One-and-a-half syndrome, which combines a horizontal gaze palsy and ipsilateral INO (Fig 7-6), is caused by a pontine abnormality that is large enough to involve the MLF and the paramedian pontine reticular formation (PPRF) or the CN VI nucleus on the same side of the brainstem. The only horizontal eye movement remaining is abduction of the eye contralateral to the lesion (ie, horizontal eye movements are lost in 1 eye, whereas they are “half” lost in the fellow eye, hence the name). Vertical gaze is preserved. A lesion that produces one-and-a-half syndrome but also involves the intra-axial portion of CN VII is termed eight-and-a-half syndrome (7 + 1.5 = 8.5). Stroke is the most common cause of this disorder.
Espinosa PS. Teaching NeuroImage: one-and-a-half syndrome. Neurology. 2008;70(5):e20.
Johkura K, Kudo Y, Amano Y, et al. Gaze palsy and exotropia in internuclear ophthalmoplegia. J Neurol Sci. 2015;353(1–2):158–160.
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.