ASTIGMATISM Flashcards
defined as a refractive condition
Astigmatism
it is the difference in refractive power between the two principal meridian of the eye
Astigmatism
astigmatism s caused by
Irregular shape of the cornea
internal cause of astigmatism are
aqueous humor, back index of the cornea, crystalline lens, vitreous humor
2 Meridians of the eye
Power meridian
Axis meridian
meridian of greatest optical power
Power meridian
meridian of least optical power
Axis meridian
2 Meridian descriptions
*Refractive power (Diopters)
* Radius of curvature (mm)
Astigmatism cuases
*Meridional differences
*Shape of posterior pole
astigmatism causes meridional differences in
*curvature and/or flattening rates of cornea and crystalline lens
*refractive index of optical components
Structural classification of astigmatism
*Corneal
*Lenticular
*Other internal astigmatism
*Total
cornea is seldom
spherical
it implies anterior
Corneal astigmatism
true corneal astigmatism must include
posterior
posterior neutralizes
10-14% of anterior astimgatism
corneal astigmatism is always in a
minus form
optical description
Corneal astigmatism
it is due to differences in
*meridional refractive powers
*refractive indices of lens layers
*surface flattening rates of lens layers
*symmetry of lens
Lenticular astigmatism
anatomical description
Corneal toricity
tilting and/or decentration of crystalline lens
Lenticular astigmatism
the posterior pole (retina) my be:
*toric in shape
*tilted
*decentered
total astigmatism is the combination of
*corneal astigmatism
*lenticular astigmatism
*other internal astigmatism
principal meridians that is 90deg apart
Regular astigmatism
principal meridians that is not 90deg apart
Irregular astigmatism
*more than 2 principal meridian
*no principal meridians
*often acquired like trauma or disease
Irregular astigmatism
Irregular astigmatism causes
*distortion(s) of refractive surfaces
*irregularities in refractive indices
*disease/degeneration
*scars
*diabetes
refractive power of the vertical meridian is the greatest
with-the-rule
correcting of with-the-rule
minus cylinder axis 0-30deg, 150-180deg
refractive power of horizontal meridian is the greatest
against-the-rule
correcting of against-the-rule
minus cylinder axis 60-120 deg
two principal meridians lie somewhere between the axes defining either WTR or ATR astigmatism
Oblique astigmatism
correcting of oblique astigmatism
minus cylinder axis 31-59deg, 121-149deg
Classification of astigmatism by magnitude
*negligible
*low
*moderate
*high
≤ 0.75D cyl
Negligible
1.00 - 1.50D cyl
Low
1.75 - 2.50D cyl
Moderate
against-the-rule to oblique/with-the-rule
Infants
> 2.00D cyl
High
with-the-rule to against-the-rule/oblique
Adults
Astigmatism components
*total
*corneal
*lenticular/internal
Astigmatism compoents relationship
Total = corneal + lneticular
Javal’s rule (1890)
Total astigmatism = corneal astigmatism - 0.50
measurement of astigmatism in front surface curvature
*keratometer
*photokeratoscopy
*computer-assisted corneal mapping system
measurement of astigmatism in back surface curvature
*Purkinje images
astigmatic refractive error that is present when a contact lens ids placed upon the cornea to correct the existing ametropia
Residual astigmatism
Physiological residual astigmatism
*Unneturalized corneal astigmatism
*Posterior corneal astigmatism
*Lenticular astigmatism
*Tilted crystalline lens
*Refractive index anomalies
*Oblique aberrations
*Misalignment of components
Residual astigmatism can create
tears or tear lens
interpret as , ‘with spherical contact lens’ unless stated otherwise
Residual astigmatism
Induces residual astigmatism
*Tilted and/or decentered CL
*Toricity and/or bitoricity of CL
*CL mislocation (rotation)
*Warpage and/or flexture of CL
Residual astigmatism formula
Residual = Total - Corneal
