Snapshot A 37-year-old woman presents to her primary care physical for diplopia. Her symptoms began 4 days ago and have progressively worsened. Her diplopia is present on horizontal gaze and improves when looking straight ahead. Medical history is unremarkable but reports an episode of right upper extremity numbness that lasted approximately 2 weeks and self-resolved. Physical examination is notable for impaired adduction on the right eye and nystagmus of the left eye with abduction. She is scheduled for an MRI of the brain for further evaluation. (Multiple sclerosis) Introduction Definition an impairment in horizontal eye movement characterized by weakened adduction of the affected eye nystagmus with abduction of the contralateral eye Oculomotor circuitry during horizontal eye movement the paramedian pontine reticular formation (PPRF) sends a signal to the abducens nucleus, which contains 2 sets of neurons neuronal axons that innervate the ipsilateral rectus muscle and cause abduction abducens interneurons decussate to form the medial longitudinal fasciculus (MLF) and innervate the medial rectus subnucleus axons from the medial rectus subnucleus innervate the ipsilateral medial rectus muscle adduction Etiology Lesions that impair the function of the MLF such as demyelinating diseases (multiple sclerosis is seen in ~70% of cases; bilateral impairment of the MLF is more common in these patients) brainstem infarction (most common cause in an older adult) brainstem and fourth ventricle tumors Presentation Symptoms diplopia with horizontal gaze Physical exam impaired adduction of the eye ipsilateral to the MLF lesion nystagmus of the contralateral eye with abduction Imaging MRI indication all patients presenting with an INO Differential Partial third nerve palsy with significant medial rectus involvement differentiating factor patients may have ptosis, pupilary dilation, and weak eye elevation Treatment Conservative observation indication INO typically resolves with demyelinating disease and there is often improvement in stroke cases prisms indication in cases where there are exotropia and no resolution of the deficit Prognosis Typically resolves over few-to-several months