Effects of Brain Herniation
Type of Herniation | Mechanism* | Findings |
|---|---|---|
Transtentorial | Compression of ipsilateral 3rd cranial nerve | Unilateral dilated, fixed pupil Oculomotor paresis |
Compression of the posterior cerebral artery | Contralateral homonymous hemianopia Absence of blinking in response to visual threat from the hemianopic side in obtunded patients | |
Compression of the contralateral 3rd cranial nerve and cerebral peduncle (indented by the tentorium to form Kernohan notch) | Contralateral dilated pupil and oculomotor paresis Ipsilateral hemiparesis | |
Compression of the ipsilateral cerebral peduncle | Contralateral hemiparesis | |
Eventually, compression of the upper brain stem and the area in and around the thalamus | Impaired consciousness Abnormal breathing patterns Fixed, unequal pupils | |
Further compromise of the brain stem | Loss of oculocephalic reflex Loss of oculovestibular reflex Loss of corneal reflexes Decerebrate posturing | |
Subfalcine (cingulate) | Trapping of one or both anterior cerebral arteries, causing infarction of the paramedian cortex | Contralateral leg paralysis |
Expansion of infarcted area | Increased intracranial pressure Increased risk of transtentorial herniation, central herniation, or both | |
Central | Bilateral, more or less symmetric damage to the midbrain | Pupils fixed in midposition Decerebrate posturing Many of the same symptoms as transtentorial herniation |
Further compromise of the brain stem | Loss of all brain stem reflexes Disappearance of decerebrate posturing Cessation of respirations Brain death | |
Upward transtentorial | Compression of the posterior 3rd ventricle | Hydrocephalus, which increases intracranial pressure |
Distortion of the mesencephalon vasculature Compression of the veins of Galen and Rosenthal Superior cerebellar infarction due to occlusion of the superior cerebellar arteries | Early: Nausea, vomiting, occipital headache, ataxia Later: Somnolence, breathing abnormalities, patchy and progressive loss of brain stem reflexes | |
Posterior fossa mass (eg, cerebellar hemorrhage) | Ataxia, dysarthria | |
Progressive brain stem compression | Increasing somnolence Respiratory irregularities Patchy but progressive loss of brain stem reflexes | |
Tonsillar | Compression of the brain stem Obstruction of cerebrospinal fluid (CSF) flow | Acute hydrocephalus (with impaired consciousness, headache, vomiting, and meningismus) Dysconjugate eye movements, nystagmus Later, abrupt respiratory and cardiac arrest |
* Not all mechanisms occur in every patient. | ||
CNS = central nervous system. | ||