AstigmatismAstigmatism

Walter Huang, ODWalter Huang, OD

Yuanpei UniversityYuanpei University

Department of OptometryDepartment of Optometry

DefinitionDefinition

When parallel rays of light enter the eye When parallel rays of light enter the eye ((with accommodation relaxedwith accommodation relaxed) and do ) and do notnot come to a single point focus on or near the come to a single point focus on or near the retinaretina

OpticsOptics

Power in the Power in the horizontalhorizontal plane projects a plane projects a verticalvertical focal line image focal line image

Power in the Power in the verticalvertical plane projects a plane projects a horizontalhorizontal focal line image focal line image

OpticsOptics

Refraction of light taking place at a toric Refraction of light taking place at a toric surface: the conoid of Sturmsurface: the conoid of Sturm

EtiologyEtiology

CorneaCornea The cornea has an unequal curvature on its The cornea has an unequal curvature on its

anterior surfaceanterior surface

EtiologyEtiology

LensLens The crystalline lens has an unequal curvature The crystalline lens has an unequal curvature

on its surface or in its layers on its surface or in its layers

EtiologyEtiology

It is due to a distortion of the cornea It is due to a distortion of the cornea and/or lensand/or lensThe refracting power is not uniform in all The refracting power is not uniform in all meridiansmeridiansThe principal meridians are the meridians The principal meridians are the meridians of greatest and least refracting powersof greatest and least refracting powersThe The amount of astigmatism amount of astigmatism is equal to the is equal to the difference in refracting power of the two difference in refracting power of the two principal meridiansprincipal meridians

ClassificationClassification

Based on etiologyBased on etiology

Based on relation between principal Based on relation between principal meridiansmeridians

Based on orientation of meridian or axisBased on orientation of meridian or axis

Based on focal points relative to the retinaBased on focal points relative to the retina

Based on relative locations of principal Based on relative locations of principal meridians or axes when comparing the meridians or axes when comparing the two eyes two eyes

Corneal AstigmatismCorneal Astigmatism

When the cornea has unequal curvature When the cornea has unequal curvature on the anterior surfaceon the anterior surface

Lenticular AstigmatismLenticular Astigmatism

When the crystalline lens has an unequal When the crystalline lens has an unequal on the surface or in its layerson the surface or in its layers

Total AstigmatismTotal Astigmatism

The The sumsum of corneal astigmatism and of corneal astigmatism and lenticular astigmatismlenticular astigmatism

Regular AstigmatismRegular Astigmatism

When the two principal meridians are When the two principal meridians are perpendicularperpendicular to each other to each other

Most cases of astigmatism are regular Most cases of astigmatism are regular astigmatismastigmatism

The three types are with-the-rule, against-The three types are with-the-rule, against-the-rule, and oblique astigmatismthe-rule, and oblique astigmatism

Irregular AstigmatismIrregular Astigmatism

When the two principal meridians are When the two principal meridians are not not perpendicularperpendicular to each other to each other

Curvature of any one meridian is not Curvature of any one meridian is not uniformuniform

Associated with trauma, disease, or Associated with trauma, disease, or degenerationdegeneration

VA is often not correctable to 20/20VA is often not correctable to 20/20

With-The-Rule (WTR) AstigmatismWith-The-Rule (WTR) Astigmatism

When the When the greatest refractive powergreatest refractive power is is within 030 of the vertical meridianwithin 030 of the vertical meridian (i.e., (i.e., between 060 and 120 meridians)between 060 and 120 meridians)

Minus cylinder axisMinus cylinder axis around horizontal around horizontal meridianmeridian

The The most commonmost common type of astigmatism type of astigmatism based on the orientation of meridiansbased on the orientation of meridians

With-The-Rule (WTR) With-The-Rule (WTR) AstigmatismAstigmatism

Against-The-Rule (ATR) Against-The-Rule (ATR) AstigmatismAstigmatism

When the When the greatest refractive powergreatest refractive power is is within 030 of the horizontal meridianwithin 030 of the horizontal meridian (i.e., (i.e., between 030 and 150 meridians)between 030 and 150 meridians)

Minus cylinder axisMinus cylinder axis around vertical around vertical meridianmeridian

