One-and-a-Half Syndrome Due to Pontine Hemorrhage

This is a 50-year-old woman who, while exercising in the gym, suddenly experienced vertigo, nausea, vomiting, tingling in the left arm, and diplopia. MRI demonstrated a brainstem hemorrhage that involved the right greater than left pons. Examination demonstrated a right horizontal gaze palsy due to right 6th nucleus involvement, and a right right internuclear ophthalmoplegia (INO) due to right medial longitudinal fasciculus (MLF) involvement. Abduction OS was the only movement that was spared, and this constellation of signs is known as the ‘one-and-a-half syndrome'. In her case, there was also a mild right lower motor neuron facial palsy from involvement of the right 7th fascicle (a common occurrence given its proximity to the 6th nucleus), which is referred to as the ‘eight-and-a-half syndrome'. There was also upbeat nystagmus in upgaze and downbeat nystagmus in downgaze, which are commonly seen with a lesion involving the MLF given the vertical gaze holding pathways that travel from medullary nuclei to the interstitial nucleus of Cajal in the midbrain. It's also possible that adjacent cell groups of the paramedian tracts (PMT) were involved causing vertical gaze holding deficits, or perhaps a combination of MLF and PMT. Her horizontal motility deficits could not be improved through use of the vestibulo-ocular reflex or with the high velocity head impulse test; therefore, the lesion is likely to be nuclear, infranuclear or internuclear as opposed to supranuclear. Since convergence signals descend from supratentorial regions to synapse on the medial rectus subnuclei in the midbrain, adduction deficits due to lesions involving the MLF and/or 6th nucleus (i.e., involvement of the pons) may be overcome by asking the patient to converge. There was a slight improvement in adduction OU in this particular patient with convergence.