Brain herniation
Introduction0:00–0:26
Brain herniation is what it’s called when some some brain tissue moves outside of the skull, or moves across or into a structure with the skull.
Brain herniation typically happens in response to increased intracranial pressure, which refers to a high pressure within the skull.
An intracranial pressure above 15 mmHg is considered high. OK - let’s start with some basic brain anatomy.
Anatomy and physiology0:26–3:21
The brain has a few regions - the most obvious is the cerebrum, which is divided into two cerebral hemispheres, each of which has a cortex - an outer region - divided into four lobes including the frontal lobe, parietal lobe, temporal lobe, and the occipital lobe.
There are also a number of additional structures - including the cerebellum, which is down below, as well as the brainstem which connects to the spinal cord.
Now zooming in, the brain and spinal cord is covered by the meninges, which are three protective layers of the brain. The inner layer of the meninges is the pia mater, the middle layer is the arachnoid mater, and the outer layer is the dura mater.
These first two, the pia and arachnoid maters, form the subarachnoid space, which houses the cerebrospinal fluid, or CSF.
CSF is a clear, watery liquid which is pumped around the spinal cord and brain, cushioning them from impact and bathing them in nutrients.
The outer membrane is the dura mater, which forms the meningeal folds, such as falx cerebri and tentorium. The falx cerebri is a meningeal fold that goes down into the longitudinal fissure that separates the hemispheres of the brain.
The free edge of the falx cerebri is in close contact with the central part of the brain called corpus callosum, which connects the left and right hemisphere.
There’s also the tentorium which is a meningeal fold located in the back of our skull, that separates the cerebrum from the cerebellum.
The free edge of the tentorium is in close contact with the brainstem, which is the region that connects the brain and the spinal cord.
The skull has a set volume and the pressure inside of the skull is kept relatively constant. In other words, the sum of the volumes of the brain, cerebrospinal fluid, and intracranial venous and arterial blood is always about the same.
So, if there’s an increase in the volume of any one of these three, there’s a compensatory decrease in the other two. For example, when a high-speeded golf ball hits you in the head, an artery could rupture within the skull.
As the artery bleeds, the blood starts to pool, it leads to what’s called a mass effect within the skull, and that mass effect increases the intracranial pressure.
To help reduce the volume and pressure back to normal, there’s less CSF production and more CSF reabsorption. Over time, if the arterial bleed continues, then it might overwhelm the bodies ability to compensate, and the intracranial pressure starts to get quite high, and that can lead to brain herniation.
Causes3:21–4:22
So brain herniation can be caused by either a focal mass effect, like the arterial bleed, or a diffuse mass effect - depending on whether the problem is in one area or involves the entire brain.
Diffuse mass effects are caused by generalized cerebral edema, which is an excessive buildup of fluid throughout the brain tissue.
Some causes of cerebral edema are an large ischemic strokes and meningitis. Depending on the size and location of the mass effect, there’s a possibility of brain herniations - and it can either be supratentorial and infratentorial herniation.
Pathology4:22–10:15
Supratentorial herniation refers to displacement of the cerebrum which is above the tentorium, and infratentorial herniation refers to herniation of the cerebellum which is below the tentorium.
Alright, so supratentorial herniations include four types of displacement and these differ by the exact part of the cerebrum that is affected.
The first type of supratentorial herniation is uncal herniation, also referred to as transtentorial herniation. In this herniation, the innermost part of the temporal lobe, called uncus, slips down towards the tentorium and puts pressure on the brainstem.
The uncus can squeeze the oculomotor nerve resulting in an oculomotor nerve palsy. In oculomotor nerve palsy the eyeball is in a “down and out” position due to a loss of innervation of muscles controlled by the nerve.
Also, the affected pupil becomes dilated and fails to constrict in response to light. The posterior cerebral artery can also be compressed, which results in ischemic stroke of the occipital part of the brain, which is responsible for processing of visual information.
This leads to homonymous hemianopia, which is a loss of vision in either the left or right halves of the visual fields of both eyes.
The vision is lost in halves that are contralateral to the posterior cerebral artery that is affected. Although there is a partial loss of vision, macular function is spared, meaning that central vision is still sharp and detailed.
This is because the part of the occipital lobe in charge of the macula is gets blood from both the posterior cerebral artery as well as the medial cerebral artery.
Uncal herniation stretches and sometimes breaks branches of the paramedian basilar artery which nourish the brain stem. That causes small linear or flame-shaped hemorrhages called Duret hemorrhages which can be seen on autopsy.
So, let’s say there’s a focal mass effect on the right side of the skull, which increases intracranial pressure and squeezes the uncus down onto the brainstem.
