Shunt Malfunction

Shunt malfunctions are the most common complications encountered with CSF shunts, occurring in up to 67 percent of patients during their lifetime. Obstruction is the most common type of shunt malfunction and most commonly occurs in the proximal tubing followed by the distal tubing, and, finally, the valve chamber. Proximal obstructions usually occur within the first two years after shunt insertion. Causes include tissue debris, chorioid plexus, clot, infection, catheter-tip migration, or following a localized immune response to the tubing. Kinking or disconnection of the tube, pseudocyst formation, or infection can cause distal obstruction. Distal obstruction is the most frequently encountered obstruction in shunts in place for longer than two years. 12

SLIT VENTRICLE SYNDROME Overdrainage and the slit-ventricle syndrome are seen in approximately 5 percent of shunted patients. Due to overdrainage, the tissues actually occlude the orifices of the proximal shunt apparatus. As ICP increases, the same occluding tissue is disengaged, allowing drainage to resume. This phenomenon is cyclical and is responsible for the episode 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, further contributing to ventricular collapse. Newer shunt systems with antisiphon devices and improved shunt valves have a lower rate of this complication.12

CLINICAL PRESENTATION Symptoms of shunt malfunction usually develop over several days although rapid deterioration within 24 h 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. As intracranial pressure increases, paralysis of upward gaze or sundowning, dilated pupils and papilledema may develop. "Sundowning" is due to impingement of the brainstem by the third ventricle as it engorges. Symptoms of slit ventricle syndrome are exacerbated or precipitated by standing or exercise due to excessive CSF drainage and relieved by lying down or the Trendelenburg position.

SHUNT EVALUATION Identification of shunt type is important although frequently difficult. Many different types exist and appropriate assessment is dependent on the apparatus implanted. Shunt function is evaluated 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 such features as valves, dome or cylinder-shaped reservoirs. Generally, testing follows intuitive expectations but may yet 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, while slow refill, defined as greater than 3 s, following compression indicates a proximal obstruction. Clinicians should realize that compression is inaccurate in identifying shunt obstruction as up to 40 percent of obstructed shunts have normal refill during manual palpation.2 In any case, further evaluation is required.

A shunt series of plain films includes an AP and lateral radiographs of the skull, and an AP view of the chest and abdomen (for ventriculoperitoneal shunts). While plain radiography will identify kinking, migration or disconnection of the shunt system, CT is required to evaluate ventricular size. Comparison to previous CT scans is needed as many shunted patients have an abnormal baseline ventricular size. In one series, using CT, or both CT and plain films, 25 percent of patients with documented shunt malfunction had no radiologic evidence of shunt malfunction.3 Therefore, in patients with suggestive clinical features, unimpressive CT and/or shunt series cannot be relied on to exclude shunt obstruction. In this instance, obtain neurosurgical consultation whenever shunt malfunction is suspected.

A shunt tap may need to be performed to make the diagnosis of shunt malfunction, rule out infection, or to alleviate life threatening increased intracranial pressure. The shunt tap should be performed by a neurosurgeon if available. 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. The scalp should be shaved. A 23-g needle or butterfly attached to a manometer is inserted into the reservoir. If no fluid returns or flow ceases, a proximal obstruction is likely. The opening pressure should be measured while occluding the reservoir outflow. An opening pressure of 20 cm or greater indicates a distal obstruction while low pressures indicate a proximal obstruction. The normal basal intracranial pressure is around 12 ± 2 cm.

MANAGEMENT Surgical intervention is generally required for shunt obstruction. As a temporizing measure intracranial pressure can be lowered by standard methods—hyperventilation and osmotic diuresis (mannitol). If these measures fail and surgical intervention is not immediately available, the emergency physician can lower intracranial pressure when the malfunction is distal by removing CSF via the reservoir as previously described. To prevent choroid plexus bleeding, CSF is removed slowly, and the process is discontinued when intracranial pressure reaches 10 to 20 cm. Stable patients with suspected obstruction require admission and neurosurgical consultation. These patients should be observed for any neurologic changes, abdominal complaints, or development of fever.

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