Ординатура / Офтальмология / Английские материалы / Pediatric Opthalmology_Mukherjee_2005

Horoptor

Horoptor is an imaginary surface in space. All points lying on the horopter curve will stimulate corresponding retinal element and will be seen as one. For every retinal point in one eye, there is a limited area around the corresponding point in the other eye. Stimulation of this area along with original point can also result in binocular single vision. This limited area is called Panum’s fusional area. Objects in front and behind the Panum’s area cause diplopia.

Projection is defined as interpretation of the position of an object in the space due to stimulation of retinal element. The foveal projection is straight ahead. A point in temporal retina is projected in the nasal field and vice-versa.

Prerequisite of binocular single vision :

1.Reasonably clear media in both eyes.

2.Accurate co-ordination between the two eyes in all directions of gaze

3.Ability of the brain to cause fusion of two slightly different images.

Grades of binocular vision are :

1.Simultaneous perception

2.Binocular fusion

3.Stereopsis

Advantages of binocular single vision are :

1.Greatest advantage of binocular single vision is stereopsis or perception of depth.

2.Binocular field of vision is larger than uniocular field.

3.Due to overlapping of the uniocular fields, defect in one field is masked by the other field.

4.Binocular single vision is greater than uniocular vision.

Lack of binocular single vision results in double vision.3 This has two components :

1.Confusion

2.Diplopia

1. Confusion. A patient without squint has a correspondence between the two foveae. The brain superimposes and fuses the two images provided they are similar enough.

In case of a manifest squint fovea of fixing eye sees the object of regard while the squinting eye sees something other than the object of regard. The brain in an effort to superimpose these two dissimilar images create a state of confusion. This confusion is the result of corresponding retinal points being stimulated by two different objects.

Confusion can not be explained well by the patient. It is very rare for a patient to describe two different images superimposed over one another.

Confusion is soon replaced by binocular diplopia.

2. Diplopia is the result of one object stimulating non corresponding retinal points i.e. fovea of fixing eye and non foveal point in the deviating eye. Pathological binocular diplopia develops only in presence of manifest squint. Esotropia cause uncrossed (homonymous)

diplopia while exotropia cause crossed (heteronymous) diplopia. Direction of diplopia depends on involvement of particular muscle or group of muscles. It is most marked in acute paralytic squint. Diplopia can be horizontal, vertical or torsional.

Diplopia is generally associated with paralytic squint, though there is misalignment of eyes in concomitant squint as well, it is not associated with diplopia because of the following mechanism :

1.Anomalous retinal correspondence

2.Suppression

3.Amblyopia

Anomalous retinal correspondence (abnormal retinal correspondence ARC)4

Normal retinal correspondence is a state where fovea of one eye corresponds with fovea of the fellow eye, nasal retina of one eye corresponds with temporal retina of the fellow eye, the two images are fused together to give a binocular single vision. Once normal retinal correspondence has been well established up to adulthood, it is not possible for ARC to develop hence ARC is seen in children.

Abnormal retinal correspondence

Abnormal retinal correspondence is an attempt to restore binocular single vision as far as possible. The binocular single vision may not be real but may resemble it. ARC is an adaptation in squint to avoid diplopia and confusion. It is not an anatomical change in the retina, it is a cortical adaptation. ARC may lead to formation of pseudo macula.2 It coexist with some suppression. Amblyopia may be present in ARC. It has various degrees of severity.

ARC develops more commonly and is more severe when the child is small, with smaller degree of squint, longer the duration of squint more severe is the ARC. It is more common in infantile esotropia, less common is exotropia and least in vertical squint.1

Clinically there are two types of abnormal retinal correspondence :

1.Harmonious

2.Unharmonious

In harmonious ARC, angle of anomaly is equal to angle of squint i.e. subjective angle of squint is zero. In unharmonious ARC, the subjective angle of squint is less than objective angle.

Objective angle of squint is the actual measurement of deviation. This is measured on synoptophore. The patient sits on the synoptophore with chin on chin rest and head touching the head band. The inter pupillary distance is adjusted. The fixing eye fixes the slide in the tube. The non-fixing eye deviates. The examiner moves the tubes of the synoptophore so that the corneal reflexes produced by the illumination from the tubes are located in the centre of the cornea. The examiner than does alternate cover test by alternatingly switching off the lights in each tube and moving the tube until there is no movement in either eye. Angle between the two eyes gives the objective angle of deviation.

In the other method the slide of lion is put in front of the fixing eye and the cage before the deviating eye, the tube in front of the fixing eye is set at zero. The other tube is moved by the examiner till the corneal reflex is brought to the centre of the deviating eye. At this point, the patient can see the lion and the cage simultaneously but can not superimpose. This gives the objective angle. To measure subjective angle the patient moves the tube to put the lion inside the cage and the corneal reflex is in the centre of the squinting eye.

