Examination of the near triad in VDU operators.

It has recently been demonstrated that the tonic level of accommodation is related to the loading of accommodative work. The aim of the present study was to investigate the effect of a sustained accommodative stimulus on the tonic level of accommodation, pupil, and convergence, i.e., the near triad, in three different groups of young (20-39 years) subjects, viz., 1. 19 professional visual display unit (VDU) workers; 2. 12 ordinary hard copy (non-VDU) workers; and 3. 19 control subjects doing different jobs from the two former groups. All had worked 25-30 h per week for over two years. Only Groups 1 and 2 had eyestrain after office work. Except for mild myopia, subjects had no eye or systemic disease upon routine examination. Accommodation was measured with an objective infrared optometer, the pupil was measured by an infrared video pupillography, and convergence was measured from the contralateral eye by photoelectronic oculography. In order to elicit tonic responses, the velocity of the target stimulus was 0.2 D (diopters) per second. Based on the data obtained from the controls, the near triad was analysed. Asymmetric response of the near triad in the right and the left eye and the existence of a hysteresis curve, especially in the pupil and in convergence, were the major findings. No abnormality was found in the control group. In the VDU group, 34.6% showed an abnormal near triad, but this also occurred in 18.5% of the non-VDU group. Even with the same complaint of eyestrain, it was concluded that VDU work produced abnormalities 1.8 times higher in the near triad produced by tonic stimulus.