Optokinetic nystagmus (OKN) and optokinetic after-nystagmus (OKAN) tests are common visual stimulation tests. OKN is induced when part of, or the entire visual field is continuously moving. It is eye movement elicited when a moving field is being tracked. OKN is sometimes confused with smooth pursuit where eye movement is produced with a single distinct target tracked. Smooth pursuit is contained within OKN because a distinct target is part of the moving field. The instructions the eye doctor gives divide OKN into two types: Schau-nystagmus when the doctor instructs the patient to look at the stripes and count them one by one; and Stier-nystagmus, where the patient is instructed to stare vacantly in front of him/her.
OKAN occurs when there is a persistent after- response when the stimulus is terminated for OKN, and the individual is in complete darkness (100%) and alert. OKAN happens in two phases, where the nystagmus is in the same direction as during OKN while in the second phase, nystagmus is in the opposite direction. It is based on various pathological eye movements. OKAN occurs roughly between experimental nystagmus on one hand, and spontaneous and provoked nystagmus on the other hand. OKN and OKAN are both present in vertical and horizontal planes.
Also Known As
There are various methods to stimulate OKN/OKAN and present a moving target in both the peripheral and central visual fields. These methods may include:
- A light bar
- A physical drum
- Stripe projector
- A large screen array or LCD projector
OKN is used to stabilize the retinal image of a moving environment. OKAN, on the other hand, helps to compensate for vestibular after-responses. It is also a motor response to a protracted central excitatory state elicited when an individual view a moving visual field. Besides, OKAN is concerned with the velocity storage mechanism, hence significant in the central vestibular system and peripheral functions. OKAN also shows perceptual errors that occur when visual stimulation is entirely absent.
Before the Procedure
An amphetamine sulphate medication is administered 30 minutes before the procedure to keep the patient alert.
Research indicates that the best OKN stimulus should be real and a physical surround that rotates. It should have a pattern without distinguishing features like a random dot pattern. Different methods are used to induce OKN and subsequently, OKAN.
In one method, the patient sits in a chair with the head fixed. The eye doctor uses an electrooculography (EOG) to record both eyes' vertical and horizontal movements. The doctor records on paper the EOG, the EOG’s time derivatives, and voltages representing stimulus parameters and stores them on FM magnetic tape. Data is analyzed by a general-purpose digital computer, after which graphs are generated. OKN is then induced using a servo-controlled, internally lit, and rim-driven rotating drum. Once OKN has been induced, the doctor records OKAN while in total darkness. Other methods may include:
A striped projector can be used to produce OKN. However, patients can end up fixating on still objects in the room.
Like the stripe projector, individuals can fixate on still objects, but this method can still elicit an OKN. The technique may fail to stimulate the entire peripheral field since the projector is not anywhere near a full field. However, it’s marginally better than a light bar. An array of large televisions arranged in a semi-circle can be used to elicit an OKN.
In this method, a physical drum is rotated around the patient to elicit OKN.
Research reveals that OKN stabilizes a moving environment’s retinal image while OKAN can compensate for vestibular after-responses. OKAN also shows perceptual errors that occur when visual stimulation is completely absent.
OKN and OKAN help detect malingering, especially in patients who pretend to have a bilateral vestibular loss to obtain a legal outcome. It is difficult to stop OKN, and an individual without OKN is said to be uncooperative. Research shows that the OKN/OKAN test may detect uncooperative subjects manifested in a missing OKN and a present OKAN.
Risks & Complications
OKN may disturb some patients who may make an active attempt to suppress it by fixating on an immobile object in the room.
Some of the methods used to elicit OKN may be faulty. Care must be taken when inducing a full field surround. Any compromised full field surround will compromise the results. However, some commercial simulators may end up producing smooth pursuit instead of an OKN. These may include:
- An array of LED lights designed to produce moving bars of light may fall short of creating a full field. These bars can easily be tracked by the individual and may instead elicit smooth pursuit.
- Projectors also do not produce a full field with the possibility that the patient will fixate on the projected image's edges.
- The drum is said to be the weakest at generating a full field and classified more as a pursuit stimulus than an OKN.
- Strips with squares containing alternating colors, busy ties, and tape measures can also elicit OKN. However, these strips also come short at providing a full field.
- OKN testing using tape can be intrinsically subjective, thus affecting accuracy.