of slit ventricle syndrome are exacerbated or precipitated when the patient stands or exercises due to excessive CSF drainage and are relieved when the patient lies down or is in the Trendelenburg position.
Identification of shunt type is important, although frequently difficult. Many different types exist, and appropriate assessment depends on the apparatus implanted. For example, many flow control valves have a high set resistance so that flow is quite slow but steady. Conversely, such a flow pattern might indicate obstruction in a low-resistance shunt.13 Evaluate shunt function by manual testing and radiologic studies. Palpation of the shunt allows the physician to locate the valve chamber. Shunt patency is evaluated somewhat differently for each type of device depending on features such as the presence of valves or dome- or cylinder-shaped reservoirs. Generally, testing follows intuitive expectations but still may prove perplexing to inexperienced clinicians. For a simple device, once the chamber is located, it is gently compressed and observed for refill. Difficulty compressing the chamber indicates distal flow obstruction, whereas slow refill, defined as refill requiring >3 seconds after compression, generally indicates a proximal obstruction. Compression is inaccurate for identifying shunt obstruction because up to 40% of obstructed shunts show normal refill during manual palpation.4 Furthermore, positive predictive value has been found to be as low as 12% for shunt pumping.13 In any case, further evaluation is required.
A shunt series of plain radiographs includes anteroposterior and lateral radiographs of the skull and an anteroposterior view of the chest and abdomen (for ventriculoperitoneal shunts). Although plain radiography will identify kinking, migration, or disconnection of the shunt system, CT is required to evaluate ventricular size (Figures 175–6 and 175–7). Compare with previous CT scans because many patients with shunts have an abnormal baseline ventricular size. In one series using either CT or both CT and plain radiography, 24% of patients with documented shunt malfunction showed no radiologic evidence of the malfunction.14 In another series, radiographs were shown to have a sensitivity of 20% and a negative predictive value of 22%; CT had a sensitivity of 83% and a negative predictive value of 95%.15 Therefore, in patients with suggestive clinical features, unremarkable findings on CT and/or radiographic shunt series cannot be relied on to exclude shunt obstruction. Thus, obtain neurosurgical consultation whenever shunt malfunction is suspected.
CT may reveal a persistent hydrocephalus despite the presence of a shunt, which suggests a malfunction. Comparison CTs are helpful when available.
Slit ventricle syndrome often presents with waxing and waning symptoms. The CT is often helpful in distinguishing it from other causes of malfunction.
Perform a shunt tap to make the diagnosis of shunt malfunction, exclude infection, or alleviate life-threatening increased intracranial pressure. Unless a CNS emergency exists, the shunt tap should be performed by a neurosurgeon to avoid damage to the valve apparatus. Emergency physicians should be prepared to perform a shunt tap if a neurosurgeon is unavailable or if a shunt tap is needed to control life-threatening increased intracranial pressure.
To perform a shunt tap, locate and sterilely prepare the site over the valve system or reservoir of the shaved scalp. A 23-gauge needle or butterfly attached to a manometer is inserted into the reservoir. If no fluid returns or flow ceases, a proximal obstruction is likely. Measure the opening pressure while the reservoir outflow is occluded. An opening pressure of ≥20 cm H2O indicates a distal obstruction, whereas low pressures indicate a proximal obstruction. The normal basal intracranial pressure is 12 ± 2 cm H2O.
Flash MRI has assumed a growing role in serial examination for shunt function to obviate some of the radiation exposure associated with CT. In certain institutions, single-shot T2-weighted MRI has become the initial imaging modality of choice. With the advent and widespread use of programmable shunts, the concern over shunt failure after MRI exists. The MRI magnetic field can change the valve-pressure setting in programmable valves. Newer programmable valves do not reprogram even at a 3-T magnetic field.6 In general, the settings of a programmable valve should be checked by the neurosurgeon after a patient undergoes MRI.