Content area
Full text
Correspondence to Dr David Edward Hale, Neurology, Johns Hopkins University, Baltimore, Maryland, USA; [email protected]
Introduction
On the top shelf of many clinics is a dusty optokinetic nystagmus (OKN) drum, and most neurologists have an OKN tape that is either long lost, at the back of a drawer or the bottom of a medical bag, out of use. This review presents six clinical scenarios where the use of OKN can be helpful diagnostically.
What is OKN?
OKN is a physiological reflexive eye movement that occurs in response to motion of the visual environment, such as looking out the window of a moving train. In 1907, Bárány described this true form of OKN as ‘Eisenbahn nystagmus’.1 In this example, slow phases move with the flow of visual information and are followed by quick phases in the opposite direction to optimise visual stability during rotational motion.2 An optokinetic tape used at the bedside is stimulating only a portion of the visual field; therefore, this situation differs from a full-field optokinetic stimulus, such as looking out of the window of a moving train. At the bedside, OKN represents a combination of the optokinetic and smooth pursuit systems.2 Therefore, with bedside testing, the slow phase of OKN represents smooth pursuit, and saccades are represented by the quick phases.3
How should I test OKN?
OKN can be elicited using an optokinetic stimulus, such as an optokinetic tape, the examiner’s fingertips, an optokinetic drum or a smartphone application. The patient is instructed to count each coloured square (or fingertip) as it moves in both horizontal and vertical directions (video 1).3 If the optokinetic stimulus is held close to the eyes, more visual field will be stimulated, but patients with convergence impairment (eg, elderly, patients with parkinsonism) may find this distance problematic. A stimulus that is positioned too close to the patient will also partially obscure the eyes, making assessment a challenge for the examiner. Therefore, placing the stimulus at eye level at a distance of 6–12 inches (from eye to stimulus) works well and is still close enough that the examiner can easily assess the nystagmus. When the direction of the optokinetic stimulus is towards the patient’s left, the examiner will see leftward slow phases (pursuit),...