Downbeat nystagmus, in which the fast phase is directed downward and is most prominent on looking laterally or down, localizes to the:

• A Dorsal midbrain
• B Cervicomedullary junction ✓
• C Cerebellar flocculus only
• D Vestibular nuclei of the pons

Explanation

Downbeat nystagmus localizes to the cervicomedullary junction, classically seen with Arnold-Chiari type I malformation, syringobulbia, and foramen magnum tumours, and also with cerebellar degeneration, lithium toxicity, and anticonvulsant excess. Disruption of the vestibulocerebellar pathways at the craniocervical junction removes inhibition of the anterior canal pathways, driving the eyes upward with a corrective downward fast phase. Dorsal midbrain lesions instead produce convergence-retraction nystagmus, which is the key distractor here.

Reference: Kanski's Clinical Ophthalmology, 9th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.