Increased Intracranial Pressure
Intracranial pressure (ICP) is the pressure in the skull that results from the volume of three essential components: cerebrospinal fluid(CSF), intracranial blood volume and central nervous system tissue. The normal intracranial pressure is between 5-15 mmHg. This is slightly lower than the mean systemic arterial pressure but considerably higher than venous pressure.
The intact cranium is essentially inexpandable containing about 1400 grams ofcentral nervous system (CNS) or brain tissue, 75 ml of blood and about 75 ml of cerebrospinal fluid (CSF). These three components of the cranial vault maintain a state of equilibrium. Their pressure and volume determine the condition of balance. According to Monro-Kellie hypothesis, any increase in one of these elements must be balanced or compensated by a proportional constriction either or both of the other two components such as decreasing the volume of cerebral blood flow, shifting CSF flow (into the spinal canal) or increasing CSF absorption. Absence of these compensatory changes results toincreased intracranial pressure. Once ICP reaches around 25 mmHg marked elevation in intracranial pressure will be noted.
CSF is formed from the blood by the choroid plexuses, which are hanging at the roof of the brain’s ventricles. From the point where it is produced, it flows through the aqueduct of Sylvius to the fourth ventricles. Three apertures (opening) are found in the fourth ventricle which serves as passageway going to the subarachnoid spaces in the brain and spinal cord. These openings are Foramina of Magendie (median aperture) and two Foramina of Luschka (lateral apertures). A presence of tumor in choroid plexus may cause an overproduction of CSF. If the passageway of CSF is obstructed or brain tissue damage during surgery occurs, elevated ICP is inevitable.
Normally, a change in CSF and blood volume occurs. For instance, during exhalation a temporary rise in intrathoracic pressure occurs. This impairs cerebral venous drainage and thereby reabsorption of CSF. An increase in ICP might likely occur, unless the blood will be expelled or the brain tissue will shrink (compensatory mechanism). If no compensation will occur, based on Monro-Kellie hypothesis, a slight increase in intracranial pressure will take place. The same process occurs during Valsalva maneuver (forcible exhalation against a closed glottis), sneezing, coughing and straining at stool. This is the main reason why people with increase ICP and at risk for cerebral hemorrhage are instructed to avoid these instances.
Presence of carbon dioxide can also increase ICP. Carbon dioxide is a potent vasodilator that dilates aretrioles (including those in the chorionic plexus in the brain) which elevates cerebral blood volume and ICP.
Etiology
CSF – hydrocephalus
- Overproduction of CSF
- Meningitis
- Subarachnoid hemorrhage
- Brain tumor
- Impediment of CSF flow
- Narrowed foramina of Magendie and Luschka
- Obstruction in the Aqueduct of Sylvius
- Arnold-Chiari disorder
- Interference with CSF absorption
- Surgery
CNS tissue
- Head injury
- Cerebral edema
Blood
- Cerebral venous sinus thrombosis
- Hematoma
- Increased carbon dioxide partial pressure
Sources:
- Medical Surgical Nursing by Smeltzer and Bare
- Pathophysiology by Nowak and Handford
