Walsh & Hoyt: Combined Unilateral Conjugate Gaze Palsy and Internuclear Ophthalmoplegia (One-and-a-Half Syndrome)

Combined lesions of the abducens nucleus or PPRF and adjacent MLF on one side of the brain stem cause an ipsilateral horizontal gaze palsy and INO, so that the only preserved horizontal eye movement is abduction of the contralateral eye; hence the name one-and-a-half syndrome. Such patients may show an exotropia when attempting to look straight ahead; the eye opposite the side of the lesion is deviated outward. This misalignment of the eyes is thought to be caused by the unopposed drives of the intact pontine gaze center to the spared abducens nucleus and is called paralytic pontine exotropia. Many patients, however, have an esotropia or no deviation in primary position (as with many cases of INO), though it may be always present when fixation is eliminated. The spared abduction saccades of the contralateral eye are followed by centripetal drift, so that a nystagmus similar to that of the abducting eye in INO is present. Occasionally, the ipsilateral horizontal vestibular responses are preserved when voluntary gaze is abolished, suggesting that the pontine lesion is more rostral in the PPRF or more discrete in the caudal PPRF, thus sparing the vestibular projections to the abducens nucleus. Although attempts at conjugate movements elicit no adduction, vergence movements may be preserved. Ocular bobbing may accompany the one-and-a-half syndrome.