Testing for Adduction Lag in Partial INO Using an Optokinetic Stimulus

In this patient we demonstrate the use of an optokinetic stimulus to elicit an internuclear ophthalmoplegia (INO). Occasionally adduction appears to be normal with an INO, and an adduction lag with horizontal saccades should be sought as a confirmatory sign. Optokinetic tape is an easy way to assess the conjugacy of saccades. It is worth noting that using optokinetic tape at the bedside is not generating true optokinetic nystagmus (OKN), but rather is a method of generating pursuit (slow phase) and saccadic (fast phase) movements. True OKN is triggered by full field visual stimulation, as in the example of watching passing scenery while on a moving train. In this patient with a left INO, when the tape is moved to the right, OKN is normal and symmetric. When the tape is moved to the left, the adducting saccades OS are clearly slower than the corresponding abducting saccades OD, which suggests a left INO.