Astigmatism

Astigmatism

Defined as a refractive condition. It is the difference in refractive power between the two principal meridians of the eye

Power meridian

meridian of greatest optical power

Axis meridian

meridian of least optical power

Refractive Power (Dioptres)

Radius of Curvature (mm)

2 Meridian descriptions

1. Meridional differences in:

• curvature and/or flattening rates of

  • cornea

  • crystalline lens

• refractive index of optical components

2. Shape of posterior pole

Astigmatism Causes

Structural

• Corneal

• Lenticular

• Other internal astigmatism

• Total

Classification of Astitgmatism

spherical

Cornea is seldom _____

Corneal

astigmatism implies anterior

posterior

True corneal astigmatism must include

10-14%

True corneal astigmatism must include posterior

• posterior neutralizes ____% of anterior astigmatism

optical

Corneal astigmatism = _____ description

anatomical

Corneal toricity = ________ description

Internal Astigmatism

In _____ Astigmatism, the posterior pole (retina) may be:

• toric in shape

• tilted

• decentred

• toric in shape

• tilted

• decentred

In Internal Astigmatism, the posterior pole (retina) may be:

?

?

?

• Corneal astigmatism

• Lenticular astigmatism

• Other internal astigmatism

Total astigmatism is the combination of

Regular Astigmatism

Principal meridians, 90^o apart

Irregular Astigmatism

• principal meridians not 90^o apart

• more than 2 principal meridians

• no principal meridians

• often acquired (i.e. secondary to)

  • trauma

  • disease

• Regular

  • with-the-rule

  • against-the-rule

  • oblique

• Irregular

Astigmatism Types

According to axis orientation (minus Cyl form)

With-the-Rule Astigmatism

Ocular astigmatism in which the refractive power of the vertical (or near vertical) meridian is the greatest

Correcting minus cylinder axis 0-30^o, 150-180^o

In WTR Astigmatism, the correcting minus cylinder axis are ___ & ___

Against-the-Rule Astigmatism

Ocular astigmatism in which the refractive power of the horizontal (or near horizontal) meridian is the greatest

Correcting minus cylinder axis 60-120^o

In ATR Astigmatism, the correcting minus cylinder axis is ___

Oblique Astigmatism

Astigmatism in which the two principal meridians lie somewhere between the axes defining either WTR or ATR astigmatism

Correcting minus cylinder axis 31-59^o, 121-149^o

In OBL Astigmatism, the correcting minus cylinder axis are ___ & ___

ASTIGMATISM REFRACTIVE CYLINDER

Classification by magnitude

?

?

?

?

Astigmatism Incidence Shift with Age

Infants

• ___ → ___

Adults

• ___ → ___

Astigmatism Components

DETERMINING THE OCULAR ASTIGMATISM FROM THE CORNEAL ASTIGMATISM

Javal’s Rule (1890)

DETERMINING THE OCULAR ASTIGMATISM FROM THE CORNEAL ASTIGMATISM

Grosvenor et al. (1988)

MEASUREMENT OF ASTIGMATISM

Front surface curvature

?

?

?

Back Surface Curvature

?