Fixation instability in amblyopia: Oculomotor disease biomarkers predictive of treatment effectiveness

Abstract

Amblyopic patients are known to have fixation instability, particularly of the amblyopic eye. The stability of the fixation is affected by the presence of nystagmus, the frequency and amplitude of fixational saccades and inter-saccadic drifts. Amblyopic patients without nystagmus have increased amplitude of the fixational saccades with reduced frequency of the physiologic microsaccades and have increased inter-saccadic drifts. Amblyopia patients who have experienced a disruption in binocularity in early infancy develop fusion maldevelopment nystagmus (FMN) previously called latent nystagmus as it is more evident during monocular viewing conditions. We have found that some amblyopic patients can have nystagmus with slow phases that are not directed nasally and without the reversal in direction on ocular occlusion, features seen in patients with FMN. The current mainstay of amblyopia treatment comprises of part-time occlusion therapy of the non-amblyopic eye. The amount of patching treatment is in the range of 2-6h/day as determined by the severity of amblyopia. Despite treatment, up to 40% of patients have residual amblyopia. We analyzed the effectiveness of part-time occlusion therapy in amblyopic patients as a function of fixation instability. We categorized amblyopic patients based on their eye movement waveforms obtained during a visual fixation task into those lacking nystagmus, those with FMN and those with nystagmus but no FMN. We did a retrospective chart review to gather information about their clinical characteristics and treatment response. We found that patients with FMN require a more prolonged duration of treatment and have a poorer recovery of stereopsis compared to patients with nystagmus but no FMN and patients lacking nystagmus. This study suggests that eye movement assessment provides valuable information in the management of amblyopia.

Keywords: Amblyopia; Fixation instability; Latent nystagmus; Microsaccades; Nystagmus.

Figure 1.

Representative eye position traces obtained during fixation in amblyopia patients without nystagmus, nystagmus no FMN, and FMN. In patients without nystagmus there is an increase in the amplitude of the fixational saccade with an increase in the inter-saccadic drift. In patients with nystagmus no FMN there is no reversal of the quick phase of nystagmus as seen in patients with FMN. In patients with FMN, there is an increase in slow phase velocity of the amblyopic eye during amblyopic eye viewing condition. Of note, in all three patients abnormalities are seen during binocular viewing condition particularly of the amblyopic eye.

Figure 2.

Clinical outcomes sub grouped by the type of amblyopia. Visual acuity improvement and patching duration are not significantly different between types. Final stereopsis is significantly better in anisometropic patients.

Figure 3.

Clinical outcomes sub grouped by the fixation eye movement waveforms. Visual acuity improvement is not significantly different between the waveform groups. However, in FMN patients the duration of patching is significantly longer, and the final stereopsis is significantly worst compared to the other two groups.