Astigmatism Flashcards
- refractive condition in which a point focus of light cannot be formed on the retina
- having multiple foci
Astigmatism
it exist because of variation in powers in the different meridians of the eye
resulting to having multiple foci
Astigmatism
Considered to be the most common refractive error corrected today
Astigmatism
meridian with least power
Axis meridian
meridian with greatest power
Power meridian
axis and power meridian are ____ degrees apart
90 degrees
- HE had a crude measurement of his own astigmatism about 1.75D
- The value remained when he immersed his
head underwater (thus eliminating the
influence of cornea) which he attributed to
the lens.
Thomas Young
First to correct astigmatism by spherocylindrical lens
George Biddle Airy
first lens was made to correct astigmatism in united states in this period
USA 1828
Produced the first distance test chart for astigmatism
created the clock dial chart
John Green
Credited for the first trial case containing cylindrical lenses
suggested the method that we now use for the location of axis
H. Knapp
Etiology / causes of astigmatism:
Different Corneal curvature
Different lenticular curvature
traction of the external muscles which exerted the scleral coat
Eccentric position of fovea in relation to visual axis
Changes in vitreous gel w/c results in a change in index of refraction
Irregularity in surface of retina
Angle alpha may cause against-the-rule astigmatism of 0.50 to 0.75 D
Symptoms of astigmatism:
- Blurred Vision
- Transient blurring of vision (due to constant accommodation)
- Headaches (frontal or temporal) - from effort to accommodate in attempt to clear the remaining blurred vision
- Tearing and smarting
- Tiring of the eyes
Signs of astigmatism:
- Tilting of the head
- Squinting of the eyes
- Wrinkling and furrowing of brows
- Holding reading material too close
Classification of Astigmatism:
- Total / Physiologic astigmatism
- Corneal Astigmatism
- Lenticular Astigmatism
type of astigmatism which 2 principal meridians are at right with each other
Regular Type astigmatism
type of astigmatism not at right angle to each other
curvature is not uniform
Irregular Type
type of astigmatism wherein curvature of the greatest power lies nearest to the vertical meridian
60deg to 120deg
With-the-Rule astigmatism / Direct astigmatism
type of astigmatism wherein curvature of the greatest power lies within
0 - 30 degrees
150 - 180 degrees
Against-the-rule astigmatism / Inverse or Perverse astigmatism
type of astigmatism wherein Greatest power lies in:
30 - 60 degrees
120 - 150 degrees
Oblique Astigmatism
astigmatism referring to both corneas:
total of degrees between 2 weakest or 2 strongest is equal to 180
Symmetric astigmatism
astigmatism referring to both corneas:
astigmatism when the two is not equal to 180 degrees
Asymmetric Astigmatism
astigmatism referring to both corneas:
both astigmatism are WTR or both are ATR
Homonymous Astigmatism
Symmetric astigmatism if both WTR
Homologous Symmetric Astigmatism
Symmetric Astigmatism if both ATR
Heterologous Symmetric Astigmatism
astigmatism referring to both corneas:
one eye is WTR and the other eye is ATR
Heteronymous astigmatism
With accommodation completely relaxed, the 2 principal meridians tend to fall beyond the retina
Correction is Plus sphere with Plus cylinder lens
Compound Hyperopic Astigmatism
with accommodation relaxed, one meridian is focus on the retina while the other would tend to focus behind the retina
Correction would consist of simple plus cylinder
Simple Hyperopic Astigmatism
with accommodation relaxed, both principal meridians would focus in front of retina
Correction will require minus sphere combined with minus cylinder
Compound Myopic Astigmatism
with accommodation relaxed, one meridian is focused on the retina while the other is in front of it
correction will require a simple minus cylinder
Simple myopic astigmatism
with accommodation relaxed, one meridian tends to focus behind the retina while the other focuses in front of retina
Mixed Astigmatism
mixed astigmatism wherein cylinder is stronger than sphere
Contrageneric mixed astigmatism
mixed astigmatism wherein cylinder is weaker than sphere
Congeneric Mixed astigmatism
