A Case of Vertebrobasilar Insufficiency with Spontaneous Downbeat Nystagmus

We report a case of a 79-year-old man who presented with positional vertigo and spontaneous downbeat nystagmus and was diagnosed as having vertebrobasilar insufficiency. Magnetic resonance angiography (MRA) and ultrasonographic evaluation of the vertebral artery revealed hypoplasia of the left vertebral artery. Caloric testing did not reveal canal paresis, but visual suppression was weakened, suggesting the presence of cerebellar and brainstem dysfunction. The video head impulse test revealed increased vestibulo-ocular reflex (VOR) gain in the anterior canal and decreased VOR gain in the left posterior canal, which was thought to be due to disinhibition of the VOR of the anterior canal due to damage to the floccules, and impairment of the VOR of the posterior canal due to damage to the pons. This led to hyperfunctioning of the anterior canal, with formation of an upward slow phase followed by a downward fast phase, resulting in the appearance of spontaneous downbeat nystagmus. The downbeat nystagmus was observed in the right lateral decubitus position in the absence of cervical twisting, but disappeared during the supine roll test in the right-head-down position with cervical twisting. This disappearance was due to a decrease in blood flow in the left vertebral artery caused by rightward twisting of the neck, which led to dysfunction of the midbrain-pontine junction and disappearance of hyperfunctioning of the VOR of the anterior canal.