ASTIGMATISM

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

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

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

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

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

With-the-Rule

This is when the vertical meridian is steepest.

Oblique This is

when the steepest curve lies in between 120 and 150 degrees and 30 and 60 degrees.

Against-the-Rule

This is when the horizontal meridian is steepest.

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.

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.

Symptoms-

Blurring of vision Asthenopic symptoms Tilting of the head- Squinting

Investigations-

Retinoscopy Keratometry

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

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 astigmatismiii.Refinement of the axisiv.Refinement of cylindrical power

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°

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

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

2. Surgical correction-

Incisional refractive procedures

-Astigmatic keratotomy(AK) 4-6D

-Limbal relaxing incision 1-2D

b. Laser based corneal refractive procedures

Photoastigmatic refractive keratotomy

Astigmatic epi-LASIK (epipolis laser in situ keratomileusis)

Astigmatic LASIK

Astigmatic C-LASIK

Irregular astigmatism

characterized by an irregular change of refractive power in different meridian

There are multiple meridian which admit no geometric analysis

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

Symptoms- Defective vision, distortion of objects and polyopiaTreatment- 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