Cornea Refraction PDF

Precorneal Tear Film The exposed surfaces of the cornea and globe are covered by the precorneal tear film, which is composed conceptually of 3 layers: 1. a superficial oily layer produced predominantly by the meibomian glands 2. a middle aqueous layer produced by the main and accessory lacrimal gla...

Precorneal Tear Film The exposed surfaces of the cornea and globe are covered by the precorneal tear film, which is composed conceptually of 3 layers: 1. a superficial oily layer produced predominantly by the meibomian glands 2. a middle aqueous layer produced by the main and accessory lacrimal glands 3. a deep mucin layer derived from the conjunctival goblet cells Maintenance of the precorneal tear film is vital for normal corneal function. In addition to lubricating the surface of the cornea and conjunctiva, tears produce a smooth optical surface; allow for the diffusion of oxygen and other nutrients; CORNEA : The cornea is a transparent, avascular tissue that measures 11-12 mm horizontally and 10-11 mm vertically and occupies the center of the anterior pole of the globe.The cornea and the aqueous humor together form a positive lens of obout 43 D IN AIR. The cornea contributes 74% or 43.25 diopters of the total diopteric power of a normal human eye and is also the major source of astigmatism in the optical system , the sensitivity of the cornea is 100 times that of the conjunctiva , sensory nerve fibers extend from the long ciliary nerves and form a subepithelial plexus ,approximately 70 to 80 branches of the long ciliary nerves enter the cornea and lose their myelin sheath 1-2 mm from the limbus , for it`s nutrition , the cornea depends on glucose diffusing from the aqueous humor and oxygen from the tear film ,lid vasculature and aqueous humor and also the peripheral cornea is supplied by the limbal circulation.The cornea composed of six layers : Epithelium ;The corneal epithelium is composed of stratified squamous epithelial cells and makes up about 5%-10% [ 0.05 mm -Bowman`s layer :Beneath the basal lamina is bowman`s membrane.It is secreted during embryogenesis by the anterior stromal keratocytes and epithelium. Is an acellular layer that consist of collagen fibrils and is 8-14 micro m thick , it is not replaced and may become opacified by scar tissue following injury.It is acellular , and it dose not regenerate when damaged. -Dua’s layer. It is 15 micrometers in thickness, fourth caudal layer, and located between the corneal Stroma and Descemet's membrane. Anatomically, it is like Stroma -Stroma : The stroma constitutes about 90% of the total corneal thickness in humans , it is composed of keratocytes , ground substance [ mucoprotein and glycoprotein ] , and collagen lamella -Descemet`s membrane :Is basement membrane of the corneal endothelium ,it is secreted by endothelium , it increases in thickness from 3 micro m at birth to 10-12 micro m in adult. -Endothelium :Is a single layer of mostly hexagonal cells , the overall number of endothelial cells decreases with age , and human endothelial cells do not proliferate in vivo ,in young adult the normal cell count is approximately 3000cells/mm. corneas with low cell density [eg ,fewer than 1000cells/mm might not tolerate intraocular surgery ,donor corneas for transplantation should have at least 2000cells/mm.The cornea is transparent because –It is avascular ,-The size of the lattice elements is smaller than the wavelength of visible light ,-The water content of the corneal stroma is 78% ,corneal hydration is largely controlled by intact epithelial and endothelial barriers and the functioning of the endothelial pump.he corneal endothelium maintain corneal clarity through 2 function : by acting as a barrier to the aqueous humor and by providing a metabolic pump. oriised The cornea is transparent for the following reasons: 1- The epithelium is not keratinized. 2- The stroma is regularly oriented. (Maurice theory) 3- The endothelium has active pump, it pushes the fluid into aqueous and it acts as barrier to prevent entrance of aqueous inside the cornea. 4- The corneal nerves are unmylinated. 5- It contains NO blood vessels. 6- It contains NO pigments (as melanin).  