• In hydrocephalus, the cerebral ventricles dilate due to a disturbed circulation of cerebrospinal fluid (CSF). It is often treated with a shunt that drains CSF out of the ventricles.
• No effective pharmacotherapy exists for hydrocephalus.
• Shunt blockage, infection or over- or underdrainage are possible. The shunt may predispose the patient to subdural haematoma, it may no longer be needed or it may, being an implant, cause subcutaneous discomfort.
• If a CSF shunt patient presents with signs and symptoms suggestive of increased intracranial pressure (Increased intracranial pressure), his/her life is in danger. The patient must be transferred without delay to a hospital where an emergency head CT or MRI scan can be performed immediately at all hours (MRI especially in children).

• Approximately 500 ml of CSF is formed each day, principally in the lateral ventricles. According to the traditional understanding, CSF is formed in the lateral ventricles and flows then from the 3^rd ventricle via an aqueduct to the 4^th ventricle, and further to the cortical surface and the subarachnoid spaces of the vertebral canal to be reabsorbed into the bloodstream via the arachnoid villi. The circulation of CSF is, however, more complicated and still inadequately known.
• The connection of the CSF circulation and hydrocephalus to the cerebral glymphatic system is for the time being unclear.
• Intracranial pressure – normally less than 10 cm H₂O – is increased in hydrocephalus, either intermittently or consistently.
• Obstructive (non-communicating) hydrocephalus develops as a result of an obstruction (caused by, for example, a tumour) in the normal circulation of CSF within the ventricles, and the ventricles proximal to the obstruction dilate.
• In communicating hydrocephalus all the ventricles dilate as the flow of CSF is either blocked or its reabsorption impaired.

• A shunt is not technically ideal but is prone to complications.
• The aim of a shunt is to divert the CSF flow from the ventricles so as to relieve hydrocephalus and render the patient symptom-free.
• The patient’s life and functional capacity may be totally dependent on the shunt for the duration of his/her life.
• In normal pressure hydrocephalus (NPH), the aim of a shunt is to alleviate the symptoms (memory problems, gait disturbance, urinary incontinence).
• A shunt consists of two silicone catheters with a valve mechanism in between them. The proximal catheter is, in most cases, inserted into the right lateral ventricle through a burr hole in the skull in the forehead or occipital region. The distal catheter is tunnelled subcutaneously to the peritoneal cavity or through an incision below the chin to the internal jugular vein and the right atrium of the heart.
• The valve usually sits behind the right ear. Valve opening pressure of an appropriate level prevents excessive flow of CSF through the shunt. The opening pressure of a magnetic valve can be adjusted non-invasively through the skin. MRI of the head may alter the opening pressure of the magnetic valve (see below).
• Pressing of the valve does not have any diagnostic significance, at least not in unexperienced hands.
• The catheters and valve are visible in plain x-rays of the chest and abdominal cavity. In the images of a low-dose CT scan the position of the catheters in the cerebral ventricles and peritoneal cavity can be seen better. The catheters are visible also in an MRI scan (without radiation burden; children). The valve causes an artefact in the images.

Shunt blockage and acute management

• Many factors may cause the proximal catheter, the valve mechanism or the distal catheter to become blocked.
  • The cause is often not identified.
  • Presence of blood or a high number of cells or proteins in the CSF
  • The catheter may become disconnected or the patient may outgrow the shunt (native x-ray along the whole course of the shunt).
  • The proximal catheter becoming adherent to the ependyma of the cerebral ventricle or the choroid plexus.
  • Adhesions or a cyst may develop around the tip of the peritoneal catheter (ultrasonography or CT scan).
• A patient who is fully dependent on the shunt is in immediate danger to life within a few hours if the shunt is blocked.
• If the clinical picture and the imaging findings indicate shunt malfunction, the patient must be immediately transferred to a neurosurgical unit for shunt revision.
• Depending on the local circumstances and organisation of care, there may be a possibility for teleconsultation by sending the digital CT or MRI scans directly to the specialist on call at the neurosurgical unit and together with him/her negotiate about the need of emergency care.
• Regional guidance should be followed concerning the emergency medical care skills required of the persons who will accompany the patient to the hospital.
• In an emergency situation, puncture of the shunt chamber with e.g. a butterfly needle may be considered in order to tap out fluid and thus relieve the pressure symptoms should the shunt be blocked and the patient become unconscious.
• In some magnetic valves the opening pressure may be altered during MRI scanning (independent of the body part the imaging is directed at). The opening pressure of the magnetic valve must be checked after MRI scanning, unless there is documented information on the valve being MRI safe. The shunt setting can also be checked by a skull x-ray. A neurosurgeon should be consulted regarding the imaging finding.

Infection

• An infection of a shunt in an adult manifests itself as bacteraemia and persistent fever.
• Other, more common infections should be considered and excluded first also in a patient with a shunt.
• An infected shunt almost always must be removed and replaced.
• If shunt nephritis develops as a result of an implanted ventriculoatrial shunt, the shunt must be replaced by a ventriculoperitoneal shunt.
• The penetration of the valve or catheter through the skin requires emergency neurosurgical treatment.

Overdrainage

• The most common symptom of shunt overdrainage is position-dependent low-pressure headache (upright position makes it worse).
• CT or MRI scan may detect subdural effusions or haematomas (see below).
• In the slit ventricle syndrome the cerebral ventricles appear collapsed. The patient may suffer from headache related to overdrainage or underdrainage.
• Changing to a magnetic valve, which allows the adjustment of the opening pressure, often alleviates the situation.

Shunt and subdural haematoma

• A shunt predisposed the patient to chronic subdural effusion or haematoma.
• Treatment may require trephination and/or adjustment of the shunt valve's opening pressure; in some cases the shunt may need to be closed.

Underdrainage in communicating hydrocephalus

• If underdrainage is suspected, the appropriate position of the shunt in its whole length is first confirmed through imaging.
• The function of the shunt may be assessed by an infusion test apparatus and/or shunt puncture within a neurosurgical unit.

Shunt no longer needed and shunt removal

• The shunt may be dispensed with, for example after an excision of a tumour or if the aetiology of the symptoms proves to be Alzheimer’s or some other neurodegenerative disease and not NPH.
• An unnecessary shunt is usually removed if the distal catheter is in the bloodstream (risk of "shunt nephritis"), if the shunt is problematic or if the patient is young.
• The shunt should be closed for a trial period before it is explanted. The proximal catheter is not necessarily removed because the pulling out may be associated with a risk of ventricular bleeding.

Shunt discomfort

• The patient may feel that the valve or the catheters are too prominent under the skin or they may be uncomfortable. Neurosurgical consultation may be warranted.