ASTIGMATISM, ANISOMETROPIA AND ANISOKONIA

PRESENTOR- Dr Vishy SrivastavaMODERATOR- Dr Alka Jasrasaria

CONTENTS1. WHAT IS ASTIGMATISM?

2. INCIDENCE

3. OPTICS

4. ETIOLOGY

5. CLASSIFICATION

6. SIGNS AND SYMPTOMS

7. DIAGNOSIS

8. TREATMENT

ASTIGMATISM- Refractive error- Difference in degree of refraction in different meridians- Image may be clearly focused on the retina in the horizontal

(sagittal) plane, but not in the vertical (tangential) plane.

Most Astigmatic Corneas have 2 curves, steeper and flatter curves

Light focus on more than one point in eye

Cause blurring of vision

INCIDENCEAbout 60% cases of refractive errors have astigmatism which needs to be corrected. Occurs with equal frequency in males and females. Approximate distribution according to degree of astigmatism is: 0.25-0.5 D - 50% 0.75-1.0 D - 25% 1.00-4.00D - 24% >4.00 - 1.0%The most common type is compound myopic followed by compound hyperopic, mixed, simple myopic & simple hyperopic.

One study reports as:With the rule -38%Against the rule -30%

Oblique -32%

OPTICS- Rays of light from one sector fall on one point & rays from another sector fall on another point i.e point focus not formed on retina Configuration of rays refracted through astigmatic surface(toric surface) is called strums conoid Two focal points formed seperated by focal interval called interval for strum Length of focal interval forms the measure of degree of astigmatismReducing these two foci will lead to correction of astigmatism

Axis Meridian

Horizontal Focal

Line

Vertical Focal Line

Circle of Least Confusion

Power Meridian

Object Source

AB D

C

Interval of

Sturm

ETIOLOGY TYPES

CORNEAL ASTIGMATISM

LENTICULARASTIGMATISM RETINAL

ASTIGMATISM- D/t abnormalities of

curvature- Most common type- E.g Keratoconus,

Chalazion,Pterygium, mild corneal opacities

Rare

CurvaturalEg Lenticonus

PositionalEg Tilting or subluxation of lens

Index eg Catract/Nuclear sclerosis/Diabetic catract

Oblique placement of macula

SEVERITY OF ASTIGMATISMMild : <1.0DModerate : 1.0 – 2.0DSevere : 2.0 – 3.0DExtremely severe : >3.0D

CLASSIFICATION ASTIGMATISM

PHYSIOLOGICAL PATHOLOGICAL

REGULAR IRREGULAR

SIMPLE COMPOUND MIXED

-MYOPIC -MYPOIC-HYPEROPIC -HYPEROPIC

BASED ON SYMMETRY

CORNEAL ASTIGMATISM LENTICULAR ASTIGMATISMD/t Irregularly shaped cornea D/t Irregularly shaped Lens

BASED ON AXIS

REGULAR OBLIQUE BIOBLIQUE IRREGULAR

WITH THE RULE AGAINST THE RULE

REGULAR ASTIGMATISM- If there is different refraction by the eye in two meridia at

right angles to each other.- Pressure of lids on the corneal surface

- Horizontal curvature of cornea is flatter than vertical- This is physiological- Corrected by spectacles- It is with the rule.

WITH THE RULE(DIRECT ASTIGMATISM)- Principal meridia is right angle to each other.- Vertical more curved than horizontal.- T/t – Concave in horizontal and convex in vertical axis.- Vertical meridian is rendered 0.25 D more convex than

horizontal by the pressure of fleshy upper eyelid

AGAINST THE RULE(INDIRECT ASTIGMATISM)- Horizontal more curved than vertical.

- T/t – Convex in horizontal axis and concave in vertical axis.

- Associated with old age.

OBLIQUE ASTIGMATISM- Principle meridia are not horizontal or vertical but are

at right angle to each other (45 & 135).- Usually symmetrical in both eyes (cylinder required at

30 in both eyes).- Or complimentary (cylinder required at 30 in one eye

and 150 in other).

BIOBLIQUE ASTIGMATISMThe two principle meridia are not at right angle to each

other. e.g. one may be at 30 & other at 100.

