Transcript ASTIGMATISM
Speaker Abdullah Al Otaibi,MD Assistant Professor Refraction: The deviation of a ray of light passing from one medium into another is called refraction. co rn ea aq ue ou s iri s lens vitreo us retina l vessel s retina macul a choroi d sclera "Refraction" refers to the way the eye focuses light, the source of everything we see. There are three basic elements that determine the eye's ability to focus light How the eye works The eye is a very intricate optical system similar to a camera. In a normal eye, light rays pass through the cornea, pupil and lens, focusing sharply and precisely on the retina. Visual Acuity The visual acuity test measures the smallest letters that you can read on a standardized chart at a distance of 20 feet. VISUAL ACUITY Snellen chart : 20/ 20 20/ 40 20/100 V.A which improve with pinhole refractive error Convex lens : Parallel rays → into a focus. Concave lens: Diverge rays of light. Anterior corneal surface produces most of the eye`s refracting power ≈ 40 Diopter. Aqueous, lens + vitreous provide the remaining ≈ 20 Diopter. In emmetropia the principal focus is on the plane of the retina. In myopia the principal focus is anterior to the retina. High axial length. Strong refractive power. Clinical features patients usually have problem in seeing at distance. The vision at near is normal. 4 myopic will see clear at ¼. 25 C.M 6 myope? V.A. improves with pinhole. Myopes squeeze their eyes to see clearly. In hypermetropia the principal focus is behind the retina, and is therefore virtual. Causes of hypermetropia: Decreased Axial length Weak refractive power . After cataract surgery , lens is removed and we have to substitute it with : Intra ocular lens. Contact lens. Glasses. ASTIGMATISM Regular: The directions of greatest and least curvature are always 90 degree apart. Corrected by a cylindrical spectacle lens. Irregular:The principal meridians are not perpendicular to one another. Example: Corneal scar due to trauma or keratoconus. Corrected by hard lens. THE DEVELOPMENTAL EVOLUTION OF THE REF. STATE Low grade hyperopia is the usual ref. State in infancy and childhood. As the child grows;changes in axial length,cornea,and lens compensate for each other toward emmetropia. ANISOMETROPIA Difference in the refractive errors of the two eyes. Uncorrected anisometropia in children may lead to amblyopia. Unilateral aphakia. Anisometropia : achild 3-y-old, OD: +3 OS:+1 Amblyopia OD. ACCOMMODATION Flxible mechanism by which the eye can change its ref power. Contracture of ciliary muscle relaxation of the zonules more biconvex state of the lens. Presbyopia y to focus the eye when viewing objects at close When an attempt is made to look at a near object, the lens ye changes shape (accommodates). This shape change a result of impulses sent to muscles inside of the eye. As closer during viewing, impulses are also sent to the external the eye to converge or turn the eyes inward in order to avoid ouble image. This convergence system allows us to keep n of objects when viewing from distance to near. These two Accomodation and Convergence work together and provide ne another to maintain one clear image when viewing objects The lens changes shape du accommodation. AGE 8 12 16 20 24 32 40 44 52 56 64 AVERAGE ACCOMODATION AMPLITUDE 14 13 12 11 10 8 6 4 2.5 2 1 Example A 20 year old c / o decrease vision for distance. V.A: 20 / 80 ; PH 20 / 30 Ref. Error. Auto.ref. -3 Auto.ref. –3 –2 x 180. Astigmatism -3 -2 -3 Pt who had difficulty in reading. V/A 20 / 100 PH 20/40. Auto.ref: +4. Convex glasses. +4 • 15 y old boy ;VA 20/100 PH 20/40 ;at 33cm 20/20 • MYOPIA • REF.ERR -3 A 50 –y- old man started to have difficulty in reading. VA 20 /20 PRESBYOPIA. CONTACT LENSES SOFT: spherical ref error . HARD:keratoconus. TORIC:spherocylidrical ref error. RISK OF CONTACT LENSES. Refractive surgery alters the curvature of the cornea to allow light rays to come to focus closer to the retina, thus improving uncorrected vision. In myopia, the central corneal curvature is flattened; in hyperopia, the central corneal curvature is made steeper; and in astigmatism, the cornea is made more spherical. RK. PRK. Lasik. up to eight incisions are placed in a spokelike pattern in the peripheral cornea using a guarded diamond blade Photorefractive keratectomy (PRK) is used for persons with low to moderate myopia. The excimer laser is used to flatten the central corneal tissue through photoablation. The excimer laser uses an argon fluoride gas mixture to create ultraviolet energy that is able to break intermolecular bonds with submicron precision. Each pulse of the laser removes 0.25 µ of corneal tissue. The corneal epithelium is removed before photoablation and generally takes 3 days to regenerate. During the procedure, laser delivery to the cornea usually lasts < 1 min. PRK can safely treat higher degrees of myopia than can radial keratotomy, with > 90% of people seeing 20/40 or better without glasses. In laser in situ keratomileusis (LASIK), a flap of corneal tissue is created with a microkeratome, is turned back, and then the stromal bed is sculpted with the excimer laser. In the great majority of cases, the flap adheres tightly to the stromal bed without the need for sutures. Because the surface epithelium is not disrupted centrally, visual recovery is rapid. Most people notice a significant improvement in their vision 1 day postoperatively. LASIK can be used to treat myopia, astigmatism, and hyperopia.