What is the most likely cause of constant vertical pendular nystagmus and abnormal smooth pursuit in a patient?

Constant vertical pendular nystagmus and abnormal smooth pursuit are indicative of dysfunction within the central nervous system, particularly affecting the areas responsible for eye movement control. In this case, a central vestibular system lesion is the most likely cause.

Vertical pendular nystagmus suggests involvement of the brainstem or cerebellum, areas that are critical for eye movement coordination and vestibular function. The abnormal smooth pursuit further supports this, as smooth pursuit relies on intact pathways in the brainstem and cortical areas to enable the tracking of moving objects. When there is a central lesion, such as in multiple sclerosis or other neurological disorders, these critical pathways can be compromised, resulting in the symptoms described.

In contrast, lesions in the peripheral vestibular system typically cause symptoms like vertigo, but do not usually manifest as constant vertical nystagmus or significant smooth pursuit deficits. Similarly, posterior semicircular canalithiasis pertains specifically to a form of benign paroxysmal positional vertigo (BPPV), which does not lead to the constant nystagmus observed in this scenario, as it would usually present with brief episodes of nystagmus during positional changes.

Vertebrobasilar insufficiency can result in dizziness and various neurological signs depending