Definition
- Optokinetic nystagmus (OKN) is a physiologic phenomenon produced by asking the patient to visually track a succession of moving stimuli. OKN eye movements involve two phases: the first is smooth pursuit eye movements in the direction of the moving target and the second is nystagmus in the contralateral direction.
1. To test OKN, use a device such as cloth tape, drum, or mobile phone application. The OKN tape ideally comprises a series of 2-inch square red patches placed 2 inches apart on a white tape that is 1 yard long. The patient should count the number of patches he/she sees as you draw the tape across the patient’s field of vision. If you sit in a moving car and count the trees that pass by, you will experience the phenomenon of OKN. This phenomenon can be disturbing to some patients and they will actively suppress the OKN response by fixating on non-moving stimuli, so be aware whether or not the patient is following your instructions to determine the reliability of the observed OKN response.
2. Notable clinical uses of OKN assessment are:
A. To evaluate the integrity of the patient’s parieto-occipital lobe (most common use). The rationale for this is explained in the discussion of the anatomy of smooth pursuit eye movements.
B. To assess visual tracking ability in patients with depressed level of consciousness.
C. To elicit slowed adduction nystagmus in internuclear ophthalmoplegia.
D. To elicit convergence-retraction nystagmus in Parinaud syndrome.
E. To assess for psychogenic causes of visual complaints.
