Which type of herniation most directly compresses the midbrain structures?

• A. Uncal herniation
• B. Tonsillar herniation
• C. Subfalcine herniation
• D. Midline shift

Answer: (A)

Uncal herniation happens when the inner portion of the temporal lobe (the uncus) herniates through the tentorial notch. As it moves, it sits directly against the midbrain, pressing on the structures there, including the cerebral peduncles and the oculomotor nerve. This near-by compression explains why this pattern most directly affects midbrain regions, producing a classic oculomotor palsy on the same side (upward and outward gaze with a dilated pupil) and potential motor signs from the crus cerebri. Other forms of herniation involve different parts of the brainstem: tonsillar herniation presses downward on the medulla and lower brainstem, subfalcine herniation shifts the cingulate gyrus and often affects the anterior cerebral artery territory, and a midline shift is a general sign of mass effect rather than a specific midbrain compression.