Refractive errors, refraction and contact lenses

1. Refractive errors, refraction and contact lenses Penny Shaw MSc, FCOptom

2. Refractive errors Types Effects

3. Hypermetropia Accommodation is the increase in power of the intraocular lens effected by contraction of the ciliary muscle • Axial length too short or refractive power too low • Light would focus behind retina • Accommodation needed to bring image into focus

4. Effects of Hypermetropia • Nothing! • Tired, irritable eyes • Headaches • Blur N and/or D – transient or permanent • Increased problems in low light

5. Myopia • Axial length too long or refractive power too high • Focal plane infront of retina • Accommodation is no use

6. Effects of myopia • Blur beyond far point (e.g. -1.00 myopia = blurred after 1m) • Glare from light sources affects night driving / flying • Occasionally headaches

7. Astigmatism Irregular astigmatism results from corneal ectasia (eg. keratoconus), scarring, surgery Bar to military flying • Irregular curvature of the refractive surface(s), usually the cornea • 2 or more focal planes • Simple/myopic/hyperopic/mixed • Accommodation is of little use

8. Effects of astigmatism • Blur D and N • Doubling or ghosting of image • Point sources spread along orientation of astigmatism • Glare in bright light • Headaches

9. Presbyopia

10. Effects of presbyopia • Blurring at near • Headaches • Eyestrain/tired eyes after near work • Difficulty refocusing to distance after near work • First noticed in dim light / poor contrast (cockpits, maps!)

11. Questions?

12. Measurement of vision

13. Snellen chart at exactly 6 metres (or other known distance) • Well illuminated (preferably internally) • Use occluder, avoid pressing on eye, squeezing eye shut or looking through fingers • Record smallest line correctly read • Note: people have good memories!

14. Recording vision • Standard testing distance: UK=6m, US=20ft • Vision recorded as the fraction: test distance/letter size • “Standard vision”: UK 6/6, US 20/20 • “Standard vision”: Each limb of the letter subtends 1’ arc at the eye • Letter size increases iaw similar triangles: e.g 6/12 letter is double the size of 6/6 letter • Can also be recorded as decimal e.g. 6/6=1.0, 6/12=0.5, 6/3=2.0

15. Recording vision Snellen PULHEEM S <6/60 8 6/60 7 6/36 6 6/24 5 6/18 4 6/12 3 6/9 2 6/6 1 6/4 1 • V = vision without correction • VA =Visual acuity with correction • PULHEEMS Recording under EE R V/VAL V/VA e.g. 7/2 4/1 R Unaided 6/60 corrects to 6/9, L Unaided 6/18 corrects to 6/6

16. Questions?

17. Lens types Convex Concave Toric Recognition

18. Convex lenses - recognition • Thicker in the middle • Magnifying effect • Face looks larger within spx frame • “Against” movement of image

19. Convex lenses - use • Correction of hyperopia and presbyopia

20. Concave lenses - recognition • Thinner in the middle • Minifying effect • Face looks smaller within spx frame • “With” movement of image

21. Concave lenses - use • Correction of myopia:

22. Toric lenses - recognition • Can be concave, convex, simple or mixed • Swivel test produces “scissor” effect

23. Toric lenses - use • Correction of astigmatism • Refraction determines the position and orientation of each focal plane

24. Spectacle lenses Spx lenses are thin, curved to improve visual comfort and appearance Convex Concave

25. Questions?

26. Refraction

27. Subjective refraction Aim To determine the lens strength needed to focus parallel light from distant object on to the retina of the relaxedeye

28. Subjective refraction • Use maximum plus to ensure relaxed accommodation • Use minimum minus to ensure accommodation is not stimulated • Clearest image with relaxed eye

29. Subjective refraction

30. Subjective refractionBest sphere • Fit trial frame correctly • Record monocular vision including Ph vision • Unaided vision: correspondence to degree of refractive error esp. myopia e.g 6/60 approx -3.00, 6/12 approx -1.00 • Uncorrected hyperopia may not blur vision

31. Subjective refractionStart with +ve lenses

32. Subjective refractionMove to –ve lenses if myopeNote unaided vision: start with appropriate strengthe.g. V6/12 start with -1.00

33. Subjective refractionBest sphere – final check • Final check with +1.00 should blur vision by ~ 3 lines • If VA remains below Ph level, consider astigmatism correction

34. Questions?

35. Contact lenses Types Aftercare Issues

37. Spectacles vs CL in aviation • Depends on A/C type • CFS mist up, restrict field of view, fall to bits, hurt • CL: Some issues mainly to do with lens dehydration. • CL generally preferred to CFS • Daily disposables preferred • Survey of Refractive correction in RAF Aircrew :2004: Shaw P, Scott RAH, Mushtaq B, Coker W • Refractive Correction in RAF Aircrew: 2006: Partner A, Scott RAH, Shaw P, Coker W

38. Lens types • Daily disposable: sph or toric designs, hydrogel/silicone hydrogel • FRP: hydrogel/silicone hydrogel replaced weekly, 2-weekly or monthly. • Durable: tailor-made hydrogels • Complex fits eg keratoconus - kerasoft (hydrogel or silicone hydrogel)

39. Modalities • Daily wear with daily disposable or FRP • Flexible wear: occasional overnight use • Continuous wear: up to 30 days • Orthokeratology (OK): overnight rigid lenses give temporary correction

40. Aftercare intervals Daily wear Extended /flexible wear • Initial fitting • 7-10 days • 1-3 months • 6 months • Initial fitting • 1 week daily wear (practice lens handling) • After 1st overnight wear • 1 week CW • 3 months • 6 months

41. Aftercare checks • Vision: stability, over refraction • Fit/comfort • Wearing times • Compliance • Lens handling • Ocular response

42. CL in aviation - advantages • Full field of view • Integration with head furniture • No misting • Aesthetics!

43. Contact lens complications(very few!) Subjective: • Drying • Excess movement • Poor/fluctuating vision • Lens supplies/storage • Solution use/storage

44. Contact lens complications Objective: • Corneal oedema/ hypoxia • Drying • CLPU

45. Contact lens complications • Poor lens hygiene • Lid reactions • MK

46. Questions?