Astigmatism Flashcards
it is the difference in refractive power between the
two principal meridians of the eye
astigmatism
The greater the curvature the:
greater the power
two meridians of the eye
power meridian
axis meridian
meridian of greatest optical power
power meridian
meridian of least optical power
axis meridian
Meridian Descriptions
Refractive Power (Dioptres)
Radius of Curvature (mm)
Causes of astigmatism
Meridional differences in curvature and/or flattening rates of cornea, crystalline lens, and refractive index of optical components
Shape of posterior pole
Astigmatism Classification
● Simple hyperopic astigmatism
● Compound hyperopic astigmatism
● Simple myopic astigmatism
● Compound myopic astigmatism
● Mixed astigmatism
Structural classification of astigmatism
Corneal
Lenticular
Other internal astigmatism
Total
most common classification of astigmatism
corneal astigmatism
Classification of astigmatism where the crystalline lens has a problem
lenticular astigmatism
Total Astigmism =
Internal + External
In corneal astigmatism cornea is seldom ________
spherical
True corneal astigmatism must include ___________
this neutralizes 10-14% of anterior astigmatism
posterior
optical description
corneal astigmaism
anatomical description
corneal toricity
Lenticular astigmatism is due to differences in
- meridional refractive powers
- refractive indices of lens layers
- surface flattening rates of lens layers
- symmetry of lens
Lenticular astigmatism is caused by tilting and/or decentration of __________________
crystalline lens
Internal astigmatism
The retina may be:
- toric in shape
- tilted
- decentred
Total Astigmatism is the combination of
● Corneal astigmatism
● Lenticular astigmatism
● Other internal astigmatism
Astigmatism where the principal meridian is 90deg apart
Regular
Astigmatism that is often acquired or decondary to trauma or disease
regular or irregular
irregular
CAUSES IRREGULAR ASTIGMATISM
- Disease/Degeneration
- Diabetes
- Distortion of refractive surface
- Irregularities of refractive indices
- Scar
Types of regular astigmatism
WTR
ATR
Oblique
Ocular astigmatism in which the refractive power of the vertical (or near vertical) meridian is the greatest
WTR
Correcting minus cylinder axis 0-30°, 150-180°
WTR
Ocular astigmatism in which the refractive power of the horizontal (or near horizontal) meridian is the greatest
ATR
Correcting minus cylinder axis 60-120°
ATR
Astigmatism in which the two principal meridians lie somewhere between the axes defining either WTR or ATR astigmatism
Oblique
Correcting minus cylinder axis 31-59°, 121-149°
oblique
ASTIGMATISM REFRACTIVE CYLINDER
Classification by magnitude
≤ 0.75 D Cyl
Negligible
ASTIGMATISM REFRACTIVE CYLINDER
Classification by magnitude
1.00 to 1.50 D Cyl
Low
ASTIGMATISM REFRACTIVE CYLINDER
Classification by magnitude
1.75 to 2.50 D Cyl
Moderate
ASTIGMATISM REFRACTIVE CYLINDER
Classification by magnitude
> 2.50 D Cyl
High
ASTIGMATISM INCIDENCE SHIFT WITH AGE
ATR -> OBLIQUE/WTR
INFANTS
ASTIGMATISM INCIDENCE SHIFT WITH AGE
ADULTS
WTR -> ATR/OBLIQUE
ASTIGMATISM COMPONENTS
TOTAL
CORNEAL
LENTICULAR/INTERNAL
JAVAL’S RULE
If ATR
Total = 1.25 (Ac) - 0.50
If WTR
Total = 1.25 (Ac) + 0.50
Grosvenor et al rule for WTR
T = Ac + 0.50
Grosvenor et al rule for ATR
T = AC - 0.50
Measurement of astigmatism
front surface curvature
keratometer
photokeratoscopy
computer-assisted corneal mapping system
Measurement of astigmatism
back surface curvature
purkinje images
\ defined as the astigmatic refractive error that is present when a contact lens is placed upon the cornea to correct the existing ametropia.
residual astigmatism
PHYSIOLOGICAL RESIDUAL ASTIGMATISM
● Unneutralized corneal astigmatism
● Posterior corneal astigmatism
● Lenticular astigmatism
● Tilted crystalline lens
● Refractive index anomalies
● Oblique aberrations
● Misalignment of component
INDUCED RESIDUAL ASTIGMATISM
● Tilted and/or decentred CL
● Toricity and/or bitoricity of CL
● CL mislocation (rotation)
● Warpage and/or flexure of CL
