Downbeat nystagmus

Abstract

Localizing significance: Downbeat nystagmus (DBN) was found in lesions in the posterior fossa close to the medullocervical (craniocervical) junction, of the floor of the 4th ventricle and of the cerebellum, especially of the cerebellar flocculi. The occurrence of DBN in a patient demands special differential-diagnostic considerations in Neuro-Ophthalmology. Both congenital and acquired diseases were found in patients with this special kind of nystagmus.

Clinical evaluation has proved a multifactorial cause of DBN. About one-third of the patients with this sign show an Arnold-Chiari malformation. In a cerebellar degeneration this vertical nystagmus type is not rare. It is essential to investigate all the family members to make diagnosis sure if a hereditary disease is suspected. Familial DBN due to spinocerebellar degeneration is one of the commonest causes of DBN. Hereditary downbeat nystagmus was described without additional neurological findings. Magnetic resonance imaging is the method of choice for investigating patients with DBN.

Clonazepam, prism therapy, or a Kestenbaum-type operation, or artificial retinal image stabilization can be helpful in patients who suffer from nystagmus-induced oscillopsia.

The mechanism of DBN remains controversial. The mechanism postulated for DBN supposes that it reflects imbalance of tonic vestibular control in the vertical canal system or, alternatively, that it results from defective smooth pursuit mechanisms in the vertical plane.

A large series of 62 patients with DBN showed that in about 40% the cause remained undiagnosed despite detailed neurological and neuroradiological investigations.

Key Words:

• downbeat nystagmus
• Arnold-Chiari malformation
• cerebellar degeneration
• tumors of the brainstem and cerebellum
• paraneoplastic syndrome
• intoxications
• multiple sclerosis
• neurosyphilis
• encephalitis and meningitis
• trauma of the skull