Optokinetic System (Optokinetic Nystagmus)
The optokinetic system maintains steady alignment of images on the retina during sustained rotation of the head (or environment). OKN is induced when a large visual image sweeps across the retina. The initial response is a slow-phase, pursuit-like movement that follows the visual scene, followed by a contraversive, involuntary, reflexive saccade. The VOR generates similar reflexive eye movements. However, the vestibular response is generated by brief head rotations that stimulate the semicircular canals and is attenuated after approximately 30 seconds as the cilia on the hair cells resume their normal position. In contrast, the optokinetic system response is a sustained response that is generated by continuous visual image motion over a large area of retina.
The vestibular and optokinetic systems act synergistically to hold the visual scene steady on the retina during sustained head rotations. For example, when a normal subject is rotated, the VOR initially stimulates the slow-phase ocular rotations in the opposite direction that hold the visual image steady on the retina. However, after approximately 30 seconds, the rotation-induced VOR is attenuated. Thereafter, the optokinetic system responds to the visual scene sweeping across the retina by providing a sustained output, which generates continuous ocular movements that hold the visual image steady on the retina.
The smooth-pursuit system and the optokinetic system are similar but distinct. The smooth-pursuit system is voluntarily activated; its function is to place the image of a small moving object onto the fovea. In contrast, the optokinetic system is involuntary and is reflexively activated when a large visual image sweeps across a large area of retina.
The optokinetic system cannot be isolated for testing in the clinical setting. In order to do so, the moving stimulus would have to fill the complete visual environment. Although practical, OKN induced by the rotating OKN drum of black-and-white stripes that clinicians use in the office subtends only a portion of the visual field and primarily tests the pursuit and saccade systems, not the optokinetic system.
Optokinetic nystagmus dysfunction
Isolating the optokinetic system response from the pursuit response to a moving target requires specialized testing equipment not typically used in the clinical setting. As noted, the OKN drum subtends a small portion of the visual field and therefore primarily tests smooth-pursuit movements in the direction of drum rotation and saccadic eye movements opposite the direction of drum rotation (eg, drum rotation toward the patient’s right side generates a rightward ocular pursuit movement followed by a leftward saccade.)
Asymmetry of the OKN drum–induced responses can be caused by a unilateral lesion of the cerebral pathways that descend from the ipsilateral parietal, middle temporal, or medial superior temporal areas to the brainstem ocular motor centers. These regions control ipsilateral smooth pursuit. Therefore, abnormal, jerky pursuits (ie, saccadic pursuits) will occur when the drum is rotated toward the side of the lesion. Asymmetry of OKN responses, with normal smooth-pursuit movements produced by drum rotation toward the patient’s left and abnormal saccadic pursuit movements produced by drum rotation toward the patient’s right, suggests a lesion in the right cerebrum. Typically, relatively large lesions of the parietal or parieto-occipital cortex are required to produce drum-induced asymmetries; these lesions are usually accompanied by a homonymous hemianopia. A lesion confined to the occipital lobe (eg, as usually occurs secondary to a stroke within the distribution of the posterior cerebral artery) also produces a homonymous hemianopia but not OKN asymmetry. Thus, the clinician may use the OKN drum to gain insight into the location and extent of a cerebral lesion that produces a homonymous hemianopia.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.