Tonic accommodation, accommodative hysteresis, and refractive error.

Accommodative hysteresis was compared in high myopes, low myopes, emmetropes, and hyperopes. Subjects consisted of 48 visually normal young adults who were fully corrected for their ametropia and equally distributed among the four refractive categories. Baseline measures of tonic accommodation, nearpoint, and farpoint were followed by a 10 min period of monocular fixation on a target which provided a stimulus equal to the subject's accommodative amplitude. The tonic measures were repeated immediately after fixation and were followed by either reassessment of the nearpoint and farpoint, or a 20 min period in darkness with reassessment of the tonic level at 5-min intervals. The period of sustained maximal focus produced significant increases in both tonic accommodation and the accommodative nearpoint in each refractive group. Although the magnitude of change was statistically equivalent across groups, a tendency to greater increases in tonic accommodation was noted for the high myopes. Although the farpoint remained relatively stable in the individual refractive groups, a slight inward shift was found in the combined sample. The tonic change diminished primarily during the first 5 min in darkness, with no significant difference in either the rate or degree of decay in the various refractive groups. The results indicate no clear association between susceptibility to accommodative hysteresis and refractive category.