Binocular and Fellow Eye Acuity Deficits in Amblyopia: Impact of Fixation Instability and Sensory Factors

Amblyopia, a neurodevelopmental disorder, is commonly assessed through amblyopic eye visual acuity (VA) deficits, but recent studies also highlight abnormalities in the fellow eye. This study quantified binocular and fellow/dominant eye VA in individuals with amblyopia and strabismus without amblyopia and examined factors influencing these measures, including fixation eye movement (FEM) abnormalities. Identifying which subsets of patients-such as those with nystagmus, concurrent strabismus, or greater fixation instability-exhibit more pronounced deficits in binocular visual acuity and binocular summation can enhance clinical decision-making by enabling tailored interventions and aiding patient counseling. Sixty-eight amblyopic, seventeen strabismic without amblyopia, and twenty-four control subjects were assessed using an adaptive psychophysical staircase procedure and high-resolution video-oculography to evaluate FEMs and fixation instability (FI). Binocular and fellow eye VA were significantly lower in amblyopia, regardless of type or nystagmus presence, whereas binocular and dominant eye VA in strabismus without amblyopia did not differ from the controls. Despite reduced binocular acuity, amblyopic and strabismic subjects exhibited binocular summation, with binocular VA exceeding fellow/dominant eye VA. Reduced binocular VA correlated with greater fellow eye VA deficits, diminished binocular summation, and increased FI in the amblyopic eye. Fellow eye VA deficits were linked to greater amblyopic eye VA deficits, an increased degree of anisometropia, higher FI, and stronger nystagmus correlation. These findings suggest amblyopia affects both visual sensory and motor systems, impacting binocular function and fixation stability, with potential consequences for everyday visuomotor tasks like reading.