Ординатура / Офтальмология / Английские материалы / Comprehensive Ophthalmology_Khurana_2007

occlusion therapy is started, it should be ensured that:

Opacity in the media (e.g., cataract), if any, should be removed first, and

Refractive error, if any, should be fully corrected.

Simplified schedule for occlusion therapy depending up on the age is as below:

Upto 2 years, the occlusion should be done in 2:1, i.e., 2 days in sound eye and one day in amblyopic eye.

At the age of 3 years, 3:1,

At the age of 4 years, 4:1,

At the age of 5 years, 5:1, and

After the age of 6 years, 6:1

Duration of occlusion should be until the visual acuity develops fully, or there is no further improvement of vision for 3 months.

Abnormal retinal correspondence (ARC)

In a state of normal binocular single vision, there exists a precise physiological relationship between the corresponding points of the two retinae. Thus, the foveae of two eyes act as corresponding points and have the same visual direction. This adjustment is called normal retinal correspondence (NRC). When squint develops, patient may experience either diplopia or confusion. To avoid these, sometimes (especially in children with small degree of esotropia), there occurs an active cortical adjustment in the directional values of the two retinae. In this state fovea of the normal eye and an extrafoveal point on the retina of the squinting eye acquire a common visual direction (become corresponding points). This condition is called abnormal retinal correspondence

(ARC) and the child gets a crude type of binocular single vision.

Tests to detect abnormal retinal correspondence include Worth’s four-dot test, titmus stereo test, Bagolini striated glass tests, after image tests and synoptophore tests (see page 327-329).

Diplopia

Binocular diplopia occurs due to formation of image on dissimilar points of the two retinae (see page 331) Causes of binocular diplopia are:

Paralysis or paresis of the extraocular muscles (commonest cause)

Displacement of one eye ball as occurs in space occupying lesion in the orbit, and fractures of the orbital wall,

Mechanical restriction of ocular movements as caused by thick pterygium, symblepharon and thyroid ophthalmopathy.

Deviation of ray of light in one eye as caused by decentred spectacles.

Anisometropia i.e., disparity of image size between two eyes as occurs in acquired high anisometropia (e.g., uniocular aphakia with spectacle correction).

Types. Binocular diplopia may be crossed or uncrossed.

Uncrossed diplopia. In uncrossed (harmonious) diplopia the false image is on the same side as deviation. It occurs in convergent squint.

Crossed diplopia. In crossed (unharmonious) diplopia the false image is seen on the opposite side. It occurs in divergent squint.

Uniocular diplopia. Though not an anomaly of binocular vision, but it will not be out of place to describe uniocular diplopia along with binocular diplopia.

In uniocular diplopia an object appears double from the affected eye even when the normal eye is closed. Causes of uniocular diplopia are:

Subluxated clear lens (pupillary area is partially phakic and partially aphakic).

Subluxated intraocular lens (pupillary area is partially aphakic and partially pseudophakic).

Double pupil due to congenital anomaly, or large peripheral iridectomy or iridodialysis.

Incipient cataract. Usually polyopia i.e., multiple images may be seen due to multiple water clefts within the lens.

Keratoconus. Diplopia occurs due to changed refractive power of the cornea in different parts.

Treatment of diplopia. Treat the causative disease. Temporary relief from annoying diplopia can be obtained by occluding the affected eye.

STRABISMUS

Definition

Normally visual axis of the two eyes are parallel to each other in the ‘primary position of gaze’ and this alignment is maintained in all positions of gaze.

A misalignment of the visual axes of the two eyes is called squint or strabismus.

Classification of strabismus

Broadly, strabismus can be classified as below:

iiiNon-specific type (exophoria which does not vary significantly in degree for any distance).

II. Latent squint (Heterophoria)

III.Manifest squint (Heterotropia)

1.Concomitant squint

2.Incomitant squint.

PSEUDOSTRABISMUS

In pseudostrabismus (apparent squint), the visual axes are in fact parallel, but the eyes seem to have a squint:

1.Pseudoesotropia or apparent convergent squint may be associated with a prominent epicanthal fold (which covers the normally visible nasal aspect of the globe and gives a false impression of esotropia) and negative angle kappa.

2.Pseudoexotropia or apparent divergent squint may be associated with hypertelorism, a condition of wide separation of the two eyes, and positive angle kappa.

