Mass effect

Mass Effect

Mass effect describes the local effect of an intracranial pathology (e.g. space occupying lesion or haemorrhage) on the brain. This can lead to midline shift or hydrocephalus.

Pathophysiology

The Monroe-Kelly doctrine is the underling principle behind mass effect. The skull is a fixed volume. Consequently, a space occupying lesion will compress/distort/displace nearby structures.
Mass effect can be caused by:
- Space occupying lesions: tumours, abscesses,
- Haemorrhage: extradural, subdural, intraparenchymal haematomas
- Infection with oedema
- Ischaemia with oedema

How to use imaging to assess mass effect:

Identify the area of mass effect and the underlying cause.
Identify any complications secondary to mass effect.
Compare against previous imaging - has the lesion and mass effect significantly changed?
Identify which structures are affected by mass effect.
Examine the ventricles; mass effect can cause CSF outflow obstruction and an obstructive hydrocephalus.

CT Findings

identifying mass effect

Identify the underlying pathology and assess the nearby structures for displacement, distortion, sulcal effacement and shift.

Mass effect - extradural haematoma

Non-contrast CT shows a left sided extradural haematoma with local sulcal effacement

obstructive hydrocephalus secondary to mass effect

Sagittal MRI shows a posterior fossa lesion causing compression of the fourth ventricle and CSF outflow, resulting in an obstructive hydrocephalus.

References

Extradural haematoma: Case courtesy of Frank Gaillard, Radiopaedia.org, rID: 3428Obstructive hydrocephalus secondary to medulloblastoma compressing CSF outflow (MRI-T1): Case courtesy of Ahmed Abdrabou, Radiopaedia.org, rID: 22688

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