Astigmatism, Classification, Sturm's conoid

Astigmatism (opthalmology)

1. ASTIGMATISM

2. DEFINITION Type of refractive error, wherein the refraction varies in different meridia . Consequently the ray of light entering in the eye cannot converge to a point focus. But form focal line

3. CLASSIFICATON Regular irregular

4. REGULAR ASTIGMATISM Refractive power change uniformly from one meridian to another

5. REGULAR ASTIGMATISM OPTICS OF REGULAR ASTIGMATISM parallel rays of light are not focused on a point but form two focal lines.

6. REGULAR ASTIGMATISM STURM’S CONOID The configuration of rays refracted through a toric surface is called the Sturm’s conoid Instead of 1focal point, there are 2 focal point in diff meridian They doesn’t converge in same point Point A - vertical rays (V) are converging more than the horizontal rays (H) – HORIZONTAL OVAL Point B - vertical rays have come to a focus while the horizontal rays are still converging- HORIZONTAL LINES Point C - vertical rays are diverging and their divergence is less than the convergence of the horizontal rays – HORIZONTAL OVAL

7. REGULAR ASTIGMATISM

8. REGULAR ASTIGMATISM Point D - divergence of vertical rays is exactly equal to the convergence of the horizontal rays – CIRCLE (called the circle of least diffusion) Point E -divergence of vertical rays is more than the convergence of horizontal rays - VERTICAL OVAL Point F - horizontal rays have come to a focus while the vertical rays are divergent – VERTICAL LINE Beyond F - both horizontal and vertical rays are diverging – VERTICAL OVAL Focal interval of Sturm refers to the distance between the two foci (B and F)

9. REGULAR ASTIGMATISM ETIOLOGY Corneal Lenticular Index Retinal TYPES (Position of 2 focal line in relation to retina) Simple Compound Mixed

10. REGULAR ASTIGMATISM TYPES (Axis and angle between two principle meridian) With-the-rule Against-the-rule Oblique Bioblique

11. REGULAR ASTIGMATISM SIMPLE ASTIGMATISM One meridian focused on retina Other one focused in Front- simple myopic Behind- simple hypermetropic

12. REGULAR ASTIGMATISM COMPOUND ASTIGMATISM Rays of light in both the meridians are focused in front – COMPOUND MYOPIC ASTIGMATISM In behind the retina – COMPOUND HYPOEMETROPIC ASTIGMATISM

13. REGULAR ASTIGMATISM MIXED ASTIGMATISM Light ray of one meridian focused in front and other focused in behind

14. REGULAR ASTIGMATISM WITH-THE-RULE-ASTIGMATISM Vertical meridian is more curved than horizontal meridian

15. REGULAR ASTIGMATISM AGAINST-THE-RULE-ASTIGMATISM Horizontal meridian is more curved than vertical

16. REGULAR ASTIGMATISM OBLIQUE ASTIGMATISM Both meridians are not in horizontal and vertical , but they are in right angle to each other

17. REGULAR ASTIGMATISM BIOBLIQUE Two principle meridian are not at right angle to each other

18. REGULAR ASTIGMATISM CLINICAL FEATURES Asthenopia – difficulty in focussing, dull ache in eye, frontal head ache, nausea Tranient blurreing vision - <1D Constant blurring of vision - >1D Spherical error – elongation of object Keeping the reading material close to the eyes

19. REGULAR ASTIGMATISM SIGN Half closure of lids Tilting of head squint

20. REGULAR ASTIGMATISM DIAGNOSIS Retinoscopy reveals different power in two different axes. Keratometry and computerized corneal topography reveal different corneal curvature in two different meridians in corneal astigmatism. Astigmatic fan test Determine the axis & magnitude of astigmatism Jackson’s cross cylinder test

21. REGULAR ASTIGMATISM TREATMENT Optical (Cylindric lens) spectacle form Contact lens (hard & toric ) Correct up to 2-3D of regular astigmatism Surgical correction is quite effective

22. IRREGULAR ASTIGMATISM Irregular change of refractive power in different meridians. There are multiple meridians which admit no geometrical analysis. ETIOLOGICAL TYPE Curvatural irregular astigmatism - patients with extensive corneal scars or keratoconus. Index irregular astigmatism - variable refractive index in different parts of the crystalline lens may occur rarely in patients with cataract.

23. IRREGULAR ASTIGMATISM SYMPTOMS Defective vision Distortion of objects Polyopia (seeing multiple images)

24. IRREGULAR ASTIGMATISM INVESTIGATION Retinoscopy - reveals irregular pupillary reflex Slit-lamp examination – reveal corneal irregularity or Keratoconus Placido’s disc test - reveals distorted circles Photokeratoscopy and computerized corneal topography – -give photographic record of irregular corneal curvature

25. IRREGULAR ASTIGMATISM TREATMENT OPTICAL TREATMENT contact lens which replaces the anterior surface of the cornea for refraction Phototherapeutic keratectomy (PTK) performed with excimer laser may be helpful in patients with superficial corneal scar responsible for irregular astigmatism. Surgical treatment It indicated in extensive corneal scarring (when vision does not improve with contact lenses) It consists of penetrating keratoplasty or deep anterior lamellar keratoplasty (DALK)

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