HETEROPHORIA

Heterophoria also known as ‘latent strabismus’, is a condition in which the tendency of the eyes to deviate is kept latent by fusion. Therefore, when the influence of fusion is removed the visual axis of one eye deviates away. Orthophoria is a condition of perfect alignment of the two eyes which is maintained even after the removal of influence of fusion. However, orthophoria is a theoretical ideal. Practically a small amount of heterophoria is of universal occurrence and is known as ‘physiological heterophoria’.

Types of heterophoria

1.Esophoria. It is a tendency to converge. It may be:

i Convergence excess type (esophoria greater for near than distance).

ii Divergence weakness type (esophoria greater for distance than near).

iii Non-specific type (esophoria which does not vary significantly in degree for any distance).

2.Exophoria. It is a tendency to diverge. It may be: i Convergence weakness type (exophoria greater

for near than distance).

ii Divergence excess type (exophoria greater on distant fixation than the near).

while hypophoria is a tendency to deviate downwards. However, in practice it is customary to use the term right or left hyperphoria depending on the eye which remains up as compared to the other. 4. Cyclophoria. It is a tendency to rotate around the anteroposterior axis. When the 12 O’clock meridian of cornea rotates nasally, it is called incyclophoria and when it rotates temporally it is called excyclophoria.

Etiology

A. Anatomical factors

Anatomical factors responsible for development of heterophoria include:

1.Orbital asymmetry.

2.Abnormal interpupillary distance (IPD). A wide IPD is associated with exophoria and small with esophoria.

3.Faulty insertion of extraocular muscle.

4.A mild degree of extraocular muscle weakness.

5.Anomalous central distribution of the tonic innervation of the two eyes.

6.Anatomical variation in the position of the macula in relation to the optical axis of the eye.

B. Physiological factors

1.Age. Esophoria is more common in younger age group as compared to exophoria which is more often seen in elderly.

2.Role of accommodation. Increased accommodation is associated with esophoria (as seen in hypermetropes and individuals doing excessive near work) and decreased accommodation with exophoria (as seen in simple myopes).

3.Role of convergence. Excessive use of convergence may cause esophoria (as occurs in bilateral congenital myopes) while decreased use of convergence is often associated with exophoria (as seen in presbyopes).

4.Dissociation factor such as prolonged constant use of one eye may result in exophoria (as occurs in individuals using uniocular microscope and watch makers using uniocular magnifying glass).

Factors predisposing to decompensation

Inadequacy of fusional reserve,

General debility and lowered vitality,

Psychosis, neurosis, and mental stress,

Precision of job, and

Advancing age.

Symptoms

Depending upon the symptoms heterophoria can be divided into compensated and decompensated.

Compensated heterophoria. It is associated with no subjective symptoms. Compensation of heterophoria depends upon the reserve neuro-muscular power to overcome the muscular imbalance and individual’s desire for maintenance of binocular vision.

Decompensated heterophoria. It is associated with multiple symptoms which may be grouped as under:

1.Symptoms of muscular fatigue. These result due to continuous use of the reserve neuromuscular power. These include:

Headache and eyeache after prolonged use of eyes, which is relieved when the eyes are closed.

Difficulty in changing the focus from near to distant objects of fixation or vice-versa.

Photophobia due to muscular fatigue is not relieved by using dark glasses, but relieved by

closing one eye.

2.Symptoms of failure to maintain binocular single vision are:

Blurring or crowding of words while reading;

Intermittent diplopia due to temporary manifest deviation under conditions of fatigue; and

Intermittent squint (without diplopia) which is usually noticed by the patient’s close relations or friends.

3.Symptoms of defective postural sensations cause problems in judging distances and positions especially of the moving objects. This difficulty may be experienced by cricketers, tennis players and pilots during landing.

Examination of a case of heterophoria

It should include the following tests:

1. Testing for vision and refractive error. It is most important, because a refractive error may be responsible for the symptoms of the patient or for the deviation itself. Preferably, refraction should be performed under full cycloplegia, especially in children.