INTRODUCTION : The visible part of the light lies in the waveband of 390 nm to 760 nm.For the eye to generate accurate visual information light must be correctly focused on the retina and this focus must be adjustable to allow equally clear vision of near and distant objects.The cornea , or actually the air/tear interface is responsible for 2/3 and the crystalline lens for 1/3 of the focusing power of the eye.These two refracting elements in the eye converge the rays of lights because :-The cornea has a higher refractive index than air ; the lens has a higher refractive index than the aqueous and vitreous -They are spherically convex. - Axial diameter of the eyeball: is the distance from tip of cornea to the center of macula, which is normally 23-25 mm (average 24 mm). - The most important two refractive surfaces of the eyeball are cornea and lens. - The power of cornea and the lens depends on the curvature of their surfaces, mostly the anterior surface of each one. - the total refractive power (converging power) of normal eye (emmetropic eye) is 60 -63diopters (Diopters) divided between the cornea and the lens. Definitely, the cornea sharing in highest portion (43D) of this refracting power while the lens having power of residual 15-20 (in non accommodative state) D only. So, the cornea is more important than the lens regarding focusing of light on the retina for the following 2 reasons: 1- the anterior surface of the cornea is steeper than that of the lens. 2- the light is transfer to the cornea passing through the air which having the least refractive index (1.00) or (least density of any other material) while the light transfer to lens passing through the aqueous humor (water) which having refractive index higher than that of the air (1.33). The refractive index for the cornea is 1.37 while the lens is 1.38. REFRACTIVE STATES OF THE EYE : An object at optical infinity can be imaged through the eye to a point focus on the retina , in front of or behind the retina.Object focusing anterior or posterior to the retina form a blurred retinal image.[ the far point is the point in space that is conjugate to the foveola of the nonaccommodating eye ] Emmetropia [ with accommodation relaxed ] is the refractive state in which parallel rays of light from a distant object are brought to focus on the retina. Ametropia refers to the absence of emmetropia , ametropia may be axial or refractive.In axial ametropia the eyeball is unusually long in myopia or short in hyperopia , in refractive ametropia the length of the eye is statistically normal but the refractive power of the eye [cornea and/or lens] is abnormal : excessive in myopia or inadequate in hyperopia , aphakia is an example of extreme refractive hyperopia ,unless the eye was highly myopic [more than 20 D] before lens removal.An ametropic eye require either a diverging or a converging lens to focus a distant object on the retina. Ammetropia: is either → Hypermetropia Or → Myopia Or → Astigmatism Hypermetropia (hyperopia), far-sightedness: [ Is a type of refractive errors in which parallel rays of light are brought to a focus some distance behind the retina when the eye is at rest. Etiological classification: 1- Axial Hypermetropia: shorter antero-posterior axial length, i.e. the eye has normal converging power (60-63D) but its axial length is less than 24 mm. 2- Curvature Hypermetropia: due to decreased curvature (flattening) of the cornea congenitally or as a result of trauma or disease e.g. corneal ulcer, microbial keratitis. 3- Index Hypermetropia: decrease in effective refractivity of the lens. * the power of the lens depends on the difference between refractive indices of the nucleus and cortex, so the more the difference the more converging power and vise versa. In index Hypermetropia, there is decrement of the difference MYOPIA : If the power of an eye is too strong for its size ,we say that the eye is myopic or nearsighted.[myopia may be axial or refractive ] In myopic eye the image of distant object focus in front of the retina [ light rays from an object at infinity converge too soon and thus focus in front of the retina ] and the myopic eye possesses too much optical power for its axial length.For correction of myopia we should place a diverging lens in front of the eye. HYPEROPIA : In hyperopic eye the image of distant object focus behind the retina , and the hyperopic eye does not possess enough optical power for its axial length , for correction of hyperopia we should place a converging lens in front the eye. Astigmatism [A=without ,stigmos=point] is an optical condition of the eye in which light rays from an object do not focus to a single point. Myopia or short-sightedness:[ That form of a refractive error where parallel rays of light come to a focus in front of the retina when the eye is at rest. - As there is increase in the refractive power of the eye, the near objects (closer than 6m) will be seen normally, while far objects (whom rays come parallel) will be focused in front of retina. Etiological classification: 1- Axial myopia: anteroposterior length is longer than normal. 