Anisometropia , Aniseikonia and Amblyopia

Anisometropia

Anisometropia is one of the binocular optical defects. Anisometropia arises because of the difference in refractive error between two eyes. A small degree of anisometropia occurs commonly when there is a small amount of astigmatic error. Each diopter difference between the refraction of the two eyes causes 2% difference between the two retinal images.

Aetiology

Congenital and development anisometropia

This occurs due to differential growth of the two eyeballs.

Acquired anisometropia

1. Uniocular aphakia occurs after caratoconous lens removal
2. From trauma to the eye
3. Due to inadvertent surgical treatment of refractive error

Vision status in anisometropia

Binocular vision: Binocular vision is present in small degrees of anisometropia. An anisometropia of about 1.5 D to 3 D is tolerated depending upon the individual.

Alternate vision: Alternating of vision occurs when one eye is emmetropic or moderately hypermetropic and the other eye is myopic. The emmetropic or moderately hypermetropic eye is used for distance vision and the myopic eye is used for near vision. These patients are usually comfortable and never have to make an effort of either accommodation or convergence.

Uniocular vision: When the refractive error is high in one eye compared to the other, then the high degree refractive error eye receives continuously blurred images compared to the other eye. Due to this the eye receiving blurred image is suppressed and develops amblyopia. This type of amblyopia is called anisometropic amblyopia.

Clinical types

Simple anisometropia In simple anisometropia one eye is emmetropic and the other eye is either myopic or hypermetropic.

Compound anisometropia In compound anisometropia both eyes have refractive error. The refractive error may be hypermetropic or myopic, but one has a higher refractive error than the other.

Mixed anisometropia When the refractive error of one eye is hypermetropic and other eye is myopic it is mixed anisometropia. This is also termed antimetropia.

Simple astigmatic anisometropia When one eye is normal and the other has simple myopic or hypermetropic astigmatism it is termed simple anisometropic astigmatism.

Compound astigmatic anisometropia When both eyes are astigmatic but of unequal degree, it is called compound astigmatic anisometropia.

Mixed astigmatic anisometropia When one eye has hypermetropic astigmatism and the other eye has myopic astigmatism.

Clinical test The visual status is assessed by using either FRIEND test or Worth’s Four Dot test.

FRIEND test

An illuminated word FRIEND is present in the Snellen’s vision box. The alternate letters in the word FRIEND are illuminated with green and red colour. The letters F, I, N are in green and R, E, D in red colour. Red and green goggles are placed in front of the eye such that red is placed over the right eye and green is placed over the left eye. The patient sees only red letters through the right eye and green letters through the left eye. From the patient’s response we can determine whether the patient is using both eyes or not.

Responses

1. If the patient reads FRIEND at once the patient has binocular vision
2. If patient reads either FIN or RED, the patient has Uniocular vision with the eye which has the corresponding glass
3. If the patient first reads FIN and then RED, then he has alternating vision

Worth four dot test

This test is similar to the FRIEND test. This test has four dots. Of these four dots, one dot is red in colour, two are green and one is white. The patient wears the red and green goggles, red in front of the right eye and green in front of the left eye and views the box with four lights.

Results

1. When all four lights are seen they have normal binocular single vision
2. When all the four lights are seen with the presence of a manifest squint then they have abnormal retinal correspondence
3. When they see only two red lights, it indicates left eye suppression. If they see only three green lights, it indicates they have right eye suppression
4. If they see two red lights, alternating with three green lights it indicates presence of alternating suppression
5. If they see five lights (two red lights and three green lights) it indicates diplopia

Treatment

Optical

• Contact lens is the best choice for anisometropia
• In children with anisometropic amblyopia and refractive error correction, occlusion therapy should also be given
• Anisometropic spectacles were given for the correction of anisometropia but these spectacles are obsolete now

Surgery

• Implanting intraocular lens for unilateral aphakia
• Removal of crystalline lens for unilateral high myopia
• Refractive corneal surgeries for unilateral myopia, astigmatism and hypermetropia

Student exercise

Answer the following

1. Define anisometropia.
2. What are the causes and types of anisometropia?
3. What are the different types of tests used for anisometropia?
4. What is the visual status in anisometropia?
5. Explain various treatment modalities for anisometropia.