2.Cover-uncover test. It tells about the presence and type of heterophoria. To perform it, one eye is covered with an occluder and the other is made to fix an object. In the presence of heterophoria, the eye under cover will deviate. After a few seconds the cover is quickly removed and the movement of the eye (which was under cover) is observed. Direction of movement of the eyeball tells the type of heterophoria (e.g., the eye will move outward in the presence of esophoria) and the speed of movement tells whether recovery is slow or rapid.

3.Prism cover test. (see page 327).

4.Maddox rod test. Patient is asked to fix on a point light in the centre of Maddox tangent scale (Fig. 13.9) at a distance of 6 metres. A Maddox rod (which consists of many glass rods of red colour set together in a metallic disc) (Fig. 13.10) is placed in front of one eye with axis of the rod parallel to the axis of deviation (Fig. 13.11).

Fig. 13.9. Maddox tangent scale.

Fig. 13.10. Maddox rod.

The Maddox rod converts the point light image into a line. Thus, the patient will see a point light with one eye and a red line with the other. Due to dissimilar images of the two eyes, fusion is broken and heterophoria becomes manifest. The number on Maddox tangent scale where the red line falls will be the amount of heterophoria in degrees. In the absence of Maddox tangent scale, the dissociation between the point light and red line is measured by the superimposition of the two images by means of prisms placed in front of one eye with apex towards the phoria.

5. Maddox wing test. Maddox wing is an instrument (Fig. 13.12) by which the amount of phoria for near (at a distance of 33 cm) can be measured. It is also based on the basic principle of dissociation of fusion by dissimilar objects.

The instrument is designed in such a way that, through its two slits, right eye sees a vertical white arrow and a horizontal red arrow and the left eye sees a vertical and a horizontal line of numbers. The patient

is asked to tell the number on the horizontal line which the vertical white arrow is pointing (this will give amount of horizontal phoria) and the number on the vertical line at which the red arrow is pointing (this will measure the vertical phoria). The cyclophoria is measured by asking the patient to align the red arrow with the horizontal line of numbers (Fig. 13.13).

6. Measurement of convergence and accommodation. It is important in planning the management of heterophorias. Near point of convergence (NPC) can be measured with the help of a RAF rule or the Livingstone binocular gauge. The normal NPC is considered to be around 70 mm.

Near point of accommodation (NPA) should be measured after the NPC. NPA can be measured with the help of a RAF or Prince’s rule. Normal NPA varies with the age of patient (see page 41).

7. Measurement of fusional reserve. It can be done with the help of a synoptophore or prism bar. The normal values of fusional reserve are as follows:

Vertical fusional reserve: 1.5°-2.5°

Fig. 13.12. Maddox wing.

Horizontal negative fusional reserve (abduction range): 3°-5°

Horizontal positive fusional reserve (adduction range) : 20°-40°

Treatment

It is indicated in decompensated heterophoria (i.e., symptomatic cases).

1.Correction of refractive error when detected is most important.

2.Orthoptic treatment. It is indicated in patients with heterophoria without refractive error and in those where heterophoria and/or symptoms are not corrected by glasses. Aim of orthoptic treatment is to improve convergence insufficiency and the fusional reserve. Orthoptic exercises can be done with synoptophore. Simple exercises to be carried out at home should also be taught to the patient.

3.Prescription of prism in glasses. It may be tried in selected cases of hyperphoria and in troublesome cases of esophoria and exophoria. Prism is prescribed with apex towards the direction of phoria to correct only half or at the most two-thirds of heterophoria.

4.Surgical treatment. It is undertaken in patients with marked symptoms which are not relieved by other measures. Aim of the surgical management is to strengthen the weak muscle or weaken the strong muscle.

Fig. 13.13. Maddox wing test.

CONCOMITANT STRABISMUS

It is a type of manifest squint in which the amount of deviation in the squinting eye remains constant (unaltered) in all the directions of gaze; and there is no associated limitation of ocular movements.

Etiology

It is not clearly defined. The causative factors differ in individual cases. As we know, the binocular vision and coordination of ocular movements are not present since birth but are acquired in the early childhood. The process starts by the age of 3-6 months and is completed up to 5-6 years. Therefore, any obstacle to the development of these processes may result in concomitant squint. These obstacles can be arranged into three groups, namely: sensory, motor and central. 1. Sensory obstacles. These are the factors which hinder the formation of a clear image in one eye. These include:

Refractive errors,

Prolonged use of incorrect spectacles,

Anisometropia,

Corneal opacities,

Lenticular opacities,

Diseases of macula (e.g., central chorioretinitis),

Optic atrophy, and

Obstruction in the pupillary area due to congenital ptosis.

