Presentation on theme: "Refraction Švehlíková G."— Presentation transcript:
1Refraction Švehlíková G. Department of Ophthalmology LF UPJS v KošiciachPrednosta: prof. MUDr. Juhás T., DrSc
2How the Eye SeesThe process of vision begins when light rays that reflect off objects and travel through the eye's optical system are refracted and focused into a point of sharp focus.For good vision, this focus point must be on the retina, where light-sensitive cells - photoreceptors capture images in much the same way that film in a camera does when exposed to light. These images then are transmitted through the eye's optic nerve to the brain for interpretation.
3refractive errorAn eye that has no refractive error when viewing a distant object is said to have emmetropia or be emmetropic.An eye that has a refractive error when viewing a distant object is said to have ametropia or be ametropic.Very few people have refraction of exactly 0.0 diopters.Approximately 55% of persons between the ages of 20 and 30 have refraction between + 1 and –1 diopters.
4refractive errorThe total refractive power of an emmetropic eye is approximately 58 diopters (D), of which 43 D come from the cornea and the remaining 15 D from the lens, aqueous, and vitreous.forms of refractive error:Myopia (nearsightedness)Hyperopia (farsightedness)Astigmatismrefractive error - blurred or distorted vision
5refractive error Causes of Refractive Errors The eye's ability to refract or focus light sharply on the retina primarily is based on three eye anatomy features:1) the overall length of the eye2) the curvature of the cornea and3) the curvature of the lens inside the eye
6Eye Length:If the eye is too long, light is focused before it reaches the retina, causing myopia.If the eye is too short, light is not focused by the time it reaches the retina. This causes hyperopia.
7Curvature of the Cornea: If the cornea is not perfectly spherical, then the image is refracted or focused irregularly to create a condition called astigmatism.A person can have myopia or hyperopia with or without astigmatism.
8Curvature of the Lens:If the lens is too steeply curved in relation to the length of the eye and the curvature of the cornea, this causes myopia.If the lens is too flat, the result is hyperopia.
9myopia Rays of light entering the eye focus in front of the retina The refractive power of the eye is too strong for the lenght of the globe – refractive myopiaThe eye is too long for the refractive power – axial myopiaEpidemiology: Approximately 25% of persons between the ages of 20 and 30 have refraction less than –1 diopters.
10myopia Forms: Simple myopia: Onset is at the age of 10–12 years Usually does not progress after the age of 20Refraction rarely exceeds 6 dioptersPathologic ( progressive, malignant )myopia:This disorder is largely hereditary and progresses continuouslyOvergrowth of the posterior 2/3 of the globeDegeneration at the retinal peripherySpecial forms:Lenticular myopia - sclerosis of the nucleus of the lens (cataract) in advanced age (Spherophakia (spherically shaped lens).Treatment: minus or concave lenses
11hyperopia Rays of light entering the eye focus behind the retina The refractive power of the eye is too week for the lenght of the globe – refractive hyperopiaThe eye is too short for the refractive power – axial hyperopiaEpidemiology: Approximately 20% of persons between the ages of 20 and 30have refraction exceeding +1 diopters
12hyperopiaSymptoms:In young patients, accommodation can compensate for slight to moderate hyperopia.However, this leads to chronic overuse of the ciliary muscle. Reading in particular can cause asthenopic symptoms – eye pain or headache, blurred vision, ...As accommodation decreases with advancing age, near vision becomes difficult. For this reason, hyperopic persons tend to become presbyopic early.Special forms of hyperopia:Absence of the lens (aphakia) due to dislocation.Postoperative aphakia following cataract surgery without placement of an intraocular lensTreatment: plus or convex lenses
13astigmatism Epidemiology: 42% of all humans have astigmatism greater than or equal to 0.5 diopters.In approximately 20%, this astigmatism is greater than 1 diopter and requires optical correctionAstigmatism is derived from the Greek word stigma (point) and literally means lack of a focal point.parallel light rays do not focus at a pointThe refracting power in one axis is not the same than that in an axis perpendicular to it
14astigmatism Classification Simple myopic astigmatism – the focus of one meridian is on the retina, other is in front of the retinaSimple hyperopic astigmatism- the focus of one meridian is on the retina, other is behind the retinacompound myopic astigmatism – both meridians are focused in front of the retinacompound hyperopic astigmatism - both meridians are focused behind the retinamixed astigmatism – one is focused in front and the other behind the retinaRegular - only two meridians approximately perpendicular to each otherIrregular - there are multiple focal pointsCausescorneal ulcerations with resulting scarring of the corneapenetrating corneal traumaadvanced keratoconusCataractTreatment: Cylinder lensesOnly regular astigmatism can be corrected with eyeglassesirregular astigmatism cannot be corrected with eyeglasses
15AccomodationAbility of the eye to bring retinal images of objects in various distances into sharp focusVarying the reftactive power of the lensLens is fixated with elastic ligaments – the zonules – at the ciliary muscle.With contraction of ciliary muscle, the zonules relaxes and the lens takes spherical shape
