A 28-year-old man complains of oscillopsia and imbalance. Examination shows sustained downbeat nystagmus, most obvious on looking straight ahead and on downgaze. MRI of the brain is most likely to demonstrate:

• A Pineal region tumour compressing the quadrigeminal plate
• B Chiari type I malformation with cerebellar tonsillar descent through the foramen magnum ✓
• C Infarction of the paramedian thalamus
• D Demyelinating plaque in the periaqueductal grey matter

Explanation

Downbeat nystagmus localises to the cervicomedullary junction at the craniocervical junction where the flocculus and vestibular nuclei pathways are affected. Arnold-Chiari type I malformation is the classic structural cause, followed by cerebellar degeneration, lithium toxicity and magnesium deficiency. B pineal region tumour produces Parinaud dorsal midbrain syndrome with convergence-retraction nystagmus, which is upward beating rather than downward. Recognising the localising value of downbeat nystagmus should prompt imaging of the foramen magnum specifically.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.