Herniations

Definition

Patients can have very high ICP, it is the variation in pressure in different compartments that causes herniations that causes the death in patients

The downwardly shifted hemisphere and basal ganglia compress and displace the diencephalon through the tentorial incisura → displaced brainstem causes stretching of the paramedian branches of the basilar artery → brainstem ischaemia → marked diencephalon and brainstem dysfunction (altered levels of consciousness and abnormal respiration)

Pupils become small with a poor reactivity to light.
A unilateral lesion can cause contralateral hemiparesis, with ipsilateral flexor and decorticate responses.
With progressive midbrain involvement, respiration becomes tachypneic and the pupils fall into a midline fixed position.
Internuclear ophthalmoplegia may arise, in which case motor examination shows bilateral decerebrate posturing.
As the pons becomes involved, respiration remains rapid and shallow.
Motor examination demonstrates flaccid extremities with bilateral extensor plantar responses.
With progressive medullary involvement, respiration slows and becomes irregular with prolonged sighs or gasps.
As hypoxia ensues the pupils dilate, and brain death follows shortly thereafter.

The uncus and hippocampal gyrus shift medially into the tentorial notch → distorting the brainstem and creating significant dysfunction.
Kernohan’s notch phenomenon

A situation where a brain injury causes weakness or paralysis on the same side of the body (ipsilateral) as the injury

A left side uncal herniation displaces the cerebral peduncle to the right side → right cerebral peduncle is pressing on the sharp right tentorial edge.

One pupil may become dilated and poorly reactive even in the presence of a normal conscious level. The pupil then fully dilates with external oculomotor ophthalmoplegia. If midbrain compression ensues, consciousness may be impaired, followed by contralateral decerebrate posturing. Occasionally, posturing or hemiparesis may occur ipsilateral to the lesion as a result of pressure on the contralateral cerebral peduncle on the edge of the tentorium cerebelli.
If the uncal syndrome is allowed to progress, extensor plantar responses appear bilaterally, along with dilation of the contralateral pupil.
Finally patients have hyperpnea, midposition pupils, impaired oculovestibular response, and bilateral decerebrate rigidity.
From this point progression is as for the central syndrome.

Expansion of one hemisphere that causes a movement of the cingulate gyrus under the falx cerebri → Cingulate herniation may compress the internal cerebral veins and/or the ipsilateral anterior cerebral artery

There is posterior and inferior displacement of the posterior aspect of the frontal lobe orbital surface over the sphenoid wing.

Small herniations involve only the orbital gyri, while larger herniations may include the gyrus rectus.

Posterior displacement of the frontal lobe → compression of the MCA against the sphenoid ridge → MCA territory infarction.

There is displacement of the temporal lobe superiorly and anteriorly across the sphenoid ridge.

Superior displacement of the temporal lobe → compress the supraclinoid internal carotid artery against the anterior clinoid process → infarction of the anterior and middle cerebral artery territories.