Astigmatism

Astigmatism

Astigmatism

Astigmatism is type of refractive eror in which refraction varies in diffrent media.
Types-regular and irregular.
Regular=refractiue power changes uniformly frm one meridian to another.
ETIOLOGY-1.Corneal astigmatism is d result of abnormalities of curvature of cornea.Most comon.
2.Lenticular astigmatism-a)curvatural due to abnormalities of curvature of lens.
b)positional due to tilting of lens as in subluxation..
c)index astigmatisms may ocur rarely.
Types of regular astigmatism-1.With rule astigmatism=when 2 principal meridia r placd at right angles to one another but vertical meredian is more curvd than horizontal.Corection requires cocave cylindes at 180 +-20 degree or convex cylindrical at 90+-20
2.Against rule astigmatism refers to condition in which horizontal meridian is more curvd than vertical.Correction requires convey cylindrical lens at 180+-20degree.
3.Oblique astigmatism-in which two principal media s nt horizontal and vertical thougi these are at right angle to 1 another.Corection requires cylindrical lens at 30 in both eyes.Or comlimentary cylindrical in 30 in 1 eye and 150 in other eye.
4.Bioblique astigmatism-two principal media r nt at right angle to each other..
Refractive type of regular astigmatism-1.Simple when rays r focussd in one meridian and either in front or behind. Simle myopic or simple hypermetropic respectively.
2.Compound-in which rays of light in both the meridia r focusd either in front or behind.
3.Mixed astigmatism in which light rays in 1 meridian r focusd in front and in other meridian behind the retina.Thus 1 meridian eye is myopic and another hypermetropic.
Symptoms-1.Defective vision 2.Blurin of objects .3.Object may b proportionaly elongated4.Asthenopic symptoms.
Signs-1.Difrent power in 2 media.
2.Oval or tilted disc
3.Head tilt-may develop torticolis to have good vision.
4.Half closure of eyelid.
INVESTIGATIONS-
1.Retinoscopy reveal difrent power in 2 difrent axis
2.Keratometry
3.Astigmatic fan test and jackson cross cylinder test.
TREATMENT-1.Optical treatment of regular astigmatism comprises following
a)spectacle with ful corection of cylindrical power and corect axis.
b)contact lenses-rigid contact lenses may corect upto 2-3 of regular astigmatism
2.Surgical correction of astigmatism is quite efective.
IRREGULAR ASTIGMATISM-
Characterisd by an irregular change of refractive power in diffrent media.
Etiological types-
1.Curvatural irregular astigmatis m found in patient of extensive corneal scar.
2.Index irregular astigmatirm due to variable refractive index in difrent parts in lens.
Symptoms-1.Defective vision
2.Distortion of objects
3.Polyopia
Investigations-1.Placido disc test reveals distorted circles.
2.Photokeratoscopy and computerisd corneal topography reveal irregular corneal curvature.
TREATMENT-1.Optical treatment consists of contact lens which replaces anterior surface of cornea.
2.Phototherapeutic keratectomy may b helpful.
3.Surgical treatment indicated in extensive corneal scarring and consists of penetrating keratoplasty.