Isolated Pupil-Sparing Third Nerve Palsy Revealing Cryptococcal Meningitis in a Patient With Diabetes Mellitus: A Diagnostic Pitfall

Isolated pupil-sparing oculomotor nerve palsy is classically attributed to microvascular ischemia, particularly in patients with diabetes mellitus. This heuristic may lead to diagnostic anchoring and delay the identification of serious underlying pathology. We report a case of an elderly diabetic patient who presented with isolated pupil-sparing oculomotor nerve palsy, a form of cranial neuropathy, and was subsequently found to have cryptococcal meningitis confirmed by positive cerebrospinal fluid (CSF) cryptococcal antigen. This case highlights that pupil sparing is not a reliable discriminator between ischemic and non-ischemic etiologies. Infectious causes such as cryptococcal meningitis can present atypically as isolated cranial neuropathy, even in the absence of classical meningeal features. Early CSF evaluation is essential in atypical neuro-ophthalmic presentations to avoid diagnostic delay.