internuclear ophthalmoplegia
A disorder of horizontal eye movements caused by lesions in the medial longitudinal fasciculus, resulting in impaired adduction.
Full Definition
Internuclear ophthalmoplegia (INO) is a distinctive eye movement disorder caused by damage to the medial longitudinal fasciculus (MLF), a brainstem tract that coordinates horizontal conjugate eye movements. The hallmark finding is impaired adduction of the affected eye during horizontal gaze, while abduction of the contralateral eye remains intact but may show nystagmus. Convergence is typically preserved, helping differentiate INO from third nerve palsy. INO can be unilateral or bilateral and is commonly caused by multiple sclerosis in young adults and brainstem stroke in older patients. The condition reflects disconnection between the sixth nerve nucleus and the contralateral third nerve nucleus.
Usage
Usage note: Always test convergence to differentiate from third nerve palsy; specify unilateral or bilateral.
In Context
- "Left internuclear ophthalmoplegia was evident on rightward gaze with preserved convergence." — Neuro-ophthalmological examination
- "Bilateral INO in a young patient raised suspicion for multiple sclerosis." — Clinical assessment