Objective Measurement of Fusional Vergence Ranges and Heterophoria in Infants and Preschool Children

Abstract

PurposeBinocular alignment typically includes motor fusion compensating for heterophoria. This study evaluated heterophoria and then accommodation and vergence responses during measurement of fusional ranges in infants and preschoolers.MethodsPurkinje image eye tracking and eccentric photorefraction (MCS PowerRefractor) were used to record the eye alignment and accommodation of uncorrected infants (n = 17; 3–5 months old), preschoolers (n = 19; 2.5–5 years), and naïve functionally emmetropic adults (n = 14; 20–32 years; spherical equivalent [SE], +1 to −1 diopters [D]). Heterophoria was derived from the difference between monocular and binocular alignments while participants viewed naturalistic images at 80 cm. The presence or absence of fusion was then assessed after base-in (BI) and base-out (BO) prisms (2–40 prism diopters [pd]) were introduced.ResultsMean (±SD) SE refractions were hyperopic in infants (+2.4 ± 1.2 D) and preschoolers (+1.1 ± 0.6 D). The average exophoria was similar (P = 0.11) across groups (Infants, −0.79 ± 2.5 pd; Preschool, −2.43 ± 2.0 pd; Adults, −1.0 ± 2.7 pd). Mean fusional vergence range also was similar (P = 0.1) for BI (Infants, 11.2 ± 2.5 pd; Preschool, 8.8 ± 2.8 pd; Adults, 11.8 ± 5.2 pd) and BO (Infants, 14 ± 6.6 pd; Preschool, 15.3 ± 8.3 pd; Adults, 20 ± 9.2 pd). Maximum change in accommodation to the highest fusible prism was positive (increased accommodation) for BO (Infants, 1.69 ± 1.4 D; Preschool, 1.35 ± 1.6 D; Adults, 1.22 ± 1.0 D) and negative for BI (Infants, −0.96 ± 1.0 D; Preschool, −0.78 ± 0.6 D; Adults, −0.62 ± 0.3 D), with a similar magnitude across groups (BO, P = 0.6; BI, P = 0.4).ConclusionsDespite typical uncorrected hyperopia, infants and preschoolers exhibited small exophorias at 80 cm, similar to adults. All participants demonstrated substantial fusional ranges, providing evidence that even 3- to 5-month-old infants can respond to a large range of image disparities.

Adaptation of horizontal eye alignment in the presence of prism in young children

“…In the current study there was a higher percentage of children (3/13) than adults (1/11) who could not fuse through BI prism. This is reasonable in that preschoolers have demonstrated a mean divergence fusional range of close to 6pd at a similar viewing distance of 80 cm (Sreenivasan et al, 2016 ). For some young children, who didn't fuse initially, ramping from 4 pd to 6 pd did help them maintain fusion.…”

“…At the moderate viewing distance (1 m), percentage of adaptation was higher in base-in than in base-out conditions for both age groups. This adaptation might be helpful in relieving stress on the divergence fusional vergence system, given that the divergence fusional vergence range was significantly smaller than the convergence range at a similar viewing distance (Sreenivasan et al, 2016 ). The mean percentage of BO adaptation in adults [48% (SD ± 0.26) – Figure 4 , Panel B] was consistent with values found in previous studies of adults (Henson & Dharamshi, 1982 ; McCormack, 1985 ; North & Henson, 1981 , 1982 ; North, Sethi, & Owen, 1990 ).…”

“…In a nonstrabismic individual, the covered eye will drift under these dissociated conditions and the magnitude of this drift is called the heterophoria (phoria). Under typical visual conditions the fast-acting disparity-driven fusional vergence system can only compensate for this heterophoria over a limited working range to achieve fusion (e.g., around 20 pd convergence and 12 pd divergence for a target at 80 cm, see Sreenivasan, Babinsky, Wu, & Candy, 2016 ); however, there is evidence that a slower acting adaptive vergence component may help compensate for larger demands in adults (Schor, 1979 ). Vergence adaptation was first observed among patients who returned to their original phoria magnitude after starting to wear a prism correction (Carter, 1965 ), and thus it is also called prism adaptation.…”

“…Children with excessive accommodative convergence are believed to be at greater risk for strabismus (Parks, 1958 ; Raab, 1982 ), but only approximately 20% of moderate to high hyperopes develop strabismus in the first years after birth (Anker, Atkinson, Braddick, Nardini, & Ehrlich, 2004 ; Atkinson et al, 1996 ; Babinsky & Candy, 2013 ; Ingram, Lambert, & Gill, 2009 ). Studies have indicated that the average phoria of typically developing infants and children is small and consistent with age (Babinsky, Sreenivasan, & Candy, 2015 ; Chen, O'Leary, & Howell, 2000 ; Lam, LaRoche, De Becker, & Macpherson, 1996 ; Sreenivasan et al, 2016 ; Walline, Mutti, Zadnik, & Jones, 1998 ). Does active vergence adaptation help maintain this phoria in typical development?…”

The Importance of the Interaction Between Ocular Motor Function and Vision During Human Infancy

“…Infants and small children are not able to follow instructions, and so data collected from them can reflect only reflex behavior. We recorded reflex alignment responses in the presence of prism driving convergence and divergence while uninstructed naive participants aged 3-5 months, 2.5-5 years, or 20-32 years viewed a naturalistic cartoon image subtending 6° vertically by 2.5° horizontally at an 80-cm viewing distance (Sreenivasan et al 2016). Recordings of realignment responses to prism indicated that, on average, the three naive groups had matched reflex fusional vergence ranges that compared well with adults from previous findings in the literature (Figure 3).…”