2. Motor obstacles. These factors hinder the maintenance of the two eyes in the correct positional relationship in primary gaze and/or during different ocular movements. A few such factors are:

Congenital abnormalities of the shape and size of the orbit,

Abnormalities of extraocular muscles such as faulty insertion, faulty innervation and mild paresis,

Abnormalities of accommodation, convergence and AC/A ratio.

3. Central obstacles. These may be in the form of:

Deficient development of fusion faculty, or

Abnormalities of cortical control of ocular movements as occurs in mental trauma, and hyperexcitability of the central nervous system during teething.

Clinical features of concomitant strabismus (in general)

The cardinal features of different clinico-etiological types of concomitant strabismus are described separately. However, the clinical features of concomitant strabismus (in general) are as below :

1. Ocular deviation. Characteristics of ocular deviation are:

Unilateral (monocular squint) or alternating (alternate squint).

Inward deviation (esotropia) or outward deviation (exotropia) or vertical deviation (hypertropia).

Primary deviation (of squinting eye) is equal to secondary deviation (deviation of normal eye under cover when patient fixes with squinting eye).

Ocular deviation is equal in all the directions of

gaze.

2.Ocular movements are not limited in any direction.

3.Refractive error may or may not be associated.

4.Suppression and amblyopia may develop as sensory adaptation to strabismus. Suppression may be monocular (in monocular squint) and alternating (in alternating strabismus). Amblyopia develops in monocular strabismus only and is responsible for poor visual acuity.

5.A-V patterns may be observed in horizontal strabismus. When A-V patterns are associated, the horizontal concomitant strabismus becomes vertically incomitant (see page 334).

Types of concomitant squint

Three common types of concomitant squint are :

1.Convergent squint (esotropia),

2.Divergent squint (exotropia), and

3.Vertical squint (hypertropia).

CONVERGENT SQUINT

Concomitant convergent squint or esotropia denotes inward deviation of one eye (Fig. 13.14). It can be unilateral (the same eye always deviates inwards and the second normal eye takes fixation) or alternating (either of the eyes deviates inwards and the other eye takes up fixation, alternately).

Fig. 13.14. Concomitant squint, (right esotropia).

Clinico-etiological types

Depending upon the clinico-etiological features convergent concomitant squint can be further classified into following types:

1. Accommodative esotropia. It occurs due to overaction of convergence associated with accommodation reflex. It is of three types: refractive, non-refractive and mixed.

i.Refractive accommodative esotropia: It usually develops at the age of 2 to 3 years and is associated with high hypermetropia (+4 to +7 D). Mostly it is for near and distance (marginally more for near) and fully correctable by use of spectacles.

ii.Non-refractive accommodative esotropia: It is caused by abnormally AC/A (accommodative convergence/accommodation) ratio. This may occur even in patients with no refractive error. Esotropia is greater for near than that for distance (minimal or no deviation for distance). It is fully corrected by adding +3 DS for near vision.

iii.Mixed accommodative esotropia: It is caused by combination of hypermetropia and high AC/A ratio. Esotropia for distance is corrected by correction of hypermetropia; and the residual esotropia for near is corrected by an addition of +3 DS lens.

2. Non-accommodative esotropias. This group includes all those primary esodeviations in which amount of deviation is not affected by the state of accommodation. It includes:

i.Essential infantile esotropia. It usually presents at 1-2 months of age. However, it may be detected shortly after birth or any time within the first 6 months of life. Previously, it was known as congenital esotropia. It is characterised by fairly large angle of squint (> 30o), alternate fixation in primary gaze and crossed fixation in lateral gaze.

ii.Essential acquired or late onset esotropia. It is a common variety of concomitant convergent squint. It typically occurs during first few years of life. It is of three types:

Basic type. In it the deviation is usually equal at distance and near.

Convergence excess type. In it the deviation is large for near and small or no deviation for distance.

Divergence insufficiency type. It is characterized by a greater deviation for distance than near.