16presbyopiaThe elasticity of the lens decreases with increasing age, and the range of accommodation decreasesPresbyopia - physiologic loss of accommodation in advancing age - begins when the range of accommodation falls below 3 diopters.depending on age presbyopia can be compensated with converging lenses of 0.5–3 diopters
17Anisometropia difference in refractive power between the two eyes Where the difference in refraction is greater than or equal to 4 diopters, the size difference of the two retinal images becomes too great for the brain to fuse the two images into oneaniseikoniaEpidemiology:Anisometropia of at least 4 diopters is present in less than 1% of the populationSymptoms:Anisometropia is usually congenital and often asymptomatic.binocular functions may remain underdevelopedwhen the correction of the anisometropia results in unacceptable aniseikonia, patients will report unpleasant visual sensations of double vision
18Correction of Refractive Errors the type and degree of refractive error – computerized automated refractometryglasses,contact lenses,refractive surgery
19Eyeglass Lenses Monofocal Spherical lenses Toric lenses (cylindrical lenses) refract light only along one axis.Spherical and toric lenses can be combined where indicatedMultifocaldifferent areas of the lens have different refractive powersBifocals: The upper portion of the lens is for the distance correction; the lower portion is for the near- field correctionPatients are able to view distant objects and read using one pair of eyeglassesProgressive addition lenses: continuously increasing refractive power
20contact lensesquality of the optical image viewed through contact lenses is higher than that viewed through eyeglassesContact lenses have significantly less influence on the size of the retinal imageThe cornea requires oxygen from the precorneal tear film.To ensure this supply, contact lens materials must be oxygen- permeable. This becomes all the more importantthe contact lens moves and permits circulation of tear fluid.Contact lenses may be manufactured from rigid or flexible materials.
21Rigid Contact LensesThese contact lenses have a stable, nearly unchanging shape.Patients take some time to become used to themToday, highly oxygen- permeable materials such as silicone copolymers are availablecorrect keratoconus
22Soft Contact LensesThe material of the contact lens, such as hydrogel is softPatients find these lenses significantly more comfortable.The oxygen permeability of the material depends on its water content, which may range from 36% to 85%Supported by the limbusThe lens is displaced only a few tenths of a millimeter when the patient blinks. This greatly reduces the circulation of tear film under the lenses.This requires to be removed at night to allow regeneration of the cornea
23contact lensesContact lenses may also be classified by wearing schedule:- daily wear contacts must be removed, cleaned and stored each night,- while extended wear contact lenses are made from materials which are safe for overnight wear- sometimes "continuous wear" is used for a type of extended wear lens that can be worn for up to 30 days
24Contacts can also be described by replacement interval Daily, weekly, bi- weekly, monthly or quarterly
25Special LensesTherapeutic contact lenses: In the presence of corneal erosion, soft contact lenses act as a bandage and thereby accelerate reepithelialization of the cornea. They also reduce pain.
26Disadvantages of Contact Lenses Contact lenses exert mechanical and metabolic influences on the cornea.Therefore, they require the constant supervision of an ophthalmologist.Contact lenses require careful daily cleaning and disinfection.This is more difficult, time-consuming, and more expensive than eyeglass care
27Contact Lens Complications Infectious keratitis (corneal infiltrations and ulcers) caused by bacteria,fungi, and protozoansGiant papillary conjunctivitis: This is an allergic reaction of the palpebral conjunctiva of the upper eyelidCorneal vascularization may be interpreted as the result of insufficient supply of oxygen to the corneaSevere chronic conjunctivitis: This usually makes it impossible to continue wearing contact lenses
28refractive surgeryis surgical (laser or conventional) configuring of the curvatures of the eye (cornea and/or lens) to allow the rays of light to be focused on the retina as a pointCategoriesCorneal refractive proceduresIntraocular refractive procedures
30refractive surgerylower refractive errors (in the range to D) are usually corrected using laser techniques to reshape the corneahigher errors are corrected using lens implant based methodsastigmatism and age are also influential in determining the most appropriate technique
31Laser Refractive surgery an eye drop anesthetic is usedA portion of the cornea is removed, or creating the flapThe eye is then positioned under an Excimer laser which has been programmed to remove microscopic amounts of corneal tissue.Removal of the tissue changes the curvature of the cornea.If the patient has myopia tissue closer to the central part of the cornea is removed to decrease the curvature or flatten the cornea.If a patient has hyperopia tissue in the periphery of the cornea is removed to increase curvature.To correct for astigmatism, selected tissue at certain angles is removed to insure that the cornea curves equally in all directions.After the laser has been used, the flap is returned to its original position.
