One-and-a-half syndrome: anatomo-clinical considerations apropos of a case

A 36 years-old man was admitted in September 1987. For 5 years he suffered from 4 recurrent episodes of throbbing headache, tinnitus, nausea, diplopia and divergent strabismus to which a facial palsy was recently added. In all episodes, the symptoms disappeared spontaneously and completely. A neuro-ophthalmological examination at admission disclosed an exotropia of the right eye, gaze paralysis to the left, paralysis of adduction of the left eye and preserved right eye abduction which triggered a rhythmic horizontal nystagmus. The upward and downward gazes and the convergence were well preserved. Moreover, there was a left peripheral facial palsy, and Babinski sign at the right side. Auditory evoked potentials were slowed at the mesencephalopontine transition. CT scan showed a low-density area with no contrast enhancement at the left pontine tegmentum and a left anterolateral atrophy of the pons. CSF examination showed increase in protein content and increase in the IgG content. Additional investigation included a dopplerometry of the cervical arteries, a panangiography and a bidimensional echocardiography which were normal. Diagnosis of one-and-a-half syndrome was made, possibly secondary to multiple sclerosis, and immunossuppressive therapy was initiated.