In Parinaud's dorsal midbrain syndrome, the characteristic finding of light-near dissociation occurs because:

• A Parasympathetic fibres to iris sphincter are selectively damaged at the ciliary ganglion
• B Bilateral optic nerve lesions reduce the afferent limb of the light reflex
• C Superior colliculus damage impairs light perception but spares accommodation
• D Pupillary light reflex fibres in the posterior commissure are disrupted while near reflex fibres passing ventrally are spared ✓

Explanation

In Parinaud's syndrome (dorsal midbrain syndrome, e.g., from pinealoma), the decussating pupillary light reflex fibres coursing through the posterior commissure are selectively compressed. The near reflex pathway, which travels more ventrally through the midbrain, is spared. This produces the classic light-near dissociation: pupils are poorly reactive to direct light but constrict briskly during the accommodative-convergence response. Additional features include upgaze palsy (superior colliculus/posterior commissure), convergence-retraction nystagmus (Koerber-Salus-Elschnig sign), and eyelid retraction (Collier's sign).

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.