Astigmatism

Report
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)

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