ACCOMMODATIVE ADAPTATION AND AGE-OF-ONSET OF MYOPIA

Tonic accommodation in an empty field and accommodative adaptation, de fined as accommodation shift after 2 min viewing of a 4 D near task, w ere measured using an infrared optometer. Subjects were classified int o three types: emmetropes (n = 18), early onset myopes (n = 18) and la te onset myopes (n = 15). The relationship between accommodative adapt ation and the onset age of myopia were compared. There was no statisti cal difference in the pre-task tonic accommodation among the three sub ject groups. However, the accommodative adaptation differed in the thr ee groups during the 5 min post-task period. The average accommodative adaptation of emmetropia, early onset myopia and late onset myopia we re 0.66, 0.27 and 1.19 D, respectively. Accommodative adaptation in th e late onset myopia group was significantly higher than in the emmetro pia group. Furthermore, the accommodative adaptation in the early onse t myopia group was significantly lower than in the emmetropia group. T wo possible aetiologies of myopia are discussed: one for the group whi ch shows low accommodative adaptation that has no relationship with th e age of onset and another for the group which shows high accommodativ e adaptation that has a relationship to an onset age of 15 years or la ter. The aetiology is also considered with regard to a potentiating ch emical effect on the synapses of the ciliary muscle.