3. Secondary esotropia. It includes:

i.Sensory deprivation esotropia. It results from monocular lesions (in childhood) which either prevent the development of normal binocular vision or interfere with its maintenance. Examples of such lesions are: cataract, severe congenital ptosis, aphakia, anisometropia, optic atrophy, retinoblastoma, central chorioretinitis and so on.

ii.Consecutive esotropia. It results from surgical overcorrection of exotropia.

DIVERGENT SQUINT

Clinico-etiological types

It can be classified into following clinicoetiological types:

1.Congenital exotropia. It is rare and almost always present at birth. It is characterised by a fairly large angle of squint, usually alternate with homonymous fixation in lateral gaze, and no amblyopia.

2.Primary exotropia. It is a common variety of exodeviation (unilateral or alternating). It presents with variable features. It may be of:

Convergence insufficiency type (exotropia greater

for near than distance),

Divergence excess (exotropia greater for distance than near) or

Basic non-specific type (exotropia equal for near

and distance).

It usually starts as intermittent exotropia at the age of 2 years. It is associated with normal fusion and no amblyopia. Stereopsis is usually absent. Precipitating factors include bright light, fatigue, illhealth and day-dreaming. If not treated in time it decompensates to become constant exotropia (Fig. 13.15).

3.Secondary (sensory deprivation) exotropia. It is a constant unilateral deviation which results from longstanding monocular lesions (in adults), associated with low vision in the affected eye. Common causes include: traumatic cataract, corneal opacity, optic atrophy, anisometropic amblyopia, retinal detachment and organic macular lesions.

4.Consecutive exotropia. It is a constant unilateral exotropia which results either due to surgical overcorrection of esotropia, or spontaneous conversion of small degree esotropia with amblyopia into exotropia.

Concomitant divergent squint (exotropia) is characterised by outward deviation of one eye while the other eye fixates.

Fig. 13.15. A patient with primary exotropia.

EVALUATION OF A CASE OF CONCOMITANT STRABISMUS

I. History

A meticulous history is very important. It should include: age of onset, duration, mode of onset (sudden or gradual), any illness preceding squint (fever, trauma, infections, etc.), intermittent or constant, unilateral or alternating, history of diplopia, family history of squint, history of head tilt or turn and so on.

II. Examination

1.Inspection. Large degree squint (convergent or divergent) is obvious on inspection.

2.Ocular movements. Both uniocular as well as binocular movements should be tested in all the cardinal positions of gaze.

3.Pupillary reactions. These may be abnormal in patients with secondary deviations due to diseases of retina and optic nerve.

4.Media and fundus examination. It may reveal associated disease of ocular media, retina or optic nerve.

5.Testing of vision and refractive error. It is most important, because a refractive error may be responsible for the symptoms of the patient or for the deviation itself. Preferably, refraction should be performed under full cycloplegia, especially in children.

6.Cover tests

i.Direct cover test (Fig. 13.16). It confirms the presence of manifest squint. To perform it, the patient is asked to fixate on a point light. Then, the normal looking eye is covered while observing the movement of the uncovered eye. In the presence of squint the uncovered eye will move in opposite direction to take fixation, while in apparent squint there will be no movement. This test should be performed for near texation (i.e., at 33 cm) distance tixation(i.e., at 6 metres).

ii.Alternate cover test. It reveals whether the squint is unilateral or alternate and also differentiates concomitant squint from paralytic squint (where secondary deviation is greater than primary).

Fig. 13.16. Direct cover test depicting left exotropia.

7. Estimation of angle of deviation

i.Hirschberg corneal reflex test. It is a rough but handy method to estimate the angle of manifest squint. In it the patient is asked to fixate at point light held at a distance of 33 cm and the deviation of the corneal light reflex from the centre of pupil is noted in the squinting eye. Roughly, the angle of squint is 15o and 45o when the corneal light reflex falls on the border of pupil and limbus, respectively (Fig. 13.17).

ii.The prism and cover test (prism bar cover test i.e., PBCT). Prisms of increasing strength with apex towards the deviation are placed in front of one eye and the patient is asked to fixate an object with the other. The cover-uncover test is performed till there is no recovery movement of the eye under cover. This will tell the amount of deviation in prism dioptres. Both heterophoria as well as heterotropia can be measured by this test.

iii.Krimsky corneal reflex test. In this test the patient is asked to fixate on a point light and prisms of increasing power (with apex towards the direction of manifest squint) are placed in front of the normal fixating eye till the corneal light reflex is centred in the squinting eye. The power of prism required to centre the light reflex in the squinting eye equals the amount of squint in prism dioptres.

