Intracranial pressure (ICP)

Intracranial pressure (ICP) is the pressure inside the skull and thus in the brain tissue and cerebrospinal fluid (CSF). The body has various mechanisms by which it keeps the ICP stable, with CSF pressures varying by about 1 mmHg in normal adults through shifts in production and absorption of CSF. CSF pressure has been shown to be influenced by abrupt changes in intrathoracic pressure during coughing (intraabdominal pressure), valsalva (Queckenstedt's maneuver), and communication with the vasculature (venous and arterial systems). ICP is measured in millimeters of mercury (mmHg) and, at rest, is normally 7–15 mmHg for a supine adult. Changes in ICP are attributed to volume changes in one or more of the constituents contained in the cranium.

Intracranial hypertension, commonly abbreviated IH, IICP or raised ICP, is elevation of the pressure in the cranium. ICP is normally 7–15 mm Hg; at 20–25 mm Hg, the upper limit of normal, treatment to reduce ICP may be needed.

One of the most damaging aspects of brain trauma and other conditions, directly correlated with poor outcome, is an elevated intracranial pressure. ICP is very likely to cause severe harm if it rises too high. Very high intracranial pressures are usually fatal if prolonged, but children can tolerate higher pressures for longer periods. An increase in pressure, most commonly due to head injury leading to intracranial hematoma or cerebral edema, can crush brain tissue, shift brain structures, contribute to hydrocephalus, cause brain herniation, and restrict blood supply to the brain. It is a cause of reflex bradycardia.

It is also possible for the intracranial pressure to drop below normal levels, though increased intracranial pressure is a far more common (and far more serious) sign. The symptoms for both conditions are often the same, leading many medical experts to believe that it is the change in pressure rather than the pressure itself causing the above symptoms.

Spontaneous intracranial hypotension may occur as a result of an occult leak of CSF into another body cavity. More commonly, decreased ICP is the result of lumbar puncture or other medical procedures involving the brain or spinal cord. Various medical imaging technologies exist to assist in identifying the cause of decreased ICP. Often, the syndrome is self-limiting, especially if it is the result of a medical procedure. If persistent intracranial hypotension is the result of a lumbar puncture, a "blood patch" may be applied to seal the site of CSF leakage. Various medical treatments have been proposed; only the intravenous administration of caffeine and theophylline has shown to be particularly useful.

The causes of increased intracranial pressure (ICP) can be divided based on the intracerebral components causing elevated pressures:

Increase in brain volume

Generalized swelling of the brain or cerebral edema from a variety of causes such as trauma, ischemia, hyperammonemia, uremic encephalopathy, and hyponatremia

Mass Effect

Hematoma

Tumour

Abscess

Infarct

Increase in Cerebrospinal Fluid

Increased production of CSF

Choroid plexus tumour

Decreased Reabsorption of CSF

Obstructive hydrocephalus

Meningeal inflammation or granulomas

Increase in Blood Volume

Increased cerebral blood flow during hypercarbia, aneurysms

Venous stasis from

Venous sinus thromboses,

Elevated central venous pressures, e.g., heart failure

Other Causes

Idiopathic or benign intracranial hypertension

Skull deformities such as craniosynostosis

Hypervitaminosis A, tetracycline use

Pathophysiology

The harmful effects of intracranial hypertension are primarily due to brain injury caused by cerebral ischemia. Cerebral ischemia is the result of decreased brain perfusion secondary to increased ICP. Cerebral perfusion pressure (CPP) is the pressure gradient between mean arterial pressure (MAP) and intracranial pressure (CPP = MAP - ICP). CPP = MAP - CVP if central venous pressure is higher than intracranial pressure. CPP target for adults following severe traumatic brain injury is recommended at greater than 60 to 70 mm Hg, and a minimum CPP greater than 40 mm Hg is recommended for infants, with very limited data on normal CPP targets for children in between.

Cerebral autoregulation is the process by which cerebral blood flow varies to maintain adequate cerebral perfusion. When the MAP is elevated, vasoconstriction occurs to limit blood flow and maintain cerebral perfusion. However, if a patient is hypotensive, cerebral vasculature can dilate to increase blood flow and maintain CPP.

What are the signs and symptoms of increased intracranial pressure?

The signs and symptoms of increased ICP include:

headache

nausea

vomiting

increased blood pressure

decreased mental abilities

confusion

double vision

pupils that don’t respond to changes in light

shallow breathing

seizures

loss of consciousness

coma

Many of these symptoms can appear with other conditions, but things like confusion and behaviour changes are common early signs of increasing intracranial pressure. Your doctor will usually use other symptoms or knowledge of your personal and family medical history to determine the cause of your symptoms.

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