Anisocoria after prone positioning: A benign finding not to be mistaken for brain herniation

Prone positioning improves survival in severe respiratory failure, particularly ARDS, but it can be followed by anisocoria—traditionally a warning sign of intracranial hemorrhage, stroke, or transtentorial herniation. This article shows that benign, position-related mechanisms can also explain the finding. In one reported case, anisocoria after about 18 hours of prone positioning reflected angle-closure glaucoma: gravitational anterior shift of the lens–iris diaphragm triggered pupillary block and a sharp rise in intraocular pressure in an anatomically predisposed patient, requiring targeted glaucoma management rather than repositioning alone. In another case, miosis with ptosis after about 8 hours reflected transient sympathetic pathway compression (Horner syndrome), which resolved completely about 45 minutes after returning to the supine position.

Anisocoria without new neurological deficits after prone positioning is usually benign and positional; repositioning should be the first diagnostic and therapeutic step. Immediate neuroimaging is reserved for patients with red flags such as altered consciousness, focal deficits, or loss of brainstem reflexes. Recognizing these mechanisms can prevent unnecessary emergency imaging and invasive interventions.

The work titled “Prone Positioning–Associated Anisocoria: A Diagnostic Pitfall in Critical Care”, was published on Eye & ENT Research (published on June 28, 2026).