Which sign is most classic for impending transtentorial herniation?

Which sign is most classic for impending transtentorial herniation?

• A. Ipsilateral blown pupil
• B. Contralateral hemiparesis
• C. Posturing
• D. Cushing reflex

Answer: (A)

When a medial temporal structure herniates through the tentorial notch (uncal or transtentorial herniation), it first compresses the third cranial nerve on the same side as the herniation. This nerve carries the parasympathetic fibers that constrict the pupil, so its compression causes a dilated, nonreactive pupil on that side—the classic early sign. You’ll often see the eye also directed down and out with ptosis, reflecting the oculomotor nerve palsy. Other findings can occur but are less specific early on. Posturing reflects brainstem dysfunction and may appear later. The Cushing reflex (hypertension with bradycardia and irregular respiration) indicates severe intracranial pressure and instability, but it’s a later, more generalized sign. Contralateral weakness can occur due to brainstem compression or shifting (and can be a misleading “false localizing” sign), but it’s not the hallmark clue of impending herniation. So, the presence of an ipsilateral blown pupil best signals impending transtentorial herniation.

When a medial temporal structure herniates through the tentorial notch (uncal or transtentorial herniation), it first compresses the third cranial nerve on the same side as the herniation. This nerve carries the parasympathetic fibers that constrict the pupil, so its compression causes a dilated, nonreactive pupil on that side—the classic early sign. You’ll often see the eye also directed down and out with ptosis, reflecting the oculomotor nerve palsy.

Other findings can occur but are less specific early on. Posturing reflects brainstem dysfunction and may appear later. The Cushing reflex (hypertension with bradycardia and irregular respiration) indicates severe intracranial pressure and instability, but it’s a later, more generalized sign. Contralateral weakness can occur due to brainstem compression or shifting (and can be a misleading “false localizing” sign), but it’s not the hallmark clue of impending herniation.

So, the presence of an ipsilateral blown pupil best signals impending transtentorial herniation.