Against-The-Rule (ATR) Against-The-Rule (ATR) AstigmatismAstigmatism

Oblique (OBL) AstigmatismOblique (OBL) Astigmatism

When the When the greatest refractive powergreatest refractive power is is within 030 of the oblique meridianswithin 030 of the oblique meridians (i.e., (i.e., between 030 and 060 or 120 and 150)between 030 and 060 or 120 and 150)

Oblique (OBL) AstigmatismOblique (OBL) Astigmatism

Simple AstigmatismSimple Astigmatism

When one of the principal meridians is When one of the principal meridians is focused focused on the retinaon the retina and the other is and the other is notnot focused focused on the retinaon the retina (with (with accommodation relaxed)accommodation relaxed)

Simple Myopic AstigmatismSimple Myopic Astigmatism

When one of the principal meridians is When one of the principal meridians is focused focused in front of the retinain front of the retina and the other and the other is focused is focused on the retinaon the retina (with (with accommodation relaxed)accommodation relaxed)

Simple Hyperopic AstigmatismSimple Hyperopic Astigmatism

When one of the principal meridians is When one of the principal meridians is focused focused behind the retinabehind the retina and the other is and the other is focused focused on the retinaon the retina (with (with accommodation relaxed)accommodation relaxed)

What Patient SeesWhat Patient Sees

One meridian is out of focusOne meridian is out of focus

Compound AstigmatismCompound Astigmatism

When When bothboth principal meridians are focused principal meridians are focused either in front or behind the retinaeither in front or behind the retina (with (with accommodation relaxed)accommodation relaxed)

Compound Myopic AstigmatismCompound Myopic Astigmatism

When When bothboth principal meridians are focused principal meridians are focused in front of the retinain front of the retina (with accommodation (with accommodation relaxed)relaxed)

Compound Hyperopic AstigmatismCompound Hyperopic Astigmatism

When When bothboth principal meridians are focused principal meridians are focused behind the retinabehind the retina (with accommodation (with accommodation relaxed)relaxed)

What Patient SeesWhat Patient Sees

Both meridians are out of focusBoth meridians are out of focus

Mixed AstigmatismMixed Astigmatism

When one of the principal meridians is When one of the principal meridians is focused focused in front of the retinain front of the retina and the other and the other is focused is focused behind the retinabehind the retina (with (with accommodation relaxed)accommodation relaxed)

Symmetrical AstigmatismSymmetrical Astigmatism

The principal meridians or axes of the two The principal meridians or axes of the two eyes are symmetrical (e.g., eyes are symmetrical (e.g., both eyes are both eyes are WTR or ATRWTR or ATR))

The The sum of the two axessum of the two axes of the two eyes of the two eyes equals equals approximately 180approximately 180

Symmetrical AstigmatismSymmetrical Astigmatism

ExampleExample OD: OD: pl -1.00 x 175pl -1.00 x 175 OS:OS: pl -1.00 x 005pl -1.00 x 005

Both eyes are WTR astigmatism, and the Both eyes are WTR astigmatism, and the sum of the two axes equal approximately sum of the two axes equal approximately 180180

Asymmetrical AstigmatismAsymmetrical Astigmatism

The principal meridians or axes of the two The principal meridians or axes of the two eyes are eyes are notnot symmetrical (e.g., symmetrical (e.g., one eye is one eye is WTR while the other eye is ATRWTR while the other eye is ATR))

The The sum of the two axessum of the two axes of the two eyes of the two eyes does does notnot equal equal approximately 180approximately 180

Asymmetrical AstigmatismAsymmetrical Astigmatism

Example:Example: OD: OD: pl -1.00 x 180pl -1.00 x 180 OS: OS: pl -1.00 x 090pl -1.00 x 090

One eye is WTR astigmatism, and the One eye is WTR astigmatism, and the other eye is ATR astigmatism, and the other eye is ATR astigmatism, and the sum of the two axes do not equal sum of the two axes do not equal approximately 180approximately 180

PrevalencePrevalence

AgeAge Infants are born with ATR astigmatism, where Infants are born with ATR astigmatism, where

the cornea is the source of the astigmatismthe cornea is the source of the astigmatism Preschool children have little or no Preschool children have little or no

astigmatismastigmatism Teenage children demonstrate a shift towards Teenage children demonstrate a shift towards

