Effect of Viewing Conditions on Fixation Eye Movements and Eye Alignment in Amblyopia

Abstract

Purpose: Patients with amblyopia are known to have fixation instability, which arises from alteration of physiologic fixation eye movements (FEMs) and nystagmus. We assessed the effects of monocular, binocular, and dichoptic viewing on FEMs and eye alignment in patients with and without fusion maldevelopment nystagmus (FMN).

Methods: Thirty-four patients with amblyopia and seven healthy controls were recruited for this study. Eye movements were recorded using infrared video-oculography during (1) fellow eye viewing (FEV), (2) amblyopic eye viewing (AEV), (3) both eye viewing (BEV), and (4) dichoptic viewing (DcV) at varying fellow eye (FE) contrasts. The patients were classified per the clinical type of amblyopia and FEM waveforms into those without nystagmus, those with nystagmus with and without FMN. Fixational saccades and intersaccadic drifts, quick and slow phases of nystagmus, and bivariate contour ellipse area were analyzed in the FE and amblyopic eye (AE).

Results: We found that FEMs are differentially affected with increased amplitude of quick phases of FMN observed during AEV than BEV and during DcV at lower FE contrasts. Increased fixation instability was seen in anisometropic patients at lower FE contrasts. Incomitance of eye misalignment was seen with the greatest increase during FEV. Strabismic/mixed amblyopia patients without FMN were more likely to demonstrate a fixation switch where the AE attends to the target during DcV than patients with FMN.

Conclusions: Our findings suggest that FEM abnormalities modulate with different viewing conditions as used in various amblyopia therapies. Increased FEM abnormalities could affect the visual function deficits and may have treatment implications.

Figure 1.

Examples of fixation eye movements during a 3 second epoch under conditions of (A) both eye viewing (BEV), (B) fellow eye viewing (FEV), and (C) amblyopic eye viewing (AEV) from a subject without nystagmus (top row), subject with fusion maldevelopment nystagmus (FMN) (middle row), and subject with nystagmus without FMN (bottom row). The x-axis represents time and the y-axis represents horizontal (solid line, black: fellow eye, and grey: amblyopic eye) and vertical (dotted line, black: fellow eye, and grey: amblyopic eye) positions. The black arrow represents the fast fixation eye movements (FEMs), whereas the grey arrows represent slow FEMs. Rightward and upward movements correspond to positive vertical axis.

Figure 2.

Cumulative sum histogram of composite amplitude (degrees) of fast fixation eye movements of fellow eye obtained during fellow eye viewing and both eye viewing in groups 0 vs. 1 (A) and groups 0, 1, and 2 (B) and amblyopic eye obtained during amblyopic eye viewing and both eye viewing in groups 0 vs. 1 (C) and groups 0, 1, and 2 (D).

Figure 3.

Cumulative sum histogram of composite eye velocity (degrees/sec) of slow fixation eye movements of fellow eye obtained during fellow eye viewing and both eye viewing in groups 0 vs. 1 (A) and groups 0, 1, and 2 (B) and amblyopic eye obtained during amblyopic eye viewing and both eye viewing in groups 0 vs. 1 (C) and groups 0, 1, and 2 (D).

Figure 4.

Top: Horizontal and vertical eye positions of fellow eye (black) and amblyopic eye (grey) during both eye viewing (BEV), fellow eye viewing (FEV), and amblyopic eye viewing (AEV). Rightward and upward movements correspond to positive vertical axis. Notice the right hypotropia and exotropia during BEV and FEV condition and left exotropia and left hypertropia during AEV. Bottom: Histogram showing range of composite eye position difference during BEV (grey), FEV (black), and AEV (white).

Figure 5.

Cumulative sum histogram of composite amplitude (degrees) of fast fixation eye movements of fellow eye and amblyopic eye obtained during dichoptic viewing across fellow eye contrasts of 100% (black), 50% (darkest grey), 25% (lighter grey), and 10% (lightest grey) in groups 0 vs. 1 (A, C) and groups 0, 1, and 2 (B, D).

Figure 6.

Cumulative sum histogram of composite velocity (degrees/sec) of slow fixation eye movements of fellow eye and amblyopic eye obtained during dichoptic viewing across fellow eye contrasts of 100% (black), 50% (darkest grey), 25% (lighter grey), and 10% (lightest grey) in groups 0 vs. 1 (A, C) and groups 0, 1, and 2 (B, D).

Figure 7.

Horizontal and vertical eye position traces (vertical axis) and time (horizontal axis) in 2 subjects obtained during dichoptic viewing at fellow eye contrasts of (A) 100%, (B) 50%, (C) 25%, and (D) 10% contrast. To the left is a subject with mixed/strabismic amblyopia without fusion maldevelopment nystagmus (FMN) that demonstrates a fixation switch indicated by the black arrow when the fellow eye is fixing at FE contrast of 25% and the grey arrow that represents the amblyopic eye picks up with fixation. The horizontal arrows on the far left represent the primary position. To the right is a subject with mixed/strabismic amblyopia with fusion maldevelopment nystagmus (FMN). Notice that through all the trials A to D the fellow eye fixates in primary position with increase in eye misalignment at lower fellow eye contrasts indicated by the vertical black arrows on the far right.

Figure 8.

Right: Horizontal and vertical eye positions of fellow eye (black) and amblyopic eye (grey) obtained from the same subject shown in Figure 4 during dichoptic viewing across fellow eye contrasts of 100% (A), 50% (B), 25% (C), and 10% (D). Rightward and upward movements correspond to positive vertical axis. Notice the right hypotropia and exotropia during all dichoptic viewing conditions. Left: Histogram showing range of composite eye position difference during dichoptic viewing across the fellow eye contrasts of 100% (A), 50% (B), 25% (C), and 10% (D).