Herniation Patterns
| Pattern | Herniated Structure | Imaging Findings | Clinical Significance |
|---|---|---|---|
| Subfalcine | Cingulate gyrus herniates under the falx cerebri | Midline shift; contralateral lateral ventricle compressed; ipsilateral lateral ventricle enlarged if foramen of Monro obstructed | ACA territory infarct (pericallosal artery compression); hydrocephalus if Monro obstructed |
| Uncal (transtentorial) | Medial temporal lobe (uncus) herniates over the tentorium | Effacement of the ipsilateral suprasellar cistern; PCA territory infarct; oculomotor nerve (CN III) compression | CN III palsy (blown pupil); contralateral PCA infarct; duret hemorrhages in brainstem (secondary) |
| Central transtentorial | Bilateral hemispheres displace downward through the tentorial incisura | Downward displacement of diencephalon and brainstem; effacement of suprasellar and perimesencephalic cisterns bilaterally; descending brainstem | Bilateral CN III involvement; rapid neurological deterioration; Cheyne-Stokes breathing → coma |
| Upward cerebellar (ascending) | Cerebellar vermis and hemispheres herniate upward through the tentorial incisura | Superior cerebellar cistern obliteration; compression of the superior vermis; effacement of the quadrigeminal plate cistern; hydrocephalus from aqueduct compression | Obstructive hydrocephalus; midbrain and superior cerebellar artery compression; tectal compression |
| Tonsillar | Cerebellar tonsils herniate through the foramen magnum | Tonsils >5 mm below the McRae line (foramen magnum); effacement of the cisterna magna; crowding of the cervicomedullary junction | Medullary compression; respiratory arrest; Cushing reflex (hypertension, bradycardia, irregular respirations) |
Midline shift >5 mm is the threshold for urgent neurosurgical notification. Duret hemorrhages — secondary brainstem hemorrhages from downward displacement — indicate irreversible injury and are seen in the central pons and midbrain tegmentum on CT or MRI.
Midline Shift Measurement
Measure at the level of the foramen of Monro on axial CT. Draw a line between the inner tables of the skull at the widest point and measure perpendicular displacement of the septum pellucidum from this midline.
| Shift | Implication |
|---|---|
| <5 mm | May not require emergent surgical decompression |
| 5–10 mm | Significant mass effect; close monitoring; neurosurgical consultation |
| >10 mm | Critical; associated with loss of consciousness; emergent decompression usually required |
Reporting Checklist — Brain Herniation
- Herniation type(s): subfalcine / uncal / central transtentorial / upward cerebellar / tonsillar
- Midline shift: measure in mm at the level of the foramen of Monro; specify direction
- Cistern status: suprasellar / perimesencephalic / quadrigeminal plate / cisterna magna — patent / effaced / obliterated
- Tonsillar position: mm below the foramen magnum (McRae line)
- Secondary injury: PCA territory infarct / ACA territory infarct / Duret hemorrhages
- Hydrocephalus: obstructive vs communicating; level of obstruction
- Causative mass: location, estimated volume, midline displacement vector
Reference
Wijdicks EF, Miller GM. MR Imaging of Progressive Neurological Deficits in Patients on Anticoagulants. Cerebrovasc Dis. 2001;11(4):221–228.