32Laser Refractive surgery Profile of the cornea after fotoablation for MYOPIAProfile of the cornea after fotoablation forHYPEROPIAIf the patient has myopia tissue closer to the central part of the cornea is removed to decrease the curvature - flatten the corneaIf a patient has hyperopia tissue in the periphery of the cornea is removed to increase curvature
33Laser Refractive surgery divided intolamellar (LASIK) andsurface ( PRK, LASEK, and Epi-LASIK)laser-assisted in situ keratomileusis (LASIK)photorefractive keratectomy (PRK)laser assisted subepithelial keratectomy (LASEK)
34PRK, LASEK, and Epi-LASIK methods of surface ablationthese procedures do not require a partial thickness cut into the stromathese methods differ in the way the epithelial layer is handled
35photorefractive keratectomy (PRK) In photorefractive keratectomy surgery the epithelium is removedThe epithelium might be removed in several ways, including excimer laser destruction, mechanical debridement with a surgical blade, abrasion with a brush, or use of alcohol to loosen the epithelium.
36Laser assisted subepithelial keratectomy (LASEK) LASEK – epitelial flapEpi-LASIK - microkeratomeIn LASEK and Epi- LASIK surgery the epithelial flap is folded backafter the ablation, this epithelial flap is placed back into its original positionthe epithelial alignment is protected from blinks and eye movements by the addition of a bandage contact lens
37LASEK The Advantages The Disadvantages No stromal flap - much fewer serious potential complications ( Lamellar Keratopathy, Interface infections.)Minimal or no induction of optical aberrationsQuicker recovery of corneal sensation and the blink reflex (4 months vs. 1 1/2 years with LASIK)Higher amounts of refractive errors can be correctedMore discomfortRecovery time is longerfinal results for LASIK and surface treatments are similar
38laser-assisted in situ keratomileusis (LASIK) a partial-thickness corneal flapmade with a microkeratomedepths of 100–200 µmfemtosecond laser has been developed, provides more accuracy in flap thickness
39LASIK The Advantages The Disadvantages Rapid visual recoveryLess postoperative discomfortInduces more optical aberrationsInduces more complicationsIf there is a complication, it is usually due to the flapOnly used for mild to moderate myopiaMuch longer time for recovery of blink reflex when the eye is dry
40Complications Corneal scarring/haze (<1-2%) Keratitis (0.5-1%) Although LASEK may carry a decreased rate of corneal haze relative to PRK, it may still develop secondary to an inflammatory response to the surgical manipulation of the corneal surface.The inflammation leads to the formation of an opacified cellular layer that appears as a white haze and restricts light from transmitting to the back of the eye, thus causing a defect in visionThe risk of scar formation increases with increasing ablation depth, and scars are common when treating more than 8 D of myopia.Keratitis (0.5-1%) Postoperative infection is more likely when epithelial coverage is incomplete or when the surgical duration is longer than average.Additionally, contact lenses may serve as a source of infection, as they may be contaminated with microorganisms. Likely, because contact lenses are not used postoperatively in LASIK, LASIK has a lower incidence of keratitis (about 0.2%).
42Complications Dry eye syndrome associated with recurrent erosions This complication is secondary to decreased corneal sensation due to corneal denervation. It may last from a few weeks to 1 year, although, on average, it lasts 1-4 weeks.Although this complication occur in LASEK and LASIK, it is more likely to be associated with a longer duration in LASIK.Overcorrection (1%, incidence similar to LASIK and PRK)Undercorrection (10-15%, incidence similar to LASIK and PRK)Macular cyst formation (<0.1%)Irregular astigmatism (<1%): This complication is secondary to decentration of the laser optical zone or uneven healing, leading typically to a wavy corneal surface.
43Intraocular refractive procedures Clear lens extractionrange: myopia/hypermetropia at any levelPrinciple = replacement of the natural lens with an intraocular lens (IOL)multifocal IOLs can be implanted to reduce spectacle dependence for near visionis identical to modern cataract surgery
44Intraocular refractive procedures Phacic intraocular lensimplantation - range: up to - 17D myopia; up to +10D hypermetropiaPrinciple = implantation of a soft flexible artificial lensis particularly suited to younger patients who are out of range for laser refractive surgery
45IndicationsThe major indications for refractive surgery include astigmatism, myopia, and hyperopia, specifically in patients who are intolerant of or who desire to be free from glasses or contact lenses
46Contraindications Autoimmune disease Unstable refractive errorRefractive error outside the range of correction (The range varies according to the surgeon's experience, the laser used, and the laser strategy; however, it is typically approximately D of myopia, 4-6 D of hyperopia, and 2-6 D of astigmatism.)KeratoconusPellucid marginal degenerationSignificant dry eye syndromeActive inflammation of external eyeAutoimmune diseaseHistory of or active herpes simplex keratitis, risc of the reactivation of the virusActive collagen vascular diseaseUncontrolled diabetesUncontrolled glaucomaPregnancy or breastfeeding