2- Curvature myopia: increased curvature of cornea, or one or both surfaces of lens. 3- Index myopia: increased refractivity of lens, e.g. nuclear sclerosis (stage before nuclear cataract), due increase the difference between refractive indices of nucleus and cortex. Clinical classification: 1- Simple (stationary): 6D, start before the age of 4y, progress even after age of 20y, real pathological process and VA cannot be corrected to 6/6. ASTIGMATISM : In astigmatism ,the optical power of the cornea in different planes is not equal, variations in the curvature of the cornea or lens at different meridians prevent the light rays from focusing to a single point.In astigmatism , light rays do not focus to a single point.Whereas simple myopic and hyperopic eyes are stigmatic optical systems that focus light rays from infinity to a single point.In an astigmatic eye there is no single focal point , but rather a set of 2 focal line.Astigmatism may be regular or irregular , regular astigmatism is with-the-rule [the more common type in children ] the vertical meridian is steepest or against-the-rule [ the more common type in older adult ] the horizontal meridian is steepest ,the oblique astigmatism is a form of regular astigmatism in which the principal meridians do not lie at , or close to , 90º and 180 º. Regular astigmatism corrected with plus or minus cylinder but in irregular astigmatism like keratoconus or traumatic corneal scar rigid contact lens may be useful. Anisometropia refers to a difference in the refractive status of the two eyes. Aniseikonia is a difference in the size or shape of the images formed in the two eyes. In unilateral aphakia spectacle correction produces an intolerable aniseikonia of about 25% ; contact lens correction produces aniseikonia of about 7% which is usually tolerated. A combination of corneal power , lens power , AC depth , and axial length determines an individual`s refractive status but corneal power and lenticular power change markedly during the first 2 years of life , as does axial length.Most newborns are born hyperopic or emmetropic and only 25% are myopic. Prevalence of refractive errors : On average , babies are born with about 3.0 D of hyperopia , in the first few months this hyperopia may increase slightly , but it declines to an average of about 1.0 D of hyperopia by age 1 , due to marked changes in corneal and lenticular power , as well as axial growth. A bout 52% of general population aged 3 years and older have refractive error and need to wear eyeglasses or contact lens.The prevalence of myopia increases steadily with increasing age it estimated 3% among those 5 -7 years old , 8% among those 8-10 years old , 14% among those 11-12 years old , and 25% among those 12-17 years old. In Chinese children and Taiwan children the prevalence may reach to 84% among those 16-18 years old.From birth to age 6 years the axial length of the eye grow by approximately 5 mm and the corneal power decrease by 4 D and the lens power decrease by 2 D and this changes keeps most eyes close to emmetropia. MYOPIA : Different subtype of myopia have been identified , physiologic myopia often called simple myopia or school myopia , is associated with normal growth of each of the refractive components of the eye , the combination of which results in mild to moderate nearsightedness.Pathologic myopia , also referred to as malignant , progressive or degenerative myopia , is caused by excessive growth in axial length while other components of the eye exhibit normal growth.No significant differences between men and women have been noted in persons with low or moderate myopia who are 20 years old or older , although may be slightly greater in males.But pathologic myopia is about 2.5 times more common in females.The prevalence of myopia begins to increase at about the age of 6 years.The prevalence of myopia begins to increase at about the age of 6 years ACCOMMODATION : As an object is brought nearer to the eye the power of the lens increase this is called accommodation.Accommodation is the mechanism by which the eye changes refractive power by altering the shape of its crystalline lens.The ability to accommodation decrease with age , reaching a critical point at about 40 when the subject experience difficulty with near vision that called presbyopia.This occurs earlier in hypermetropes than myopes.This problem is overcome with convex reading lenses. - Accommodation: contraction of Ciliary muscle in order to increase curvature of lens (and so increase its refractive power more than 17D) to visualize objects closer than 6 meters (near objects). - In order to see near objects, there will be contraction of Ciliary muscles which lead to decrease the tone of Zonule and their will be increase in the curvature of lens and increasing in the refractive power of lens (>17D). - Amplitude of accommodation: is the difference in the converging power of the eye between maximum accommodation and un accommodated eye (rest) , which depends on contraction power of Ciliary muscles and elasticity of lens capsule, and both of them decrease with advancing in age. The Amplitude of accommodation is decrease with advancing age as the following: * Early in life: it is 14D, so the child can focus an object located 7 cm away from the eyes, i.e. the range of lens refractive power can be increase from 17 D normally up to 31D. * At age of 36y: due to atrophy of muscles and loss of lens elasticity (sclerosis), the amplitude of accommodation will be decrease to 6 D only, so the nearest object to the person which can be focused is at a distance of about 15cm away from eye. * At age of 45y: the amplitude of accommodation is 4D only, and the nearest focus point is 25cm away from eye. * At age of 60y: amplitude of accommodation is 1D only, and the nearest focus point is 1meter. - Presbyopia: is a recession of near point with age (from 7 cm early in life to 1 meter at 60 year) due to decreased amplitude of accommodation (from 14 D early in life to 1 D at age of 60 year).  