A brainstem lesion of any type that involves the medial longitudinal fasciculus (MLF) can cause internuclear ophthalmoplegia (INO). This primarily affects conjugate horizontal gaze and classically manifests as impaired adduction ipsilateral to the lesion and abduction nystagmus contralateral to the lesion. Here, we describe the anatomy of the MLF and review the clinical features of INO. We also describe conjugate horizontal gaze palsy and some of the ‘INO-plus’ syndromes.
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Contributors The manuscript is written by JDV and reviewed by GTP with corrections. Diagrams are produced by JDV. Animations are produced by Emma Plant. MR images are provided by Dr Indran Davagnanam (UCL Neuroradiology) and Professor Janice Holton (UCL Brain Bank).
Competing interests None declared.
Provenance and peer review Commissioned; externally peer reviewed. This paper was reviewed by Christian Lueck, Canberra, Australia.
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