Theme 8: Astigmatism

transcript

• Concept of Astigmatism• Formation of the retinal image• Quantifying Astigmatism• Classifying Astigmatism• Retinal Image of the astigmatic eye• Neutralizing Astigmatism

Concept of astigmatism

Astigmatism: Refraction Anomaly that appears when the eye presents different meridian powers in some of its refractive surfaces

The retinal image is blurry but with a distinct size in different directions

CAUSES OF ASTIGMATISMAstigmatism of curvature: Non spherical surfaces (First surface of the cornea)Astigmatism of IndexIrregular Astigmatism

Low Astigmatism 0.2D < A < 1D

Medium Astigmatism A < 3D

High Astigmatism A > 3D

THE IMAGE OF A POINT IS NOT A POINT

In general, the retinal image is an ellipse

Near farAstigmatic ametropia

Light rays

Formation of the retinal image

CONOID OF STURM

SPHERICAL AMETROPIA

ASTIGMATISM

Formation of the retinal image

CONOID OF STURM

FOCAL INTERVAL OF STURM Sturm Interval: 0.37mm for 1D of Astigmatism

CIRCLE OF LEAST CONFUSION

Quantifying Astigmatism

RPtRe'H

zyyzPPRRA

Astigmatic ametropia

Light rays

Notation for Astigmatism

Notation for astigmatism is done by analogy to the

spherical-cylindrical lenses

spherical-cylindrical lens: (Sph) (Cyl)

(Sph) Spherical lens power(Cyl) Cylindrical lens power Angle which the axis of the

cylindrical lens forms with the horizontal

Astigmatism: (Sph) (Cyl) (Ry) (Rz-Ry) y

(Rz) (Ry-Rz) z

(Transposed)

R90=2D R0=1D A=-1

(+2) (-1) 90º

(+1) (+1) 0º

Transposed

Classifying Astigmatism

POSITION OF THE FOCALS WITH RESPECT TO THE RETINA

Astigmatism can be classified under two criteria:

CURVATURE OF THE PRINCIPAL MERIDIANS

POSITION OF THE FOCALS WITH RESPECT TO THE RETINA

Compound hyperopic Rz > 0

Ry > 0

Simple hyperopic Rz > 0

Ry = 0

Rz > 0

Ry < 0

Simple Myopic

Rz = 0

Ry < 0

Compound Myopic

Rz < 0

Ry < 0

CURVATURE OF THE PRINCIPAL MERIDIANS

Direct: Greatest curvature in the vertical meridian (90º30º) Py>Pz Ry<Rz A>0

Inverse: Greatest curvature in the horizontal meridian (0º30º) Py<Pz Ry>Rz A<0

Oblique: Greatest curvature in the meridian situated at (45º15º)

Direct

Py>Pz Ry<Rz A>0 Py<Pz Ry>Rz A<0

Inverse

Retinal Image of the astigmatic eye

Point ObjectConoid of Sturm

In function of where the image forms on the eye, the image will have a distinct form

If the retina is on the focal of the vertical meridian the image will be a horizontal line

POINT OBJECTConoid of Sturm

If the retina coincides with the focal point of the horizontal meridian the image will be a vertical

If the retina coincides with the circle of least confusion the image will be a circle.

In general, the retinal image of a point object will be an ellipse whose greater axis corresponds to the direction of meridian that is perpendicular to the

more emmetropic one.

if the retina is found in the zone marked in the figure, the retinal image will be an ellipse with the greater semi axis in the horizontal direction

(perpendicular to the more emmetropic meridian)

POINT OBJECTConoide de Sturm

If the retina is found in the zone marked in the figure, the retinal image will be an ellipse with

the greater semi axis in a vertical direction. (perpendicular to the most emmetropic

meridian)

EXTENSIVE OBJECTThe clearer meridian is perpendicular

to the more emmetropic meridian

The vertical meridian is emmetropic The image of a point object is a horizontal line.

The horizontal segment of the cross is the clearest

The horizontal meridian is emmetrope. The image of a point object is a vertical line.

The vertical segment of the cross is the clearest.

The horizontal and vertical meridians have the same ametropia.

The image of a point object is a circle.The two arms of the cross have the same clarity.

The vertical meridian is the more emmetropic. The image of a point object is an ellipse with the

greater semi axis in the horizontal direction.The horizontal segment of the cross is clearer.

The horizontal meridian is more emmetropic. The image of a point object is an ellipse where the

vertical is the greater semiaxis The clearer arm of the cross is the vertical one.

Neutralizing Astigmatism

Goal of neutralization: The person sees objects at infinity clearly

without the need to accommodate

Principle of neutralization: The image of an object positioned at infinity through a lens should form on the remote

point of the eye (for each meridian)

The focal point image of the lens should be situated on the remote point of the eye (for each meridian)

Cylindrical lens

Near far

Spherical Cylindrical Lens (Sph) (Cyl)

(Pvpy) (Ppvz-Ppvy) y

(Ppvz) (Ppvy-Ppvz) z (Transposed)

Theme 8: Astigmatism

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