MANAGEMENT : -Eyeglasses ; is the safest method for correction of refractive errors. -Contact lens : a-Hard contact lens , for correction of high astigmatism , keratoconus , irregular astigmatism , high myopia and hyperopia. b-Soft contact lens , for correction of low grade astigmatism , myopia and hyperopia [ high or low degree ]. -Keratorefractive surgery : a-RK [radial keratotomy ] for correction of myopia and astigmatism [ 1-4 D] b-PRK [ photorefractive keratectomy ] for mild to moderate myopia [ less than 7 D ] and up to 6 D of hyperopia. c-LASIK [ laser in situ keratomileusis ] for correction of up to 15 D of myopia and up to 6 D of hyperopia d-ICR [intracorneal ring ] for correction up to 3 D of myopia e-Phakic IOL F-intra corneal lens CONTRAINDICATION TO LASIK : -Keratoconus -Ophthalmic HZ -Active inflammation of the ant or the post segment -Thin cornea -Marked pupil decentration -Large diameter pupil with high ametropia -Pregnancy Keratoconus It is a progressive disorder in which the cornea assumes an irregular conical shape. The onset is at around puberty with slow progression thereafter and become stationary at any time (age). In keratoconus, there is progressive and irregular changes in the cornea making it more convex and make it have a more conical shape, also there is severe thinning of the cornea that the cornea may reach 1/3 its original thickness (about 0.16mm). These deformities will affect the visual acuity (as the cornea is the most important focusing power) leading to myopia and irregular astigmatism. Presentation: 1- Unilateral impairment of vision due to progressive myopia and regular astigmatism, which subsequently becomes irregular. 2- Frequent changes in spectacle prescription or decrease tolerance to contact lens (due changes in the shape of cornea). 3- The fellow eye usually has normal vision with negligible astigmatism at presentation because of the asymmetrical nature of the condition. Most [not all] of the cases are bilateral. Signs of keratoconus: The hallmarks are: 1- Central or paracentral stromal thinning. 2- Apical protrusion. 3- Irregular astigmatism. 1- Direct ophthalmoscope from a distance of one foot shows an oil droplet reflex. 2- Retinoscpoy (used for diagnosis of refraction errors) shows an irregular reflex. 3- Slit-lamp shows very fine, vertical, deep stromal striae "Vogt's lines" due to protrusion of cornea. 4- Later, there is progressive corneal thinning as little as one third of normal thickness, associated with poor visual acuity (irregular myopic astigmatism). 5- Bulging of the lower lid in down gaze "Munson sign". 6- Acute hydrops: It is an acute influx of aqueous into the cornea as a result of a rupture in Descemet's membrane and Dua’s layer àsudden decrease in visual acuity associated with discomfort and watering (lacrimation). The break usually heals within 6-10 weeks and the corneal oedema clears, but a variable amount of stromal scarring may develop. Use corneal topography Lasek or PRK - YouTube.mp4 Management: 1- Spectacles: In early cases to correct regular and mild irregular astigmatism. 2- Rigid contact lenses: For higher degree of irregular astigmatism, they only reshape the cornea, there refractive power is zero. 3- Keratoplasty: In advanced progressive disease, especially with significant corneal scarring. 4- intracorneal ring segment (Intacs) implantation: which is done by using laser or mechanical channel creation. These rings which are inserted inside these corneal channels can change the abnormality in the curvature of cornea and causing moderate visual improvement. 5- corneal collagen cross-linking: is done by using riboflavin (vitamin B2) drops to photosensitizer the eye followed by exposure to ultraviolet-A light. This a newer treatment which offers promise of stabilization or reversal of ectasia in at least some patients. It can combined with Intacts insertion. Note: lasik is contraindicated in correction of myopia and astigmatism induced by keratoconus because it lead to more thinning and ectasia of the cornea and sever deterioration of vision later on. Keratoplasty (Corneal transplantation, Grafting) It is an operation in which abnormal corneal host tissue is replaces by healthy donor cornea, it is either Full-thickness (Penetrating Keratoplasty) or Partial thickness (Lamellar Keratoplasty and endothelial Keratoplasty)