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Theme 8: Astigmatism. Concept of Astigmatism Formation of the retinal image Quantifying Astigmatism Classifying Astigmatism Retinal Image of the astigmatic eye Neutralizing Astigmatism. Concept of astigmatism.

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## Theme 8: Astigmatism

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**Theme 8:Astigmatism**• 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**Concept of astigmatism**• CAUSES OF ASTIGMATISM • Astigmatism of curvature: • Non spherical surfaces (First surface of the cornea) • Astigmatism of Index • Irregular Astigmatism Low Astigmatism 0.2D < A < 1D Medium Astigmatism A < 3D High Astigmatism A > 3D**Concept of astigmatism**THE IMAGE OF A POINT IS NOT A POINT In general, the retinal image is an ellipse Near far Astigmatic 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**Astigmatic ametropia Light rays**Quantifying Astigmatism**• Notation for Astigmatism Notation for astigmatism is done by analogy to the spherical-cylindrical lenses**Quantifying Astigmatism**• Notation for Astigmatism • 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**Quantifying Astigmatism**• Notation for Astigmatism • 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**Astigmatism can be classified under two criteria: POSITION OF THE FOCALS WITH RESPECT TO THE RETINA CURVATURE OF THE PRINCIPAL MERIDIANS**Classifying Astigmatism**POSITION OF THE FOCALS WITH RESPECT TO THE RETINA CLC**Classifying Astigmatism**Compound hyperopic Rz > 0 Ry > 0 CLC**Classifying Astigmatism**Simple hyperopic Rz > 0 Ry = 0 CLC**Classifying Astigmatism**Mixed Rz > 0 Ry < 0 CLC**Classifying Astigmatism**Simple Myopic Rz = 0 Ry < 0 CLC**Classifying Astigmatism**Compound Myopic Rz < 0 Ry < 0 CLC**Classifying Astigmatism**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)**Classifying Astigmatism**Inverse Direct Py>Pz Ry<Rz A>0 Py<Pz Ry>Rz A<0**Retinal Image of the astigmatic eye**Point Object Conoid of Sturm In function of where the image forms on the eye, the image will have a distinct form**Retinal Image of the astigmatic eye**POINT OBJECT Conoid of Sturm If the retina is on the focal of the vertical meridian the image will be a horizontal line**Retinal Image of the astigmatic eye**POINT OBJECT Conoid of Sturm If the retina coincides with the focal point of the horizontal meridian the image will be a vertical line**Retinal Image of the astigmatic eye**POINT OBJECT Conoid of Sturm If the retina coincides with the circle of least confusion the image will be a circle.**Retinal Image of the astigmatic eye**POINT OBJECT Conoid of Sturm 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.**Retinal Image of the astigmatic eye**POINT OBJECT Conoide 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 the horizontal direction (perpendicular to the more emmetropic meridian)**Retinal Image of the astigmatic eye**POINT OBJECT Conoid of 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)**Retinal Image of the astigmatic eye**EXTENSIVE OBJECT The clearer meridian is perpendicular to the more emmetropic meridian CLC The vertical meridian is emmetropic The image of a point object is a horizontal line. The horizontal segment of the cross is the clearest**Retinal Image of the astigmatic eye**EXTENSIVE OBJECT The clearer meridian is perpendicular to the more emmetropic meridian CLC The horizontal meridian is emmetrope. The image of a point object is a vertical line. The vertical segment of the cross is the clearest.**Retinal Image of the astigmatic eye**EXTENSIVE OBJECT The clearer meridian is perpendicular to the more emmetropic meridian CLC 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.**Retinal Image of the astigmatic eye**EXTENSIVE OBJECT The clearer meridian is perpendicular to the more emmetropic meridian CLC 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.**Retinal Image of the astigmatic eye**EXTENSIVE OBJECT The clearer meridian is perpendicular to the more emmetropic meridian CLC 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)**Neutralizing Astigmatism**The focal point image of the lens should be situated on the remote point of the eye (for each meridian)**Neutralizing Astigmatism**Near far Cylindrical lens Near far**Neutralizing Astigmatism**Spherical Cylindrical Lens (Sph) (Cyl) (Pvpy) (Ppvz-Ppvy) y (Ppvz) (Ppvy-Ppvz) z (Transposed)

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