Downbeat nystagmus is an abnormal eye movement caused by a lesion in the cerebellum, often due to structural problems such as an Arnold-Chiari malformation or tumors. Other causes include head injuries, medication, and poor diet. Treatment options include medication, glasses with prisms, and surgery.
Downbeat nystagmus is an abnormal eye movement pattern characterized by a slow upward drift in the position of both eyes followed by a rapid downward jerk. This pattern of nystagmus indicates an abnormality in the area where the spinal cord enters the skull and joins the brainstem, specifically in the cerebellum. A lesion in the cerebellum interrupts the nerve impulses that normally inhibit the eye’s upward movement. Although the eyes may blink down when the patient looks straight ahead, the nystagmus is usually worse when looking straight down. Patients with bearish nystagmus report a sensation of a moving environment, called oscillopsia.
Some structural lesions can cause downbeat nystagmus. The most common structural problem leading to downbeat nystagmus is an Arnold-Chiari type I malformation. This malformation is characterized by the movement of parts of the cerebellum through the foramen magnum, the opening at the base of the skull through which the spinal cord runs. When the displaced cerebellum enters the foramen, the narrow space compresses the cerebellum, which leads to dysfunction of the eye movement control centers and corresponding bearish nystagmus.
Tumors in the foramen magnum area can also compress the cerebellum, producing bearish nystagmus. Masses in this area can include meningiomas and cerebellar hemangiomas. Head injuries involving the back of the head and upper neck can also produce enough swelling to cause downbeat nystagmus. Any cerebellar degenerative process caused by stroke, multiple sclerosis, infection, or inherited degeneration can contribute to nystagmus.
Other causes of negative nystagmus include certain medications, such as lithium, alcohol, and antiepileptic drugs. A poor diet can contribute to the formation of a nystagmus in case of alcoholism, tube feeding and in case of magnesium deficiency. In some cases of negative nystagmus, affected patients have circulating proteins, called antibodies, in the bloodstream that inhibit the activity of a critical enzyme in the brain, glutamic acid decarboxylase. The antibodies interfere with the function of nerve cells in the cerebellum.
Negative nystagmus can be treated with oral medications, such as baclofen, gabapentin, and clonazepam. Ophthalmologists sometimes prescribe glasses with prisms that bend light entering the eye and stimulate the eyes to converge or turn inward. The convergence of the eyes dampens the movements of the nystagmus. If magnetic resonance imaging (MRI) indicates a treatable structural lesion, neurosurgeons may operate to remove masses or decompress the foramen magnum region. Most treatments for nystagmus are unsuccessful or only partially effective.
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