Monocular and Binocular Visual Function Deficits in Amblyopic Patients with and without Fusion Maldevelopment Nystagmus

Abstract

Purpose: The aim of the study is to examine the association between amblyopia type and the presence of nystagmus on binocular and monocular functions of the fellow (FE) and amblyopic eye (AE).

Methods: We recruited 19 controls and 44 amblyopes (anisometropes=13, strabismic=10, mixed=21). We measured visual, grating, and vernier acuities and high/low spatial frequency (SF) contrast sensitivities in each eye using a staircase method. Stereoacuity was measured with the Titmus fly test. We recorded fixation eye movements (FEM) using high-resolution video-oculography. Subjects were classified as having either no nystagmus (n=18), fusion maldevelopment nystagmus syndrome (FMNS) (n=12), or nystagmus without any structural anomalies that does not meet the criteria for FMNS or infantile nystagmus (n=14).

Results: Analysis of visual function by clinical amblyopia type showed that patients with strabismus/mixed amblyopia (F (2,54)=9.5, p<0.001) were more likely to have poor stereopsis while controlling for AE grating acuity deficit. The FE of patients with anisometropia had greater contrast sensitivity deficits at low (F (2,43)=4.4, p=0.018) and high SF (F (2,42)=10.1, p<0.001). Analysis of visual function by FEM characteristics (low SF: (F (3,43)=4.3, p=0.010) and high SF: (F (3,42)=7.1, p=0.001) showed that the FE of patients with FMNS had worse low and high SF contrast sensitivities, whereas those without FMNS had greater contrast sensitivity deficits only at high SF compared to controls. Patients with FMNS (F (3,54) = 12.9, p<0.001) were more likely to have poor stereopsis while controlling for AE grating acuity deficit compared to patients without FMNS. All amblyopic patients had worse high SF contrast sensitivity of the AE irrespective of type or FEM characteristics (Type = F (2,43)=8.8, p=0.001; FEM characteristics= F (3,43)=5.1, p=0.004).

Conclusion: The presence of FMNS in patients with strabismic/mixed amblyopia is associated with poor/absent stereopsis. FE deficits vary across amblyopia type. Like FEM abnormalities, visual function deficits are seen in the FE of patients with and without nystagmus.

Keywords: amblyopia; contrast sensitivity; fellow eye; fusion maldevelopment nystagmus; stereopsis.

Figure 1

Gaze positions as a function of time in patients without nystagmus (A), with FMNS (B), and with nystagmus but not FMNS (C) as they fixated on a target monocularly with the amblyopic (top) and fellow (bottom) eyes. Note the large drift amplitudes in the subject without nystagmus. FMNS can be distinguished by the reversal in slow phase direction that occurs when the eye under cover is changed. Red - right horizontal, magenta – right vertical, blue – left horizontal, cyan – left vertical. Rightward and upward movements correspond to the positive vertical axis.

Figure 2

Mean and standard error of the mean for amblyopic eye log percentage Michelson contrast thresholds measured at 4 (black bars) and 14 (white bars) cycles per degree for subjects grouped by clinical type (A) and FEM characteristic (B). Single asterisks and daggers denote significant (p<0.05) differences relative to the control and mixed/strabismic groups, respectively, after controlling for AE grating acuity and applying Bonferroni correction.

Figure 3

Mean and standard error of the mean for fellow eye log percentage Michelson contrast thresholds measured at 4 (black bars) and 14 (white bars) cycles per degree for subjects grouped by clinical type (A) and FEM characteristics (B). Single asterisks denote significant (p<0.05) differences relative to the control group after controlling for FE grating acuity and applying Bonferroni correction.

Figure 4

Mean and standard error of the mean for stereopsis in log arcseconds grouped by type (A) and FEM characteristic (B). Single asterisks denote statistically significant (p<0.05) differences compared to the groups indicated by the brackets after controlling for AE grating acuity and applying Bonferroni correction.