Fusional Maldevelopment Nystagmus Syndrome (Latent Nystagmus)

Fusional maldevelopment nystagmus (FMN) syndrome, or latent nystagmus (LN), is an early-onset, conjugate, horizontal, jerk nystagmus that is accentuated by monocular fixation. Eyes with FMN are stable during fusion. However, when 1 eye is occluded (thereby eliminating fusion), the fixating eye slowly drifts toward the nose, followed by a corrective abducting saccade. The eye under the occluder is moving in the same direction and with the same amplitude as the fixating eye (ie, the nystagmus is conjugate). Thus, the fast phase of both eyes beats toward the viewing eye and away from the occluded eye. The fast phase immediately reverses direction upon occlusion of the fixating eye as the fellow eye takes up fixation. Therefore, when a standard occluder is used to measure visual acuity in patients with FMN, acuity is often degraded by the induced nystagmus. Partial optical blurring of 1 eye (with a high-plus lens or filter) may not induce FMN and therefore may permit better visual acuity measurements in the fellow eye.

FMN may occur when both eyes are open if 1 eye is suppressed, a condition termed manifest latent nystagmus (MLN). In other words, the nystagmus is damped with fusion but develops spontaneously whenever suppression occurs; the clinician may not need to occlude an eye to induce this form of nystagmus.

FMN occurs with any condition that disrupts binocular development in the first 6 months of life, most commonly infantile esotropia. Other associated conditions include severe anisometropia, constant infantile exotropia, monocular cataract, corneal opacities, and unilateral microphthalmos. If 1 eye has severe structural damage with poor vision (such as unilateral microphthalmos), then the fellow eye with normal vision will always be the fixating eye and the FMN will always beat toward that eye (because the abnormal eye with poor vision is unable to take up fixation). Patients may maintain a head turn toward the fixating eye. This head posture places the fixating eye in adduction, thereby damping the nystagmus in order to improve vision.

Patients with FMN often have other associated signs of abnormal binocular development, such as dissociated vertical deviation, oblique overaction, and smooth pursuit asymmetry (in which the fixating eye is able to pursue a target moving toward the nose but has difficulty pursuing a target moving away from the nose).

Eye movement recordings show that FMN usually has a constant-velocity or decelerating slow phase, in contrast to the increasing exponential waveform of INS. FMN and INS may coexist.

Dell’Osso LF, Schmidt D, Daroff RB. Latent, manifest latent, and congenital nystagmus. Arch Ophthalmol. 1979;97(10):1877–1885.

Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.