Intracranial pressure monitoringDefinition:
Intracranial pressure monitoring uses a device, placed inside the head, which senses the pressure inside the skull and sends its measurements to a recording device.
ICP monitoring; CSF pressure monitoring
How the test is performed:
There are three ways to monitor pressure in the skull (intracranial pressure):
- A thin, flexible tube threaded into one of the two cavities, called lateral ventricles, of the brain (intraventricular catheter)
- A screw or bolt placed just through the skull in the space between the arachnoid membrane and cerebral cortex (subarachnoid screw or bolt)
- A sensor placed into the epidural space beneath the skull (epidural sensor)
The intraventricular catheter is thought to be the most accurate method, but if immediate access is needed, a subarachnoid bolt is typically used. If no qualified brain surgeon (neurosurgeon) is available to place a bolt, then an 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 liquid (cerebrospinal fluid or CSF) that protects the brain and spinal cord. 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 get in place when there is increased intracranial pressure, since the ventricles change shape under increased pressure and are often quite small because the brain expands 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 outermost membrane protecting the brain and spinal cord (dura mater).
The epidural sensor is placed through a burr hole drilled in the skull, just over the epidural covering. Since no hole is made in the epidural lining, this procedure is less invasive than other methods, but it cannot remove excess CSF.
Lidocaine or another local anesthetic will be injected at the site where the incision will be made. You will most likely get a sedative to help you relax. First the area is shaved and cleansed with antiseptic. After the area is dry, 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 to 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 to expose the subarachnoid space and the bolt is screwed into the bone. This allows 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 effective and accurate at sensing 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 (as with any surgery) you will have to sign a consent form.
How the test will feel:
If the procedure is done while you are 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 feel a prick on your scalp like a bee sting 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 it cuts through the skull. The amount of time this takes will depend on the type of drill that is used. You will also feel a tugging sensation as the surgeon sutures the skin back together after the procedure.
Your health care provider may prescribe mild pain medications for relief. You will not receive strong pain medications, so that your doctor can check for signs of brain function. Neurologic problems are common with increased intracranial pressure.
Why the test is performed:
This test or procedure is done to measure the intracranial pressure and to learn if you are at risk for injury from increased intracranial pressure. It also provides a sterile access for draining excess CSF.
Normally, the ICP ranges from 1 to 15 mm Hg.
Note: mm Hg = millimeters of mercury
Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.
What abnormal results mean:
Intracranial pressure monitoring is usually done in cases of severe head injury. It also may be done after surgery to remove a tumor or repair damage to a blood vessel (vascular lesion) if the surgical team is concerned about brain swelling.
Elevated intracranial pressure can be treated by draining CSF through the catheter. It also may be treated by changing ventilator settings (for people who are in critical condition and on a respirator) or by giving certain medications through a vein (intravenous).
Intracranial pressure monitoring is crucial to identify the problem and treat it right away. Raised intracranial pressure means that both nervous system (neural) and blood vessel (vascular) tissues are being compressed. If left untreated, it can result in permanent neurologic damage. In some cases, it can be fatal.
What the risks are:
Brain herniation or injury from the increased pressure despite the monitor
- Damage to the brain tissue with continued neurologic effects
- Inability to find the ventricle and accurately place catheter
- Risks of general anesthesia
References: Fletcher JJ, Nathan BR. Cerebrospinal fluid and intracranial pressure. In: Goetz, CG, ed. Textbook of Clinical Neurology. 3rd ed. Philadelphia, Pa: Saunders Elsevier; 2007: chap 26.