Cerebral Herniation: Imaging Essentials

A practical radiology review of Cerebral Herniation: Imaging Essentials, focused on imaging findings, differential diagnosis, reporting points, and high-yield teaching pearls.

Definition & Pathophysiology

• Brain tissue shift across compartments
• ICP ↑ (tumor, edema, hemorrhage) → herniation
• ICP ↓ (CSF loss) → paradoxical herniation
• Monro-Kellie: brain + CSF + blood = constant volume

Classification

• Intracranial
• Subfalcine
• ⬇ Transtentorial (descending: lateral/central)
• ⬆ Transtentorial (ascending)
• ⬇ Tonsillar
• Extracranial (post-craniectomy)

6-Step Radiology Approach

• Clinical context
• Anatomic landmarks
• Mass effect direction
• Displaced structures
• Indirect signs
• Complications

Subfalcine Herniation (Most Common)

• Cingulate gyrus → under falx
• ↔ Midline shift (septum pellucidum)

Prognosis

• <5 mm → good
• 15 mm → poor
• ACA compression → leg weakness
• Contralateral ventricle dilatation

⬇ Descending Transtentorial Herniation (DTH)

• Lateral (Uncal)
• Uncus ↓ tentorial notch
• CN III compression → blown pupil
• PCA compression → occipital infarct
• Aqueduct compression → hydrocephalus
• Kernohan notch → ipsilateral weakness
• Posterior
• Parahippocampal gyrus ↓
• Parinaud syndrome (tectal compression)
• Central
• ⬇ Diencephalon + midbrain + pons
• Cistern effacement (key sign)
• Coma → death

⬆ Ascending Transtentorial Herniation

• Cerebellum ↑ through incisura
• Brainstem anterior displacement
• “Spinning top” midbrain
• PCA/SCA infarcts
• Hydrocephalus

⬇ Tonsillar Herniation

• Cerebellar tonsils ↓ foramen magnum
• Brainstem compression → respiratory arrest
• McRae line measurement
• 4th ventricle compression → hydrocephalus
• PICA infarct
• Extracranial Herniation
• Brain through skull defect
• “Mushroom cap” appearance
• Venous infarction risk
• Paradoxical Herniation
• ICP < atmospheric pressure
• Brain shifts away from craniectomy
• Emergency
• Complications (High-Yield)
• Vascular compression → infarcts (ACA, PCA, PICA)
• CN compression (esp. CN III)
• Hydrocephalus
• Duret hemorrhage → terminal sign
• Imaging Pearls
• CT = first-line (ER)
• MRI → better posterior fossa & soft tissue
• Basal cisterns = critical landmarks
• Cistern effacement = early warning

Key Takeaways

• Rapid diagnosis and direct communication are time critical.
• Multiple herniation patterns can coexist.

Reporting midline shift and basal cisterns

Record midline shift in millimeters as displacement of the septum pellucidum from the expected midline and state the measurement level. The number alone does not define clinical severity. Add subfalcine passage of the cingulate gyrus, lateral ventricular compression, contralateral ventricular enlargement, and sulcal effacement to the same assessment.

Review the suprasellar, ambient, and quadrigeminal cisterns separately. Replace a vague statement such as "basal cisterns are effaced" with the specific cistern involved and any brainstem deformation. Example impression: Right hemispheric mass effect produces 8 mm leftward midline shift, compression of the right lateral ventricle, and subfalcine herniation. The right ambient cistern is effaced and the midbrain is displaced leftward, supporting associated right uncal herniation.