Intracranial pressure (ICP) is the pressure exerted by fluids such as cerebrospinal fluid (CSF) inside the skull and on the brain tissue. ICP is measured in millimeters of mercury (mmHg) and at rest, is normally 7–15 mmHg for a supine adult. This equals to 9–20 cmH2O, which is a common scale used in lumbar punctures. 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. Changes in ICP are attributed to volume changes in one or more of the constituents contained in the cranium. CSF pressure has been shown to be influenced by abrupt changes in intrathoracic pressure during coughing (which is induced by contraction of the diaphragm and abdominal wall muscles, the latter of which also increases intra-abdominal pressure), the valsalva maneuver, and communication with the vasculature (venous and arterial systems). Intracranial hypertension (IH), also called increased ICP (IICP) or raised intracranial pressure (RICP), refers to elevated pressure in the cranium. 20–25 mmHg is the upper limit of normal at which treatment is necessary, though it is common to use 15 mmHg as the threshold for beginning treatment.
Signs and symptoms of raised intracranial pressure In general, symptoms and signs that suggest a rise in ICP include headache, vomiting without nausea, ocular palsies, altered level of consciousness, back pain and papilledema. If papilledema is protracted, it may lead to visual disturbances, optic atrophy, and eventually blindness. The headache is classically a morning headache that may wake the person up. The brain is relatively poorly supplied by oxygen as a result of mild hypoventilation during the sleeping hours leading to hypercapnia and vasodilation. Cerebral edema may worsen during the night due to the lying position. The headache is worse on coughing, sneezing, or bending, and progressively worsens over time. There may also be personality or behavioral changes. In addition to the above, if mass effect is present with resulting displacement of brain tissue, additional signs may include pupillary dilatation, abducens palsies, and Cushing's triad. Cushing's triad involves an increased systolic blood pressure, a widened pulse pressure, bradycardia, and an abnormal respiratory pattern. In children, a low heart rate is especially suggestive of high ICP. Intracranial hypertension syndrome is characterized by an elevated ICP, papilledema, and headache with occasional abducens nerve paresis, absence of a space-occupying lesion or ventricular enlargement, and normal cerebrospinal fluid chemical and hematological constituents. Irregular respirations occur when injury to parts of the brain interfere with the respiratory drive. Biot's respiration, in which breathing is rapid for a period and then absent for a period, occurs because of injury to the cerebral hemispheres or diencephalon. Hyperventilation can occur when the brain stem or tegmentum is damaged. As a rule, patients with normal blood pressure retain normal alertness with ICP of 25–40 mmHg (unless tissue shifts at the same time). Only when ICP exceeds 40–50 mmHg does CPP and cerebral perfusion decrease to a level that often causes loss of consciousness. Any further elevations may lead to brain infarction and brain death. In infants and small children, the effects of ICP differ because their cranial sutures have not closed. In infants, the fontanels (soft spots on the head where the skull bones have not yet fused) bulge when ICP gets too high. ICP correlates with intraocular pressure (IOP) but seems to lack the accuracy necessary for close management of intracranial pressure in the acute post-traumatic period. Papilledema (swelling of the optic disc) can be a reliable sign of elevated ICP. Unlike other conditions that may result in the swelling of the optic disc, it is in the case of papilledema that vision may go largely unaffected.
Causes of abnormal intracranial pressure
Increased ICP Causes of increased intracranial pressure can be classified by the mechanism in which ICP is increased:
Mass effect such as brain tumor, infarction with edema, contusions, subdural or epidural hematoma, or abscesses all tend to deform the adjacent brain. Generalized brain swelling can occur in ischemic-anoxia states, acute liver failure, hypertensive encephalopathy, hypercarbia (hypercapnia), and Reye hepatocerebral syndrome. These conditions tend to decrease the cerebral perfusion pressure but with minimal tissue shifts. Increase in venous pressure can be due to venous sinus thrombosis, heart failure, or obstruction of superior mediastinal or jugular veins. Obstruction to CSF flow and/or absorption can occur in hydrocephalus (blockage in ventricles or subarachnoid space at base of brain, e.g., by Arnold–Chiari malformation), extensive meningeal disease (e.g., infection, carcinoma, granuloma, or hemorrhage), or obstruction in cerebral convexities and superior sagittal sinus (decreased absorption). Increased CSF production can occur in meningitis, subarachnoid hemorrhage, or choroid plexus tumor. Idiopathic or unknown cause (idiopathic intracranial hypertension, a common cause in otherwise well people especially younger women) Craniosynostosis 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.
Drug-induced intracranial hypertension Drug-induced intracranial hypertension (DIIH) or medication-induced intracranial hypertension is a condition of higher than normal intracranial pressure with the main cause being a drug. This condition is similar to idiopathic intracranial hypertension, however the etiology in this instance is a drug. The most frequent symptoms are headaches, pulsatile tinnitus, diplopia, and impairment of visual acuity. The only observable signs of the condition may be papilledema and bilateral sixth cranial nerve (abducens) palsies.
Low ICP
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