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34 year old man with headache, vertigo, clumsiness, and unsteady vision
Digital Journal of Ophthalmology 1996
Volume 2, Number 1
August 14, 1996
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Melanie Ryan-Graham, MD | Massachusetts Eye and Ear Infirmary, Harvard Medical School, Boston, MA
Simmons Lessell, MD | Massachusetts Eye and Ear Infirmary, Harvard Medical School, Boston, MA
Diagnosis and Discussion
Downbeat Nystagmus Secondary to Bony Cervicomedullary Junction Abnormality

Clinical Course
Given the patient's progressive and disabling symptoms of bidirectional vertical oscillopsia, vertigo, headache, dysmetria, and gait ataxia, as well as the precarious situation of HAVING the medulla rest on a protuberant bony abnormality, surgical intervention was recommended. After transoral decompression of the foramen magnum with removal of the clivus protuberance, there was considerable recovery. At a one year follow up examination, the patient reported oscillopsia only after vigorous exercise. Examination revealed trace nystagmus on lateral gaze, a normal gait, and normal reflexes.

Discussion
Downbeat nystagmus is a primary position vertical jerk nystagmus that is often accompanied by vestibular and cerebellar signs such as vertigo, ataxia, and oscillopsia. It is characterized by conjugate eye movements which occur in primary postion and are not affected by fixation. The nystagmus increases in amplitude in lateral gaze, is greatest in down and lateral gaze, and dampens in up gaze. There may be a torsional component in lateral gaze.

Studies have suggested that DBN is a pathologic central vestibular nystagmus caused by imbalance of the tonic activity of the vertical vestibulo-ocular reflex pathways. This disturbance of neural input and integration is thought to upset the usual balance of information that maintains fixation. It is hypothesized that an abnormal tone of the semicircular canal reflexes in the sagital plane causes a nystagmus of primary position with a slow vertical drift of the eye away FROM a target, and a fast saccade returning the eye to the target (7). For example, a reduction in a tonic inhibitory stimulus to the superior rectus or an increase in a tonic excitatory stimulus to the inferior rectus could cause an upward drift of the eye, and than a downward corrective saccade, to produce DBN.

The lesions causing the central vestibular imbalance of DBN are proposed to be either of the brainstem or of the vestibulo-cerebellum, which is comprised of the cerebellar flocculus and nodulus. These lesions lead to the disturbance of the connections between the cerebellum, medulla, and the anterior and posterior semicircular canals. Studies have shown that structural lesions of the fourth ventricle between the vestibular nuclei cause disruption of the tonic excitatory fibers to the inferior rectus muscles, and that a bilateral lesions of the flocculus lead to an increase in the tonic excitatory input to the superior rectus muscle (8). The diagnosis of DBN warrants an MRI to identify possible pathology of the cervicomedullary junction, and an investigation of possible iatrogenic, toxic, metabolic and paraneoplastic etiologies as suggestied by history and examination.

Treatment is directed at correction the underlying abnormality, whether it be via withdrawal of toxic drugs and substances, surgical decompression, treatment of neoplastic process, or correction of vitamin deficiency. Clonazepam has been used to decrease DBN and oscillopsia with some success, although it has side effect of drowsiness and fatigue (9). A recent study reports the efficacy of intravenous scopolamine in reducing nystgmus and oscillopsia and improving visual acuity and motion perception (10). This study's findings are supported by an earlier study by Dietertich which reports a transient worsening of DBN with injection of physostigmine, a cholinergic agonist (11). Dieterich also reports the efficacy of baclofen. Fresnel prisms and strabismus surgery to dampen the nystagmus by promoting a null point position have also been described (12).

Downbeat nystagmus with secondary oscillopsia is a potentially curable entity. Careful evaluation and therapeutic intervention may lead to improvement of disability and recognition of a potentially fatal disease.
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