Fig. 13.17. Hirschberg corneal reflex test.

iv. Measurement of deviation with synoptophore.

All types of heterophorias and heterotropias (both objective and subjective angle of squint) can be measured accurately with it. In addition, many other tests can also be performed with this instrument (for details see pages 329).

8. Tests for grade of binocular vision and sensory functions. Normal binocular single vision consists of three grades. Sensory anomalies include disturbances of binocular vision, eccentric fixation, suppression, amblyopia, abnormal retinal correspondence and diplopia. A few common tests for sensory functions are as follows:

i. Worth’s four-dot test.: For this test patient wears goggles with red lens in front of the right and green lens in front of the left eye and views a box with four lights – one red, two green and one white (Fig. 13.18).

Interpretation.:

If the patient sees all the four lights in the absence of manifest squint, he has normal binocular single vision (Fig. 13.18A).

In abnormal retinal correspondence (ARC) patient sees four lights even in the presence of a manifest squint.

If the patient sees only three green lights, he has right suppression (Fig. 13.18D).

When the patient sees only two red lights, it indicates left suppression (Fig. 13.18C).

Fig. 13.18. Worth’s four-dot test.

When he sees three green lights and two red lights, alternately, it indicates presence of alternating suppression.

If the patient sees five lights (2 red and 3 green), he has diplopia (Fig. 13.18E).

ii.Test for fixation. It can be tested with the help of a visuoscope or fixation star of the ophthalmoscope. Patient is asked to cover one eye and fix the star with the other eye. Fixation may be centric (normal on the fovea) or eccentric (which may be unsteady, parafoveal, macular, paramacular, or peripheral (Fig. 13.19).

iii.After-image test. In this test the right fovea is stimulated with a vertical and left with a horizontal bright light and the patient is asked to draw the position of after-images.

Interpretation:

A patient with normal retinal correspondence will

draw a cross (Fig. 13.20A).

An esotropic patient with abnormal retinal correspondence (ARC) will draw vertical image to the left of horizontal (Fig. 13.20B).

An exotropic patient with ARC will draw vertical image to the right of horizontal (Fig. 13.20C).

Central Fixation

Fig. 13.19. Types of fixation.

iv. Sensory function tests with synoptophore.

Synoptophore (major amblyoscope) consists of two tubes, having a right-angled bend, mounted on a base (Fig. 13.21). Each tube contains a light source for illumination of slides and a slide carrier at the outer end, a reflecting mirror at the right-angled bend and an eyepiece of +6.5 D at the inner end (Fig. 13.22). The two tubes can be moved separately or together by means of knobs around a semicircular scale. Synoptophore is used for many diagnostic and therapeutic indications in orthoptics.

Synoptophore tests for sensory functions include:

Estimation of grades of binocular vision

(page 318).

Detection of normal/abnormal retinal correspondence (ARC). It is done by determining the subjective and objective angles of the squint. In normal retinal correspondence, these two angles

are equal. In ARC, objective angle is greater than the subjective angle and the difference between these is called the angle of anomaly. When the angle of anomaly is equal to the objective angle, the ARC is harmonious. In unharmonious ARC angle of anomaly is smaller than the objective angle.

(v) Neutral density filter test. In this test, visual acuity is measured without and with neutral density filter placed in front of the eye. In cases with functional amblyopia visual acuity slightly improves while in organic amblyopia it is markedly reduced when seen through the filter.

TREATMENT OF CONCOMITANT STRABISMUS

Goals of treatment. These are to achieve good cosmetic correction, to improve visual acuity and to maintain binocular single vision. However, many a time it is not possible to achieve all the goals in every case.

Treatment modalities. These include the following:

1.Spectacles with full correction of refractive error should be prescribed in every case. It will improve the visual acuity and at times may correct the squint partially or completely (as in accommodative squint).

2.Occlusion therapy. It is indicated in the presence of amblyopia. After correcting the refractive error, the normal eye is occluded and the patient is advised to use the squinting eye. Regular followups are done in squint clinic. Occlusion helps to improve the vision in children below the age of 10 years.