Astigmatism, Aniseikonia & Anisometropia

Presenter : Dr. Om Patel

Moderator: Dr. Minal Kaur

REFRACTION

Refraction of light occurs when light passes from one

medium to another of different refractive index.

REFRACTIVE PHYSIOLOGY

Light rays are focused on the retina because they are refracted

passing through the cornea and lens (Snell’s Law)

Corneal refractive power is constant

Lens refractive power is modifiable with accommodation

EMMETROPIA

Adequate correlation between axial length and refractive power Parallel light rays fall on the retina (relaxed accommodation)

FoveaLight rays

ASTIGMATISM

• Type of refractive error wherein the light fails to come to a single focus on the retina

• Unequal refraction of light in different meridia

• Resulting in blurred vision

• Based on axis of the principal meridia:

• Regular astigmatism – principal meridia are perpendicular

• Irregular astigmatism - principal meridia cannot be defined

E.g. Keratoconus, Corneal scars

TYPES OF ASTIGMATISM

Regular astigmatism

The refractive power changes regularly from one meridian to the other

Aetiology

Corneal – curvatural abnormalities

Lenticular – rarer

- Curvatural: congenital abnormalities in curvature

- Positional: d/t tilting or oblique placement of the lens

- Index : DM and cataract

Types of Regular astigmatism

• With-the-rule astigmatism

• Against-the-rule astigmatism

• Oblique astigmatism

• Bi-oblique astigmatism

With the rule astigmatism

• Vertical meridian is steeper than horizontal meridian

• Eyes see vertical lines more sharply than horizontal lines

• Requires concave cylinders at 180 ͦ+ /- 20 ͦ

• Or convex cylinders at 90 ͦ +/- 20 ͦ

Against the rule astigmatism

• Horizontal meridian is steeper than vertical meridian

• Eyes see horizontal lines more sharply than vertical lines

• Requires concave cylinders at 90 ͦ +/-20 ͦ

• Or convex cylinders at 180 ͦ+/-20 ͦ

Oblique astigmatism

• Two principle meridians lies somewhere between the axis defining either with-the-rule or against-the-rule astigmatism

• Complementary (45 ͦin one meridian,135 ͦ in other meridian)

Bi-oblique astigmatism

• Principle meridia are not at right angle

• One at 30 ͦ& other at 100 ͦ

• Rarely seen

Refractive types of regular astigmatism

• Simple

Rays are focused on retina in one meridian or either in front or behind in the other.

• Compound

Rays of light in both meridia are focused either in front or behind the retina.

• Mixed

Light rays in one meridian are focused in front and in the other meridian behind the retina.

EXERCISE

1. -1.00 DC * 180

- SIMPLE WTR

2. -1.00 DS / -2.00 DC * 170

- COMPOUND WTR

3. +1.00 DS / -3.00 DC *180

- MIXED WTR

4. +1.00 DS *165

- SIMPLE ATR

5. +1.00 DS / +2.00 DC *180

- COMPOUND ATR

6. -1.00 DS / -2.00 DC * 135

- OBLIQUE

Optics of regular astigmatism• Parallel rays of light are not brought to focus on a point

but form focal lines

• Configuration of rays refracted from an astigmatic surface– STURM’S CONOID

• Distance between to focal lines– focal interval of Sturm (measures the degree of astigmatism)

Symptoms Blurring of vision

• Transient blurring of vision

• Relieved by rubbing or closing eyes

• Tries to focus only on one meridian, meridian near to emmetropia (mostly vertical) is chosen

• Point of light appears tailed off

• A line appears as a succession of strokes fused into blurred image

Asthenopic symptoms

• Includes tiredness of eyes

• Headache(mild frontal ache to violent explosions of pain)

• Dizziness

• Irritability, fatigue

• Symptoms more common in patients with

Low astigmatism

Hypermetropic astigmatism

Symptoms

• Tilting of head- to reduce image distortion.

• Half closure of lids – high astigmatism in an effort to make a stenopaeic slit

• Rubbing of eyes, hyperaemia of lid margin recurrent styeand chalazia

Investigations-Retinoscopy

• Shows different power in two different axis

KERATOMETER

Bausch and Lomb keratometer Javal and schiotz keratometer

JACKSON’S CROSS CYLINDER TEST

It is a 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.

COMPUTERISED CORNEAL TOPOGRAPHY• Corneal topography

system or videokeratography implies computerised video assisted technique that provides detailed information about shape of corneal surface

Treatment Options

• Spectacles

• Single vision glasses with cylinder

• Contact lenses

• Toric soft contact lenses

• Rigid gas permeable contact lenses

• Refractive surgery

• Photorefractive keratectomy (PRK)

• Laser in-situ keratomileusis (LASIK)

• Penetrating Keratoplasty (PK)

• Corneal Cross-Linking With Riboflavin and Ultraviolet Irradiation

AAO GUIDELINES

• In children, give the full astigmatic correction

• In adults, try the full astigmatic correction first. Give warning and encouragement. If problems are anticipated, try a walking-around trial with trial frames before prescribing

• To minimize distortion, use minus cylinder lenses and minimize vertex distances

• Spatial distortion from astigmatic spectacle corrections is a binocular phenomenon. Occlude one eye to verify that this is indeed the cause of the patient's complaints

AAO GUIDELINES

• If necessary, reduce distortion still further by rotating the cylinder axis toward 180 or 90 (or toward the old axis) and or by reducing the cylinder power. Balance the resulting blur with the remaining distortion, using careful adjustment of cylinder power and sphere.

