Abstract
Purpose: :
To compare the adaptive properties of accommodation and vergence in early onset myopes with emmetropic controls. Empirical measures had been taken over a series of experiments where children viewed a (cartoon clip) target at 33cm through either +/- 2D near adds or a 10 pd base out prism for 20 minutes.
Methods: :
Data were selected from two of our previous studies, but subjects were balanced to show normal near phorias for the plus add study and normal tonic vergence levels for the prism study. This controlled for previously found effects resulting from phoria magnitude and direction. The following designs had been used.Children were between the ages of 7 - 14 years. 11 emmetropes and 10 myopes viewed through near adds for 20 min. Then 14 emmetropes and 14 myopes viewed a DOG target through 10pd base out prism for 20 min. For both studies accommodation was measured dynamically at 25Hz (Power Refractor, Multichannel systems). Responses were averaged over 5 sec and sampled at regular intervals over 20 minutes. Phorias and tonic vergence were measured with a Thorington technique . Tonic accommodation was measured at 4M using a DOG target (0.5cpd) before and after viewing which defined accommodative adaptation (AA). Vergence adaptation (VA) was determined by the pattern of recovery of the initial phoria through the near adds or the 10 pd prism over the 20 min viewing period. Response AC/A and CA/C measures were taken prior to testing.
Results: :
Statistical significance was set at p<0.05. Myopes showed significantly greater AA viewing through negative adds (-0.56D±0.08 vs -0.30±0.03) and during prism viewing (-0.33D±0.1 vs 0.02D±0.16). No change in AA was found with plus add viewing. The degree of phoria recovery (VA) was significantly reduced in myopes (54.7%±3.8 vs. 81.8%±2.2) for the plus add but not for the minus add. A similar pattern of significantly reduced VA in myopes was found following prism viewing (57%±2.5 vs. 70%±2.4). The response AC/A was significantly elevated in myopes (6.17pd/D±0.46 vs 4.26pd/D±0.3) but no significant difference was found between the CA/C measures.
Conclusions: :
Myopes show greater AA and reduced VA compared to emmetropes when accommodation and fusional convergence demands are increased. The AC/A and CA/C findings are not readily explained from the differing adaptation profiles.
Keywords: accommodation • vergence • refractive error development