The difference between the objective and subjective angle is the angle of anomaly. In normal retinal correspondence (NRC) the two angles are equal.

In harmonious ARC, the subjective angle is zero.

Various methods available to diagnose ARC4

1.Bagolini glasses (striations)

2.Worth four dot test

3.Synoptophore

4.Titmus fly test

5.After image test

6.Vertical prism test

7.Binocular convergence test

Abnormal retinal correspondence does not develop in all cases of squint always.

The conditions where ARC may develop are :

1.Small angle of non paralytic esotropia

2.Infantile esotropia

3.Esotropia developing under three years of age

4.Congenital alternating esotropia

5.Residue in post operative status

Conditions where ARC may not develop :

1.Concomitant divergent squint

2.Concomitant convergent squint after 15 years

3.Esotropia with large degree of deviation

Management of ARC

It is not always possible to treat ARC. It may be so mild that it is not worth the trouble to treat it. In early stages occlusion of the sound eye may help. In cases of alternate squint, alternate occlusion should be tried in children. Orthophoric treatment may be required.

Suppression

Suppression is an in-built central faculty that develops in young children with manifest squint. It is meant to ward off diplopia. It develops in the squinting eye that gets a blurred image. The brain learns to ignore the blurred image. It is a binocular feature, disappears if the fixing eye is closed. Suppression is mostly uniocular but may be alternating. Suppression is

not limited to the fovea. There are larger areas where suppression is present. These areas are called suppression scotomas. They are seen both in esotropia as well as exotropia. Esotropic suppression scotomas extends nasally while the exotropic suppression scotoma spread towards temporal side.

There are two types of suppression :

1.Facultative

2.Obligatory

1.Facultative suppression. It is that type of suppression that develops in the eye when it deviates. The moment the eye takes up fixation, the suppression disappears. This is met with in alternating deviation.

2.Obligatory suppression. In this, suppression is present even when the squinting eye takes up fixation. It is most commonly seen in monocular esotropia. In such cases, in initial stages the suppression is facultative which becomes obligatory later.

Suppression is diagnosed by :

1.Worth four dot test

2.Red glass test

3.Bagolini’s glasses

Eccentric fixation

This is a uniocular feature in which an object is fixed by a retinal point other than fovea without change in the principle visual direction. It develops in long standing esotropia and microtropia. There are various types of eccentric fixation according to location of the retinal point that acts as eccentric point as per findings of visuscope.5

Visuscope is a modified direct ophthalmoscope that has an in-built star which can be projected on the retina. The instrument is used as any other direct ophthalmoscope with dilated pupil. To do the test one eye is closed. The beam of the visuscope is focussed on the retina, this casts a shadow of the star on the retina. The patient is asked to fix the star. In normal eye, the star will be superimposed on the macula but not in eyes with eccentric fixation. As per position of the star, the eccentric fixation can be classified as—Para foveal, para macular, para central, centrocecal. In erratic fixation, the star seems to jump from one place to another near the macula.

Squint

In normal person the visual axes of the two eyes are parallel in primary position and all direction of gaze except convergence and divergence, there too the angle of convergence or divergence of one eye is equal to that of the fellow eye. This ideal condition is calledorthophoria. The position of rest in a normal person is slight divergence.

Orthophoria like many physiological conditions is exception than rule. All such non orthophoric need not have manifest deviation. In contrast to this, a person may be orthophoric but looks to have squint due to anatomic configuration of the eye, lids or orbit. Such conditions are called pseudo squint.

Causes of pseudo squint are :

1.Prominent epicanthic fold—This gives an appearance of convergent squint.

2.Hypertelorism—This gives an appearance of divergent squint.

3.Facial asymmetry results in pseudo vertical squint.

4.Hypermetropia due to large positive angle kappa gives an appearance of apparent divergent squint.

5.Myopia due to large negative angle kappa gives an appearance of convergent squint.

An apparent convergent squint worsens true convergent squint and neutralises divergence squint. Similarly an apparent divergent squint worsens appearance of divergence.

In pseudo convergent squint, obliteration of epicanthic fold abolishes squint.

In pseudo squint, the corneal light reflex is always in the centre of the cornea. The eyes do not move under cover test or alternate cover test. Pseudo convergent squint gradually disappears as the child grows and epicanthic fold fades away. But this is not true for hypertelorism that is a permanent feature. Pseudo squint due to errors of refraction may disappear with correction.

It is a wrong notion that a true manifest squint will disappear as the child grows. In fact with age, the child adapts a compensatory head posture that may mask the squint to a casual observer.

