Human Eye: Optical Conditions There are a number of optical conditions of the eye which require correction. Some are refractive errors and some are age-related conditions. The refractive errors, i.e. myopia and hyperopia with or without astigmatism, as well as astigmatism itself uncomplicated by myopia or hyperopia will be dealt with in detail in the following sections. Presbyopia, an age-related condition, with and without ametropia, will be dealt with separately.
Ametropia An emmetropic eye focuses light from a distant object on the retina resulting in a clear image. An ametropic eye is one in which light from distant objects is not focused on the retina. The light rays may be focused either in front of or behind the retina.
Myopia Myopia is the refractive error resulting from lights rays from a distant object being brought to a focus in front of the retina. The greater the refractive error the further in front of the retina this focus is located. This situation means that, regardless of the accommodative state, unaided clear distance vision cannot be achieved. Applying accommodation only makes the situation worse by bringing the focus position further forward thereby increasing the blur perceived. NOTE: In this and subsequent diagrams, a simplistic approach to the optics of each situation is taken. In the interests of clarity, light rays are shown for an unrealistic pupil size. Thus light appears to pass through the iris, an impossibility in the real world. Further, light is shown being refracted by the anterior corneal surface only, rather than being refracted progressively by each surface. In the case of hyperopia, light rays are shown being focused behind the retina, also an impossibility in the real world since light cannot pass through the posterior pole of the eyeball. When illustrating the forms of astigmatism, even the crystalline lens is omitted from the diagrams.
Myopia lecture By Sumayya Naseem
In the name of ALLAH, most gracious, most merciful
AIMS & OBJECTIVES OF TODAY’S LECTURE • Myopia and its etiology • Mechanisms of production • Clinical types • Signs & symptoms • Complications • Diagnosis • Correction • Prevention
HUMAN EYE:OPTICAL CONDITIONS Emmetropia Ametropia Index Axial Presbyopia Curvature Myopia Hyperopia Astigmatism Parallel incident light is Parallel incident light is We get 2 focal points focused in front of the retina focused behind the retina
• EMMETROPIA Parallel incident rays come to a focus on the retina when the accomodation is fully relaxed so far point is at infinity.
• AMMETROPIA Parallel incident rays do not come to a focus on the retina when the accomodation is fully relaxed so far point is not at infinity.
MYOPIA (short sightedness, hypometropia) A type of refractive error in which Parallel incident light is brought to a focus in front of the retina when eye is at rest. So far point is at finite distance.
AETIOLOGY • Hereditary (Genetic Factor) • Role of diet • Theory of excessive near work • Reading posture • Racial (Chinese and Japanese highest) • Environmental Factors
MECHANISMS OF PRODUCTION 1. Axial myopia • Increased length of eyeball • 1mm=3D 2. Curvature myopia • Increased curve of cornea e.g. conical cornea • Increased curve of lens e.g. lenticonus • 1mm=6D
3. Positional myopia• Anterior displacement of lens e.g. trauma4. Index myopia• Increase in ref index of lens e.g. nuclear sclerosis in diabetes• Increase in ref index of aqueous humor• Decrease in ref index of vitreous humor e.g. vitreous liquefication5. Myopia due to excessive accomodation• Patients with excessive accomodation
MYOPIA: REFRACTIVE vs AXIAL Refractive Myopia Axial Myopia (Eye too long) (Optics of the eye too strong)
CLINICAL VARIETIES OF MYOPIA 1. Congenital myopia 2. Simple or developmental myopia 3. Pathological or degenerative myopia 4. Acquired myopia
CLINICAL VARIETIES OF MYOPIA 1. Congenital myopia • Present since birth • Diagnosed at 2-3 years • Common in children born with Marfan’s syndrome • Mostly unilateral and anisometropic • Rarely bilateral • Usually 8-10 D and constant • May be associated with aniridia, megalocornea and congenital separation of retina
2. Simple myopia• Commonest• Not associated with any eye disease• Error usually does not exceed 6D• Usually begins at age of 7 to 10 years• Stabilizes around midteens
CLINICAL PICTURE Symptoms• Poor vision for distance
• Asthenopic symptoms (due to dissociation b/w accomodation and convergence)• Change in physiological outlook of children
SIGNS • Myopic eyes are large • Anterior chamber is deeper than normal • Pupil is dilated and sluggish • Fundus is normal • Does not exceed 6D • Normal near vision • Defective distant vision
3. PATHOLOGICAL MYOPIA • Rapidly progressing error • Associated with degenerative changes in posterior segment • Rapid axial growth of eyeball • Usually Hereditary in nature • Can exceed till 30 D • Ref error increases 4D yearly • Also known as degenerative myopia.
CLINICAL PICTURE Symptoms • Defective distant vision • Defective near vision if degeneration starts • Muscae volitantes (Flying Flies) • Night blindness
CHOROIDAL AND RETINAL CHANGES• Degeneration of choroid• Choroidal hemorrhage• White atrophic patches• Tilted disc• Posterior staphyloma• Myopic crescent (temporally)• Forster-Fuchs flecks (subretinal neovascularisation & pigmented lesion at or near the fovea)
4. ACQUIRED MYOPIASome causes are as follows:1. Index myopia (diabetic, nuclear sclerosis)2. Curvature myopia (conical cornea & corneal ectasias)3. Positional myopia (ant. Subluxation of lens)4. Consecutive myopia (surgical overcorrection)
5. Night myopia (as pupil dilates) 6. Drug induced myopia(pilocarpine, steroids)7. Pseudo myopia (excessive & spasm of accomodation)
REFRACTION PROCEDURE • VA with and without correction monocularly • Pinhole VA • Cover test with and without correction • Quick ophthalmoscopy • Retinoscopy Subjective verification: • Duochrome test • Muscle balance- Maddox rod for distance
DISTANCE VA CHART University of WaterlooBailey-Lovie Chart distance VA chart
RETINOSCOPIC FINDINGSDuring retinoscopy of amyopic patient, ‘against’movement of reflex is seen ascompared to retinoscopiclight or streak.Against movement isneutralized by negative orconcave lenses.