• Residual astigmatism at any position of the cylinder axis may be minimized with the Jackson cross cylinder test for cylinder power.

• If distortion cannot be reduced sufficiently by altering the astigmatic spectacle correction , consider contact lenses (which cause no appreciable distortion) or iseikoniccorrections.

Irregular astigmatism

• Characterized by an irregular change of refractive power in different meridia

Etiological types

1. Corneal irregular astigmatism – corneal scars or keratoconus.

2. Lenticular irregular astigmatism

3. Retinal irregular astigmatism – due to distortion of the macular area

Symptoms: Defective vision , distortion of objects and polyopia.

Treatment• Optical – prescribing contact lenses which replace the

ant. Corneal surface.

• Surgical – indicated in extensive corneal scarring and consists of PK.

ANISEIKONIA

• Anomaly of binocular vision

• Ocular images are unequal in size or shape or both

• Its importance lies in causation of eye strain which is difficult to assess

ETIOLOGY

• Inherent

• Acquired anisometropia of high degree

Optical aniseikonia

• Displacement of retinal elements towards nodal point in one eye

Retinal aniseikonia

• Asymmetrical simultaneous perception in spite of equal size of two images

Cortical aniseikonia

CLINICAL TYPES

Symmetrical

i. Spherical.

Image may be magnified or minified equally in both meridia

ii. Cylindrical

Image is magnified or minified symmetrical in one meridian

A – Spherical B – Cylindrical C – Prismatic D – Pincushion E – Barrel distortion F – Oblique distortion

Asymmetrical i. Prismatic

Image difference increases progressively in one direction

ii. Pincushion

Image distortion increases progressively in both directions, as seen with high plus correction in aphakia

iii. Barrel

Image distortion decreases progressively in both direction, as seen with high minus correction

iv. Oblique

Image size remains the same but there occurs an oblique distortion of shape

Symptoms

• Asthenopic symptoms

Occurs when the differences in image size of the two images is between 0.75 to 5.0%

Headache , difficulty reading, photophobia, difficulty of fixation, vertigo, etc

• Disturbances of binocular vision

Diplopia occur only if the difference exceeds 5%

• Disturbances in depth perception and spatial disorientations

• Suppression of one eye

Treatment

• Optical aniseikonia

- Unilateral aphakia is best corrected by IOL implantation

- Contact lenses are a better choice than spectacles for correcting anisometropic aniseikonia

- Refractive corneal surgery

• Retinal aniseikonia - Corrected by treating the causative disease

ISEIKONIC LENSES

• Lenses which cause magnification without introducing any appreciable refractive power by changing the direction of the pencils of light passing through them

ANISOMETROPIA

• Total refraction of the two eyes is unequal

• Difference of 1D in two eyes cause a 2% difference in the size of the two retinal images

• 5% size difference / 2.5D - well tolerated

• 2.5-4D – individual sensitivity

• >4D – not tolerated

• Alternate vision when one of the two eyes is used at a time

• Occurs when one eye is emmetropic or moderately hypermetropic and the other myopic

• Patient uses former eye for distant vision and latter for near vision

• Anisometropia leads to development of squint

UNIOCULAR VISION

• If defect in one eye is higher, it may be excluded from vision at an early stage in life

• The more ametropic eye becomes amblyopic

• This amblyopia from disuse is known as amblyopia ex anopsia

ETIOLOGY

Congenital and developmental

Acquired

Uniocular aphakia

Wrong power IOL

Trauma

Keratoplasty

CLINICAL TYPES

Simple

One eye normal, other myopic / hypermetropic

Compound

Both eyes either myopic / hypermetropic

Mixed

One eye myopic, other eye hypermetropic

DIAGNOSIS

Retinoscopy under cycloplegia

State of vision- FRIEND TEST / WORTH’S FOUR DOT TEST

FRIEND Test

F, I, N - GREEN

R,E,D - RED

1. The patient wears red green goggles and is seated at a distance of 6m from the chart

2. Binocular single vision- will read FRIEND at once

3. Uniocular vision – will read either FIN or RED

4. Alternate vision– will read FIN at one time and RED at other time

WORTH’S FOUR DOT TESTSees all four lights in

absence of manifest squint-normal binocular vision

ARC- sees four lights in presence of manifest squint

Sees 2 red lights- LE suppression

Sees 3 green lights- RE suppression

Sees 2 red and 3 green alternately- alternate suppression

Sees 5 lights(2red, 3 green)- diplopia

TREATMENT

Glasses• In children prescribe full refractive difference regardless

of age, presence of strabismus or not, degree of anisometropia

• The corrective spectacles can be tolerated up to a maximum difference of 4D,after that diplopia occurs.

• So in children where best corrected visual acuity is required in both eyes, contact lenses are preferred

• In adults, the more ametropic eye is under corrected

• In adults with alternating vision the condition is usually left alone

CONTACT LENSES

• Advised for higher degrees of anisometropia and for children

• Anisometropic spectacles-

In these spectacles margin of the stronger lens is made weaker, thus minimizing the annoyance of peripheral prismatic effect of conventional lenses

OTHER MODALITIES

• Intraocular lens implantation for uniocular aphakia

• Refractive corneal surgery for unilateral myopia,astigmatism,hypermetropia

• Phakic refractive lenses(PRL) - for 4 to 10D

• Refractive lens exchange(RLE) - for more than 10D

THANK YOU