Astigmatism This diagram represents the imaging of an astigmatic system (convex sphero-cylindrical lens or an astigmatic eye). Below the optical representation are the cross-sections that would be found if a screen was placed perpendicular to the optical axis at the positions shown. The line foci, ellipses and their orientations are shown, along with the circle of least confusion.
The figure shows the imaging of a positive spherical lens, of radius of curvature r s . The principal meridia are seen at 90 and 180 . The principal meridia of a lens are always positioned at right angles to each other, whatever the orientation of the lens in front of the eye. The power of a spherical lens is related to the radius of curvature of the lens surfaces and the material from which the lens is made. At each surface the power can be calculated as follows: F = (n’-n) / r where n’ is the refractive index of the media that the incident light is entering into and n is the refractive index of the media that the lens is coming from. Often the lens is in air, so the power equation at the front surface would read: F 1 =(n’ - 1) / r 1 where F 1 is the power of the front surface, n’ is the refractive index of the lens material and r 1 is the radius of curvature of the front surface. The power equation at the back surface of the lens would read as follows: F 2 = (1-n’) / r 2 where F 2 is the power of the back surface of the lens and r 2 is its radius of curvature.
The figure shows a cylindrical lens. Light incident normal to the lens surface will pass through undeviated. Light incident towards the edge of the lens will meet a curved surface and will cone to a point focus along the optic axis. If light incident on the upper portion of the lens is considered, it can be seen that rays grazing the edge of the lens will also be refracted to a single point focus, but the point focus will not be coincident with the point focus on the optic axis. This is due to there being no condensing power in the vertical plane of the lens. If any series of rays between those illustrated were traced, a series of point foci would be seen to fall between the two extremes drawn forming a line image for the point object.
The dioptric separation of the line foci is known as the Interval of Sturm and this distance should be equivalent to any axial astigmatism that the cylindrical lens is to be used to correct.
CONTENTS OF TODAY’S LECTURE What is Astigmatism? Incidence Optics Etiology Classification Signs & Symptoms Diagnosis Treatment
ASTIGMATISMAstigmatism is a refractive error of the eye in which thereis a difference in degree of refraction in different meridians(i.e. the eye has different focal points in different planes.) For example, the image may be clearly focused on theretina in the horizontal (sagittal) plane, but not on theretina in the vertical (tangential) plane.Astigmatism causes difficulties in seeing fine detail, and insome cases vertical lines (e.g., walls) may appear to thepatient to be leaning over.
WHAT IS ASTIGMATISM?Most astigmatic corneas have two curves, a steepercurve and a flatter curve. This causes light to focus on more than one point in the eye, resulting inblurred vision. The cornea is normally spherical, although in people withastigmatism, it may be shaped like a rugby ball or ovalinstead of tennis ball.
OPTICS OF ASTIGMATISMIn astigmatism, the rays of light from one sectorfall on one point & rays from another sector fall onanother point. In other words, a point focus oflight cannot be formed upon the retina.The configuration of rays refracted through theastigmatic surface (toric surface) is called sturmsconoid.
ASTIGMATISM Vertical Focal Line Circle of Least C Confusion Horizontal Focal LinePower Meridian A Axis Meridian B DObjectSource Interval of Sturm
OPTICS OF ASTIGMATISMThus, there are 2 focal points separated fromeach other by a focal interval, called as intervalof sturm.The length of this focal interval is the measureof the degree of astigmatism & the correction ofthe error can only be accomplished by reducingthese two foci in to one.
INCIDENCENo eye is perfectly stigmatic as almost all individuals have aminor degree of physiological astigmatism.About 60% cases of refractive errors have astigmatism whichneeds to be corrected.Occurs with equal frequency in males and females.Approximate distribution according to degree of astigmatismis: 0.25-0.5 D 50% 0.75-1.0 D 25% 1.00-4.00D 24% >4.00 1.0%
INCIDENCEThe most common type is compound myopicfollowed by compound hyperopic, mixed, simplemyopic & simple hyperopic.One study reports as: With the rule 38% Against the rule 30% Oblique 32%
Severity of Astigmatism The severity of astigmatism can be classified as follows: Mild Astigmatism < 1.00 diopter Moderate Astigmatism 1.00 to 2.00 diopters Severe Astigmatism 2.00 to 3.00 diopters Extreme Astigmatism > 3.00 diopters
CLASSIFICATION1. Astigmatism - Based on asymmetry of structure• Corneal astigmatism - astigmatism due to an irregularly shaped cornea• Lenticular astigmatism - astigmatism due to an irregularly shaped lens
CLASSIFICATION2. Astigmatism - Based on axis of the principal meridiansa. Regular astigmatism Against-the-rule astigmatism With-the-rule astigmatismb. Oblique astigmatismc. Bioblique astigmatismd. Irregular astigmatism
a. Regular astigmatism:The astigmatism is said to be regular if there is differentrefraction by the eye in two meridia at right angles to eachother.Can be corrected with spectacles.Normally, horizontal curvature of cornea is flatter thanvertical & this is attributed to the pressure of lids on thecorneal surface. This is physiological. So, vertical corneashould be more curve than horizontal.On this basis, it has two types: With the rule (WTR) & against the rule (ATR)
With-the-rule (direct astigmatism):Principle meridia are at right angle to each other.Vertical curve is more than horizontal.Concave cylinder is prescribed in horizontal axis(180) and convex are prescribed in vertical axis(90).Normally the vertical meridian is rendered 0.25 Dmore convex than horizontal by the pressure offleshy upper eyelid.
