Medial Longitudinal Fasciculus (MLF) Lesion
The most likely structure involved is the medial longitudinal fasciculus (MLF), specifically on the left side (Answer D). This clinical presentation is classic for left internuclear ophthalmoplegia (INO), a hallmark manifestation of multiple sclerosis affecting the brainstem 1, 2.
Clinical Reasoning
The patient's presentation demonstrates the pathognomonic triad of INO:
- Failure of left eye adduction (cannot move medially) when attempting rightward gaze 1, 2
- Abduction nystagmus of the right eye during rightward gaze 2, 3
- Diplopia specifically when looking to the right 4, 3
The MLF is the final common pathway for all conjugate horizontal eye movements, connecting the abducens nucleus (cranial nerve VI) on one side to the contralateral oculomotor nucleus (cranial nerve III) 4, 3. A left MLF lesion disrupts signals traveling from the right abducens nucleus to the left medial rectus subnucleus, preventing left eye adduction during rightward gaze 1, 2.
Why Other Options Are Incorrect
- Frontal cortex (A): Would cause conjugate gaze deviation with both eyes moving together, not dissociated eye movements 1
- Left thalamus (B): Does not produce this specific pattern of ocular motility dysfunction 1
- Sixth cranial nerve (C): A right sixth nerve palsy would prevent right eye abduction, not left eye adduction, and would not produce abduction nystagmus 1, 3
Multiple Sclerosis Context
In young adults with MS, demyelinating plaques typically affect the pons where the MLF is located 1, 5. The American College of Radiology identifies MS as the leading cause of INO in patients under 50 years 1, 5. INO is one of the most frequent brainstem presentations of MS, with medial longitudinal fasciculus lesions causing characteristic failure of ipsilateral eye adduction and contralateral abduction nystagmus 2, 3.
Diagnostic Confirmation
High-resolution T2-weighted MRI of the brainstem is mandatory to confirm MLF lesions 1, 5. The American Academy of Ophthalmology recommends this imaging specifically to visualize demyelinating plaques in the pontine tegmentum 1, 6. In MS patients with INO, MRI typically reveals hyperintense lesions at the mid-pontine level where the MLF courses 3, 7.
Associated Right-Sided Weakness
The concurrent right-sided weakness indicates additional MS lesions beyond the MLF, likely involving corticospinal tracts elsewhere in the CNS 8. This multifocal involvement is characteristic of MS pathophysiology, with dissemination in space being a diagnostic criterion 8, 2.
Management Implications
Immediate neuroimaging with high-resolution brainstem MRI is indicated, and neurology consultation should be obtained to determine the underlying etiology and initiate disease-modifying therapy for MS 1. While vasculopathic INO in older adults typically resolves within 6 months, MS-related INO in younger patients requires long-term disease management 1, 2.