Brain Herniation

Herniations:

The volume of the intracranial contents is fixed by the skull. as a result, the introduction of additional tissue or fluid (as may occur in a space-occupying lesion, cerebral oedema, or hydrocephalus) raises intracranial pressure, which may lead to life-threatening herniation of the brain through openings of the skull. The major herniations are as follows:

• Subfalcine: cingulate gyrus herniates under the falx (may compromise the anterior cerebral artery).

• Transtentorial: medial temporal lobe (uncus) passes over the free edge of the tentorium (may lead to distortion of the adjacent midbrain and pons and tearing of feeding vessels [Duret haemorrhages] or compress the posterior cerebral artery)

• Tonsillar: cerebellar tonsils herniate through the foramen magnum (may result in compression of the medulla and compromise cardio-respiratory centres)

Subfalcine herniation: occurs in a supratentorial intracerebral expanding lesion; expansion of a unilateral cerebral hemisphere lesion results in the cingulate gyrus herniating under the free margin of the falx cerebri above the corpus callosum.

Transtentorial herniation: is the most important herniation associated with a supratentorial-expanding lesion. The medial part of the ipsilateral temporal lobe (anteromedial hippocampus) is squeezed through the tentorial opening.

Depending on the supratentorial pressure several sequelae may be seen:

• Uncal notch: a definite notch forms on the inferior aspect of the brain due to compression of the herniating brain against the edge of the tentorium cerebella.

• Fixed dilated pupil: (unilateral or bilateral) with oculomotor nerve (III) may be damage.

• Compression of the posterior cerebral artery: close proximity to the tentorium and herniating portion of the brain  lumen may be obliterated  infarction occurs in the medial occipital lobe (unilateral or bilateral).

• Contralateral midbrain compression: midbrain pushed against contralateral ridge edge of the tentorium (forms groove – Kernohan’s notch on midbrain)

• Compression of the aqueduct: distortion and partial obstruction of the lumen of the aqueduct  CSF accumulates within the ventricles which exacerbates the problem ( pressure).

• Midbrain herniation: rapidly expanding supratentorial mass shifts the midbrain down through the incisura; causes shearing of vessels  Duret haemorrhages (upper pons and midbrain), associated compromised vital centres and rapid death.

Upward transtentorial herniation: occurs with space occupying lesions in the posterior fossa (i.e. cerebellar lesions). The midbrain and cerebellar hemispheres move upwards through the incisura  aqueduct becomes distorted and compressed, blocking CSF outflow   supratentorial pressure

Cerebellar tonsillar herniation: cerebellar tonsils move downward through the foramen magnum; caused by:

• Subtentorial expanding lesion (posterior fossa mass)

• Transmitted supratentorial pressure

• Coning – post lumbar puncture in the patient with raised intracranial pressure

Intracranial pressure is increased with the obstruction of CSF through the fourth ventricle and exit foramina; the tonsils compress the medulla and interfere with the cardio-respiratory centres  rapid demise of the patient.

Transcalverial herniation: herniation of the brain through any defect in the skull (usually seen in severe penetrating injuries of the skull and brain)

Category: Medicine Notes , Pathology Notes