2.1 Introduction

A shunt is a tubular system, for cerebrospinal fluid (CSF) diversion, that is composed of a proximal catheter, valve, distal catheter, and sometimes a reservoir. It allows for the alleviation of elevated intracranial pressure (ICP), particularly hydrocephalus, by diverting CSF from the ventricular system to a distal terminus. The most common terminus for a shunt is the peritoneal cavity: however, the pleura or right ventricle of the heart are often used as a secondary option as well. ▶ Fig. 2.1 depicts a ventriculoperitoneal shunt system. The etiologies of hydrocephalus are many and varied and are beyond the scope of this chapter.

Once a neurosurgeon implants a shunt, many complications may arise in both an acute setting or many years after the initial surgery. The most common complication of a shunt is malfunction. The reason for the malfunction is of vital importance to the clinician in order to ascertain how to properly treat and correct the underlying etiology for the “shunt failure.” A detailed history and physical examination, including an examination for papilledema is key. Of note, patients who present with shunt failure, either in the ambulatory or emergency setting, will most often re-present with their initial symptoms prior to having the shunting procedure, many of which are associated with elevated ICP including papilledema, headache, vomiting, diplopia, impaired up-gaze, coma, seizure, or imbalance.

In the process of working up a shunt malfunction, one must determine whether it is due to infection, hardware failure (e.g., fractured hardware), proximal catheter obstruction, obstruction of the shunt valve, or distal catheter obstruction. Critical to this workup is the following imaging: computed tomography (CT) of the head or magnetic resonance imaging (MRI) of the brain, anteroposterior (AP) and lateral skull X-rays, chest X-ray, and abdominal X-ray (if the distal terminus is in the peritoneum). Having prior imaging from previous encounters, in particular one that allows visualization of the ventricular system is extremely useful. The skull film enables the clinician to determine what type of valve the shunt system has (▶ Fig. 2.2), whether it has a reservoir, and where they are in relation to skull anatomy. Additionally, the X-rays should be reviewed for any fractures of the hardware, disconnections, or kinking of the tubing that could lead to malfunction (▶ Fig. 2.3). One may also undertake a nuclear shunt study, or “shunt-o-gram,” which entails injection of a radioisotope into the shunt system to determine if there is proper flow. If prior imaging and a nuclear shunt study are unavailable, then the provider must rely on clinical judgment. A shunt tap or externalization can be performed for diagnostic and therapeutic purposes in the setting of shunt malfunction.

2.2 Relevant Anatomy and Physiology

A shunt system allows for diversion of CSF from the ventricles of the brain to other areas of the body. When the pressure in the ventricular system, or ICP, is greater than that of the opening pressure setting of the valve, fluid will flow from the proximal ventricular catheter through the system to the terminus of the distal catheter. The body then absorbs this excess of CSF in the cavity into which it is deposited. A more detailed description of CSF physiology can be found in the ventriculostomy chapter (see Chapter 1 External Ventricular Drain).

Most often, the system begins with a proximal ventricular catheter either frontally, near Kocher’s point, or posteriorly near Keen’s or Frazier’s point (see Chapter 1 External Ventricular Drain). This proximal catheter is then attached to a valve, with or without a reservoir, and a distal catheter is connected to the valve or reservoir. The shunt valve most often sits on the flat surface of the skull. This is important for shunt programming and shunt tapping in order to identify the valve by palpation. The course of the distal catheter most commonly runs in the anterior lateral portion of the neck, riding over the sternocleidomastoid and then continuing over the clavicle prior to depositing either in the pleura or peritoneum. For those patients who have a ventricular atrial shunt, the distal catheter is placed in either the facial vein or the internal jugular vein within the neck. It is of vital importance to know the course of the distal catheter, especially if one is to externalize the shunt.

2.3 Indications—Shunt Tap (▶ Fig. 2.3)

Patients who have shunts may have several reasons to have their shunt systems evaluated. Often times, if one is concerned as to whether the shunt is working properly or not a shunt tap may be most useful.

The placement of a small gauge needle, 23-gauge butterfly needle most preferably, allows for accessing the shunt system. By doing so the clinician must realize that there is a risk of introducing potential infection to a sterile environment. This risk must be weighed carefully when considering the need for a shunt tap in evaluating a shunt system.

There are two primary reasons to tap a shunt: first, to obtain access to the CSF, and second, to determine if there is proper flow of CSF. These two categories can be further subdivided. By obtaining access to CSF via shunt tapping, one can easily take a CSF sample to be evaluated for microbiological purposes for infectious workup. With this access to the shunt system one can also use this opportunity to administer intrathecal medications (i.e., antibiotics or medications).

During workup for possible shunt failure, information from a shunt tap can be most useful in helping to determine if there is proper flow or if there is an obstruction within the system. This can be done either by injecting a radioisotope into the system or by manually evaluating the flow (this will be discussed later in this chapter).

2.4 Contraindications—Shunt Tap

The following are relative contraindications to performing a shunt tap:

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  Scalp infection

  
  
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  Nonaccessible shunt valve or lack of shunt reservoir

  
  
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  Bacteremia