Identifying the pre-myope
There are four key principles for assessing risk of myopia onset:
- Family history – one myopic parent increases risk by three-fold, while two myopic parents doubles this risk again1
- Visual environment – less than 90 minutes a day spent outdoors increases risk, especially if combined with more than 3 hours a day spent on near work activities (outside of school time)2
- Binocular vision – Children with higher accommodative convergence (AC/A) ratios, typically seen with esophoria, have an increased risk of myopia development within one year of over 20 times.3 Accommodative lag may also be a risk factor but there is conjecture.4 Intermittent exotropia has also been associated with onset of myopia.5
- Current refraction – the most significant risk factor of this lot for future myopia is if a child exhibits 0.50D or less of manifest hyperopia at age 6-7. This risk is independent of family history and visual environment.6
In addition to this, the fastest rate of refractive change in myopic children occurs in the year prior to onset,7 so the child who is less hyperopic than age normal should be closely monitored, especially if concurrent risk factors are evident.
Identifying the myopia progressor
- Age - the younger a child becomes myopic, the faster they will progress, with children 7 years of age progressing by at least 1D per year with this halving by age 11-12.8
- Family history - children with two myopic parents have been shown to be the fastest progressors in single vision spectacle and atropine corrections, and children with one myopic parent progress less than the former but more than the child without such family history.9, 10
- Visual environment – near work at less than 20cm working distance and durations of longer than 45 minutes have been linked with more myopia progression.
- Ethnicity - Asian ethnicity has been linked to faster myopia progression8, 11
- Binocular vision – watch for esophoria, accommodative lag and intermittent exotropia. In myopia control studies of progressive addition spectacle lenses (PAL), children with esophoria in single vision spectacle control groups were found to progress more quickly, 12 and children with a larger baseline accommodative lag in the PAL groups showed statistically greater treatment effect.13 Children with lower baseline accommodative amplitude have shown a greater myopia control response to orthokeratology contact lens wear compared to normal accommodators.14 Finally, while the effect of controlling IXT on controlling myopia has not yet been studied, 50% of children with intermittent exotropia (IXT) are myopic by age 10 and 90% by age 20.5
Essentially, any myopic child is a progressor until proven otherwise! The institution of a myopia control strategy as early as possible is evidence based practice, especially by age 9. The Clinical Myopia Profile,free to download, provides a simple format for explaining these risk factors for onset and progression to parents, and to decide on a management strategy.