Physiologically nystagmus occurs during self-rotation in order to hold images of the visual world steady on the retina and maintain clear vision.
Optokinetic nystagmus (involuntary, conjugate, jerk nystagmus that is seen when a person gazes into a large moving field) | Fig (d)
Cortical and subcortical pathways contribute to the response, which is driven by the retinal image slip velocity.
Vestibular nystagmus (occurs during self-rotation even in darkness) | Fig (d)
Vestibular labyrinth (which project to the vestibular nuclei and cerebellum) contribute to the response
Can also be induced by irrigating the ears with warm or cold water
Oscillations are typically conjugate, horizontal and jerky.
Congenital nystagmus (CN) (slow phases are of an increasing exponential velocity form)
Manifest latent nystagmus (MLN) (slow phases are decreasing/linear)
In addition to its distinguishing slow phase, the fast phase of MLN always beats toward the viewing eye.
MLN is also closely associated with presence of strabismus and dissociated vertical divergence
Strongly visually driven
Largely dependent on the attentional state of the patient
Many forms of acquired nystagmus can be attributed to disturbances of the three mechanisms that normally ensure steady gaze—visual fixation, the vestibulo-ocular reflex, and the mechanism that makes it possible to hold the eyes at an eccentric eye position (e.g. far right gaze)
3 main control mechanisms for maintaining steady gaze:
Gaze-holding system (neural integrator) (operates whenever the eyes are required to hold an eccentric gaze position)
Failure of any of these control systems will bring about a disruption of steady fixation.
2 types of abnormal fixation can result:
Nystagmus: Essential difference between them lies in the initial movement that takes the line of sight off the object of regard. In nystagmus, a slow drift or ‘slow phase’ often due to a disturbance of one of the three mechanisms for gaze stability occurs
Saccadic intrusions/oscillations: Inappropriate fast movement that moves the eyes off target
Pendular nystagmus: Eyes oscillate like a sine wave
Jerk nystagmus: Drifts in one direction with corrective fast phases
Oscillopsia (jumping images)
Tendency to fall
Brainstem-related symptoms (for example, swallowing or speaking difficulties)
Cerebellar symptoms (for example, coordination problems of the extremities)
Inner ear symptoms (for example, hearing loss or tinnitus).