MRI-Negative Internuclear Ophthalmoplegia: A Diagnostic Challenge Emphasizing Clinical Localization.

Internuclear ophthalmoplegia (INO) is a distinct ocular motility disorder localizing to the medial longitudinal fasciculus (MLF). Because the MLF is a microscopic, highly myelinated tract, small inflammatory lesions can cause significant functional conduction blocks that fall below the resolution threshold of typical 1.5T or 3T MRI. This leads to clinical radiographic dissociation, resulting in a significant diagnostic challenge when pathognomonic physical findings are present despite negative neuroimaging. A 16-year-old female presented with acute binocular diplopia and bilateral INO. Physical examination revealed pathognomonic bilateral failure of adduction, dissociated nystagmus, and impaired convergence. While the brain MRI demonstrated periventricular white matter hyperintensities, it failed to reveal a distinct lesion within the MLF. Despite the lack of definitive radiological localization in the brainstem, the clinical findings were conclusive for a demyelinating process. The patient was treated empirically with high-dose intravenous methylprednisolone to facilitate blood-brain barrier stabilization and address suspected inflammatory demyelination. This case underscores the diagnostic hierarchy in neurology, where pathognomonic physical findings must supersede nonspecific imaging in highly localizing brainstem disorders. Early recognition is vital, especially in pediatric populations where bilateral INO serves as a "red flag" for multiple sclerosis or other central demyelinating events.