Midline Shift After Head Trauma

The brain is naturally balanced between the left and the right hemispheres. On a computed tomography (CT) scan that looks down at the brain from the top of the head, there is a groove that runs between both sides of the brain that is midline to the body. The spinal cord emerges at the middle-base of the brain and continues down the center of the back.

A midline shift occurs when something pushes this natural centerline of the brain to the right or to the left. It is a concerning sign after head trauma.

Midline Shift and Intracranial Pressure

The brain maintains a natural pressure level at all times. Normal pressure within the skull is 7-15 mm/Hg. This baseline pressure is created by fluid, tissue and blood flow within the bony skull.

Head trauma can immediately and significantly increase intracranial pressure (ICP). If there is a powerful blow to the head, blood vessels rupture and bleed into and around the brain. Since the heart continues to pump fresh blood into the brain, the extra blood that is leaking out of broken blood vessels begins to accumulate. This raises overall brain pressure and the growing collection of blood, called a hematoma, begins to push against brain tissue.

Other causes of increased ICP after head trauma include brain swelling around the site of injury, a condition called hydrocephalus which is a collection of fluid in the ventricles of the brain, and infection.

A midline shift occurs when the pressure exerted by the buildup of blood and swelling around the damaged brain tissues is powerful enough to push the entire brain off-center. This is considered a medical emergency and is an ominous sign.

Diagnosis

The most common test to identify a midline shift is the CT scan. However, in some cases, a CT scan is not possible because the patient is unstable, or because frequent measurements are wanted to track the progress of a bleed. In these situations, bedside sonography can also be used to diagnose and track the development of a midline shift.

There are three important structures evaluated when determining the presence of a midline shift: the septum pellucidum, the third ventricle, and the pineal gland.

• The septum pellucidum: a thin membrane that runs directly down the center of the brain
• The third ventricle: a space filled with cerebrospinal fluid that lies deep in the center of the brain
• The pineal gland: a small gland that lies just behind the third ventricle

The locations of these three brain structures serve as reference points on a radiologic scan. If any of them are out of alignment, this indicates that pressure on one side of the brain is pushing the brain out of position.

Treatment

The most important treatment when a midline shift is present is relieving the pressure that is pushing the brain off-center. If a collection of blood is the cause, such as a subdural hematoma, surgery will be needed to remove the blood clot and stop the bleeding.

Antibiotics are used to treat infection and steroids may be used to decrease inflammation and swelling.

Prognosis

A number of studies have examined the effects of midline shift on long-term outcomes. Since a midline shift happens due to bleeding and pressure, the amount of bleeding, the location of damage, and the overall level of pressure experienced by the brain are all important considerations.

When the brain moves, this causes trauma to other structures as they are pulled and pushed out of their natural position. The greater the midline shift, the more serious the complications and the greater the risk of death.