Introduction
Background
The brain is an organ of immense complexity and importance to life. In the cranium, dural reflections and bony landmarks divide the brain into anatomic regions. Brain herniation represents mechanical displacement of normal brain relative to another anatomic region secondary to mass effect from traumatic, neoplastic, ischemic, or infectious etiologies.
Herniations of the brain are divided into 5 major categories, as follows:
- Transtentorial herniation
- Subfalcine/cingulate herniation
- Foramen magnum/tonsillar herniation
- Sphenoid/alar herniation
- Extracranial herniation
Each category of herniation is associated with a specific neurologic syndrome.
Pathophysiology
Transtentorial herniation
Transtentorial herniation is a downward or an upward displacement of the brain through the tentorium at the level of the incisura. A descending transtentorial herniation occurs when the supratentorial brain herniates downward through the incisura. Conversely, an ascending transtentorial herniation occurs when the infratentorial brain herniates upward through the incisura.
Descending transtentorial herniation occurs more often than ascending herniations and includes the subcategory of uncal herniation. Mass effect in the cerebrum pushes the supratentorial brain through the incisura; this displacement may lead to a host of neurologic symptoms, as discussed below in Clinical Details.
Ascending transtentorial herniation is usually caused by a posterior fossa tumor with mass effect that pushes the infratentorial brain through the incisura. This results in the distortion of the midbrain, flattening of the posterior quadrigeminal plate, and narrowing of the bilateral ambient cisterns. Extra-axial and intra-axial hematomas of the posterior fossa are less common causes. [see the eMedicine article Posterior Fossa Tumors.]
Subfalcine/cingulate herniation
Subfalcine herniation occurs when the supratentorial brain is displaced underneath the falx secondary to mass effect.
Foramen magnum/tonsillar herniation
Foramen magnum herniation occurs when the infratentorial brain is displaced through the foramen magnum secondary to mass effect.
Sphenoid/alar herniation
Sphenoid/alar herniation results from the supratentorial brain sliding either anteriorly or posteriorly over the wing of the sphenoid bone. An anterior herniation occurs when the temporal lobe herniates anteriorly and superiorly over the sphenoid bone. Conversely, a posterior herniation occurs when the frontal lobe herniates posteriorly and inferiorly over the sphenoid bone.
Extracranial herniation
Extracranial herniation occurs with displacement of brain through a cranial defect.
Frequency
United States
Various causes of brain herniation have been identified; the frequency of occurrence largely depends on the particular etiology.
International
Various causes of brain herniation have been identified; the frequency of occurrence largely depends on the particular etiology.
Mortality/Morbidity
Mortality and morbidity vary with the causes and treatments of brain herniation.
Presentation
Transtentorial herniationDescending transtentorial herniation can cause various symptoms. Compression of ipsilateral cranial nerve III may lead to ipsilateral dilatation of the pupil and abnormal extraocular movements. Compression of the ipsilateral corticospinal tracts in the brainstem may cause contralateral hemiparesis because these tracts decussate at the level of the medulla. Ipsilateral hemiparesis also can occur if there is sufficient mass effect to cause the contralateral cerebral peduncle (Kernohan notch) to be compressed against the adjacent incisura.
Other complications include unilateral or bilateral occipital lobe infarction from compression of the posterior cerebral artery. Brainstem hemorrhages are another complication caused by compression or kinking of pontine perforating vessels. Compression on the midbrain may cause hydrocephalus.
Ascending transtentorial herniation
Ascending transtentorial herniation causing brainstem compression can cause nausea and vomiting, which may progress rapidly to coma if rapid changes occur in the intracranial anatomy. A slow-growing mass in the posterior fossa results in slow changes in the intracranial anatomy; these do not often present as an acute emergency.
Subfalcine/cingulate herniation
Subfalcine herniation does not necessarily indicate severe clinical symptoms. This type of herniation may lead to the clinical findings of headache, and symptoms may progress to contralateral leg weakness or ipsilateral frontal lobe infarction secondary to compression of the anterior cerebral artery.
Foramen magnum/tonsillar herniation
Acute compression of the brainstem may result in obtundation and death. However, patients with an Arnold-Chiari I malformation may present with a paucity of symptoms, or they may present with dysesthesia in the extremities with cervical flexion. This is referred to as Lhermitte phenomenon. (See the eMedicine article Chiari I Malformation.)
Sphenoid/alar herniation
Associated clinical symptoms are usually minimal, although sphenoid herniations are often associated with other types of herniations.
Extracranial herniation
This finding usually results from a traumatic or surgical cause. The herniated region of the brain may become ischemic, leading to infarction.
Preferred Examination
For transtentorial herniation, computed tomography (CT) scanning or magnetic resonance imaging (MRI) is useful for evaluation. MRI can provide axial, as well as sagittal and coronal, views.
For subfalcine/cingulate herniation, CT scanning or MRI is again useful for evaluation, with MRI able to provide axial, sagittal, and coronal views.
For foramen magnum/tonsillar herniation, MRI provides the best visualization on sagittal and coronal views. However, because patients with this type of herniation often present acutely, axial CT scanning enables visualization of this condition.
For sphenoid/alar herniation, MRI provides the best visualization on parasagittal images. However, axial CT scanning or MRI can demonstrate anterior displacement of the ipsilateral middle cerebral artery, which is an indirect sign of sphenoid herniation.
For extracranial herniation, CT scanning or MRI is useful for evaluation.
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References
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Further Reading
Keywords
brain displacement, mass effect, transtentorial herniation, subfalcine/cingulate herniation, subfalcial herniation, foramen magnum/tonsillar herniation, sphenoid/alar herniation, extracranial herniation, uncal herniation
