Clinical findings before the onset of myopia in youth .2. Zone of clear single binocular vision

Background. Many theories of myopia etiology suggest differences in accommodation and convergence in children who become myopic. Several studies have found differences in accommodation and convergence in myopes and emmetropes. Only one previous study examined accommodation and convergence before youth onset myopia. Methods. A cohort of initially emmetropic children was given eye and vision examinations at 6-month intervals for 3 years. The data collected included fusional vergence ranges at 4 m and 40 cm, and relative accommodation findings with a target at 40 cm. These clinical accommodation and vergence test findings were compared in children who became myopic (became-myopic group) to children who did not (remained-emmetropic group). The mean test findings were used to construct a zone of clear single binocular vision (ZCSBV) for each group. Results. The midpoint between the 40-cm fusional vergence range blur points was more convergent in the became-myopic group (+5.8 Delta compared to +3.2 Delta in the remained-emmetropic group; p < 0.004). The positive relative accommodation (PRA) finding was less in magnitude in the became-myopic group (-1.46 D) than in the remained-emmetropic group (-2.04 D), the difference being significant at the 0.02 level. Conclusions. The ZCSBV near blur points were more convergent in the children who became myopic as indicated by the more convergent position of the midpoint of the near fusional vergence range. The lower value for the PRA test in the became-myopic group duplicates the same result reported in a previous study for retrospective longitudinal private practice data.