People who work in occupations that involve intensive near work are thought to have a higher chance of developing myopia than other people. For example, microscopists in the United Kingdom have a higher prevalence of myopia than that of the general community. The prevalence of myopia in Hong Kong is extremely high (71 %) and Hong Kong Chinese people are particularly susceptible to myopia development and progression due to environmental factors. It is possible that this environmental susceptibility may lead to Hong Kong Chinese microscopists developing even greater levels of myopia. We found that the prevalence of myopia in Hong Kong microscopists (n=47, mean age=31 years) was higher than that of United Kingdom microscopists (87 % c.f. 71 %) and similar aged people within the general Hong Kong population (87 % c.f. 71 %; −4.45 D c.f. -3.00 D). However, while in most microscopists (83 % of 36 microscopists followed for a two-year period) the amount of myopia and vitreous chamber depth increased over a two year monitoring period (−0.11 D, 0.06 mm), the increase was not clinically significant. We hypothesised that the slower myopia progression rate in Hong Kong microscopists may be the result of their older average age (Hong Kong microscopists: 31.7 years c.f. United Kingdom microscopists: 29.7 years). When a person looks into a microscope, excessive accommodation occurs even though the microscope is designed to render the magnified image at optical infinity (zero accommodation and vergence demand). This over accommodation is called instrument myopia. It is possible that this over accommodation is linked to the myopia development and progression that occurs in users of these instruments. We found that instrument myopia remained consistent with different viewing conditions and microscope settings (inexperienced microscopists, n=20, mean age: 24.1 years, mean spherical refractive error: −2.83 D). The magnitude of instrument myopia was not correlated with either the age or refractive error of the microscope user, while it was lower in those users with greater experience (inexperienced microscopists: 1.03 D c.f. experienced microscopists: 0.43 D). As the Hong Kong microscopists (n=10, mean age: 31.2 years, mean spherical refractive error: −3.39 D) who partook in this study were experienced (6.3 years spent working in this field), this may have contributed to the lower myopia progression that was observed. Studies to determine the main contribution to the phenomena of instrument myopia were also conducted. Instrument myopia was not correlated with convergence when looking into microscope (r= −0.224, p=0.342), near phoria (r=0.351, p=0.129), AC/A ratio (r= −0.135, p=0.571), the convergence induced by the excessive accommodative response (r= −0.028, p=0.906), lag of accommodation (r=0.065, p=0.785) and tonic accommodation (r=0.142, p=0.551). We suggest that the main contribution to instrument myopia during microscopy is proximal accommodation due to the awareness of the closeness, caused by the height of the microscope (i.e. the distance between the viewer and the table where the microscope is placed), during microscopy. For example, we found that the magnitude of instrument myopia increased significantly (from 0.64 D to 1.16 D) when the height of the microscope decreased from 50 cm to 35 cm. In conclusion we have added, through direct observation, to the understanding of the characteristics of instrument myopia. Guidelines for new microscopists aimed at minimising the amount of instrument myopia that is experienced have been developed. This information might help to reduce the amount of myopia progression in commencing microscopists.
| Identifer | oai:union.ndltd.org:ADTP/264953 |
| Date | January 2004 |
| Creators | Ting, Wai Ki |
| Publisher | Queensland University of Technology |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
| Rights | Copyright Wai Ki Ting |
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