Transcript Astigmatism
Astigmatism Walter Huang, OD Yuanpei University Department of Optometry Definition When parallel rays of light enter the eye (with accommodation relaxed) and do not come to a single point focus on or near the retina Optics Power in the horizontal plane projects a vertical focal line image Power in the vertical plane projects a horizontal focal line image Optics Refraction of light taking place at a toric surface: the conoid of Sturm Etiology Cornea The cornea has an unequal curvature on its anterior surface Etiology Lens The crystalline lens has an unequal curvature on its surface or in its layers Etiology It is due to a distortion of the cornea and/or lens The refracting power is not uniform in all meridians The principal meridians are the meridians of greatest and least refracting powers The amount of astigmatism is equal to the difference in refracting power of the two principal meridians Classification Based on etiology Based on relation between principal meridians Based on orientation of meridian or axis Based on focal points relative to the retina Based on relative locations of principal meridians or axes when comparing the two eyes Corneal Astigmatism When the cornea has unequal curvature on the anterior surface Lenticular Astigmatism When the crystalline lens has an unequal on the surface or in its layers Total Astigmatism The sum of corneal astigmatism and lenticular astigmatism Regular Astigmatism When the two principal meridians are perpendicular to each other Most cases of astigmatism are regular astigmatism The three types are with-the-rule, againstthe-rule, and oblique astigmatism Irregular Astigmatism When the two principal meridians are not perpendicular to each other Curvature of any one meridian is not uniform Associated with trauma, disease, or degeneration VA is often not correctable to 20/20 With-The-Rule (WTR) Astigmatism When the greatest refractive power is within 030 of the vertical meridian (i.e., between 060 and 120 meridians) Minus cylinder axis around horizontal meridian The most common type of astigmatism based on the orientation of meridians With-The-Rule (WTR) Astigmatism Against-The-Rule (ATR) Astigmatism When the greatest refractive power is within 030 of the horizontal meridian (i.e., between 030 and 150 meridians) Minus cylinder axis around vertical meridian Against-The-Rule (ATR) Astigmatism Oblique (OBL) Astigmatism When the greatest refractive power is within 030 of the oblique meridians (i.e., between 030 and 060 or 120 and 150) Oblique (OBL) Astigmatism Simple Astigmatism When one of the principal meridians is focused on the retina and the other is not focused on the retina (with accommodation relaxed) Simple Myopic Astigmatism When one of the principal meridians is focused in front of the retina and the other is focused on the retina (with accommodation relaxed) Simple Hyperopic Astigmatism When one of the principal meridians is focused behind the retina and the other is focused on the retina (with accommodation relaxed) What Patient Sees One meridian is out of focus Compound Astigmatism When both principal meridians are focused either in front or behind the retina (with accommodation relaxed) Compound Myopic Astigmatism When both principal meridians are focused in front of the retina (with accommodation relaxed) Compound Hyperopic Astigmatism When both principal meridians are focused behind the retina (with accommodation relaxed) What Patient Sees Both meridians are out of focus Mixed Astigmatism When one of the principal meridians is focused in front of the retina and the other is focused behind the retina (with accommodation relaxed) Symmetrical Astigmatism The principal meridians or axes of the two eyes are symmetrical (e.g., both eyes are WTR or ATR) The sum of the two axes of the two eyes equals approximately 180 Symmetrical Astigmatism Example OD: OS: pl -1.00 x 175 pl -1.00 x 005 Both eyes are WTR astigmatism, and the sum of the two axes equal approximately 180 Asymmetrical Astigmatism The principal meridians or axes of the two eyes are not symmetrical (e.g., one eye is WTR while the other eye is ATR) The sum of the two axes of the two eyes does not equal approximately 180 Asymmetrical Astigmatism Example: OD: OS: pl -1.00 x 180 pl -1.00 x 090 One eye is WTR astigmatism, and the other eye is ATR astigmatism, and the sum of the two axes do not equal approximately 180 Prevalence Age Infants are born with ATR astigmatism, where the cornea is the source of the astigmatism Preschool children have little or no astigmatism Teenage children demonstrate a shift towards WTR astigmatism Older adults show a shift towards ATR astigmatism Prevalence Gender In general, there are no significant differences between males and females Prevalence Ethnicity Higher prevalence in North Americans, Latinos Asian infants tend to be WTR astigmatism Caucasian infants tend to be ATR astigmatism Incidence General trend For older adults, the average rate of change towards ATR astigmatism is less than or equal to 0.25D every 10 years Visual Acuity Theoretically, at NO distance does an uncorrected astigmat have a sharp retinal image Clinically, if astigmatism is small (less than 0.50DC), the patient may not notice blur Visual Acuity Simple or compound myopic astigmatism Accommodation may make the retinal image even more blurry Simple or compound hyperopic astigmatism Accommodation may improve VA to some extent Mixed astigmatism VA is relatively good May not need much accommodation Spherical and Astigmatic Ametropia Uncorrected VA 20/30 Spherical Astigmatism (D) Refractive Error (D)* 0.50 1.00 20/40 0.75 1.50 20/60 1.00 2.00 20/80 1.50 3.00 20/120 2.00 4.00 20/200 2.50 >4.00 Spherical and Astigmatic Ametropia Spherical refractive error (D)* Myopia or absolute hyperopia When multiplied by a factor of two, it equals astigmatism (D) Symptoms Distorted vision at distance and near Letter confusion Asthenopia or ocular fatigue Due to constantly squinting to clear up distorted vision Headaches Squinting Signs Decreased visual acuities at distance and near Clinical Tests Visual acuity tests – distance and near Autorefraction Keratometry Retinoscopy Most reliable source of information for cylinder power and axis Monocular subjective refraction, including Jackson cross cylinder Management Cylindrical lenses and spherocylindrical lenses in spectacles and contact lenses for simple astigmatism and compound astigmatism, respectively Refractive surgery Management Spectacles Single vision glasses with cylinder Management Contact lenses Toric soft contact lenses Toric rigid gas permeable contact lenses Management Refractive surgery Photorefractive keratectomy (PRK) Laser in-situ keratomileusis (LASIK)