A high degree of fusion is required to keep the eyes in near orthophoric condition. A breakdown of fusion will cause one of the eyes to deviate. This is called strabismus or squint and is defined as a state when only one of the visual axes is directed towards the point of fixation and the other deviates away.

There are two possibilities :

In one condition, the fusion is maintained under strain and as soon as the fusion is broken, the eyes deviates only to return to normal position when the fusion is restored. This state is called heterophoria or latent squint.

In the other type of squint, the fusion is already broken down or is absent resulting into a visible deviation of eye. This is called heterotropia.

In between the two are the conditions that cause intermittent heterotropia.

The deviation may not be equal for near and distances. It may vary during the day, or may be present from time to time.

Classification of squint

Varieties of squints are caused due to diverse causes with great variation in signs and symptoms making it impossible to have an uniform classification of squint. Most practical is to divide ocular deviation into :

1.Latent : (apparent) heterophoria

2.Manifest : heterotropia

In case of manifest squint, it is essential to find out if the deviation is

1.Pseudo squint or

2.Real squint

Pseudo squint does not require any specific treatment and does not develop sequel of manifest squint.

Once it has been decided that the child really has a manifest squint, the next step is to decide if the squint is

1.Concomitant

2.Non comitant

Non comitant is further divided into

1.Paralytic

2.Restrictive (obstructive)

A flow chart given below is helpful

Latent squint (Heterophoria)

Heterophoria is that binocular state of ocular balance where the two eyes are kept in physiological alignment (orthophoric) for distance as well as near and in all direction of gaze so long the child is able to maintain binocular fusion.

Once the fusion has been broken, any of the eyes will deviate only to come to near orthophoric position when the fusion has been restored.

Fusion can be broken in following conditions :

1.One eye is occluded—Cover test

2.Dissimilar images fall on two foveae—Maddox rod test

3.The two images have different colour—Red glass test

4.Separate images are being projected on two foveae—Maddox wing test

Etiology of heterophoria is not well understood. Some of the following conditions may cause heterophoria singly or in combination.

1. Uncorrected error of refraction :

(a) Hypermetropia is more associated with esophoria than myopia. (b) Myopia is more associated with exophoria than hypermetropia.

2.Muscular anomaly : (a) Faulty origin

(b) Faulty insertion

(c) Poorly developed muscle

3.Abnormality of the orbit

4.Defective innervation of extra ocular muscles, muscles in each eye getting different tonic innervation.

5.Debilitating disease

6.Fatigue

7.Drugs, toxins, psychomatic disorder

8.Age—Children are more likely to develop esophoria.

9.Heredity

Types of heterophoria

Heterophoria is not a manifest condition, neither the child nor the parents are aware of it so long the fusion is maintained. Its presence is elicited by cover test and its extent is evaluated by various tests. Maddox rod, Maddox wing, red glass test on Maddox tangent, prism test, synoptophore etc.

As per direction of the eye when fusion is broken i.e. cover test. Heterophoria can be :

1.Esophoria. The eye deviates nasally under cover and move temporally to take up orthophoric position when cover is removed.

2.Exophoria. When the eye deviates out under cover and moves inward when cover is removed.

3.Hyper phoria/hypo phoria. One eye has a tendency to turn upwards in relation to the other and called hyper phoric. Logically there is a possibility that one eye can deviate down in relation to the other eye and should be termed as hypo phoria but in practice the term has been given up in favour of right hyperphoria or left hyper phoria.

Here it is worth noting that esophorias and exophorias are not specified as right or left phoria.

In horizontal phorias the amount of deviation in each eye is the same for a given distance but may change in relation to distant or near fixation.

Cyclophoria

This is the rarest form of phoria. In this there is tendency for one eye to wheel rotate in relation to the other. If the upper pole of the eye in question dips inwards, it is called incyclophoria and when it dips outward it is called excyclophoria.

There is no condition when the two eyes deviate in different direction under cover.

Diagnosis of heterophoria Cover test

This simple test which does not require any special instrument denotes presence and type of heterophoria. The child is made to look at a distant object, than any of the eyes is covered and the other is made to fix. After a few seconds the occluder is removed and the movement of the eye is noted. Movement in any direction denotes presence of heterophoria. If the eye moves out to take up fixation means that the eye has moved in under cover, in a convergent position and the condition is called esophoria.

If the eye moves in, it is called exophoria. If the eye moves down, it is hyperphoria. If the upper pole dips medially, it is incyclophoria, if the upper pole moves out it is excyclophoria.

The test is repeated for near. It is better to do the test with correcting glasses also, if phoria is still present with correction, the exact number that abolishes deviation should be noted. If movement still persists with best correction, other method of treatment should be considered.

