Slide 1

Theme 8: Astigmatism Concept of Astigmatism Formation of the retinal image Quantifying Astigmatism Classifying Astigmatism Retinal Image of the astigmatic eye Neutralizing Astigmatism Slide 2 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 Slide 3 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 Slide 4 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 Slide 5 Formation of the retinal image CONOID OF STURM SPHERICAL AMETROPIAASTIGMATISM Slide 6 Formation of the retinal image CONOID OF STURM FOCAL INTERVAL OF STURM Sturm Interval: 0.37mm for 1D of Astigmatism CIRCLE OF LEAST CONFUSION Slide 7 Quantifying Astigmatism Astigmatic ametropia Light rays Slide 8 Quantifying Astigmatism Notation for Astigmatism Notation for astigmatism is done by analogy to the spherical-cylindrical lenses Slide 9 Quantifying 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 Notation for Astigmatism Slide 10 Quantifying Astigmatism Astigmatism: (Sph) (Cyl) (R y ) (R z -R y ) y (R z ) (R y -R z ) z (Transposed) R 90 =2D R 0 =1D A=-1 (+2) (-1) 90 (+1) (+1) 0 Notation for Astigmatism Transposed Slide 11 Classifying Astigmatism POSITION OF THE FOCALS WITH RESPECT TO THE RETINA Astigmatism can be classified under two criteria: CURVATURE OF THE PRINCIPAL MERIDIANS Slide 12 Classifying Astigmatism POSITION OF THE FOCALS WITH RESPECT TO THE RETINA CLC Slide 13 Classifying Astigmatism Compound hyperopic R z > 0 R y > 0 CLC Slide 14 Classifying Astigmatism Simple hyperopic R z > 0 R y = 0 CLC Slide 15 Classifying Astigmatism Mixed R z > 0 R y < 0 CLC Slide 16 Classifying Astigmatism Simple Myopic R z = 0 R y < 0 CLC Slide 17 Classifying Astigmatism Compound Myopic R z < 0 R y < 0 CLC Slide 18 Classifying Astigmatism CURVATURE OF THE PRINCIPAL MERIDIANS Direct: Greatest curvature in the vertical meridian (90 30) P y >P z R y 0 Inverse: Greatest curvature in the horizontal meridian (0 30) P y R z A Classifying Astigmatism Direct P y >P z R y 0P y R z A