ICP monitoring

Alternative names
Intracranial pressure monitoring

Definition
A sensing device placed inside the head that senses the pressure intracranially and sends its measurements to a recording device.

How the test is performed
There are three ways of monitoring intracranial pressure:

     
  • Intraventricular catheter (a catheter threaded into one of the lateral ventricles of the brain)  
  • A subarachnoid screw or bolt (a screw or bolt placed just through the skull in the space between the arachnoid and cerebral cortex)  
  • Epidural sensor (a sensor placed into the epidural space beneath the skull)

The intraventricular catheter is thought to be the most accurate, but if immediate access is necessary, a subarachnoid bolt will probably be used. If there is not a qualified neurosurgeon to place a bolt, then a epidural sensor will probably be used.

To insert an intraventricular catheter a burr hole is drilled through the skull and the catheter is inserted through the brain matter into the lateral ventricle which normally contains cerebrospinal fluid. Not only can the intracranial pressure (ICP) be monitored, but it can be lowered by draining cerebral spinal fluid (CSF) out through the catheter. This catheter may be difficult to place with increased intracranial pressure, since the ventricles change shape under increased pressure and are often quite small secondary to brain expanding around them from injury and swelling.

A subarachnoid screw or bolt is a hollow screw that is inserted through a hole drilled in the skull and through a hole cut in the dura mater.

The epidural sensor is placed through a burr hole drilled in the skull, just over the epidural covering. Since the epidural lining is not perforated this procedure is less invasive, but it has the disadvantage of not being able to withdraw excess CSF.

Lidocaine or other local anesthetic will be injected at the site where the incision will be made, and you will most likely receive a sedative to help you relax. First the area that is to be incised is shaved and cleansed with Betadine and alcohol.

After drying, an incision is made and the skin is pulled back until the skull is visible. A drill is then used to cut through the bone and expose the epidural tissue.

If an epidural sensor is used, it is then inserted between the skull and epidural tissue. If a bolt is used, an incision is made through the dura and arachnoid tissue exposing the subarachnoid space and the bolt is screwed into the bone. This allows for the sensor to record from the subdural/subarachnoid space.

If an intraventricular catheter is used, it is threaded through the brain matter into one of the lateral ventricles. This type of catheter is most effective and accurate in its ability to sense intracranial pressure measurements.

How to prepare for the test
If you need this procedure done, you will be in the hospital and most likely in an intensive care unit. If you are conscious your health care provider will explain the procedure and the risks and a consent form must be signed.

How the test will feel
If the procedure is performed under general anesthesia you will feel nothing until you wake from the anesthesia. At that time you will feel the normal side effects of anesthesia, plus the discomfort of the incision made in your skull.

If the procedure is performed under local anesthesia you will fell a prick on your scalp like a bee sing as the local anesthetic is injected. You may feel a tugging sensation as the skin is cut and pulled back to expose the bone. You will hear a drill sound as they cut through the skull bone. This will take a varying amount of time depending on the type of drill that is used. You will also feel a tugging sensation as they suture the skin back together after the procedure.

Your health care provider may prescribe mild pain medications for relief, but he will not want to mask neurologic signs with strong pain medications. With increased intracranial pressure neurologic deficits are common.

Why the test is performed
This test or procedure is performed to measure the intracranial pressure and determine if you are at risk for developing injury from increased intracranial pressure. It is also meant to provide a sterile access for draining excess CSF.

Normal Values
Normally, the ICP ranges from 1 to 15 mm Hg.

Note: mm Hg = millimeters of mercury

What abnormal results mean

Intracranial pressure monitoring is usually done in cases of severe Head injury or after surgeries in cases where a tumor or vascular lesion has been removed or repaired and the surgical team is concerned about brain swelling.

Elevated intracranial pressure can be treated by draining CSF through the catheter, as well as by changing ventilator settings (for those patients who are in critical condition and on a respirator) or by the application of certain intravenous medications.

Intracranial pressure monitoring is crucial in identifying the problem and allowing for immediate treatment. Raised intracranial pressure signifies compression of both neural and vascular tissues and if left untreated can result in permanent neurologic damage and in some cases can be fatal.

What the risks are

     
  • infection  
  • bleeding  
  • damage to the brain tissue with residual neurologic effects  
  • risks of general anesthesia  
  • inability to locate ventricle and accurately place catheter

 

Johns Hopkins patient information

Last revised: December 5, 2012
by Potos A. Aagen, M.D.

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