Intracranial pathological processes may cause mass effect - displacement or compression of the brain
Mass effect can be caused by intracranial masses, haemorrhage, and oedema
The brain is a soft tissue structure located inside the cranial vault: a finite space confined by bones of the skull. As the intracranial volume cannot change, any intracranial lesion which is 'space-occupying' may increase intracranial pressure and displace the soft tissues of the brain. This is known as 'mass effect'.
Intracranial pathological processes, such as masses and haemorrhage, can cause mass effect. Surrounding cerebral oedema often worsens mass effect, and in the case of infarcts, which are not in themselves 'space-occupying', the mass effect is solely due to oedema.
Stages of mass effect
Intracranial space-occupying lesions cause mass effect in predictable stages. Effacement of the sulci adjacent to the lesion is followed by partial or complete effacement of the adjacent ventricles. Effacement of the sulci and ventricles may extend across the whole hemisphere. This is followed by displacement of midline structures, and then effacement of the contralateral ventricles and sulci.
In extreme cases CT may demonstrate herniation of structures through the incisura tentorii (the gap at the top of the tent normally occupied by the brain stem and basal cisterns), or coning (extrusion of the posterior fossa structures through the foramen magnum). These uncommon features are associated with extremely poor outcome.
A space occupying lesion - below the level of this CT slice - is causing mass effect with effacement of the sulci over the whole left cerebral hemisphere
Compare with the contralateral side which shows normal sulci
This image shows a small intracerebral bleed with surrounding oedema
The combination of the blood and oedema is causing mass effect: effacement of the adjacent sulci and partial effacement of the adjacent lateral ventricle
A large acute left subdural haematoma is causing severe mass effect
The left hemisphere sulci and lateral ventricle are effaced
The midline structures are shifted to the right
The contralateral sulci are effaced
The right lateral ventricle is distorted - effaced anterior horn and focal hydrocephalus of the posterior horn
Effacement of the basal cisterns
As well as causing mass effect from one side to the other, an intracranial lesion may cause mass effect downwards into the posterior fossa. This may manifest as effacement of the basal cisterns.
