Fusion Maldevelopment Nystagmus Syndrome (Latent Nystagmus)
Fusion maldevelopment nystagmus syndrome (FMNS) (latent nystagmus) is a conjugate, horizontal jerk nystagmus and a marker of fusion maldevelopment, which occurs as a result of infantile-onset strabismus or (less commonly) decreased vision in 1 eye. When either eye is occluded, a conjugate jerk nystagmus develops, with the direction of the fast-phase component toward the uncovered eye. Left jerk nystagmus occurs upon covering the right eye, and right jerk nystagmus upon covering the left (Video 13-3). This is the only nystagmus that reverses direction depending on which eye is fixating. The nystagmus damps when the fixating eye is in adduction, so the preferred head turn also reverses direction with change of fixation (Fig 13-2). Amplitude, frequency, and velocity of the nystagmus can also vary depending on which eye is fixating.
Fusion maldevelopment nystagmus syndrome (latent nystagmus).
Courtesy of Robert W. Hered, MD.
Figure 13-2 Fusion maldevelopment nystagmus syndrome (latent nystagmus). A, A right head turn occurs during fixation with the right eye. B, The head turn reverses direction during fixation with the left eye. The nystagmus damps with the fixating eye in adduction.
(Courtesy of Edward L. Raab, MD.)
Fusion or binocular viewing damps FMNS, and disruption of fusion (eg, by occlusion) increases it. FMNS may manifest even when both eyes are open if only 1 eye is being used for viewing (eg, the other eye is suppressed or amblyopic); this is sometimes referred to as manifest latent nystagmus. Electronystagmographic evaluation of both fully latent and manifest forms of FMNS shows similar waveforms, with a slow phase of constant or exponentially decreasing velocity (see Fig 13-1B). FMNS is distinct from infantile nystagmus syndrome that has a latent component, worsening when 1 eye is covered. Like other hallmarks of infantile strabismus with which it is associated (dissociated vertical deviation and oblique muscle overaction), FMNS becomes more prominent with age.
Richards M, Wong A, Foeller P, Bradley D, Tychsen L. Duration of binocular decorrelation predicts the severity of latent (fusion maldevelopment) nystagmus in strabismic macaque monkeys. Invest Ophthalmol Vis Sci. 2008;49(5):1872–1878.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.