By Marcello Cherchi, MD PhD
For clinicians
When the eye is directed to an eccentric point of regard and then maintained in that orientation, there may be nystagmus. Bárány(Bárány 1906) credits John Hunter as the first to have described such nystagmus in 1786 (apparently referring to the discussion on p. 212 of the chapter entitled, “The use of the oblique muscles”) (Hunter 1786).
This nystagmus at eccentric gaze can be physiologic or pathologic, and the literature exhibits some terminological inconsistency in this regard. Some authors use the term “end-point nystagmus” for non-pathologic cases, and “gaze-evoked nystagmus” for pathologic ones. Other authors refer to any nystagmus at eccentric gaze as “gaze-evoked nystagmus.” Beyond this terminological inconsistency, the boundary between physiologic and pathologic is not clearly established.
In healthy individuals, when the eyes are horizontally eccentric one may observe transient, low amplitude nystagmus whose fast phase is centrifugal (i.e., in the same direction of gaze holding, beating away from primary position of gaze) and slow phase is centripetal (i.e. drifting towards primary position of gaze). This phenomenon is sometimes referred to as “end-point nystagmus” (Abel et al. 1978; Eizenman et al. 1990; Romano et al. 2017; Shallo-Hoffmann et al. 1990). Some authors (Abel et al. 1978) characterize end-point nystagmus according to when (after assuming eccentric gaze) it begins and whether it is sustained. For instance, Eizenman and colleagues (Eizenman et al. 1990) describe end-point nystagmus that begins immediately or within a few seconds after the eye assumes an eccentric position as “sustained” if it persists as long as the eye is maintained in that eccentric position and “unsustained” if it does not persist. If the nystagmus “starts after a considerable delay (more than 30 sec)” after the eye assumes an eccentric position, they refer to it as “fatigue” end-point nystagmus.
The amplitude of end-point nystagmus in healthy subjects appears to be affected by several variables. First, the greater the eccentric position of the eye, the greater the amplitude of the end-point nystagmus; moreover, the amplitude increases linearly only up to 20 degrees of eccentricity, after which the increase in amplitude becomes non-linear (Bertolini et al. 2013). Second, there is usually some asymmetry in the amplitude of the nystagmus between the abducting and adducting eye (Romano et al. 2017). Third, the amplitude of end-point nystagmus varies depending on whether the subject is fixating on a visual target, or simply trying to maintain gaze at a remembered (though not currently visible) target (Eizenman et al. 1990).
Given this variability, there does not exist a single “cutoff value” that delimits the normative range. We regard with caution literature that appears simply to stipulate a normative range; for instance, Rett (Rett 2007) states that gaze-evoked nystagmus >4˚/sec is pathological.
Perverted nystagmus refers to nystagmus whose fast phase is in some direction other than centrifugal, such as down beat nystagmus on extreme lateral gaze, or torsional nystagmus on upgaze. Perverted nystagmus at eccentric gaze is generally regarded as pathological.
References
Abel LA, Parker L, Daroff RB, Dell’Osso LF (1978) End-point nystagmus. Invest Ophthalmol Vis Sci 17: 539-44.
Bárány R (1906) Über die vom Ohrlabyrinth ausgelöste Gegenrollung der Augen bei Normalhörenden [On the counter-rolling of the eyes triggered by the labyrinth in people with normal hearing]. Archiv für Ohrenheilkunde 68: 1-30.
Bertolini G, Tarnutzer AA, Olasagasti I, Khojasteh E, Weber KP, Bockisch CJ, Straumann D, Marti S (2013) Gaze holding in healthy subjects. PLoS One 8: e61389. doi: 10.1371/journal.pone.0061389
Eizenman M, Cheng P, Sharpe JA, Frecker RC (1990) End-point nystagmus and ocular drift: an experimental and theoretical study. Vision Res 30: 863-77. doi: 10.1016/0042-6989(90)90055-p
Hunter J (1786) Observations on certain parts of the animal oeconomy, London
Rett D (2007) Gaze-evoked nystagmus: a case report and literature review. Optometry 78: 460-4. doi: 10.1016/j.optm.2007.02.016
Romano F, Tarnutzer AA, Straumann D, Ramat S, Bertolini G (2017) Gaze-evoked nystagmus induced by alcohol intoxication. J Physiol 595: 2161-2173. doi: 10.1113/JP273204
Shallo-Hoffmann J, Schwarze H, Simonsz HJ, Muhlendyck H (1990) A reexamination of end-point and rebound nystagmus in normals. Invest Ophthalmol Vis Sci 31: 388-92.
