A mass effect with fixed, dilated pupils most likely indicates which type of herniation?

• A. Uncal (lateral) herniation compressing CN III.
• B. Central transtentorial herniation.
• C. Cingulate herniation.
• D. Tonsillar herniation.

Answer: (A)

The main concept is that a fixed, dilated pupil with a mass effect points to compression of the oculomotor nerve (CN III) from a herniating temporal lobe, or uncus, pressing against the tentorial edge. The parasympathetic fibers that constrict the pupil lie on the surface of CN III, so when the uncus is pushed medially, these fibers are compressed first. Loss of parasympathetic input causes the pupil to dilate and become nonreactive to light. At the same time, the eye often turns “down and out” because the lateral rectus and superior oblique muscles (innervated by CN VI and IV) act unopposed. This constellation—ipsilateral fixed dilated pupil with an oculomotor pattern—is classic for uncal herniation.

The other patterns don’t fit this specific mechanism. Central transtentorial herniation tends to produce more diffuse brainstem involvement and less specifically a CN III palsy with a fixed dilated pupil. Cingulate herniation usually affects the medial aspects of the frontal lobes and can impair behavior or leg function via the anterior cerebral artery, not producing a unilateral fixed pupil. Tonsillar herniation compresses the brainstem and cerebellar structures, leading to rapidly evolving brainstem signs and respiratory/cardiac failure rather than a focal CN III palsy with a dilated pupil.