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: pl -1.00 x 175

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

Asymmetrical Astigmatism
Example:

OD: pl -1.00 x 180

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

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
Refractive Error
(D)*
Astigmatism (D)
0.50
1.00
20/40 0.75
1.50
20/60
20/80
20/120
20/200
1.00
1.50
2.00
2.50
2.00
3.00
4.00
>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)