astigmatism,strum's conoid,astigmatism types,

1. ASTIGMATISM
PRESENTED BY,
MR.SUBHAM KUMAR
BACHELOR OF OPTOMETRY 2ND 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. 1. 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
2. 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
3. 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
4. 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
a. Curvatural
b. Positional
c. 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

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

23. 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

24. INVESTIGATION
Retinoscopy
Keratometry
Astigmatic fan test
Jackson cross cylinder test

25. TREATMENT
– Spectacles (spherocyl.)
– Contact lenses(toric lens)
SURGICAL TREATMENT
– Lasik
– PRK

26. 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.

27. ETIOLOGY OF IRREGULAR
ASTIGMATISM
corneal irregular astigmatism
Lenticular irregular astigmatism
Retinal irregular astigmatism

28. 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

29. 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.

30. SYMPTOMS
Defective vision
Distortion of object
Polyopia
Aberration

31. TREATMENT
 Special contact lens such as RGP, Rose-K and Miniscleral
contact lenses etc
Penetrating keratoplasty

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