Dissociated Nystagmus With Internuclear Ophthalmoplegia
Lesions of the medial longitudinal fasciculus (MLF) produce an internuclear ophthalmoplegia (INO) (see Chapter 7). Isolated slowing of adduction of the eye ipsilateral to an MLF lesion is the primary feature required to establish a diagnosis of INO. In addition, nystagmus of the abducting eye often occurs when gaze is directed to the side opposite the lesion. One explanation for this pattern of dissociated nystagmus is the development of increased neural pulsing in an attempt to overcome the adduction weakness. According to Hering’s law, the increased neural signaling is also delivered to the contralateral yoke muscle, causing excessive saccadic movements in the contralateral lateral rectus muscle.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.