Astigmatism

astigmatism,strum's conoid,astigmatism types,

1. ASTIGMATISM PRESENTED BY, MR.SUBHAM KUMAR BACHELOR OF OPTOMETRY 2 ND YEAR 2017 RIDLEY COLLEGE OF OPTOMETRY, JORHAT,ASSAM.

2. CONTENTS DEFINITION OCCURANCE STURM’S CONOIDS ETIOLOGY TYPES ETIOLOGY OF REGULAR ASTIGMATISM REGULAR ASTIGMATISM TYPES OF REGULAR ASTIGMATISM

3. CONTENTS WITH THE RULE ASTIGMATISM AGAINST THE RULE ASTIGMATISM OBLIQUE ASTIGMATISM BI-OBLIQUE ASTIGMATISM REFRACTIVE TYPES SIGN AND SYMPTOMS

4. CONTENTS INVESTIGATION TREATMENT IRREGULAR ASTIGMATISM ETIOLOGY OF IRREGULAR ASTIGMATISM DIFFERENCE BETWEEN REGULAR AND IRREGULAR ASTIGMATISM SYMPTOMS TREATMENT REFERENCES THANK YOU

5. DEFINITION A refractive condition that is the result of two principal meridians of the eye having different refractive power

6. OCCURRENCE Astigmatism occurs when the cornea or the lens ,has a slightly different surface curvature in one direction from the other There is a steep meridian and a flat meridian Varying refractive index

7. STURM’S CONOID In a toric surface, one principle meridian is more curved than the second principle meridian. The principle meridian with minimum curvature,and therefore with minimum power is called base curve of a toric lens. The configuration of rays refracted through a toric surface is called strum’s conoid . The shape of bundle of the light rays at different levels in strum’s conoid is as follows:-

8. At point A, the vertical rays(V) are converging more than the horizontal rays (H). So ,the selection here is horizontal oval or an oblate ellipse At point B ( first focus ) the vertical rays have come to a focus while the horizontal rays are still converging and so they form a horizontal line At point C the vertical rays are diverging and their divergence is less than the convergence of the horizontal rays so a horizontal oval is formed here At point D , the divergence of vertical rays is exactly equal to the convergence of the horizontal rays form the axis . So here the section is a circle , which is called the circle of least diffusion

9. 5. At point E , the divergence of vertical rays is more than the convergence of horizontal rays, so the section here is a vertical oval. 6. At point F (second focus) the horizontal rays have come to a focus while the vertical rays are divergent and so a vertical line is formed here. 7. Beyond F (as point G ) both horizontal and vertical rays are divergent and so the section will always be a vertical oval or prolate ellipse The distance between two foci(B and F) is called the focal interval of sturm .

10. ETIOLOGY Asymmetry of the cornea, or the lens or the posterior retina Any variation in refractive index Tilting of the lens Any eye mass lesion The weight of the upper eyelid resting on the eyeball Any healed aberration or injury of the cornea which can be due to any infection or trauma

11. TYPES Regular Astigmatism Irregular Astigmatism

12. ETIOLOGY OF REGULAR ASTIGMATISM Corneal Astigmatism Lenticular astigmatism Curvatural Positional Index Retinal astigmatism

13. TYPES OF REGULAR ASTIGMATISM With the rule astigmatism Against the rule astigmatism Oblique astigmatism Bi-oblique astigmatism

14. WITH THE RULE ASTIGMATISM Vertical curvature is more steeper than the horizontal

15. NOTES -Vertical meridian is more myopic than horizontal Near vertical is usually defined as between 70˚-110˚ i.e ±20˚ of the vertical The axis of the negative cylindrical correction in with the rule astigmatism will be between 0˚-20˚ or 160˚-180˚ i.e ±20˚ of the horizontal

16. AGAINST THE RULE ASTIGMATISM The horizontal curvature is greater than the vertical curvature i.e the correction of astigmatism will require the prescription of a concave cyl at 90˚ or a convex cyl at 180˚

17. NOTE:- Horizontal meridian is more myopic than vertical The axis of the correcting negative cylinder will be between70˚-110˚ i.e within ±20 of the vertical

18. OBLIQUE ASTIGMATISM The power meridian is neither horizontal nor vertical

19. The principal meridian are more than 30˚ from vertical or horizontal The axis of the negative correcting cylindrical axis will be between 20˚-70˚and 110˚-160˚

20. BI-OBLIQUE ASTIGMATISM The two principle meridian are not at right angle to each other i.e one may be at 30˚ and the other at 100˚

21. REFRACTIVE TYPES Simple astigmatism Compound astigmatism Mixed astigmatism

22. SIGNS AND SYMPTOMS Blurring of vertical , horizontal or diagonal lines Eyestrain or fatigue Headaches Distortion in portions of the visual field Squinting NOTES :- The astigmatic patients tend to squeeze the eyes producing a narrow stenopic slit effect

23. INVESTIGATION Retinoscopy Keratometry Astigmatic fan test Jackson cross cylinder test

24. TREATMENT Spectacles ( spherocyl .) Contact lenses( toric lens) SURGICAL TREATMENT L asik PRK

25. IRREGULAR ASTIGMATISM It can defined as if the parallel rays of light coming from infinity is focused in more than two points is called Irregular Astigmatism.

26. ETIOLOGY OF IRREGULAR ASTIGMATISM corneal irregular astigmatism Lenticular irregular astigmatism Retinal irregular astigmatism

27. DIFFERENCE BETWEEN REGULAR AND IRREGULAR REGULAR It is the condition in which there are two principle meridians separated by 90 degree This is the most common form of astigmatism IRREGULAR It is the condition in which there are more than two principle meridians which are not separated by 90 degree This is less common form of astigmatism

28. REGULAR It can be corrected by a cylinder or spherocylindrical lens IRREGULAR Irregular astigmatism will not be corrected by spherocylindrical lens, Rather it is correct by special contact lens ie RGP, Miniscleral lens etc.

29. SYMPTOMS Defective vision Distortion of object Polyopia Aberration

30. TREATMENT Special contact lens such as RGP, Rose-K and M iniscleral contact lenses etc Penetrating keratoplasty

31. REFERENCE Optic and refraction …….. BY A.K KHURANNA Page no 24,79-83 Refraction and lens prescription ….. BY MONICA CHAUDHARY