Which CN palsy is the closest differential to Divergence Insufficiency?

• A. CN 3 Palsy
• B. CN 6 Palsy
• C. CN 4 Palsy
• D. CN 5 Palsy

Answer: (B)

Divergence insufficiency occurs when the eyes have a reduced ability to diverge, so the muscle system most central to the problem is the lateral rectus and its nerve supply. The sixth cranial nerve powers the lateral rectus, which abducts the eye and enables divergence. When this nerve’s function is impaired, divergence is directly limited, producing a pattern that closely resembles divergence insufficiency on examination. That direct link to lateral rectus function makes a sixth nerve palsy the most relevant differential, because other cranial nerve palsies involve different muscles or systems and don’t reproduce the same divergence deficit. Third nerve palsy disrupts multiple muscles including the medial rectus and levator, often with ptosis; fourth nerve palsy affects the superior oblique and yields vertical/torsional diplopia that varies with gaze; fifth nerve involvement relates to sensation or mastication, not ocular divergence. So the closest differential to divergence insufficiency is a sixth nerve palsy, reflecting a primary issue with the muscle responsible for divergence.