Intracranial Regulation

Increased intracranial pressure, or ICP, is a life-threatening condition characterized by increased pressure within the skull.

Now, increased ICP can occur if there’s an increase in the components of the brain, including brain tissue, cerebrospinal fluid, or CSF, or the blood supplying the brain. An increase in brain components might develop in the case of cerebral edema, which can be due to infections, trauma, or hypoxia; space-occupying lesions such as brain tumors, abscesses, or intracranial hemorrhage; or disruptions in CSF circulation, like with hydrocephalus.

Alright, so, when it comes to regulating ICP, remember that the mature skull is a rigid structure that can’t expand. So, to maintain a normal ICP, there must be a balance between the volume of brain tissue, CSF, and blood. If there’s an increase in any one of these three, there should be a compensatory decrease in the other two, which is known as the Monro-Kellie hypothesis. Normally, the volume of the brain remains relatively stable, so ICP can be regulated by changes in CSF and blood volume. When needed, CSF production can be decreased, or reabsorption can be increased, to help normalize ICP. Similarly, cerebral blood volume can be decreased either by cerebral vasoconstriction or increasing venous blood drainage out of the skull.

Now, the compensatory mechanisms that keep ICP within a normal range have limits and can be overwhelmed. When this happens, ICP starts to increase, which puts pressure on the ventricles and cerebral blood vessels. In turn, this decreases cerebral blood flow and cerebral perfusion. Decreased cerebral perfusion means that less oxygen is delivered to the brain tissue, which causes neurons to swell and die, further increasing ICP. At the same time, hypercarbia, or a build-up of carbon dioxide can occur, causing vasodilation, which also contributes to increased ICP.

If not corrected, increased ICP can lead to complications like papilledema, or swelling of the optic disc, which is the point where the optic nerve leaves the retina; or brain herniation, which occurs when a part of the brain is pushed through rigid spaces into another space of the skull or even out of it. Brain herniation is a life-threatening emergency, and if it’s not promptly treated, it can be fatal.

Now, early clinical manifestations of increased ICP include altered mental status, nausea and vomiting, headache, sluggish pupillary reaction to light, and even seizures. Additionally, papilledema may cause visual abnormalities, such as double vision or even vision loss.

On the other hand, late clinical manifestations include hypertension, bradycardia, and irregular breathing; these signs are referred to as Cushing’s triad, which indicate advanced brainstem dysfunction. There could be fixed or dilated pupils, as well as loss of brainstem reflexes such as the gag reflex, swallowing reflex, or pupillary and corneal reflexes. The level of consciousness could progress to coma.

Lastly, there may be abnormal posturing, such as decerebrate or decorticate posturing. Decerebrate posturing is where the arms are stiffly extended and abducted, and the wrists are pronated, with flexed fingers; while the legs are extended, with the feet in plantar flexion. With decorticate posturing, the arms are adducted and flexed on the chest, with flexed wrists and fingers, while the legs are extended and internally rotated, with plantar flexion of the feet.