Proximal vergence likely allows for intermittent exotropia (IXT) control, particularly among individuals with divergence or pseudo-divergence excess, according to a study published in Ophthalmic and Physiological Optics. These findings suggest that neither accommodative convergence alone nor overaccommodation secondary to fusional convergence efforts control ocular alignment.
Researchers included individuals with IXT (n=29; mean age, 10.5 years) and age-matched control group participants (n=24; mean age, 11.0 years) from the Indiana University School of Optometry and the Seattle Children’s Hospital eye clinics for the study. The cohort consisting of individuals with IXT included participants with divergence or pseudo-divergence excess (n=8), convergence insufficiency (n=5), and basic IXT (n=16). The investigators performed eye-tracking and eccentric photorefraction assessments in binocular and monocular viewing conditions and determined gradient response accommodative convergence/ accommodation (AC/A) ratios and fusional vergence ranges as participants watched a movie at viewing distances between 33 and 80 cm.
Individuals with IXT and control group participants demonstrated similar accommodation in monocular and binocular conditions when viewing targets at 33 and 80 cm (0.06 vs -0.15 meter angles [MA]; P =.44 and -1.80 vs -2.00 MA; P =.47, respectively). During monocular viewing, individuals with IXT demonstrated more divergent positions (-4.20 vs -2.10 MA; P <.001) but similar refractive states compared with control group participants at 80 cm (P =.46). The team noted similar results in positioning and refractive states at 33 cm (P <.001 and P =.49, respectively) in monocular conditions, the report shows.
Individuals with IXT demonstrated a significantly greater accommodative convergence response compared with control group participants (1.81 vs 0.98 diopters [D]; P =.002), but the vergence change was not significant between the 2 groups (P =.07).
At 80 cm, spontaneous deviations were observed among the IXT group, ranging from 1 to 6 MA. At the closer viewing distance, only 3 participants had spontaneous deviations.
“Understanding the measured response relationship between [IXT and orthophoria] provides a deeper understanding of strategies used by patients with IXT to control their ocular alignment at different viewing distances, and can guide the development of more effective treatment options by revealing the roles of the different motor responses,” according to the study authors.
Study limitations include broad inclusion criteria which created a heterogeneous sample of individuals with IXT and short viewing distances which may have limited spontaneous deviations noted at longer distances.
Mestre C, Neupane S, Manh V, Tarczy-Hornoch K, Candy TR. Vergence and accommodation responses in the control of intermittent exotropia. Ophthalmic Physiol Opt. Published online January 24, 2023. doi:10.1111/opo.13093