Differential Fixation and Eye Alignment Patterns in Strabismus with and Without Amblyopia Across Viewing Conditions

Fixation instability (FI) and vergence instability (VI) in amblyopia and strabismus are associated with disrupted physiologic fixation eye movements (FEMs). This study examined how viewing conditions affect FEM patterns in strabismic subjects with and without amblyopia. FEMs of the non-dominant/amblyopic and dominant/fellow eyes were recorded using video-oculography during both-eye viewing (BEV), fellow/dominant-eye viewing (FEV/DEV), and amblyopic/non-dominant-eye viewing (AEV/NDEV) in strabismic subjects with amblyopia (SA, n = 56), without amblyopia (S, n = 19), and controls (C, n = 25). FI, VI, fast FEM amplitudes, slow FEM velocities, and time-based control of eye deviation were analyzed. The SA group showed the greatest FI in the amblyopic eye during AEV compared with the fellow eye during FEV, whereas minimal inter-ocular FI differences were observed in the S group and controls. Under monocular viewing, both SA and S groups exhibited increased FI in the non-viewing eye and higher VI than controls. Regression analyses indicated that visual acuity deficits primarily influenced viewing-eye FI and FEM dynamics, while strabismus mainly affected non-viewing-eye FI and slow FEMs. C and S groups showed the least eye deviation during BEV, whereas the SA group showed the least eye deviation-but the highest VI-during AEV, indicating a distinct pattern of incomitance. Distinct FEM patterns shaped by viewing conditions may reflect underlying visuomotor control mechanisms and serve as biomarkers for AI (artificial intelligence)-based classification.