This is a short article about the process and considerations for fitting orthokeratology in presbyopia.
Before we fit orthokeratology to presbyopes, we need to remember that contrast sensitivity will be reduced as we are degrading the optical image in order to improve near optics. In order to maintain stereopsis, we always want to make the near visually corrected eye no worse than 6/12 (20/40) and binocularly achieve at least 6/9 (20/30) N6 or better. The aim is to provide freedom from spectacles. In many cases, we can achieve 6/7.5 (20/25) N6 - N4 binocularly.
Things to consider when attempting presbyopic fitting with orthokeratology are:
- All orthokeratology moulds induce some multifocal effect.
- Minus correction orthokeratology lenses perform like centre distance multifocal soft contact lenses.
- Plus orthokeratology lenses perform like centre near multifocal soft contact lenses
- Review the binocularity of the patient to determine whether orthokeratology will work to achieve a good balance between distance and near vision
- Altering treatment zone size affects the multifocal effect. Smaller OZD introduces more plus closer to the pupil centre.
- Altering the eccentricity of the back optic zone affects the multifocal effect. Higher e-values introduces higher amounts of plus in the reverse curve.
- Ortho-K monovision reduces distance vision less than conventional monovision in spectacles and soft contact lenses
- Residual refraction is not as important as visual acuity when assessing the post orthokeratology maps
Always perform at least two binocularity checks when assessing orthokeratology for presbyopia. Examples include:
- The hole in the card test - Where the practitioner asks the patient to look through a hole in a card at distance and then close each eye in turn. The eye that still remains centrally located when viewing the distant object is the dominant one. There is also a near version of this test.
- Plus to blur - This test involves adding plus power monocularly whilst wearing the distance prescription. The eye that allows for good distance vision whilst the reading add is in front, is the near eye for target correction.
In order to increase the multifocal effect of the orthokeratology mould, we can manipulate both the treatment zone diameter and the treatment zone eccentricity.
Optic Zone Diameter
Smaller treatment zones for myopic orthokeratology create more reading effect but can also create more flare and glare. Always review the pupil size.
Optic Zone Eccentricity
An increase in eccentricity in myopic orthokeratology will improve reading but can degrade distance vision, this is because the eccentricity controls the steepness away from the very centre of treatment.
Monovision is the main way to achieve more near correction. It is unwise to decrease the visual acuity of the reading eye worse than 6/12 (20/40) in order to maintain binocularity and stereopsis.
Information required by a distributor for troubleshooting presbyopic orthokeratology:
- Vision: distance and near monocular.
- Pre and post corneal topographical maps.
- Monocular refraction to achieve the optimal balance for both distance and near acuity.
- NaFl photo of the lens on the eye.