Acute Periaqueductal Midbrain Demyelinating Syndrome Presenting with Binocular Diplopia and Oculomotor Palsy: A Case Report

Abstract

Background: Inflammatory demyelinating disorders of the central nervous system may present with overlapping clinical and radiological features, particularly when the initial event is confined to the brainstem. Isolated periaqueductal midbrain involvement presenting with binocular diplopia and oculomotor palsy can create diagnostic uncertainty among neuromyelitis optica spectrum disorder, myelin oligodendrocyte glycoprotein antibody-associated disease, and multiple sclerosis, especially when disease-specific antibody testing is unavailable. Aims: To describe an acute periaqueductal midbrain demyelinating syndrome presenting with binocular diplopia and oculomotor palsy and to discuss the diagnostic challenge posed by neuromyelitis optica spectrum disorder (NMOSD), myelin oligodendrocyte glycoprotein antibody-associated disease (MOGAD), and multiple sclerosis (MS). Presentation of Case: A previously healthy 54-year-old man presented with a one-week history of binocular diplopia, ptosis, restricted ocular movements, vertigo, and gait ataxia. Brain magnetic resonance imaging showed T2/FLAIR hyperintense periaqueductal mesencephalic lesions without diffusion restriction and with subtle gadolinium enhancement. Cerebrospinal fluid contained 4 leukocytes/mm3; mycobacterial studies, including GeneXpert MTB/RIF Ultra, were negative. Serum aquaporin-4 IgG and MOG-IgG testing was unavailable. Orthoptic follow-up documented residual ocular motor incomitance consistent with left oculomotor palsy. Dimethyl fumarate was started at 120 mg twice daily for 7 days and then 240 mg twice daily, with vitamin B supplementation. At one month, diplopia had resolved and the oculomotor deficit, vertigo, and ataxia had markedly improved. Discussion: The clinical localisation and periaqueductal MRI pattern were compatible with an inflammatory brainstem syndrome and raised suspicion of NMOSD, but the available data did not satisfy a definitive antibody-defined diagnosis. MOGAD, MS, and metabolic mimics remained relevant differential diagnoses. Clinical improvement cannot be attributed causally to dimethyl fumarate. Conclusion: Acute binocular diplopia and oculomotor palsy may be the presenting manifestations of a focal inflammatory midbrain lesion. Antibody testing and longitudinal clinical and radiological follow-up are essential when the initial phenotype is isolated to the brainstem.