Basic Introduction to Brain Herniation Syndromes

Understanding brain herniation syndromes requires a quick review of the basics of brain anatomy. The central nervous system consists of the brain and the spinal cord. The brain rests inside the bony skull under 3 layers of protective tissue, the dura mater, arachnoid mater, and pia mater. The dura mater is the outermost layer of tough tissue.

The dura mater folds in on itself to project into the brain at two different points. One of these areas is in between the left and right hemispheres of the brain. This dural fold is called the falx cerebri.

The other area of dura mater projection is at the back of the skull between the occipital lobe of the cerebrum and the cerebellum beneath it. This posterior dural fold is called the tentorium cerebelli.

The falx cerebri and tentorium cerebelli are important anatomically because they create the potential for brain herniation. The other main area that is involved in brain herniation syndromes is called the foramen magnum. The foramen magnum is an actual hole in the bottom of the bony skull. The brain stem descends from the bottom of the brain through the foramen magnum and then turns into the spinal cord to continue down the back.

Parts of the brain can herniate down over these firm dural folds or through the hole of the foramen magnum. This occurs when the pressure inside the skull increases. This pressure is commonly known as the intracranial pressure.

The intracranial pressure can be increased by a great number of things. Most commonly, increased intracranial pressure is due to lesions that take up space in the brain causing a "mass effect". These can include brain tumors, hematomas (large bruises), and abscesses (pockets of infection).

There are four main types of brain herniation syndromes. These include the cingulate, central, uncal, and cerebellar tonsillar herniations described below:

-Subfalcine (or cingulate) herniation:

A section of brain herniates under the falx cerebri. This can cause compression of the anterior cerebral artery.

-Downward Transtentorial (or central) herniation:

The thalamic area herniates down over the tentorial notch. This can lead to decorticate posturing during which the individual's body is in an extended position but the arms and wrists flex in response to pain.

The paramedian arteries that branch off of the basilar artery may rupture due to excessive stretching. This will cause a characteristic brain bleed known as Duret hemorrhages. The result is usually fatal.

-Temporal Transtentorial (or uncal) herniation:

The medial part of the temporal lobe herniates down over the tentorial notch. This can lead to pressure on the brainstem and decerebrate posturing, often beginning unilaterally and progressing to involve both sides. Decerebrate posturing is full extension of the arms, legs, and back.

Uncal herniation can also create dilated pupils on the same side as the lesion. This is due to stretching of cranial nerve III.

Paresis (slight or partial paralysis) may also be present on the ipsilateral side (same side as the lesion). However, this is known as a false localizing sign because it is actually due to compression damage on the side opposite the lesion to an area known as the crux cerebri. This area is where the bulbar and corticospinal tracts run on their way to the spinal cord.

-Cerebellar Tonsillar herniation:

Part of the cerebellum herniates through the foramen magnum. Neck stiffness, known as nuchal rigidity, is a common finding. Irregular respiration can quickly lead to cessation of breathing due to direct pressure on the medulla.

There are several types of brain herniation syndromes, each with specific signs and symptoms that provide important clues to an accurate and timely diagnosis.