A stroke or demyelinating disease involving the medial longitudinal fasciculus (MLF) will cause the patient to experience diplopia, most noticeable when he or she attempts to look to the side opposite the lesion. EOM testing reveals an ipsilateral adduction weakness (medial rectus muscle only—not a CN III palsy).
EXAMPLE If a patient has experienced a right (MLF) stroke, the right medial rectus muscle will not function when he or she attempts a leftward gaze and the patient will not be able to adduct the right eye. The left eye will abduct but may experience some nystagmus on leftward gaze. The patient will be able to look to the right without difficulty unless there is bilateral involvement. The patient will also be able to look up and down, revealing that the superior and inferior rectus muscles are functioning and that this is not a CN III palsy.
1. Same as with any newly diagnosed CVA.
2. Consider demyelinating disease if the patient is young.
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