Midline shift is a term used to describe a shift of the brain structures to the side of the head. Normally the brain is symmetric from side to side and there are structures that are directly in the midline. This is a term that reflects raised pressure inside the skull which can be from a mass, bleed, infection or anything that shifts the midline structures to the side and causes the brain to look asymmetric.
Shift of the midline structures on head CT reflects raised intracranial pressure which is a medical emergency. Many symptoms can accompany raised intracranial pressure and midline shift to include: headache, visual problems, decreased consciousness, nausea/vomiting and even death. The symptoms are non specific which can prompt extensive testing. When symptoms are thought to originate from the nervous system, a head CT is often ordered.
A head CT showing midline shift will often look asymmetric and the normally midline structures will be shifted to the side. This is often measured between the septum pellucidum, which is a thin membrane between the lateral ventricles in the midline and the attachment of the falx cerebri along the front and back part of the skull which is also along the midline. If there is measurable distance between the septum pellucidum and falx attachments, then there is midline shift. Often the reason for the midline shift will also be seen on a head CT.
Some of the common causes include a bleed, mass, infection, stroke to include some. A bleed along the surface of the brain or subdural hemorrhage will shift the brain to the other side and cause midline shift. A brain tumor either inside the brain or along the covering will also cause this finding. A big stroke in the hemispheres will produce swelling which will shift the brain and cause midline shift.
When there is raised intracranial pressure and midline shift, a neurosurgeon will often become involved. This is a dangerous condition because parts of the brain can herniate and this can cause injury such as stroke, loss of function of the brain from pressure and obstruction of the ventricles. Quick diagnosis and treatment is therefore essential.