The compressed part of the brainstem is called the left cerebral peduncle and it’s rich in motor fibers that travel from the cortex on the left side of your brain to muscles on the right side of your body.
Eventually, this injury of the left cerebral peduncle will result in weakness on the same side of the body as the side of the focal mass effect, also called ipsilateral weakness.
Now, normally damage to brain tissue results in weakness on the contralateral side, so the Kernohan’s notch causes a false localizing sign.
The second type of supratentorial herniation is central herniation, and it’s when the diencephalon and parts of the temporal lobes slip under the free edge of the tentorium.
The diencephalon is a part of the brain that processes sensory information and emotions, as well as regulates hormone production - so all of these processes can be affected.
Central herniation can also cause dilated and fixed pupils and paralysis of upward eye movement, which leads to the sunset eyes sign.
That’s where the eyeballs are in a downward position, and part of the lower pupil is covered by the lower eyelid. Just like in uncal herniation, duret hemorrhages can be seen in central herniation.
The third type of supratentorial herniation is cingulate or subfalcine herniation. In this herniation, the innermost part of the frontal lobe, called the cingulate gyrus, gets squeezed below the free edge of the falx cerebri to the opposite side of the skull.
The displaced part of the brain can compress the anterior cerebral artery and that can cause an ischemic stroke. Finally, a cingulate herniation is often a forerunner of other types of supratentorial herniation.
The fourth type of supratentorial herniation is transcalvarial herniation, which is also called an external herniation. In this herniation the brain squeezes out of the skull through a fracture or surgical site during an operation.
Now, the other category of herniations are infratentorial herniations. There are two types - upward herniation and tonsillar herniation.
In upward herniation there’s a displacement of the cerebellum upward, through a notch in the tentorium cerebelli. In tonsillar herniation, parts of the cerebellum, called cerebellar tonsils slip down through an opening in the skull called the foramen magnum.
This is particularly dangerous because the displaced cerebellum can push onto the brainstem and affect neurons responsible for breathing and cardiac function.
The most common signs of the tonsillar herniation are headache and neck stiffness. The level of consciousness may also decrease and give rise to flaccid paralysis where there’s reduced muscle tone.
Symptoms10:15–10:54
On a physical exam, increased intracranial pressure should be suspected in individuals with a decreased level of consciousness, focal neurological signs, and papilledema.
Papilledema is the swelling of the optic disc, which is the point where the optic nerve fibers leave the retina. Also, these individuals may have increased blood pressure, irregular breathing, and bradycardia.
These signs are referred to as a Cushing’s triad and they represent a physiological response of the central nervous system to increased intracranial pressure.
Diagnosis10:54–11:43
Lumbar puncture is classified as a relative contraindication in individuals with suspected increased intracranial pressure due to possible herniations.
In order to confirm the presence of a mass effect that may be causing increased intracranial pressure, medical imaging such as CT or MRI can be used.
If CT and MRI fails to prove any mass lesions within the skull, lumbar puncture can be performed. Once the needle is inserted into the spinal canal, it’s attached to a manometer, which is a device that measures the CSF pressure.
Normal opening pressure on lumbar puncture is 100-180 mm of H2O or 8-15 mmHg, above that is considered increased intracranial pressure Treatment of increased intracranial pressure and brain herniation is aimed at reducing the pressure within the skull.
Treatment11:43–12:26
It may include the treatment of the underlying cause, such as surgical removal of a tumor, abscess, or hematoma. Furthermore, it may include external ventricular drain, which requires a placement of a drain through a hole in the skull to get rid of excessive cerebrospinal fluid or decompressive craniectomy, which is where a part of the skull is removed to help relieve the pressure.
Another way of treating increased intracranial pressure includes using osmotic therapy, such as mannitol, which helps remove excess water from the body.
Review12:26–13:43
All right, as a quick recap. Brain herniation typically results from increased intracranial pressure.
There are two main classes of herniation: supratentorial, which refers to a displacement of the cerebrum, and infratentorial, which refers to a herniation of the cerebellum.
Furthermore, supratentorial herniations are subdivided into four types: uncal herniation, also referred to as transtentorial herniation which affects the innermost part of the temporal lobe - the uncus, central herniation, which affects the diencephalon and parts of the temporal lobes; cingulate or subfalcine herniation, which affects the cingulate gyrus; and transcalvarial herniation, which occurs when the brain squeezes out of the skull.
On the other hand, infratentorial herniations are subdivided into upward herniation and tonsillar herniation. In upward herniation, there’s a displacement of the cerebellum upward, through a notch in the tentorium cerebelli, while in tonsillar herniation, parts of the cerebellum slip down through an opening in the skull called the foramen magnum.
Diagnosis is done by a CT or MRI imaging, and treatment includes a decompressive craniectomy and osmotic therapy.
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