Astigmatism is a condition where the eye's refractive power varies in different meridians, causing blurred vision. It is caused most commonly by corneal irregularities, and results in light rays forming an image called the Sturm's conoid instead of focusing on the retina. Astigmatism can be regular, irregular, or oblique depending on the orientation of the steepest meridians. It is classified and treated based on whether the refractive error causes light to focus in front of, on, or behind the retina. Treatment options include corrective lenses, contact lenses, and refractive surgeries like astigmatic keratotomy.

1. ASTIGMATISM

2.  Term astigmatism (from a, meaning
"privativate" or "lacking," and stigma,
meaning "a point") was suggested to
describe this anomaly by Dr. William
Whewell (1794-1866)
 The refractive power of the astigmatic
eye varies in different meridians.
 The image is formed as a Sturm's
conoid

3.  The configuration of rays refracted through a
toric surface is called the Sturm’s conoid.
 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.
 The distance between the two foc (B and F) is
called the focal interval of Sturm

4.  Astigmatism may be classified as
follows:
 As regular or irregular
 With respect to the contributing
ocular component
 By orientation
 With respect to the refractive error

5.  If the principal meridians are at 90°
to each other, this is called regular
astigmatism.
 If the principal meridians are at 90°
to each other but do not lie at or near
90° and 180°, the term oblique
astigmatism is used.
 If the principal meridians are not at
90° to each other, this is called
irregular astigmatism

6.  Corneal astigmatism is the result of abnormalities
of curvature of cornea. It constitutes the most common
cause of astigmatism.
 Lenticular astigmatism is rare. It may be:
 i. Curvatural due to abnormalities of curvature of lens as
seen in lenticonus.
 ii. Positional due to tilting or oblique placement of lens
as seen in subluxation.
 iii. Index astigmatism may occur rarely due to variable
refractve index of lens in different meridia.
 Retinal astigmatism due to oblique placement of
macula may also be seen occasionally

7.  With-theRule
 This is when
the vertical
meridian is
steepest.
 Againstthe-Rule
 This is when
the
horizontal
meridian is
steepest.
 Oblique
 This is
when the
steepest
curve lies
in
between
120 and
150
degrees
and 30
and 60
degrees.

8.  Retina a = Compound hypermetropic
astigmatism – rays in all meridians
come to a focus behind the retina.
 Retina b = Simple hypermetropic
astigmatism – rays in one meridian
focus on the retina, the other focus
lies behind the retina.

9.  Retina c = Mixed astigmatism – one line
focus lies in front of the retina, the other
behind the retina.
 Retina d = Simple myopic astigmatism –
one line focus lies on the retina, the
other focus lies in front of the retina.
 Retina e = Compound myopic
astigmatism – rays in all meridians come
to a focus in front of the retina.

10. Symptoms



Blurring of vision
Asthenopic symptoms
Tilting of the headSquinting

11. Investigations Retinoscopy
 Keratometry

12.  Jackson cross cylinder test Used to confirm the cylinder power & for
refining axis of the cylinder
 Combination of two cylinders of equal
strength, but with opposite sign placed
with their axis at right angles to each
other and mounted in a handle
 Commonly used cross cylinders are of
±0.25 and ±0.50D

13.  Following steps are used in cross
cylinder refraction:
i. Adjust the sphere to the most plus
or least minus that gives the best
visual acuity
ii. Discovering the astigmatism
iii.Refinement of the axis
iv.Refinement of cylindrical power

14.  Astigmatic fan test The fan block test consists of series of radiating
lines spaced at 10° interval & arranged after the
manner of the rays of rising sun
 There is a central panel carrying a ‘V’ & two sets
of mutually perpendicular lines (the blocks)
 The V & block simultaneously can be rotated
through 180°

15.  Steps of fan & block technique:
-Obtain best visual acuity using sphere only
-Add positive sphere equal to half of estimated
amount of astigmatism
-Refer patient to fan chart, ask which group of
lines appear clearest & darkest
-Directing attention to maddox arrow
-Directing attention now to blocks, add negative
cylinder at appropriate axis until both blocks
equally clear

16. Treatment Optical treatment
-comprises prescribing the appropriate
cylindrical lens, discovering after
accurate refraction
-correction may be in the form of
spectacles, hard contact lenses, toric
contact lenses

17.  2. Surgical
correctionIncisional refractive
procedures
-Astigmatic
keratotomy(AK)
4-6D
-Limbal relaxing
incision 1-2D

18. b. Laser based corneal refractive
procedures
Photoastigmatic refractive
keratotomy

19.  Astigmatic epi-LASIK (epipolis
laser in situ keratomileusis)
 Astigmatic LASIK
 Astigmatic C-LASIK

20. Irregular astigmatism
 characterized by an irregular change of
refractive power in different meridian
 There are multiple meridian which
admit no geometric analysis

21. Aetiological types Corneal irregular astigmatism found in patients
with extensive corneal scars or keratoconus
 Lenticular irregular astigmatism seen due to
variable refractive index in different parts of
crystalline lens & rarely during maturation of
cataract
 Retinal irregular astigmatism due to distortion
of macular area due to- scarring or tumours of
retina and choroid pushing macular area

22. Symptoms Defective vision, distortion of objects and
polyopia
Treatment Optical treatment consists of prescribing
contact lens which replaces the anterior surface
of the cornea for refraction
 Surgical treatment
-indicated in extensive corneal scarring (when
vision does not improve with contact lenses)
-consists of penetrating keratoplasty