The "one-and-a-half" syndrome was first described by C. Miller Fisher ("Some Neuroophthalmological Observations." J Neurol Neurosurg Psychiatry 1967;30:383). It is a lesion (eg, vascular or demyelinating) of the pon-tine reticular formation and the ipsilateral medial longitudinal fasciculus.
Clinically, the ipsilateral eye has no movement in or out. The contralateral eye can abduct only but cannot cross the midline toward the nose.
(The pontine reticular formation lesion on the right, for example, will abolish conjugate gaze to the right for both eyes, the MLF lesion will abolish adduction for the right eye, and the only remaining lateral movement is abduction of the left eye.) Vertical movements are normal, as are the pupils.
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