IRREGULAR ASTIGMATISM- Irregular change of refractive power in different meridia

- Multiple meridia which admit no geometrical analysis

- Cannot be corrected by spectacles

- Occurs due to corneal scars, during maturation of cataract, etc

BASED ON FOCUS OF PRINCIPAL MERIDIA

SIMPLE COMPOUND MIXED

SIPMLE SIMPLE HYPEROPIC MYOPIC COMPLUND

COMPOUNDHYPEROPIC MYOPIC

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

i.e one focus on the retina and other in front the retina or Hyperopic i.e one focus on the retina and other behind the 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 two foci fall on the retina.- The condition is known as compound hyperopic if both

foci are at back of retina.- +4DS with +2DC at 90 is example of compound

hyperopic astigmatism.

- The condition is known as compound myopic if both foci are at front of retina. -4 DS with -2DC at 90 is example of compound myopic astigmatism

MIXED ASTIGMATISMOne of the two foci lies at back while other at front of the retina.

It can be with-the-rule or against-the-rule.

-4 DS with +2DC at 90 is an example of mixed astigmatism.

If cyl power is less than spherical power, then it is not mixed but compound astigmatism.

RESIDUAL ASTIGMATISM- Largest element of astigmatism is due to anterior corneal surface, while other component like Posterior corneal surface, lens and refractive indices constitute residual astigmatism.- RESIDUAL ASTIGMATISM = TOTAL – CORNEAL ASTIGMATISM

SIGNS AND SYMPTOMS1.BLURRING OF VISONTransient BOV in low astigmatismRelieved by closing/Rubbing eyesCircles elongate to oval, point of light appears to be tailed off2.ASTHENOPIC SYMPTOMSMore marked in low astigmatism and severe in Hyperopic AstigmatismTiredness of eyesHeadacheDizziness, Fatigue, Irritability3. TILTING OF HEADIn high oblique Astigmatism

4.HALF CLOSURE OF LIDS

In high Astigmatism To make a stenopaic slit and cutting out the rays from one meridian.

5.READING MATERIAL HELD TOO CLOSE

Achieve blur but large image just like myope.

6.BURNING AND ITCHING SENSATION

May be seen in low astigmatism.

DIAGNOSISVA with and without correction monocularly

Pinhole VA

Retinoscopy

Keratometry– For corneal curvature

Keratoscopy with placido’s disc

Computerised corneal topography/ videograph – For determining corneal curvature

Subjective verification:Jackson cross cylinder – For Power and Axis

Astigmatic fan & block – Fogging technique using astigmatic fan test.

Trial & error technique (axis then power)

Maddox V

Stenopaeic slit

JACKSON CROSS CYLINDER - Combination of equal strength but oppositeAxis placed at right angle to each other

DISCOVERING ASTIGMATISM-Cross cylinder placed at 900 and 1800 -If preferred flip found, a cylinder is added with axis parallel to respective plus or minus axis until two flip choices matchREFINEMENT OF AXIS- 0.50D Cross cylinder placed with its handle parallel to axis and patient asked to tell any changes in VA-If no difference- Correct axis placed-If VA attained in one position, “plus” correcting cylinder rotated in the direction of plus componentREFINEMENT OF CYLINDER POWER-0.25D cross cylinder placed with axis parallel to axis of cylinder.-In first position, cylindrical correction enhanced by 0.25D and in 2nd diminished by same amount-If VA not improved- correct power of cylinder

ASTIGMATIC DIAL WITH FOGGING OBTAIN BCVA WITH SPHERE & OTHER EYE OCCLUDED

FOG THE EYE BY + SPHERE TO FOCUS ALL MERIDIA ANTERIOR TO RETINA

PATIENT ASKED TO LOOK AT ASTIGMATIC DIAL AND IDENTIFY DARKEST AND SHARPEST LINE

ADD MINUS CYLINDER OF PROGRESSIVELY INCREASING POWER WITH AXIS PERPENDICULAR TO DARKEST AND SHARPEST LINE TILL ALL LINES ARE EQUAL

THIS CAUSES VERTICAL FOCAL LINES TO MOVE BACK TO THE POSITION TO HORIZONTAL FOCAL LINES i.e INTERVAL OF STRUM IS COLLAPSED- SWITCH TO DISTANCE VISION CHART AND REDUCE PLUS SPHERE TILL PATIENT ACHIEVE MAXIMUM CLARITY