Once the direction of deviation has been known by cover test, the next step is to find out the degree of deviation that can be done by :

1.Prism and cover test

2.Maddox rod test

3.Red glass test

4.Synoptophore test

5.Prism dissociation test

6.Maddox wing test

7.Double Maddox rod test

8.Maddox double prism

Measurement of phoria for distance

1.Prism and cover test. Prism of increasing strength with apex towards the deviation is placed in front of one eye. The child fixes with the other eye. Cover uncover test is performed till all the movements in the eye under cover are abolished. This gives the amount of deviation in prism diopters. This test is equally useful in both heterophoria and heterotropia.

2.Maddox rod test. The child is made to sit in a semi darkened room 6 meters away and in front of the Maddox tangent. The Maddox rod is traditionally put in front of the right eye and position of the red streak noted on the scale, both horizontal and vertical. In orthophoria, the streak passes through the central light of the scale, otherwise it passes away from the

central light, the distance of streak from the central light is directly read from the scale inscribed on the Maddox tangent. The test fails to differentiate between phoria and tropia.

3.Red glass test. This is similar to Maddox rod test. In this test, instead of a Maddox rod, a red glass is used. The procedure is the same as in Maddox rod test. If the vision is unequal, the glass is put in front of the better eye to prevent suppression of the poorer eye. The red glass should be dark enough so that the patient sees only central light of the Maddox scale. This test fails to differentiate between phoria and tropia.

4.Synoptophore test. Synoptophore is a versatile instrument that not only give degree of deviation but also can be used for cover test, alternate cover test, degree of deviation, measure range of fusion, suppression and retinal correspondence. It can also be used for orthoptic treatment for both phorias and tropias.

5.Prism dissociation test. This is less frequently used test in which a 6 Pd prism, base down is held before one eye and a rotary prism is held in front of the other eye. In presence of horizontal phoria, the patient will have both vertical and horizontal diplopia. The rotary prism is rotated to align the images horizontally one above the other. The power of the prism that causes horizontal alignment is the degree of heterophoria.

Measurement of phoria for near

Phorias for near can be measured roughly by cover test, prism and cover test, but the most accurate measurement is obtained by Maddox wing. It not only gives information about horizontal or vertical phoria but also measures cyclophoria for near.

Detection and measurement of cyclophoria

Careful observation during cover test gives a rough idea about cyclophoria. It is better measured by :

1.Maddox wing

2.Double Maddox rod, one usual red and other white

3.Maddox double prism

Double Maddox rod test

In this test a red Maddox rod and a white Maddox rod is put in front of each eye, in the trial frame with their axes at 90°. This results into two horizontal lines one red and other white. The patient fixes a white bulb at six metres. If there is no cyclophoria, both the lines will be parallel to floor. If they are not parallel, the glass causing oblique image is moved to make the streak parallel. The axis of rotation gives the amount of cyclo deviation.

Maddox double prism test

Maddox double prism consists of two prism each 4 prism diopter in strength mounted on a lens ring in such a way that they are base to base with apex away. The double prism is put in the trial frame in front of one eye, the other eye is left uncovered without any lens or prism. The patient is given a white sheet of paper which has a line drawn in the middle. The patient keeps the paper at usual reading distance in such a way that the black line is horizontal. The two prisms in the trial lens separate the line away from each eye. Thus a patient without cyclophoria will see three parallel lines. The central line is the line seen by the eye without the

prism. The other two lines one above and one below the central line are formed by the two prisms. If the central line is oblique to the two parallel lines, the patient has cyclophoria or cyclotropia.

Symptoms of heterophoria6

Most of the time the child is not aware of heterophoria and has no symptoms. The child is generally able to overcome the deviation without much effort. The parent may notice the deviation occasionally when the child is either daydreaming or fatigued. So long the fusion is well maintained, the phoria is said to be compensated. The symptoms arise only in decompensated phorias.

Factors that precipitate decompensation are :

1.Inadequate fusional reserve

2.General debility

3.Prolonged near work

Horizontal phorias are least disturbing, symptoms are more in vertical phorias and most troublesome in cyclophoria.

Common symptoms are :

1.Asthenopia and asthenopia related : (a) Burning

(b) Redness (c) Itching

(d) Stye, chalazion

2.Visual

(a) Blurring of vision (b) Running of letters (c) Intermittent diplopia

3.Others—Dizziness, vertigo, nausea

Esophoria

Here the eyes move in on dissociation. It is more common in children. Hypermetropic children have more tendency to develop esophoria than emmetropic or myopic.

Type of esophoria

1.Convergence excess type—Here esophoria is more for near than for distance.

2.Divergence weakness—Here esophoria is greater for distance than for near.

3.In between the above two there is not much change in near or distant deviation.

Management

1. First and foremost treatment is correction of any error of refraction under cycloplegia and prescription of glasses for constant use.