Intracranial herniation is the movement of part of the brain from one space to another with resultant damage. It usually occurs following a critical increase in intracranial pressure caused by an expanding lesion, e.g. tumor or hematoma. However, it may be inadvertently precipitated by withdrawing cerebrospinal fluid (CSF) at lumbar puncture.
This is an abnormal accumulation of fluid in the cerebral parenchyma. It may be the result of breakdown of the blood–brain barrier (vasogenic edema), parenchymal cell membrane injury (cytotoxic edema) or a combination of the two. Possible causes include:
- ischemia, e.g. from infarction
- trauma, e.g. from head injury
- inflammation, e.g. encephalitis or meningitis
- cerebral tumors (primary or secondary)
- metabolic disturbances, e.g. hyponatremia or hypoglycemia.
Hydrocephalus is an increase in the volume of CSF within the brain resulting in the expansion of the cerebral ventricles and eventual increase in intracranial pressure. It can occur by one of three mechanisms:
- Obstruction to flow of CSF (the most common form)
- Impaired absorption of CSF at arachnoid villi (rare)
- Overproduction of CSF by choroid plexus neoplasms (very rare)
Hydrocephalus can be classified into non-communicating and communicating forms.
Non-communicating (obstructive hydrocephalus)
This can be congenital or acquired. Congenital hydrocephalus occurs in 1 per 1000 births, due to Arnold–Chiari and Dandy–Walker malformations and cerebral aquaduct stenosis. Acquired obstructive hydrocephalus can result from any lesion that obstructs the CSF within the ventricles preventing flow into the subarachnoid space, such as:
- tumors—especially of the posterior fossa, as the fourth ventricle aqueducts are easily obstructed
- hemorrhage—intraventricular or in the posterior fossa.
Caused by an impairment outside the ventricular system in the absence of any CSF flow obstruction, e.g. meningitis or subarachnoid hemhorrage. Normal pressure hydrocephalus (intermittent pressure hydrocephalus) is a particular form of communicating hydrocephalus. This is a rare condition of progressive dementia associated with ventricular dilatation. Random sampling shows normal CSF pressure, but continuous monitoring reveals intermittent increases.
Secondary or compensatory hydrocephalus
In this special type of hydrocephalus, an increase in CSF occurs as a compensatory measure following loss of brain tissue, e.g. due to infarction or atrophy. There is no associated increase in CSF pressure.