Adduction lag
MLF lesion
LesionMLF (medial longitudinal fasciculus)
TriadAdduction deficit + abducting nystagmus
Named forSide of the adduction deficit
ConvergenceOften preserved (posterior INO)
Adducting eye lags, the other eye beats — one MLF lesion, two very different causes.
INO — two clinical patterns
| Young / bilateral | Older / unilateral | |
|---|---|---|
| Typical cause | MS demyelination | Brainstem stroke |
| Laterality | Bilateral | Unilateral |
| Localization | MLF (CN VI nucleus → contralateral CN III) | MLF (small perforator infarct) |
| Assoc. syndrome | One-and-a-half (MS plaque) | One-and-a-half (lacunar infarct) |
◆ Signature clue
Triad = ipsilateral adduction deficit + contralateral abducting nystagmus + convergence often preserved in posterior INO (impaired convergence → more anterior/midbrain lesion). Named for the side of the adduction deficit.
Diagnosis
Clinical triad; MRI brainstem for the MLF lesion (between CN VI nucleus & contralateral CN III). Add ipsilateral gaze palsy → one-and-a-half syndrome.
Management
Treat the cause — MS (demyelination) vs brainstem stroke. No specific ocular therapy; diplopia often recovers.