Ventriculoperitoneal Shunt Tap

Ventriculoperitoneal (VP) shunts, which are used to treat hydrocephalus ^[1] , shunt cerebrospinal fluid (CSF) from the lateral ventricles of the brain into the peritoneum. Tapping or aspirating the shunt is performed for both diagnostic reasons (eg, evaluate for shunt infection and blockage) and therapeutic reasons (eg, allows fluids to be drawn off to alleviate symptoms from a blocked shunt).

The ventricles of the brain are a communicating network of cavities filled with CSF and located within the brain parenchyma. The ventricular system is composed of 2 lateral ventricles, the third ventricle, the cerebral aqueduct, and the fourth ventricle. The choroid plexuses located in the ventricles produce CSF, which fills the ventricles and subarachnoid space, following a cycle of constant production and reabsorption. For more information about the relevant anatomy, see Ventricles of the Brain.

The exact role of shunt taps in diagnosing shunt malfunctions is still under debate; ^[2] therefore, the patient's neurosurgeon should be consulted before this procedure is carried out. The nonspecialist should be aware of this procedure, as it can be lifesaving. ^[3]

Diagnostic

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• Suspected ventriculoperitoneal (VP) shunt infection or meningitis

• Suspected VP shunt blockage

Therapeutic

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• Severely raised intracranial pressure in the presence of a VP shunt

Absolute

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• Infection over the entry site

Relative

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• Coagulopathy

• Lack of shunt imaging or information

Because shunt aspiration removes cerebrospinal fluid (CSF) from the supratentorial compartment, increased intracranial pressure is not a contraindication.

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• Local anesthesia is generally not necessary; many practitioners believe that the act of infiltrating the skin and subcutaneous tissue with local anesthetic is more painful than the shunt aspiration itself.

• However, in young children, topical local anesthetic cream prior to the procedure may be useful. For more information, see Topical Anesthesia.

An image of the equipment needed is shown in the image below.

• Sterile gloves

• Povidone-iodine (Betadine) or chlorhexidine (Hibiclens) solution

• Sterile fenestrated drape

• Syringe, 3-5 mL

• Butterfly needles, 23 or 25 gauge (ga)

• Three-way stopcock

• Cerebrospinal fluid (CSF) manometer

• Gauze swabs

• Wound dressing

• Numbered specimen tubes for CSF (3-4)

  Equipment needed for a ventriculoperitoneal (VP) shunt tap.

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• The patient should be supine with the head oriented so that the ventriculoperitoneal (VP) shunt reservoir lies uppermost.

• The reservoir is usually located on the right side of the head and feels like a smooth dome under the skin.

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• Except in the presence of immediately life-threatening indications, a neurosurgeon (preferably the one who placed the shunt) should be consulted before a ventriculoperitoneal shunt is tapped.

• Explain the procedure, benefits, risks, and complications to the patient, patient’s representative, or both.

• Obtain informed consent in accordance with hospital protocol.

• Review the shunt series imaging or previous imaging to locate the shunt reservoir.

• If hair overlies the reservoir, parting and gelling the hair away from the shunt, as shown below, is often enough. Otherwise, the area around the reservoir can be shaved with a razor. In children, this should be avoided, if possible.

  Scalp preparation for tap.

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• Clean the skin with an antiseptic (eg, povidone-iodine) solution. Circle outward from the entry site or radially with each swab.

• Apply a sterile fenestrated drape over the reservoir site as shown below.

  Fenestrated drape applied to patient.

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• Insert a small (23 ga or smaller) butterfly needle perpendicular to the skin into the reservoir, as shown below. Once entered, a drop in resistance is felt. Advance slowly until the bevel of the needle is fully inside the reservoir; ^[4] then, hold the needle securely as the cerebrospinal fluid (CSF) enters the tubing.

  Shunt tap with syringe attached.

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• If flow is poor, slightly adjust the angle of the needle or alter the needle's depth. Note that poor flow is strongly correlated with proximal shunt obstruction. ^{[5, 6]}

• Measure the opening pressure using a manometer (shown below). If no manometer is available, the tubing of the butterfly needle can suffice. It is measured vertically from the top of the CSF column to the level of the reservoir or to the level of the tragus of the ear. State which landmark was used in the record.

  Cerebrospinal fluid (CSF) manometer attached to tubing for opening pressure reading.

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• Slowly collect 5 mL of CSF into separate containers for analysis, as depicted below.

  Collection of CSF samples.

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• Withdraw the needle from the reservoir and apply gentle pressure over the entry site with gauze, as shown in the image below, for at least 2 minutes.

  Gauze applied to site of shunt tap.

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• Send the CSF sample for cell count, protein level, glucose level, Gram stain, and culture. ^[7] A paired blood glucose level sample should also be taken.

  CSF samples.

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• Strict aseptic technique is essential in this procedure, as the consequence of introducing infection is usually replacement of the whole shunt.

• Allow the cerebrospinal fluid (CSF) to drip into the tubes through the end of the needle or 3-way tap.

• Aspiration can result in the choroid plexus being sucked up into the shunt tip, precipitating shunt blockage. This is especially true in slit ventricles.

• A shunt tap should only be considered after performing a CT or MRI scan of the brain and shunt series imaging. Other common infections should also be excluded, since shunt infections are unlikely more than 3 months after placement.

• Manometry and documentation of the flow rate is important. ^[8] High opening CSF pressures (>25 cm H₂ O above the level of the ventricle) are associated with distal shunt obstruction in approximately 90% of cases. ^[6] Poor flow is associated with proximal shunt obstruction in over 90% of cases. ^{[5, 6]}

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• Infection: The risk of introducing infection from a tap is very low. ^{[3, 9]} However, the consequences can mean shunt revision. Infection is mostly due to skin flora entering the shunt, and the most common organism responsible is Staphylococcus epidermidis. An aseptic technique is important to avoid this.

• Bleeding: A small risk exists of bleeding from subcutaneous vessels during the tap. Using a small butterfly needle and checking for coagulopathy before the procedure minimizes this risk.

• Cerebrospinal fluid (CSF) leak from puncture site: Using a small needle and applying pressure after the procedure can help prevent this complication.

• Ventricular collapse: Rapid aspiration of CSF from a shunt can lead to collapse of the ventricles, especially in slit ventricles. Collapse can, in turn, precipitate shunt blockage, as choroid plexus may be aspirated into the proximal tip of the shunt. It can also cause rupture of bridging veins to the brain parenchyma, creating a subdural hematoma. These are rare complications.

• Misplaced tap: Unfamiliarity with the shunt type can mean that the wrong section of tubing is punctured or that components adjacent to the reservoir have been damaged by the needle. Even with a butterfly needle, the possibility of serious trauma such as an intracranial hemorrhage exists. ^[4] This highlights the importance of correct localization and familiarity. In difficult cases, ultrasound assistance may be required, although it is rarely used. ^[10]