We intend to provide detailed anatomical description of the brain regions involved in the different hernias through images and clinical cases,
with special emphasis on their complications.
Subfalcine Herniation (1,
2). It´s the most common hernia (figures 2,
3,
4)
- What´s going on?: The ipsilateral cingulate gyrus is pushed under the rigid midline falx due to an increase in pressure in any of the cerebral hemispheres.
- Symptoms: non-specific.
Decreased level of consciousness that is directly proportional to the degree of midline shift.
- Which structures are involved?:
- Corpus callosum: depression of the ipsilateral corpus callosum and elevation or compression of the contralateral.
- Lateral ventricle: compression of the ipsilateral ventricle and dilatation of the contralateral.
- Key points on CT: Displacement of the cingulate gyrus/septum pellucidum.
- Compression of the ipsilateral anterior cerebral artery with necrosis of the cingulate gyrus or extensive infarction
- Hydrocephalus due to obstruction of the foramen of Monro (figure 4)
Transtentorial Herniation
Descending or Uncal (5,
6,
7,
8,
9,
10,
11)
- What´s going on?: the uncus and the adjacent part of the temporal lobe (supratentorial structures) are displaced caudally across the tentorial incisura into the infratentorial compartment.
May be unilateral or bilateral.
- Symptoms: coma,
ipsilateral mydriasis,
homonymous hemianopsia
- Which structures are involved?: mesencephalon,
ipsilateral oculomotor nerve,
ipsilateral posterior cerebral artery and sometimes basilar artery.
- Key points on CT: widening of the ipsilateral cerebellopontine angle and obliteration of the suprasellar ipsilateral cistern.
- Compression of the brainstem (midbrain ipsilateral haemorrhage or extensive ischemia).
- Duret haemorrhage (perforating branches from the basilar artery and/or draining veins are damaged with resultant parenchymal haemorrhage) (figure 9).
- Kernohan phenomenon (contralateral midbrain compressed against the tentorium).
- Compression of the ipsilateral posterior cerebral artery so ischemia of the visual cortex so homonymous hemianopsia.
- Compression of the ipsilateral oculomotor nerve à ipsilateral mydriasis.
Bilateral uncus hernia (figure 10,
11)
- Complete obliteration of the cerebellopontine angle.
- Midbrain effaced and displaced inferiorly.
Ascending (12,
13)
- What´s going on?: superior displacement of the cerebellum through the tentorial notch.
- Which structures are involved?: cerebellum,
IV ventricle.
- Flattening or reversal of the smile-shaped quadrigeminal cistern.
- Obliteration of the quadrigeminal and superior cerebellar cistern and IV ventricle.
- Infarctions in the territory of posterior cerebral and superior cerebellar arteries (occipital lobe infarction) (3)
- Hydrocephalus by compression.
Transalar (Subfalcine + Transtentorial) (figure 14,
15,
16,
17)
- What´s going on?: herniation of brain matter in and around the middle cranial fossa across the greater sphenoid wing and can be ascending or descending.
Compression of structures against the
sphenoid bone results in the symptoms described below.
It is the result of the combination of subfalcine and transtentorial herniations.
o Ascending (figure 14,
15): superior and anterior displacement of the temporal lobe across the sphenoid ridge so can compress the internal carotid artery and result in infarction of the anterior and middle cerebral artery. Anterior displacement of the middle cerebral artery.
o Descending (figure 16,
17): posterior and inferior displacement of the frontal lobe over the sphenoid wing so Middle cerebral artery infartc.
Posterior displacement of the middle cerebral artery.
- Which structures are involved?: frontal and temporal lobe,
middle and anterior cerebral artery and internal carotid artery.
- Key points on CT: Look the position of the middle cerebral artery.
- Potential complications: vascular accident.
Tonsilar (18)
- What´s going on?: inferior descent of the cerebellar tonsils below the foramen magnum because of a downward pressure.
- Symptoms: coma,
apnea,
hypertension and neck stiffness.
- Which structures are involved?: cerebellar tonsils,
brainstem,
medulla oblongata and the upper cervical spinal cord.
When pushed in a downward direction,
the cerebellar tonsils cause compression of the medulla oblongata and the upper cervical spinal cord.
- Key points on CT: effacement of the CSF cisterns surrounding the brainstem and inferior descent of the cerebellar tonsils below the foramen magnum.
- Potential complications: altering the vital life-sustaining functions of the pons and medulla,
such as the respiratory and cardiac centers.
Extracranial (19)
- What´s going on?: herniation of brain tissue external to the calvaria through a skull bone defect,
which may be post-traumatic or post-surgical.
- Key points on CT: osseous defect.
- Potential complications: if the craniectomy defect is too small,
the swollen brain may herniate through the defect.
This can result in compression of cortical veins and lead to venous infarction and contusion of the brain at the craniectomy margins.