Pathophysiology
Cerebral perfusion pressure (CPP), the pressure of blood flowing to the brain, is normally fairly constant due to autoregulation, but for abnormal mean arterial pressure (MAP) or abnormal ICP the cerebral perfusion pressure is calculated by subtracting the intracranial pressure from the mean arterial pressure: CPP = MAP − ICP . One of the main dangers of increased ICP is that it can cause ischemia by decreasing CPP. Once the ICP approaches the level of the mean systemic pressure, cerebral perfusion falls. The bodys response to a fall in CPP is to raise systemic blood pressure and dilate cerebral blood vessels. This results in increased cerebral blood volume, which increases ICP, lowering CPP further and causing a vicious cycle. This results in widespread reduction in cerebral flow and perfusion, eventually leading to ischemia and brain infarction. Increased blood pressure can also make intracranial hemorrhages bleed faster, also increasing ICP.Severely raised ICP, if caused by a unilateral space-occupying lesion (e.g. a hematoma) can result in midline shift, a dangerous sequela in which the brain moves toward one side as the result of massive swelling in a cerebral hemisphere. Midline shift can compress the ventricles and lead to hydrocephalus. Monro–Kellie hypothesis
The pressure–volume relationship between ICP, volume of CSF, blood, and brain tissue, and cerebral perfusion pressure (CPP) is known as the Monro–Kellie doctrine or hypothesis.The Monro–Kellie hypothesis states that the cranial compartment is inelastic and that the volume inside the cranium is fixed.
The cranium and its constituents (blood, CSF, and brain tissue) create a state of volume equilibrium, such that any increase in volume of one of the cranial constituents must be compensated by a decrease in volume of another. *This concept only applies to adults, as the presence of fontanelles and open suture lines in infants that have not yet fused means there is potential for a change in size and intracranial volume. The principal buffers for increased volumes include CSF and, to a lesser extent, blood volume. These buffers respond to increases in volume of the remaining intracranial constituents. For example, an increase in lesion volume (e.g., epidural hematoma) will be compensated by the downward displacement of CSF and venous blood.The Monro–Kellie hypothesis is named after Edinburgh doctors Alexander Monro and George Kellie. Diagnosis
The most definitive way of measuring the intracranial pressure is with transducers placed within the brain. A catheter can be surgically inserted into one of the brains lateral ventricles and can be used to drain CSF (cerebrospinal fluid) in order to decrease ICPs. This type of drain is known as an external ventricular drain (EVD). This is rarely required outside brain injury and brain surgery settings.In situations when only small amounts of CSF are to be drained to reduce ICPs (e.g. in IIH), drainage of CSF via lumbar puncture can be used as a treatment. Non-invasive measurement of intracranial pressure is being studied. Treatment
The treatment for ICP depends on the cause.