Aniseikonia

Aniseikonia is one of the binocular optical defects. Aniseikonia (A = not + iso = equal+ konia = image) is a condition in which the size and shape of images of the two eyes are unequal. Aniseikonia of 3% or more becomes clinically significant.

Aetiology

Optical aniseikonia: This occurs due to inherent or acquired anisometropia of high degree.

Retinal aniseikonia: Retinal aniseikonia may develop due to:

• Widely separated arrangements of the visual elements
• Any retinal oedema causing separation of retinal elements

Clinical Types

The difference in the images can be classified as:

1. Symmetrical: It is the difference in the image size perceived in each eye

    a) Overall - the difference is the same in all dimensions

    b) Meridional - the difference is greater in one meridian compared to the other

2. Asymmetrical aniseikonia: it is the difference in the image shape perceived in each eye

    a) Regular: progressive increase or decrease in size across the visual field

Symptoms

• Headache
• Asthenopia
• Photophobia
• Reading difficulty
• Nausea
• Vertigo
• Diplopia
• Distorted space perception

Testing of aniseikonia

Space eikonometer

The degree of aniseikonia is exactly measured using space eikonometer. This instrument is expensive.

Rule of thumb

• If aniseikonia is associated with anisometropia which is of refractive then the difference in image size will be about 1.5% per diopter of anisometropia
• If anisometropia is due to axial then the difference in image size will be about 1% per diopter.

Treatment

Optical aniseikonia

These treatments are available for aniseikonia which arises due to anisometropia

• Contact lenses
• Implanting IOL for unilateral aphakia
• Refractive corneal surgery
• Aniseikonic spectacles - these are expensive and difficult to make

Retinal aniseikonia

- Due to any causative disease, treating the cause corrects the aniseikonia

Student exercise

Answer the following

1. What is aniseikonia?
2. What are the causes for aniseikonia?
3. What are the clinical types of aniseikonia?
4. What is the rule of thumb for testing aniseikonia?
5. What are the treatment modalities of aniseikonia?

Amblyopia

Amblyopia means reduced vision in a normal anatomical eye. No organic cause can be detected for amblyopia.

Amblyopia develops during early childhood. Children under nine years of age whose vision is still developing are at high risk for amblyopia. Generally the younger the child, the greater the risk.

There are many reasons for amblyopia and they are as follows:

• Squint/strabismus
• Large difference in the power of each eye
• Cataract
• Severe ptosis
• Premature birth
• Heredity
• Any disease that affects the eye

Amblyopia develops because one eye is turned as in squint, and two different pictures are sent to the brain. In a young child, the brain learns to ignore the image of the deviated eye and see only the image of the better eye. Similarly when there is difference in refractive error of each eye the blurred or defocused image formed by the eye with more refractive error is ignored by the brain.

For the retina to capture the images, it needs adequate light and visual stimulus. This being absent due to cataract, either in one eye or both, results in amblyopia.

Amblyopia can often be reversed if detected and treated early.

As soon as amblyopia is detected, active measures should be taken to treat it. Cooperation of the patient and parents is required to achieve good results. If left untreated or not treated properly the reduced vision becomes permanent and cannot be improved by any means.

Mechanism of amblyopia

- Abnormal binocular interaction
- Vision deprivation

Characteristics of an amblyopic eye

• Reduction in visual acuity
• Eccentric fixation
• Crowding phenomenon
• Visual acuity is better when the test letters are viewed singularly rather than in a series

Types of amblyopia

• Strabismic amblyopia
• Anisometropic amblyopia
• Ametropic amblyopia
• Stimulus deprivation amblyopia
• Meridional amblyopia

Strabismic amblyopia

Strabismic amblyopia is seen in patients having squint since birth, unilateral constant squint, who strongly use one eye for fixation. It is more common in esotropes.

Example:	A patient has right esotropia
Vision:	Right eye: 2/60 nig nip
	Left eye : 6/6 nil glass

In this case the patient will prefer only the left eye for fixation.

Anisometropic amblyopia

Anisometropic amblyopia occurs in an eye having a higher degree of refractive error than the other eye. It’s occurs more in hyperopes than in myopes.

