In uncal (transtenorial) herniation, which brain structure is pushed under the tentorium?

• A. Medial temporal lobe
• B. Frontal lobe
• C. Parietal lobe
• D. Occipital lobe

Answer: (A)

Uncal transtentorial herniation occurs when pressure pushes the medial part of the temporal lobe—the uncus—downward and through the tentorial notch. The tentorium separates the cerebral hemispheres from the cerebellum, and the uncus protrudes under this edge in this type of herniation. Because the uncus sits on the medial temporal lobe, this movement is described as the medial temporal lobe being pushed under the tentorium. This displacement commonly compresses the oculomotor nerve on the same side, causing a dilated pupil and ptosis, and can affect nearby vessels and brain structures. So the structure being pushed under the tentorium is the medial temporal lobe (the uncus).