One of the most important indicators of increased intracranial pressure due to mass effect is midline shift.
Any intra-axial or extra-axial lesion (tumour, haemorrhage, abscess, etc) has the potential to exert mass effect on the brain parenchyma and cause lateral shift of the midline structures.
Midline shift is measured in millimeters, as the perpendicular distance between a midline structure (usually the septum pellucidum) and a line designated the midline.
The midline is assumed to be coplanar with falx cerebri, and is best represented as a line drawn between the anterior and posterior attachments of the falx to the inner table of the skull.
Care must be taken if there is existing asymmetry of the ventricles or the falx. If the falx is not straight, a line between the free edges of the anterior and posterior falx can be used instead. The superior sagittal sinus can also be used to indicate the posterior falcine attachment provided it is truly midline and not coursing to one side as is seen sometimes with a dominant transverse sinus.
Before the advent of cross sectional imaging, midline shift was assessed by displacement of the calcified pineal gland on a frontal radiograph of the skull.
The amount of midline shift is one of the strongest indicators of neurosurgical prognosis and can be associated with other signs of increased intracranial pressure such as: