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Information about Astigmatism

Published on March 2, 2014

Author: RohitRao2



Astigmatism in short


 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-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.

 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 headSquinting

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 astigmatism iii.Refinement of the axis iv.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 correctionIncisional 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 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

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