Gaze-evoked Nystagmus and Its Diferential Diagnosis

DEFINITIONS Gaze-evoked Gaze-evoked nystagmus (GEN) is a jerk nystagmus, not present in primary position, elicited by attempted maintenance of eccentric eye position, GEN may have a linear or a decreasing-velocity exponential slow phase; the latter has sometimes been referred to as gaze-paretic nystagmus but, unless it is due to a muscle or nerve paresis, the GEN terminology is preferred. Distinction should be made between nystagmus that is truly gaze evoked and that which is gaze modulated, Many types of nystagmus vary with gaze angle but are present in primary position and are not, therefore, GEN, Gaze-modulated Gaze-modulated nystagmus is a Jerk nystagmus, present in primary position, that is increased by lateral gaze. The slow phases may be linear, decreasing-velocity or increasing-velocity exponentials. Examples of gazemodulated nystagmus are vestibular nystagmus (VN), latent/manifest latent nystagmus (LMLN) and congenital nystagmus (CN). Both VN and LMLN vary in intensity with gaze angle according to Alexander's law; the nystagmus increases as gaze is directed in the direction of the fast phases. CN increases as gaze is directed away from the null and if the null is near primary position, CN may be mistaken for GEN (especially inthe absence of a patient history or in a case of head trauma seen inthe emergency room). These types of nystagmus increase with lateral gaze but, since their genesis is not due to gaze off primary position, they are not types of GEN, Overlap Nystagmus that is modulated according to Alexander's law may be classified type I, II or III depending on the field of gaze in which the nystagmus first appears. Thus, a type I nystagmus (appearing only in lateral gaze) may easily be mistaken for GEN. The interrelationship between the various types of gaze-evoked and gazemodulated nystagmus Is summarized in Figure 1.