astigmatism Flashcards
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
astigmatism
causes of astigmatism
- the cornea has unequal curvature on its anterior surface
- the lens has unequal curvature on its surface or layers
- due to distortion of cornea or the lens
- 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
classifications of astigmatism
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
when the cornea has unequal curvature on the anterior surface
corneal astigmatism
when the crystalline lens has unequal curvature on the surface or on its layer
lenticular astigmatism
The sum of corneal astigmatism and residual astigmatism
total astigmatism
(Total Astigmatism = corneal + residual)
under total astigmatism
posterior corneal surface
lenticular surface
lenticular zonule
3 types of regular astigmatism
with the rule
against the rule
oblique
when the two principal meridians are perpendicular to each other
regular astigmatism
When the greatest refractive power is within 30 of the vertical meridian (i.e., between 60 and 120 meridians)
The most common type of astigmatism based on the orientation of meridians
Minus cylinder axis around horizontal meridian
with the rule 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
irregular 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 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 myopic astigmatism
When one of the principal meridians is focused behind the retina and the other is focused on the retina (with accommodation relaxed)
simple hyperopic astigmatism
When both principal meridians are focused either in front or behind the retina (with accommodation relaxed)
compound astigmatism
when both principal meridians are focused in front of the retina with accommodation relaxed
compound myopic astigmatism
When both principal meridians are focused behind the retina (with accommodation relaxed)
compound hyperopic 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)
mixed 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:
OD: pl -1.00 x 175
OS: pl -1.00 x 005
symmetrical 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
older adults shift towards __ astigmatism
ATR
infants are born with ___ astigmatism, where the cornea is the source of astigmatism
ATR
teenagers shift towards ______ astigmatism
WTR
preschool children have little to no astigmatism
asian infants are ____ astigmatism
WTR
caucasian infants are ____ astigmatism
ATR
presence of astigmatism symptoms
distorted vision at distance and near
letter confusion
asthenopia (ocular fatigue)
headaches
squinting
common cause of ocular fatigue
frequent squinting to clear distorted vision
presence of astigmatism signs
decreased visual acuity at distance and near
clinical tests for astigmatism
monocular subjective refraction
retinoscopy
keratometry
autorefraction
VA tests for distance and near
it is the most reliable source of information for cylinder power and axis
retinoscopy
astigmatism management
spectacle
contact lens
refractive surgery
refractive surgeries
Photorefractive keratectomy (PRK)
Laser assisted in-situ keratomileusis (LASIK)
CL types for astigmatism
Toric soft contact lenses
Toric rigid gas permeable contact lenses
is a refractive error of the eye in which there is a difference in degree of refraction in different meridians, the eye has different focal points in different planes
astigmatism
Most astigmatic corneas have two curves, ________ and _____ . This causes light to focus on more than one point in the eye resulting in blurred vision. The cornea is normally spherical although In people with astigmatism it may be shaped like a rugby ball or oval instead of tennis ball In astigmatism the front surface of
steeper curve
flatter curve
