Left MLF Lesion Causes Ipsilateral INO
A left medial longitudinal fasciculus (MLF) lesion causes internuclear ophthalmoplegia (INO) on the same (left) side, characterized by impaired adduction of the left eye during conjugate horizontal gaze. 1, 2, 3
Anatomical Basis and Clinical Findings
The MLF is the final common pathway for conjugate horizontal eye movements, and a lesion produces a characteristic pattern of eye movement abnormalities on the ipsilateral side 4:
Primary Features of Left MLF Lesion:
- Adduction paresis of the left (ipsilateral) eye during attempted conjugate lateral gaze to the right 2, 3
- Abduction nystagmus in the right (contralateral) eye when looking to the right 2, 3
- Preserved convergence (distinguishes INO from third nerve palsy) 2
Additional Associated Findings:
- Skew deviation with the left eye typically positioned higher than the right 2
- Dissociated vertical-torsional nystagmus with downbeat component in the left eye 2, 5
- Ipsiversive torsional nystagmus (toward the left side of the lesion) may occur 5
Vestibulo-Ocular Reflex Deficits
Beyond the classic INO findings, left MLF lesions produce specific VOR abnormalities 4:
- VOR gain from the left lateral semicircular canal is reduced to approximately 0.48 for the adducting (left) eye and 0.81 for the abducting (right) eye 4
- Vertical VOR deficits occur, particularly from the contralateral posterior SCC (gain ~0.29) 4
- These deficits cause oscillopsia during head movements 4
Diagnostic Approach
MRI is the gold standard for confirming MLF lesions in patients presenting with INO 1, 3:
- High-resolution T2-weighted images of the brainstem should be obtained 6
- MRI can identify the specific location and etiology of the MLF lesion 1, 3
Common Etiologies by Age:
- Young adults (<50 years): Multiple sclerosis is the primary consideration 6, 1
- Older adults: Brainstem stroke/ischemia is most common 6, 1
- Other causes: Trauma, tumors, infection, Arnold-Chiari malformation 3
Clinical Pearls
When INO presents with intense headache in a young adult, consider vertebral artery dissection as the vascular etiology 1. This is particularly important because bilateral INO can result from vertebral artery dissection affecting both MLF tracts 1.
The American Academy of Ophthalmology guidelines emphasize that abnormalities in the rostral pons and midbrain (where the MLF is located) result in contralateral hypotropia and head tilt when considering skew deviation patterns 6.