Optokinetic nystagmus

Optokinetic nystagmus (OKN) is a normal, physiologic type of nystagmus sometimes affected by disease. (1-4) OKN is conjugate nystagmus induced by a succession of moving visual stimuli. Clinical testing entails requesting the patient "count" a series of targets, such as squares on an OKN tape. Although OKN is more complex, it can be viewed for clinical purposes as testing pursuit ipsilateral to the direction of target movement and contralateral saccades. In normal, alert individuals, an OKN stimulus induces brisk nystagmus with the fast phase in the direction opposite tape movement. Responses in one direction are compared with responses in the other direction.

Patients with hemianopias due to occipital lobe disease have a normal OKN response, despite their inability to see into the hemifield from which the tape originates. Because of interruption of the OKN pathways, patients with hemianopias due to disease of the optic radiations in the deep parietal lobe have abnormally blunted or absent OKN responses. The patient is unable to generate contraversive saccades into the blind hemifield. With asymmetric OKNs, a lesion is more likely parietal and more likely to be nonvascular, i.e., a tumor (Cogan’s rule). An asymmetric OKN response has a positive LR of 5.7 for detecting parietal lobe disease. (5)

OKN testing has other uses as well. It may be used to crudely check visual acuity and may provide a clue to the presence of psychogenic visual loss. OKN testing can demonstrate the slowed adducting saccades of a subtle INO, and sometimes accentuate the nystagmus in the abducting eye. OKN forced upward saccades may induce convergence retraction nystagmus in patients with Parinaud’s syndrome. OKN abnormalities may be seen early in PSP.

Movies drawn from the NeuroLogic Exam and PediNeuroLogic Exam websites are used by permission of Paul D. Larsen, M.D., University of Nebraska Medical Center and Suzanne S. Stensaas, Ph.D., University of Utah School of Medicine. Additional materials were drawn from resources provided by Alejandro Stern, Stern Foundation, Buenos Aires, Argentina; Kathleen Digre, M.D., University of Utah; and Daniel Jacobson, M.D., Marshfield Clinic, Wisconsin. The movies are licensed under a Creative Commons Attribution-NonCommerical-ShareAlike 2.5 License.

References

1. Buttner U and Kremmyda O. Smooth pursuit eye movements and optokinetic nystagmus. Dev Ophthalmol. 2007; 40:76-89.

2. Campbell WW. DeJong's the neurologic examination, 7th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2013.

3. Campbell WW. Clinical signs in neurology: a compendium. Philadelphia: Wolters Kluwer Health, 2016.

4. David NJ. Optokinetic nystagmus. A clinical review. J Clin Neuroophthalmol. 1989;9:258-66.

5. McGee S. Evidence based physical diagnosis. 3 ed. Philadelphia: Elsevier/Saunders, 2012.