Cerebral oedema is accumulation of tissue fluid in between the cells of the nervous system.
Seen after damage from many different causes, it is the result of breakdown of the blood-brain barrier due to ischaemia, trauma, inflammation, and metabolic disorders. This breakdown also occurs around tumours.

Severe cerebral swelling is associated with a rise in the pressure within the skull (raised intracranial pressure).
Expanding intracranial lesions cause raised intracranial pressure

The cranial cavity is divided into three spaces by the falx and tentorium cerebelli.
If a lesion expands within the brain substance, there is only a limited amount of room within the skull to accommodate it.
Initially, reduction in the size of the ventricles and sub-arachnoid space occurs, but once this volume is used, further increase in the size of a lesion is associated with increase in intracranial pressure.

Cerebral herniation

Swellings within the brain are particularly dangerous when they lead to rapid local expansion of one part, causing it to shift from one brain compartment to another, a process termed cerebral herniation. 

There are four types of cerebral herniation:

Transtentorial herniation is caused by lesions expanding in one cerebral hemisphere. There is herniation of the medial part of the temporal lobe down over the tentorium cerebelli to compress the upper brain stem.
The third cranial nerve becomes first stretched, then compressed on the side of the lesion, giving rise to a fixed dilated pupil.
Branches of the posterior cerebral artery are also compressed as the brain herniates, causing secondary infarction of the occipital lobe.
As the midbrain is distorted by compression, small vessels are torn and secondary haemorrhage occurs into the brain stem leading to death.

Cerebellar tonsillar herniation is caused by expanding lesions in the posterior fossa. There is herniation of the lower part of the cerebellum (cerebellar tonsils), which pushes down into the foramen magnum and compresses the medulla; this process is also known as coning.
As the medulla is compressed, it causes cessation of respiration and death.
This may be precipitated by performing a lumbar puncture in a person with a mass in the brain. Withdrawal of CSF allows a pressure gradient to develop and there is rapid coning with death.
Lumbar puncture should never be performed until the possibility of a mass lesion in the skull has been excluded.

Cingulate gyrus (subfalcial) herniation is caused by a lesion in one of the cerebral hemispheres, resulting in movement of the cingulate gyrus beneath the falx cerebri.
This is often associated with compression of the adjacent anterior cerebral artery, leading to secondary cerebral infarction.

Diencephalic herniation is caused by generalized swelling of both cerebral hemispheres. There is compression of the ventricles, with descent of the thalamus and midbrain through the tentorial hiatus.
This causes tearing of vessels in the midbrain, with secondary haemorrhage.