Hypermetropic amblyopia - More than 2-3 Diopters

Myopic amblyopia - More than 5 Diopters

Example 1:	Right eye	6/6 nil glass
	Left eye	6/60 with + 3.0DSPH
		6/24 nig nip
	(Left eye - Anisohypermetropia)

Example 2:	Right eye	3/60 with	-9.00SPH
			-1.0 x 90
		6/36 nip
	Left eye 6/9p with – 0.75Dsph		6/6 (Right anisomyopia)

Ametropic amblyopia

Occurs in patients with bilateral uncorrected high refractive error.
Hyperopia of more than + 5.0D Myopia of more than - 10.0D

Example 1:	Right eye 5/60 with + 7.0Dsph 6/36 nip
	Left eye 4/60 with + 8.0Dsph 6/36p nip
Example 2:	Right eye 6/60 with - 11.0Dsph 6/18
	Left eye 2/60 with - 12.0Dsph 6/18/ nip

Stimulus deprivation amblyopia

It is caused by an eye being deprived of visual stimulus. It is important to alleviate the cause as soon possible.

Example
1. Ptosis (Drooping of upper eyelid)
2. Corneal opacity
3. Cataract

Meridional amblyopia

Occurs in patients with uncorrected astigmatic refractive error. It can be bilateral.

Example :
Right eye 6/12 with - 1.0 X 180 6/6
Left eye 6/60 with -4.0 X 180 6/18 nip
(Left eye meridional amblyopia)

Treatment of amblyopia

The earlier the intervention, the better the prognosis for amblyopia. Patients have a better prognosis when treated before 5 years of age. After 8 years of age, however the chance of significantly improving the vision in amblyopia is small.

Visual development and amblyopia

1. Critical period - One week to 3 - 4 months of age
2. Visual plasticity - Birth to 7 year of age
3. Extended plasticity - more than 10 years of age

Treatment

If any refractive error is present give new correction
- Occlusion
- Atropine therapy (penalisation)

Correction of refractive error

If there is any refractive error we have to give full cycloplegic correction before starting treatment.

Occlusion

Occlusion refers to closure of the normal eye with patch or ground glass, thus forcing the child to use the amblyopic eye to stimulate visual development. It can be either occlusion of light or both light and forms.

Total occlusion

With the help of direct patching of the eye both light and form occlusion is done.

Partial occlusion

With the help of cello tape or ground glass over the normal eye, only the form of objects are not seen, but the light is seen.

Full time occlusion

The ratio adapted is 3:1. Three days occlusion to normal eye and 1 day occlusion to amblyopic eye.This type of patient uses ground glass (or) cello tape.

Part time occlusion

Patient does patching for a few hours only 6hrs/day to normal eye.

Occlusion is done from a few hours to full time depending upon the age of the patient and type and severity of the amblyopia.

Follow up

Patients who are patching their eyes need periodic follow up. Duration of treatment may extend from months to years. If the patient is not coming for follow up, but continuously patching, then normal eye can become amblyopic. Follow up is very important for occlusion patients. Follow up period depends upon the eye and type of occlusion therapy.

Atropine therapy (Penalisation)

Topical atropine 1% is used to dilate pupil and paralyse accommodation. This is used to blur the normal eye. Atropine therapy (penalisation) is used to selected cases only.

Surgery

In case of stimulus deprivation amblyopia, early surgery is needed.
- Ptosis correction
- Squint correction
- Cataract removal with IOL

Student exercise

I. True or false

1. Amblyopia can be treated by surgical correction
2. Occlusion therapy is more effective in childhood
3. Strabismus amblyopia occurs due to change in the refractive status of the eye
4. In occlusion therapy the good eye is patched.
5. Stimulus deprivation amblyopia can be due to corneal opacity

II Match the following

1. Amblyopia - Refractive error
2. Meridional - Ptosis
3. Stimulus deprivation - Normal eye
4. Ametropia - Lazy eye
5. Occlusion - Astigmatism

III Answer the following

1. What is amblyopia?
2. What is occlusion therapy?
3. What are the types of amblyopia?
4. What is meridional amblyopia?
5. What is the treatment for amblyopia?