NYSTAGMUS is a repetitive to-and-fro movement of the eyes caused by slow drifiting of the eyes away from the desired position.
If nystagmus causes motion of the retinal image that exceeds a few degrees per second, visual blur occurs, due to loss of foveal stability. In patients with acquired nystagmus there may be oscillopsia.
Normal mechanisms that maintain foveal stability include: 1) Fixation by which the visual system detects retinal drift, programs corrective eye movements and suppresses unwanted saccades 2) Vestibulo-ocular reflex that keeps the eyes on a target despite head movements 3) eccentric gaze-holding performed by neural integrators in the brainstem.
Nystagmus may have jerk or pendular waveforms. The slow phase of jerk nystagmus can be linear, decreasing or increasing in velocity. Jerk nystagmus has fast re-setting phases that are corrective. Pendular nystagmus has no corrective fast phases, just slow oscillations.
Some advances have been made in downbeat and acquired pendular nystagmus.
DOWNBEAT NYSTAGMUS (DN): is a vertical jerk nystagmus considered a central vestibulat nystagmus. It initiates with slow drifts of the eye upward, and increases in down and inferolateral gaze. DN is localized to the flocculus and paraflocculus of the vestibulocerebellum. Some cases of DN originate from mediullary lesions.
A common cause of DN is craniocervical junction disease such as Chiari malfromation, and herditary cerbellar degnerations. Up to 35% of cases may be idiopathic.
A common mechanism for DN may be a tone imbalance in the central vestibular pathways for vertical eye movements, which can be either semicircular canals or the otoliths. In particular there may be an imbalance between the anterior semicircular canals that mediate upward motion of the vestibulo-ocular reflex. These are inhibited by the cerebellum, but the downward component is not. Cerebellar lesions disinhibit upward eye movement and allow slow drift of the eyes upward.
A treatment for downbeat nystagmus is potassium channel blockers such as 4-aminopyridine (4-AP) and 3,4 diaminopyridine (3,4 DAP). e.g. Amypra 10 mg bid. Potassium channels are abundant in cerebellar Purkinje cells. Inhibiting the potassium channels with potassium channel blockers can lead to increased excitability and firing of the Purkinje cless that may restore the norma inhibition of the vertical eye movements. 4-AP and 3,4 DAP may be more effective for cerebellar degenerations rather than focal cerebellar lesions. Adverse effects include seizures and cardiac arrythmias. 4-AP may be better tolerated by patients.
Consider Antibodies against GAD (anti-GAD-Ab) (Glutamic acid decarboxylase (GAD) is the enzyme that catalyzes the conversion of glutamic acid to the neurotransmitter gamma-amino butyric acid.)
ACQUIRED PENDULAR NYSTAGMUS (APN): is charactized by slow ocular oscillations with no quick phases and is commonly seen with multiple sclerosis or oculopalatal tremor (previously known as oculopalatal myoclonus).
Contrasting these two entities, the Acquired pendular nystagmus in MS has 3-5 Hz frequency, more planes of movement affected, a more regular waveform, and may be more responsive to treatment. Oculopalatal tremor has a 2-4 Hz movement, a more vertical movement with some torsion superimposed, a more irregular waveform, and is less responsive to treatment.
Acquired pendular nystagmus is more likely due to unstable neural integrators, rather than vision loss/optic neuropathy from MS. As such treatment is directed at drugs that work on neural transmitters, which are required for the neural integrator to function.
Gabapentin, has been described for treatment of APN. Gabapentin blocks a certain subunit of voltage gated calcium channels which are abundant on cerebellar Purkinje cells and might decrease the discharge of the Purkinje cells and restore the integrity of the neural integrators.
Memantine, a glutamate antagonist has also been described for APN. However memantine might exacerbate other MS symptoms.
An optical treatment for APN involves a sensor detecting the nystagmus waveform, that is then fed into a computer that rotates lenses in front of the patient to cancel the nystagmus but allow clear vision. Perhaps a video system can be developed that will capture the visual environment and modulated such that it mimics the nystagmus.