A Bilateral Internuclear Ophthalmoplegia Associated with Progressive Supranuclear Palsy: A Case Report

Purpose: We report a rare case of bilateral internuclear ophthalmoplegia (INO) associated with progressive supranuclear palsy (PSP).Case summary: An 83-year-old man presented with progressive binocular diplopia over 2 years. He had no history of systemic diseases, including diabetes mellitus, hypertension, or thyroid disorders, and the diplopia did not exhibit diurnal variation. The patient reported slowness of movement and frequent falls during ambulation which had begun 3–4 years earlier. He had no history of musculoskeletal disorders or neurological diseases such as a stroke. Brain magnetic resonance imaging demonstrated moderate midbrain atrophy consistent with PSP and mild atrophy of the frontal and temporal lobes. Ophthalmologic examination revealed visual acuity of 1.0 in the right eye and 0.8 in the left eye with normal fundus findings. Alternate cover testing exhibited 40 prism diopters of exotropia at both near and distance. Ocular motility assessment demonstrated limitations of adduction (–4), elevation (–3), and depression (–3), with absent convergence. Preservation of the doll’s eye reflex indicated a supranuclear lesion.Conclusions: Our case highlights PSP as a rare cause of bilateral INO. In older patients presenting with progressive diplopia, a comprehensive assessment including detailed ocular motility examination, thorough history of extrapyramidal symptoms, and neuro-ophthalmologic evaluation is essential to identify a potential underlying neurodegenerative disorder such as PSP.