sturm's conoid, astigmatism

1. Dr samarth mishra

2. . STURM’S CONOID:
 It is an optical condition in which refractive power of
cornea and lens is not the same in all meridians ,
therefore instead of single focal point there are two
focal points separated by focal interval, this is called
sturm’s conoid.
 The distance between two focal points is called
sturm’s conoid interval.

3. .
 In a toric surface,one principal meridian is more
curved than the second principle meridian.
 The principle meridian with minimum curvature , &
therefore minimum power is called BASE curve of
a toric lens.
 The configuration of rays refracted through a toric
surface/astigmatic surface is called sturm’s
conoid.

5. .
 At point A, the vertical rays [ V ] , the vertical rays
are converging more than the horizontal rays [ H ];
so the section here is horizontal oval or an oblate
ellipse.

6. .
 At point B ( first focus), the vertial rays have come
to a focus while the horizontal rays are still
converging & so they form a horizontal line.
(horizontal line)

7. .
 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.
(horizontal oval)

8. .
 at point D, the divergence of vertical rays is
exactly equal to the convergence of the
horizontal rays from the axis.
 So here the section is a circle which is called
the circle of least diffusion.
(circle of least diffusion)

9. .
At point E ,the divergence of vertical rays is more
than the convergence of horizontal rays; so the
section here is a vertical oval.
(divergence of vertical rays>> convergence of horizontal rays)

10. .
 At point F; ( second focus), the horizontal rays
have come to a focus while the vertical rays are
divergent.
 So a vertical line is formed here.
(vertical line at second focus)

11. .
 Beyond f ( as at point G ) : both horizontal and
vertical rays are diverging and so the section will
always be a vertical oval or prolate ellipse.
 The distance between the two foci (B & F) is called
the focal interval of sturm.

12. . the shape of bundle of the light rays at different
levels in a sturm’s conoid is as follows:

13. .
 Etiology:
 Corneal causes.
 It occurs due to abnormality of curvature of cornea.
(Most common cause of astigmatism.)
 E.g eyelid pressure, pterygium, corneal scars,
corneal degeneration, keratoconus,mild corneal
opacities.

14. .
Lenticular causes:
 It is comparatively rare.
It may be-
 Curvatural --- lenticonus
 Positional-----congenital tilting & traumatic
subluxation of lens.
 Index----developing cataract/ nuclear sclerosis/
index astigmatism.

15. . FOCUS OF STURM’S CONOID AC/TO THE TYPES OF
ASTIGMATISM:
 REGULAR ASTIGMATISM: when the refractive power
changes uniformly from one meridian to another( i.e
there are two principal meridia)
 The parallel rays of light are not focussed on a point
but form two focal lines.

16. .
 Types:
 with the rule astigmatism: in this type the two
principal meridia are placed at right angles to one
another. But the vertical meridian is more curved
than the horizontal.
 This is called “with the rule astigmatism” as similar
condition exists normally( the vertical meridian is
normally rendered 0.25D more convex than the
horizontal meridian by the pressure of eyelids.

17. .
 against the rule astigmatism: the horizontal meridian
is more curved than the vertical meridian.
 oblique astigmatism: type of regular astigmatism
where the two principal meridia are not the horizontal
and vertical though these are at right angle.
 bioblique astigmatism: principal meridia are not at
right angle to one another.

18. . Simple astigmatism: the rays are focused on the
retina in one meridian and either in front(simple
myopic astigmatism)/behind( simple hypermetropic
astigmatism).

19. .
 Compound astigmatism: rays of light in both the
meridia are focused either in front or behind the
retina and the condition is labelled as compound
myopic / compund hypermetropic astigmatism.

20. .
 Mixed astigmatism: condition wherein the light rays in one
meridian are focused in front and in other behind the retina.
Such patients have comparatively less symptoms as “circle of
least diffusion “ is formed on the retina.

21. .

22. .
.

23. . Clinical features:
-blurring of vision
-asthenopic symptoms.
-tilting of head.
-headache.
-half closure of lids.
-squinting.
-burning and itching

24. .
 IRREGULAR ASTIGMATISM:
 Chr by an irregular change of refractive power in
different media.
 Types:
-corneal irregular astigmatism: corneal scars,
keratoconus.
-lenticular irregular astigmatism: d/t variable refractive
index in diff. parts of crystalline lens. Seen during
maturation of cataract.
-retinal irregular astigmatism: d/t distortion of macular
area.

25. .
 Clinical features:
-defective vision
-distortion
-polyopia ( seeing multiple objects)

26. . surgical t/t of astigmatism:
Incisional refractive procedure;
Astigmatic keratotomy:
-making transverse or arcuate cuts in the mid periphery
perpendicular to the steepest corneal meridian.
-incised meridian flattens while the meridian perpendicular
to it steepens by nearly the same amount.
-transverse or arcuate incision can be given.

28. . Limbal relaxing incision:
-To correct mild(-1 to -2) astigmatism.
-Incision made at limbus, so , optical quality of cornea is
preserved.
- easy and safe.

29. . Laser ablation corneal refractive procedures:
Photoastigmatic refractive keratotomy:
-uses a cylindrical rather than a spherical ablation
pattern to remove a tissue in a chosen meridian.
-axis of astigmatism should be marked with the patient
seated, because it may shift when the patient
reclines.

30. .
Astigmatic epi-LASIK: preferred over astigmatic PRK.
Astigmatic LASIK:astigmatismof 0.5 to 10.0D is
amenable to correction with LASIK.
Astigmatic C-LASIK: presently the best technique to
treat corneal astigmatism

31. . MANAGEMENT OF POST-KERATOPLASTY
ASTIGMATISM:
SUTURE REMOVAL:
-suture removal in steep meridia may improve a
varying degree of both regular & irregular
astigmatism.
-near a tight suture ,the keratoscopic mires are closer
together and may demonstrate a ‘V’ indentation
vector.

32. .
RELAXING INCISIONS:
-arcuate incisions along the steeper meridian in the
donor cornea 0.5mm central to the host-graft
junction correct an astigmatism of 3.5-8.5D.
-two relaxing incisions involving 70% of corneal depth
are made 180 deg. apart.

33. .
Relaxing incisions with compression sutures:
-after making relaxing incisions , two or three 10-0 nylon
sutures are applied at the graft host junction 90 deg.
away from the steepest meridian.

34. .corneal wedge resection:
-to correct an astigmatism of 10-20D before repeating
the penetrating keratoplasty.
-corneal wedge of 1.0-1.5mm wide base and 90 deg. in
extent is made.
-gap is sutured by five to seven deep interrupted 10-0
nylon or prolene sutures.

35. .
Ruiz procedure:
-if a corneal resection fails/ patient has a highly myopic
spherical equivalent.
-if significant anisometropia exists such as post-keratoplasty
eye with more myopic eye.
-deep horizontal keratotomy incisions are made with a
guarded diamond blade in a ‘step ladder pattern’ along
the axis of steepest corneal meridian.
-it is imp. To ensure that the horizontal and radial incision
donot intersect (as this causes gaping and poor wound
healing)

36. Ruiz procedure

37. .
 Toric IOL:
Toric IOLs refer to astigmatism correcting
intraocular lenses used at the time of cataract
surgery to decrease post-operative
astigmatism.
Patient should have a visually significant
cataract and astigmatism.

38. . The toric lenses currently available are designed to
correct regular corneal astigmatism. Patients with
irregular astigmatism do not fare as well.