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Astigmatism

 

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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

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

Etiology

*      Cornea

n  The cornea has an unequal curvature on its anterior surface

*      Lens

n  The crystalline lens has an unequal curvature on its surface or in its layers

n  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, against-the-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

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

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)

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)

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)

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

*      Example

n  OD:   pl -1.00 x 175

n  OS:    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

*      Example:

n  OD:   pl -1.00 x 180

n  OS:    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

n  Infants are born with ATR astigmatism, where the cornea is the source of the astigmatism

n  Preschool children have little or no astigmatism

n  Teenage children demonstrate a shift towards WTR astigmatism

n  Older adults show a shift towards ATR astigmatism

*      Prevalence- Gender

n  In general, there are no significant differences between males and females

*      Prevalence- Ethnicity

n  Higher prevalence in North Americans, Latinos

n  Asian infants tend to be WTR astigmatism

n  Caucasian infants tend to be ATR astigmatism

*      Incidence

*      General trend

n  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

*      Simple or compound myopic astigmatism

n  Accommodation may make the retinal image even more blurry

*      Simple or compound hyperopic astigmatism

n  Accommodation may improve VA to some extent

*      Mixed astigmatism

n  VA is relatively good

n  May not need much accommodation

*      Spherical and Astigmatic Ametropia

*      Spherical and Astigmatic Ametropia

*      Spherical refractive error (D)*

n  Myopia or absolute hyperopia

n  When multiplied by a factor of two, it equals astigmatism (D)

Symptoms

*      Distorted vision at distance and near

*      Letter confusion

*      Asthenopia or ocular fatigue

n  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

n  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

*      Spectacles

n  Single vision glasses with cylinder

*      Contact lenses

n  Toric soft contact lenses

n  Toric rigid gas permeable contact lenses

n  Management

*      Refractive surgery

n  Photorefractive keratectomy (PRK)

n  Laser in-situ keratomileusis (LASIK)

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