Internuclear ophthalmoplegia (INO)

- In  INO, when the patient’s gaze is directed away from the side of the lesion, the ipsilateral (adducting) eye will not adduct and the contralateral (abducting) eye demonstrates horizontal nystagmus.
- Surprisingly, patients usually do not complain of diplopia
- lesion at medial longitudinal fasciculus (MLF)
- Causes:

• Multiple sclerosis —likely cause in adulthood/middle age; often bilateral. (bilateral INO is nearly pathognomonic of MS)
• Vascular brainstem lesion —likely cause in the elderly or people with vascular risk factors; often unilateral.
• Pontine glioma —more likely cause in children.
• Inflammatory encephalitis affecting the brainstem (e.g. autoimmune, infective)
• Myasthenia gravis (unusual)

- One and a half syndrome is internuclear ophthalmoplegia combined with a conjugate gaze paralysis in the other direction. One eye fails to adduct on attempted lateral gaze (‘the half’) and the other eye neither adducts nor abducts (‘the one’). The eye that can abduct may exhibit horizontal nystagmus when it does so. The responsible lesion is an extensive paramedian pontine lesion that involves the MLF and either the CN6 nucleus or the PPRF (parapontine reticular formation). Often ischemic.