Optokinetic nystagmus (OKN) is a complex ocular motor reflex that allows people to adequately follow the moving objects while keeping the head steady. OKN occurs because the eyes are seeking to keep a moving image stationary on the fovea. OKN strips are a test where the drifting visual stimuli across the screen are projected in order to check client´s visuomotor function from birth. These targets usually provoke pursuit eye movements alternating with resetting saccades. OKN depends on several visual parameters including contrast, spatial frequency content, stimulus speed, direction and area, and then again depends on the integrity of numerous neural pathways in the brain. Groups of people who may be impaired are individuals with neurological lesions (e.g. premature or strabismic infant) and the cognitively elderly.
Equipment and set-up:
The test is held next to the client´s head so they are able to see the screen. By clicking on the screen, the speed of the rotating drum increases. Swipe to left or right changes the colour combination from black/white to red/white and vice versa. Swipe up or down will remove or add the centre dot. First test horizontally, then rotate the screen for 90 degrees and test vertical movements. The client´s task is to watch the display centre. In case that nystagmus cannot be inducted in any direction, it may help to ask the client to count how many dark bars pass the display centre. The clinician´s task is to look for any asymmetric OKN.
Interpreting the results:
Horizontal asymmetry is often a sign of a unilateral, posterior hemisphere lesion. The direction of the rotating drum associated with a poor or absent OKN shows the side of the lesion (e.g. If a poor response is obtained on target movement on the left, the lesion probably exists in the left side). Horizontal asymmetry can also occur with brainstem lesion, internuclear ophthalmoplegia or subtle lesion of a medial longitudinal fascicle. Some up and down asymmetry in vertical OKN is commonly seen also in normal subjects, so interpretation should be cautious. Convergence and retraction jerks should be looked for, often when downward moving strips stimulate upward saccades. By doing so, convergence or retraction nystagmus is provoked as a sign of upper brainstem disease.