Penny ShawMSc, FCOptom
Types Effects
HypermetropiaAxial length too short or
refractive power too lowLight would focus behind
retinaAccommodation needed
to bring image into focus
Accommodation is the increase in power of the intraocular lens effected by contraction of the ciliary muscle
Effects of Hypermetropia
Nothing!Tired, irritable eyesHeadachesBlur N and/or D – transient or permanentIncreased problems in low light
Myopia
Axial length too long or refractive power too high
Focal plane infront of retinaAccommodation is no use
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
Astigmatism
Irregular curvature of the refractive surface(s), usually the cornea
2 or more focal planesSimple/myopic/
hyperopic/mixedAccommodation is of
little use
Irregular astigmatism results from corneal ectasia (eg. keratoconus), scarring, surgery
Bar to military flying
Effects of astigmatismBlur D and NDoubling or ghosting of
imagePoint sources spread along
orientation of astigmatism Glare in bright lightHeadaches
Presbyopia
Effects of presbyopiaBlurring at nearHeadachesEyestrain/tired eyes
after near workDifficulty refocusing to
distance after near work
First noticed in dim light / poor contrast (cockpits, maps!)
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 readNote: people have good memories!
Recording visionStandard testing distance: UK=6m, US=20ftVision 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 eyeLetter size increases iaw similar triangles: e.g 6/12
letter is double the size of 6/6 letterCan also be recorded as decimal e.g. 6/6=1.0,
6/12=0.5, 6/3=2.0
Recording visionSnellen PULHEEM S
<6/60 86/60 76/36 66/24 56/18 46/12 36/9 26/6 16/4 1
V = vision without correctionVA =Visual acuity with correctionPULHEEMS Recording under EE R V/VA L V/VA
e.g. 7/2 4/1
R Unaided 6/60 corrects to 6/9, L Unaided 6/18 corrects to 6/6
ConvexConcaveToricRecognition
Convex lenses - recognition
Thicker in the middleMagnifying effectFace looks larger
within spx frame“Against” movement
of image
Convex lenses - use
Correction of hyperopia and presbyopia
Concave lenses - recognition
Thinner in the middle
Minifying effectFace looks smaller
within spx frame“With” movement of
image
Concave lenses - use
Correction of myopia:
Toric lenses - recognition
Can be concave, convex, simple or mixed
Swivel test produces “scissor” effect
Toric lenses - use
Correction of astigmatism
Refraction determines the position and orientation of each focal plane
Spectacle lenses
Spx lenses are thin, curved to improve visual comfort and appearance
Convex
Concave
Subjective refraction
Aim
To determine the lens strength needed to focus parallel light from distant object on to the retina of the relaxed eye
Subjective refraction
Use maximum plus to ensure relaxed accommodation
Use minimum minus to ensure accommodation is not stimulated
Clearest image with relaxed eye
Subjective refraction
Subjective refractionBest sphere
Fit trial frame correctlyRecord monocular vision including Ph visionUnaided 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
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
Types Aftercare
Issues
Spectacles vs CL in aviation Depends on A/C typeCFS mist up, restrict field of view, fall to
bits, hurtCL: Some issues mainly to do with lens
dehydration. CL generally preferred to CFSDaily 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
Lens typesDaily disposable: sph or toric designs,
hydrogel/silicone hydrogel FRP: hydrogel/silicone hydrogel
replaced weekly, 2-weekly or monthly. Durable: tailor-made hydrogelsComplex fits eg keratoconus -
kerasoft (hydrogel or silicone hydrogel)
ModalitiesDaily wear with daily disposable or FRPFlexible wear: occasional overnight useContinuous wear: up to 30 daysOrthokeratology (OK): overnight rigid lenses
give temporary correction
Aftercare intervalsDaily wear Extended /flexible wear
Initial fitting7-10 days1-3 months 6 months
Initial fitting1 week daily wear
(practice lens handling)After 1st overnight wear1 week CW3 months 6 months
Aftercare checks
Vision: stability, over refraction
Fit/comfortWearing times ComplianceLens handlingOcular response
CL in aviation - advantages
Full field of viewIntegration with head furnitureNo mistingAesthetics!
Contact lens complications (very few!)
Subjective:
Drying Excess movement Poor/fluctuating vision Lens supplies/storage Solution use/storage
Objective:
Corneal oedema/ hypoxia
DryingCLPU
Contact lens complications
Contact lens complications
Poor lens hygieneLid reactionsMK