Mass Effect

Mass effect (from the English mass — weight, volume and effect — result, consequence) is a radiological and clinical term used to describe the pathological physical displacement, deformation, and compression of healthy brain structures by any additional volumetric formation.

Etiology and pathophysiology

Since the adult human skull is incapable of expanding, the appearance of any additional volume within it (such as a large hematoma, tumor, abscess, or area of severe swelling) inevitably requires the release of space. The pathological volume begins to act like a press.

Initially, it expels fluids from the cranial cavity (cerebrospinal fluid and venous blood). When this reserve is depleted, the volume begins to deform the brain tissue itself. Healthy structures are shifted to the opposite side, the brain’s midline markers become distorted, and the internal ventricles (fluid-filled cavities) are compressed and closed.

Clinical significance

The presence and severity of mass effect is the primary criterion that a radiologist evaluates when interpreting a computed tomography. It is measured in millimeters by the degree of displacement of the so-called midline brain structures (for example, the septum pellucidum or the pineal gland).

Clinically, mass effect manifests as worsening headache, focal neurological symptoms, and depressed consciousness. Displacement of midline structures by more than 5–7 mm is considered critical and usually serves as a direct indication for emergency surgical intervention to remove the pathological volume and save the patient’s life.