Against-the-rule (indirect astigmatism):Principle meridia are at right angle to each other.Horizontal curve is more than Vertical.Convex cylinder is prescribed in horizontal axis(180) and concave are prescribed in vertical axis(90).Usually associated with old age.
b. Oblique astigmatism: A type of astigmatism in which principle meridia are not horizontal or vertical but are at right angle to each other (45 & 135). Usually symmetrical in both the eyes (cylinder required at 30 in both the eyes) Or complementary (cylinder required at 30 in one eye & 150 in other eye)
c. Bioblique astigmatism: In this type of astigmatism, the two principle meridia are not at right angle to each other. e.g. one may be at 30 & other at 100.
d. Irregular astigmatism:o It is characterized by an irregular change of refractive power in different meridia.o There are multiple meridia which admit no geometrical analysis.o Cannot be corrected by spectacles.o It occurs due to corneal scars, during maturation of cataract, etc.
CLASSIFICATION3. Astigmatism - Based on focus of the principal meridians Simple astigmatism Simple hyperopic astigmatism Simple myopic astigmatism Compound astigmatism Compound hyperopic astigmatism Compound myopic astigmatism Mixed astigmatism
SIMPLE ASTIGMATISM In simple astigmatism, one of the foci falls on retina & other focus falls in front or behind retina. This leads to one meridian being emmetropic & other being myopic (one focus on the retina & other focus falls in front of retina) or hyperopic (one focus on retina & other focus behind retina), so called as simple myopic astigmatism & simple hyperopic astigmatism respectively. It can be with-the-rule or against-the-rule. -2 D cyl at 90 is example of simple myopic astigmatism. +2 D cyl at 90 is example of simple hyperopic astigmatism.
COMPOUND ASTIGMATISM• Neither of the two foci fall on the retina.• The condition is known as compound hyperopic if both foci are at back of retina.• The condition is known as compound myopic if both foci are at front of retina.• It can be with-the-rule or against-the-rule.• -3 DS with -2DC at 90 is example of compound myopic astigmatism.• +3 DS with +2DC at 90 is example of compound hyperopic astigmatism.
MIXED ASTIGMATISM In mixed astigmatism, one of the two focilies at back while other at front of the retina.It can be with-the-rule or against-the-rule.-3 DS with +8DC at 90 is an example ofmixed astigmatism.
NOTE:If cyl power is less than spherical power, then it isnot mixed but compound astigmatism.For example, -3DS with +1DC at 180 sounds as if itis mixed astigmatism, but actually is compoundastigmatism, as cyl is less than sphere.
RESIDUAL ASTIGMATISM The largest element of the total astigmatism is due to anterior corneal surface. While the other components like: Posterior corneal surface Lens Refractive indices constitute the residual astigmatism. RESIDUAL ASTIGMATISM= TOTAL – CORNEAL ASTIGMATISM
SIGNS & SYMPTOMS Type of the symptoms produced, depends upon the type of astigmatism:1. Blurring of vision: Transient blurring of vision in low astigmatism. Relieved by closing/rubbing the eyes. Circles elongate into ovals. A point of light appears tailed off. A line appears as a succession of strokes fused into a blurred image.
SIGNS & SYMPTOMS2. Asthenopic symptoms: More marked in patients with low astigmatism (more accommodative effort) Severe in hyperopic astigmatism (more accommodative effort) Tiredness of eyes Headaches (from mild frontal ache to explosions of pain) Nervous disturbances: Dizziness Fatigue Irritability
SIGNS & SYMPTOMS3. Tilting of the head: Some patients with high oblique astigmatism, mayhold the head tilted to one side to reduce imagedistortion. Some children may even develop scoliosis.(The condition of side-to-side spinal curves is called scoliosis. On an X-ray, the spine of an individual with scoliosis looks more like an "S" or a"C" than a straight line.)
SIGNS & SYMPTOMS4. Half closure of the lids: Seen in patients with high astigmatism. This is to make a sort of stenopaeic slit & cutting out the rays from one meridian.. This also causes Asthenopic symptoms.
SIGNS & SYMPTOMS5. Reading material is held too close: Reading material is held too close to the eyes by the patient to achieve blur but large image just like a myope.
SIGNS & SYMPTOMS6. Burning & itching: May be seen in patients with low astigmatism B/c of rubbing the eyes Falling of eye lashes Hyperemia Styes& chalazia
DIAGNOSIS VA with and without correction monocularly Pinhole VA Retinoscopy Keratometry Keratoscopy with placido’s disc Computerised corneal topography/videographSubjective verification: Jackson cross cylinder Astigmatic fan & block Trial & error technique (axis then power) Maddox V Stenopaeic slit
The typical spiral pattern of keratoconus progression. Incolor-coded topographic images, red represents steepercorneal curvature, and the spectrum of yellow, green,and blue represents progressively flatter curvatures.
SPECTACLESAstigmatism is corrected optically with a cylindricallens.A combination of a spherical lens and a cylindricallens (spherocylindrical lens) is used to correct aspherical error with an astigmatic error.Cyl has power (curvature) in one meridian and nopower in the other meridian.The axis of the cylinder is lined up with the axis ofastigmatism to correct the astigmatic powerdifference.
Spherical LensSpherical surfaces Optical Axis Principal Meridian
Cylindrical Lens Optical Axis Principal Meridian
CONTACT LENSESVarious types of contact lenses are used: Soft Hard Rigid gas permeable Hybrid (hard center & soft periphery, used in keratoconus)Depending upon the degree of astigmatism: Spherical Toric Bitoric
Eugene Kalt, MD, first to propose the use of a contact lens for keratoconus.
REFERENCES Theory and practice of optics and refraction by A K Khurana Duke Elders Practice of refraction (Tenth edition) Clinical Optics by Elkington, Frank and Greaney (Third edition) Text book of ophthalmology (Volume : 1) by Jaypee publishersand many websites.