ATIGMATIC FAN TEST

STENOPAIC SLIT

TREATMENT1. SPECTACLES Cylindrical lens used Combination of Spherical and Cylindrical used to correct spherical with astigmatic error2. CONTACT LENSES Soft Hard Rigid gas permeable Hybrid (hard center & soft periphery, used in keratoconus)

Depending upon the degree of astigmatism:SphericalToricBitoric3. LASER AND REFRACTIVE SURGERIESPhotorefractive keratectomy PRKRelaxing incisions (transverse & arcuate keratotomy)Wedge resectionCompression suturesContinuous sutures ( astigmatism low)Interrupted sutures (astigmatism high)Orthokeratology (hard lens)Keratoplasty (keratoconus)

ANISOMETROPIA• Condition in which the two eyes have unequal refractive power.• Difference of 1 D in two eyes - 2 % difference in size of the two

retinal images• 5% size difference / 2.5 D - Well tolerated .• 2.5 – 4 D - Individual sensitivity • >4 D - not tolerated

ETIOLOGY CONGENITAL- D/t differential growth of the eyeballs.

ACQUIRED- D/t uniocular aphakia after removal of crystalline lens or due to implantation of IOL of wrong power.

CLASSIFICATION

ABSOLUTE RELATIVE - The refractive power of two - Total refraction of the two eyes can be equal, eyes is unequal. but the axial length may be different - This lead to clear retinal image but a difference in the size of the retinal images

TYPES OF ANISOMETROPIA• Simple• Compound • Mixed• Simple astigmatic anisometropia• Compound astigmatic anisometropia• Mixed astigmatic anisometropia

1. SIMPLE ANISOMETROPIAOne eye Myopic and other either myopic(Simple Myopic) or a Hypermetropic (Simple Hypermetropic)

2. COMPOUND ANISOMETROPIABoth eyes are either hypermetropic or myopic , but one eye is having higher refractive error than the other3. MIXED ANISOMETROPIA In this , one eye is myopic and the other is hypermetropic. This is also called antimetropia

4. SIMPLE ASTIGMATIC ANISOMETROPIAWhen one eye is normal and the other has either simple myopic or hypermetropic astigmatism.

5. COMPOUND ASTIGMATIC ANISOMETROPIA When the both eyes are astigmatic but of unequal degree.

Status of vision in anisometropiaThree possibilities are there :1. Binocular single vision - In small degree of

anisometropia.2. Uniocular vision - When refractive error in one eye is of

high degree. 3. Alternate vision - When one eye is hypermetropic and

other myopic , then hypermetropic eye is used for distant vision and myopic for near.

1.Spectacles. The corrective spectacles can be tolerated up to maximum difference of 4D.After that diplopia may occurs.2.Contact lenses for higher degrees of anisometropia .3.IOL implantation for uniocular aphakia.4.Refractive corneal surgery for unilateral high myopia , astigmatism and hypermetropia.

TREATMENT

ANISOKONIAAnomalies of binocular vision.Ocular images are unequal in size or shape or both . Its importance lies in case of eye strain which is difficult to assess .

ETIOLOGY

OPTICAL ANISOKONIA RETINAL ANISOKONIA CORTICAL ANISOKONIA

-Inherited/Acquired Displacement of retinal image Assymetrical simultaeous-Anisokonia of towards nodal point perception in spite of equal higher degree size of two images.

CLINICAL TYPES• Spherical : Image may be magnified or minified equally in both

meridia.• Cylindrical : Image is magnified of minified symmetrical in one

meridia.• Asymmetrical • Prismatic : Image difference increases progressively in one direction • Pincushion : Image distortion increases progressively in both

direction as seen with high plus correction in aphakia.• Barrel : Image distortion decreases progressively in both direction

as seen in high minus lenses.• Oblique : Image size remain the same but there occurs oblique

distortion of shape.

SYMPTOMS• Asthenopic symptoms : Occurs when the difference in image size

of the two images between 0.75 to 5%• Headache , difficulty in reading, photophobia, difficulty in fixation,

vertigo • Disturbance of binocular vision.• Diplopia occur only if the difference exceeds 5%.• Disturbance in the depth perception and spatial disorientation.• Suppression of one eye

TREATMENT• Optical < IOL in Aphakia.

contact lenses. Refractive surgery.

• Retinal Aniseikonia : corrected by treating the causative disease.

• Cortical Aniseikonia : Difficult to treat .

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