Published on March 2, 2014
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
Astigmatismus (griech. ‚Punktlosigkeit‘) steht für: Hornhautverkrümmung, ein Brechungsfehler des Auges, siehe Astigmatismus (Medizin) ein optischer ...
In the analysis of this form of astigmatism, it is most common to consider rays from a given point on the object, which propagate in two special planes.
Übersetzung für astigmatism im Englisch-Deutsch-Wörterbuch dict.cc.
Astigmatism is an optical defect in which vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused ...
WebMD explains astigmatism, a common eye condition which is easily corrected.
Bei den Acuvue Oasys for Astigmatism handelt es sich um spezielle Kontaktlinsen für Menschen, die unter einer Hornhautverkrümmung leiden.
AIR OPTIX ® for Astigmatism sind torische Kontaktlinsen, die eine Hornhautverkrümmung korrigieren und eine konstante, ...
Wolfram schrieb am So, 26. Jul 2015 zu 1 Day Acuvue Moist Astigmatism 30er: Nie wieder Johnson & Johnson beim Kundenservice wird man ausgelacht bei einem ...
Astigmatismus (gr. A = nicht, Stigma = Punkt, also die Punktelosigkeit), auch Stabsichtigkeit oder Hornhautverkrümmung genannt, bezeichnet in der ...
Hornhautverkrümmung (med. auch "Astigmatismus" genannt) ist eine Fehlsichtigkeit des Auges. Sie führt zu (teilweise) unscharfem Sehen. Anders als bei ...