Reflex motion seen during retinoscopy “ w ith ” “ n e u tr a lity ” “ a g a in s t”
OPTICAL CORRECTION OF MYOPIA Options are as follows: • Spectacles and LVD’s • Contact lenses • Refractive surgeries • Laser Others: • Visual hygiene • Prophylaxis • General measures
SPECTACLES • Minus lenses (CONCAVE) are used to correct myopia • In high numbers glasses are not cosmetically attractive and minify actual pt eye size for others • Myopes are usually kept under corrected so that there accomodation is not stimulated. Otherwise, they will complain of Asthenopic symptoms
ORTHOKERATOLOGY• Orthokeratology is defined as, the reduction, modification, or elimination of visual defect by the programmed application of contact lenses
• Orthokeratology uses hard Contact Lenses to remould the Cornea, to reduce or correct Myopic (short-sighted) and Astigmatic (irregular surface) errors of the eye.• In some ways this is similar to the use of dental braces by an Orthodontist to straighten crooked teeth.• The main difference is that if a tooth position is corrected for some months it will stay in the new position. However the Cornea is highly elastic, and always returns to its original shape.• For this reason the lenses are worn nightly or on alternate nights after the ideal Corneal shape has been achieved and removed in the morning giving perfect vision without the need for spectacles or contact lenses.
• Once the desired level of V.A has been achieved a of retainer lens wear is initiated until cornea reaches the level of stability new shape cornea. Lens wear is then gradually reduced to the minimum, required to attain good functional vision through out the day.• The amount of ametropia that can be corrected using orthokeratology is: –1 to –6 D myopia with 1.5 D of WTR astigmatism and 0.75 D of ATR astigmatism.
SURGERYClear lens extraction:• For myopia of greater than 15-20 D, cataract surgical procedure is applied and non- cataractous lens is removed and intra ocular lens of calculated power is inserted.Phakic IOL:• IOL is also placed in A/C or P/C of phakic eyes to correct the refractive error.
Photorefractive keratectomy PRK:• Uses Excimer laser to change ant. curvature of cornea.• Tissue is ablated centrally 3.5-4mm and surface curvature is reduced.• After scarring, haloes, glare and reduction of best VA are the complaints of patient.
LASER ASSISTED IN SITU KERATOMILEUSIS LASIK• A mechanical keratotome is used to dissect through the superficial corneal stroma and fashion a lamellar circular flap of uniform thickness.• The bared corneal stroma is reshaped using Excimer laser and hinged flap is replaced.• Better than PRK because of little scarring and better correction predictability.
EPIKERATOPHAKIA • This uncommon surgical technique creates a new corneal surface with a different surface curvature by attaching a lenticule of pre- shaped donor corneal stroma to the surface of host cornea. • The eye is not entered and procedure is easily reversed by removal of lenticule.
KERATOMILEUSIS • It is the use of microkeratotome to remove lamella of ant. corneal stroma which is then reshaped on a cryolathe before being replaced. • High degrees of myopia till 15D can be corrected in this way. • Keratophakia is developed as a modification of keratomileusis and is used for aphakia.
PREVENTION & CONTROL OF MYOPIA• Many people believe that too much close work, such as reading or sitting too close to the television, causes nearsightedness. But there was little evidence to support this belief. However, one study suggested that people in professions that involve extensive reading have higher degrees of nearsightedness.• With regular instillation, topical 0.05% atropine is an effective agent for controlling myopia progression in a majority of school aged children.
• Nutritional Factors• Since the eye has a collagenous structure, it seems likely that the same nutrients which strengthen collagen might also be helpful in keeping the eye from becoming elongated. Calcium, magnesium, boron, silica, selenium, manganese and vitamin D all come to mind, as well as vitamin C. A strong ocular structure would likely be less prone to becoming elongated, as occurs in myopia. Low levels of calcium, fluoride and selenium were found to be related to increased risk of progressive myopia in an exploratory study.• Vitamin E, can slow the progression of myopia in children. Myopia in children was also significantly related to lower consumption of protein, fat, vitamins B1, B2 and C, phosphorus, iron, and cholesterol.
VISUAL HYGEINE• We should insist that our children use good lighting and good posture when reading, take frequent eye rest breaks during long study periods, and encourage them to be physically active.• Environmental visual stress may be lessened by taking these precautions while reading: frequently stretching and moving the eyes and looking away from the reading material at distant objects, removing distance eyewear(-) or using reading glasses for near tasks.• As it is usually hereditary in nature, so family marriages should be avoided.
REFERENCES • Theory and practice of optics and refraction by A K Khurana • Duke Elders Practice of refraction (Tenth edition) • Clinical Optics by Elkington, Frank and Greaney (Third edition) • www.visionlaser.com • www.orthokeratology.com and many other websites.