WTR astigmatismWTR astigmatism Older adults show a shift towards ATR Older adults show a shift towards ATR

astigmatismastigmatism

PrevalencePrevalence

GenderGender In general, there are no significant differences In general, there are no significant differences

between males and femalesbetween males and females

PrevalencePrevalence

EthnicityEthnicity Higher prevalence in North Americans, Higher prevalence in North Americans,

LatinosLatinos Asian infants tend to be WTR astigmatismAsian infants tend to be WTR astigmatism Caucasian infants tend to be ATR Caucasian infants tend to be ATR

astigmatismastigmatism

IncidenceIncidence

General trendGeneral trend For older adults, the average rate of change For older adults, the average rate of change

towards ATR astigmatism is less than or towards ATR astigmatism is less than or equal to 0.25D every 10 yearsequal to 0.25D every 10 years

Visual AcuityVisual Acuity

Theoretically, at NO distance does an Theoretically, at NO distance does an uncorrected astigmat have a sharp retinal uncorrected astigmat have a sharp retinal imageimage

Clinically, if astigmatism is small (less than Clinically, if astigmatism is small (less than 0.50DC), the patient may not notice blur0.50DC), the patient may not notice blur

Visual AcuityVisual Acuity

Simple or compound myopic astigmatismSimple or compound myopic astigmatism Accommodation may make the retinal image even Accommodation may make the retinal image even

more blurrymore blurry

Simple or compound hyperopic astigmatismSimple or compound hyperopic astigmatism Accommodation may improve VA to some extentAccommodation may improve VA to some extent

Mixed astigmatismMixed astigmatism VA is relatively goodVA is relatively good May not need much accommodationMay not need much accommodation

Spherical and Astigmatic AmetropiaSpherical and Astigmatic Ametropia

Uncorrected VAUncorrected VA Spherical Spherical Refractive Error Refractive Error

(D)*(D)*

Astigmatism (D)Astigmatism (D)

20/3020/30 0.500.50 1.001.00

20/4020/40 0.750.75 1.501.50

20/6020/60 1.001.00 2.002.00

20/8020/80 1.501.50 3.003.00

20/12020/120 2.002.00 4.004.00

20/20020/200 2.502.50 >4.00>4.00

Spherical and Astigmatic AmetropiaSpherical and Astigmatic Ametropia

Spherical refractive error (D)*Spherical refractive error (D)* Myopia or absolute hyperopiaMyopia or absolute hyperopia When multiplied by a factor of two, it equals When multiplied by a factor of two, it equals

astigmatism (D)astigmatism (D)

SymptomsSymptoms

Distorted vision at distance and nearDistorted vision at distance and nearLetter confusionLetter confusionAsthenopia or ocular fatigueAsthenopia or ocular fatigue Due to constantly squinting to clear up Due to constantly squinting to clear up

distorted visiondistorted vision

HeadachesHeadachesSquintingSquinting

SignsSigns

Decreased visual acuities at distance and Decreased visual acuities at distance and nearnear

Clinical TestsClinical Tests

Visual acuity tests Visual acuity tests –– distance and near distance and near

AutorefractionAutorefraction

KeratometryKeratometry

Retinoscopy Retinoscopy Most reliable source of information for cylinder Most reliable source of information for cylinder

power and axispower and axis

Monocular subjective refraction, including Monocular subjective refraction, including Jackson cross cylinderJackson cross cylinder

ManagementManagement

Cylindrical lenses and spherocylindrical Cylindrical lenses and spherocylindrical lenses in spectacles and contact lenses lenses in spectacles and contact lenses for simple astigmatism and compound for simple astigmatism and compound astigmatism, respectivelyastigmatism, respectively

Refractive surgeryRefractive surgery

ManagementManagement

Spectacles Spectacles Single vision glasses with cylinderSingle vision glasses with cylinder

ManagementManagement

Contact lensesContact lenses Toric soft contact lensesToric soft contact lenses Toric rigid gas permeable contact lensesToric rigid gas permeable contact lenses

ManagementManagement

Refractive surgeryRefractive surgery Photorefractive keratectomy (PRK)Photorefractive keratectomy (PRK) Laser in-situ keratomileusis (LASIK)Laser in-situ keratomileusis (LASIK)