Intracranial pressure monitoring

Injury to the brain will often result in brain swelling. As the brain is encased in the skull, limited swelling can be accommodated until the brain is no longer able to maintain normal function. There are two potential negative consequences from this swelling: ischemia due to compression of the brain tissue resulting in lack of blood and oxygen, and herniation of the brain. The three main components in determining ICP is the blood circulation in the brain, cerebrospinal fluid (CSF), and the brain tissue itself. This relationship is dictated by the Monro-Kellie doctrine, which states that as the brain swells, intracranial pressure (ICP) rises and cerebral perfusion decreases. As the brain swelling exceeds a certain point called the critical closing pressure (CrCP), the arterioles feeding the brain oxygen-rich blood will collapse, and the brain becomes deprived of blood. == Methods ==

Under normal conditions, regular movements such as leaning forward, normal heartbeat and breathing can cause changes to the ICP. Intracranial monitoring accounts for this by averaging measurements over 30 minutes in non-comatose patients. Readings between 7-15mmHg are considered normal in an adult, 3-7mmHg in children, and 1.4-6mmHg in infants. Drawbacks to EVDs are the difficulty to place in comparison to other methods -- especially in the setting of brain swelling or anatomical variation in ventricle size – and once placed, are at increased risk of blockage from blood, air bubbles, or other debris. Intraparenchymal pressure monitor There are three types of intraparenchymal pressure monitors (IPM), also called bolts: fiber optic, strain gauge, and pneumatic sensors. Fiber optic monitors use changes in light reflected back from a mirror at the end of the cable to reflect changes in the ICP. Strain gauge monitors use a diaphragm that is bent by surrounding pressure, which is then converted into electrical signals used to calculate changes in ICP. Pneumatic sensors are fitted with a balloon which measures the surrounding pressure, thereby measuring the ICP. IPMs are as equally accurate as EVDs, but cannot be recalibrated after placement, which is a major clinical limitation of this method of intracranial pressure monitoring. Risks of IPMs are similar to risks of EVDs as both require a surgical procedure. However, placement of IPMs is still considered less invasive than placement of EVDs. Additionally, placement of IPMs do not require the precision needed for EVD placement, and they are less affected by structural changes to the brain such as brain swelling or midline shift. IPMs can be placed not only in the parenchyma but also in the ventricular, subarachnoid, subdural, or epidural spaces. Generally, IPMs are chosen when EVD placement is unsuccessful or if CSF drainage is determined to likely not be necessary. Continuous brain tissue oxygen tension (PbO2) This method of intracranial pressure monitoring requires placement of an oxygen probe into the penumbra, the area surrounding the injury that is most at risk of secondary injury from hypoxia. The probe measures levels of oxygen in the area, with levels under 15mmHg treated with increasing oxygen levels in the body. Non-invasive There are many noninvasive methods for intracranial pressure monitoring such as transcranial doppler (TCD), and optic nerve sheath diameter (ONSD). While none of these methods have been able to have the accuracy, reliability, and independent validation of invasive methods, they may eventually be used in determining the severity of injury and if there is a need for more invasive measures. == References ==