The Medial Longitudinal Fasciculus and Internuclear Opthalmoparesis: There’s More Than Meets the Eye

Background and purpose The classic sign of a lesion in the medial longitudinal fasciculus is internuclear opthalmoplegia. However, clinical presentation may vary depending on the type of pathology and the lesion location. The purpose of this study was to identify and classify the different lesions of the medial longitudinal fasciculus on MRI and review their clinical presentations. We also offer an overview of the pertinent imaging anatomy of the medial longitudinal fasciculus. Materials and methods Patients with an abnormality affecting the medial longitudinal fasciculus were identified retrospectively using the keyword 'medial longitudinal fasciculus' included in radiology reports between 2010 and 2018 using the Nuance mPower software (Nuance Communications, Burlington, MA). The brain MRI examinations of these patients were reviewed by two neuroradiolgists. Detailed description of lesion location within the medial longitudinal fasciculus, pathology type, additional lesions, and clinical presentations were recorded along with pertinent demographic information. Results Five men and four women were identified with lesions in the medial longitudinal fasciculus on brain MRI. Five patients demonstrated demyelination in the medial longitudinal fasiculus and four patients demonstrated findings of an acute cerebrovascular accident. Two-thirds of medial longitudinal fasiculus lesions were located in the upper, mid, or lower pons with the remaining located in the midbrain. Of the patients presenting with a cerebrovascular accident, there was little to no additional evidence of acute stroke elsewhere in the brain. All patients were clinically symptomatic with 89% of patients demonstrating extraocular muscle dysfunction at presentation and 78% of patients experiencing dizziness. Additional symptoms included headache, weakness, and gait instability. Conclusions Lesions involving the medial longitudinal fasiculus may not always present with the classic sign of internuclear opthalmoplegia. Variations in lesion location may result in diplopia rather than internuclear opthalmoplegia, and additional brain lesions may produce clinical symptoms that confound extraocular muscle dysfunction. Lesions affecting the often-overlooked vestibular and otolithic reflexes, which run in the medial longitudinal fasiculus, may result in dizziness/weakness and mask the classic internuclear opthalmoplegia symptoms. The radiologist should carefully inspect the medial longitudinal fasiculus in all patients regardless of the supportive clinical history of extraocular muscle dysfunction since symptoms may be more general than classically described.