Brainstem or cerebellar lesion: This is often the most common cause of vertical nystagmus due to the involvement of the brainstem or cerebellum, which are critical for controlling eye movements.

Multiple sclerosis: This condition can cause lesions in the brainstem or cerebellum, leading to vertical nystagmus.
Stroke: A stroke affecting the brainstem or cerebellum can result in vertical nystagmus.
Wernicke's encephalopathy: This condition, often related to thiamine deficiency, can cause vertical nystagmus among other neurological symptoms.

Brainstem stroke or hemorrhage: Although less common, missing a brainstem stroke or hemorrhage could be catastrophic due to the high risk of rapid deterioration and death.
Central nervous system infection (e.g., meningitis, encephalitis): Infections in the CNS can present with a wide range of symptoms, including vertical nystagmus, and require prompt treatment to prevent serious complications or death.
Increased intracranial pressure: Conditions causing increased intracranial pressure, such as a tumor or hydrocephalus, can lead to vertical nystagmus and are critical to diagnose early to prevent severe outcomes.

Congenital nystagmus: Although typically horizontal, some forms of congenital nystagmus can have a vertical component.
Paraneoplastic syndromes: Rarely, vertical nystagmus can be a presenting symptom of a paraneoplastic syndrome associated with certain cancers.
Toxic or metabolic disorders: Certain toxins or metabolic disorders can affect the brainstem or cerebellum, leading to vertical nystagmus, though these are less common causes.