BASIC COMPONENTS OF CSF SHUNTS

Hydrocephalus is a term used to describe different conditions involving accumulation of different fluids in the intracranial ventricular system. Over the last decades, ventricular shunts, whose function is to "carry" this excess fluid to an alternative anatomical cavity, have become established as the main treatment for hydrocephalus.

In order to understand the complications that can arise after their implantation, it is interesting to know first of all what components can be found in a shunt system under normal conditions and what their usual positioning would be.

In general, shunt systems are composed of three main elements: a ventricular catheter, the "valvular system" itself and a distal catheter [Figure 1].

 The first will conduct cerebrospinal (or other) fluid from the ventricles to the valve, and is usually placed through a trephine hole located in the right frontal region (Kocher's point), with the distal end ideally located in the ipsilateral foramen of Monro (usually on the right side, as this part of the brain is assumed to be less eloquent) [Figure 2] .

The second will regulate the amount of fluid that comes out (like a tap that opens or closes) and allows this fluid to go in one direction only. The third will collect the amount of liquid that has passed and transfer it to an anatomical cavity, the most frequent being the peritoneal cavity [Figure 2d]. In the presence of previous extensive surgery on the abdominal cavity or infectious processes at that level, other destinations may be chosen such as the right atrium or the pleural cavity (the latter being rare nowadays).

Thus, the failure of any of these components can lead to the complications that we will see later and that will be reflected in the imaging tests.

COMPLICATIONS AND VALVE DYSFUNCTION [Figure 3]:

Ventricular shunt failure is estimated to be up to 40-50% after two years following implantation. In the case of ventriculo-peritoneal shunts, which as mentioned above are the most frequently implanted, the rate would be between 25-40% one year after surgery, reaching 70% at ten years.

IMMEDIATE POST-SURGICAL COMPLICATIONS [Figure 4]:

A. HEMORRHAGE.

As with all neurosurgical interventions, post-surgical bleeding is a potential complication to be taken into account. It can be caused by rupture of small cortical veins or subependymal vessels during insertion of the ventricular catheter, and is more frequent in patients with coagulation disorders or antiplatelet/anticoagulant treatment that cannot be suspended before the operation. Bleeding may be located within the ventricular system itself (intraventricular haemorrhage) or intraparenchymal (more frequently in the catheter trajectory).

The test of choice for its detection is a cranial CT scan, which will provide the diagnosis in most cases.

B. PNEUMOCEPHALUS.

It is not uncommon to find small amounts of intra- or extra-axial air secondary to shunt implantation, often without pathological significance or clinical relevance, so they are often visualised incidentally in the imaging tests performed as a post-surgical control. Tension pneumocephalus is a rare entity, although it should be kept in mind.

C. PNEUMOTHORAX.

Pneumothorax can occur either when attempting to cannulate a jugular line for placement of a ventriculoatrial valve or when cannulating a ventriculoperitoneal valve in the thoracic portion. These intraoperative complications are infrequent but should be considered.

MECHANICAL COMPLICATIONS:

A. OBSTRUCTION [Figure 5]:

It is estimated that obstruction of the proximal catheter can occur in up to 50% within 2 years of surgery, being less frequent in the distal catheter. The most common cause in the proximal catheter is occlusion of the "pores" of the catheter tip by waste particles from the choroid plexus or by small blood clots that may form at the implantation of the system.

Obstruction could also occur, anywhere along the trajectory, in the event of kinking of the catheter ("kinking" in the literature), which does not allow sufficient fluid to pass through the catheter.

In both cases, imaging would identify an abnormal increase in ventricular size compared to previous controls.

A special form of obstruction occurs when a pseudocyst forms at the tip of the distal catheter in the ventriculoperitoneal shunts. Pseudocysts are CSF collections, which form at the tip of the catheter due to the presence of peritoneal adhesions, and which would impede the proper outflow of fluid through the catheter [Figure 6]. It can be visualised with abdominal ultrasound and abdominal CT as a fluid-dense collection surrounding the distal catheter.

 B. CATHETER DISCONNECTION AND RUPTURE [Figure 5]:

These usually occur due to defects in materials or technical errors during surgery. Ruptures tend to occur more frequently in the neck region, as this is the area of greatest movement. Similarly, it is more common in young people with valves implanted during childhood, since due to growth, the catheters are subjected to greater mechanical stress, reaching a point where they can no longer increase in length.

The diagnosis of these entities is often clinical, as collections form in the area where the catheter has been disconnected. However, imaging not only shows these collections and where they come from, but also allows us to pinpoint the source of the problem.

DRAINAGE-RELATED COMPLICATIONS:

A. SHUNT MALFUNCTION [Figure 7].

The malfunction of the valve body will give us clinical manifestations similar to those presented by the patient prior to valve implantation. On imaging, its characteristics are similar to those found in the case of obstruction, an increase in ventricular size compared to previous controls. Other secondary signs would be: transependymal oedema, edema around the catheter or collections adjacent to the catheter.

B. OVERDRAINAGE AND VENTRICULAR SLIT [Figure 8].

Chronic overdrainage is a common condition, occurring in up to 50% of children with shunt systems, being more common in ventriculo-peritoneal shunts. Clinically, it manifests as a postural headache, when adopting the standing position, improving when the patient is in decubitus position.

On imaging, we will find a collapsed ventricular system.

Another possible manifestation of excessive valvular drainage is the appearance of subdural collections. These occur when the lateral ventricles collapse too quickly (due to excessive CSF drainage), and the brain is not sufficiently elastic to fill the space.

INFECTION [Figure 9]:

Infection of the shunt system occurs most frequently in the first 6 months after surgery, with skin flora germs generally being involved. It is estimated that infection in ventriculo-peritoneal shunts is approximately 10%.

On imaging, it will manifest itself, both on CT and MRI, as an anomalous enhancement of the ventricular and leptomeningeal system epidermis, suggesting the presence of ventriculitis or meningitis, respectively. Similarly, if they evolve, they may form collections such as abscesses, with their typical CT and MRI features.

VENTRICULAR LOCULATIONS [Figure 10]:

Ventricular loculations refer to CSF cavities that are not communicating with the rest of the ventricular system, and therefore a single ventricular catheter will not be sufficient to evacuate excess CSF. These isolated cavities can form in cases of intraventricular haemorrhage or ventriculitis.

Within this section we also find what is known as the isolated fourth ventricle. This ceases to be connected to the lateral ventricles, causing it to increase in size, leading to symptoms of intracranial hypertension and potential compression of the brainstem.

In CT or MRI we can see cavities or parts of the ventricular system increased in size with respect to other portions of the same that may appear collapsed. In addition, a certain component of transependymal edema associated with the isolated cavity may be seen.

OTHER COMPLICATIONS:

The accumulation of CSF in normal anatomical cavities can give rise to ascites, in the case of accumulation in the peritoneum. Sometimes the distal catheter remains in position causing accumulation of cerebrospinal fluid in the subcutaneous tissue [Figure 11].

Foreign body reactions have also been described in relation to the catheter material [Figure 12].