Ординатура / Офтальмология / Английские материалы / Clinical Ophthalmology A Systematic Approach 7th Edition_Kanski, Bowling_2011

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Krimsky and prism reflection tests

Corneal reflex assessment can be combined with prisms to give a more accurate approximation of the angle in a manifest deviation.

1Krimsky test involves placement of prisms in front of the fixating eye until the corneal light reflections are symmetrical (Fig. 18.32). This test reduces the problem of parallax and is more commonly used than the prism reflection test.

2Prism reflection test involves the placement of prisms in front of the deviating eye until the corneal light reflections are symmetrical.

Fig. 18.32 Krimsky test

(Courtesy of K Nischal)

Cover–uncover test

The cover–uncover test consists of two parts:

1Cover test to detect a heterotropia. It is helpful to begin the near test using a light to observe the corneal reflections and to assess the fixation in the deviating eye. It should then be repeated for near using an accommodative target and for distance as follows:

aThe patient fixates a straight-ahead target.

bIf a right deviation is suspected, the examiner covers the fixing left eye and notes any movement of the right eye to take up fixation.

cNo movement indicates orthotropia (Fig. 18.33A) or left heterotropia (Fig. 18.33B).

d Adduction of the right eye to take up fixation indicates right exotropia and abduction right esotropia (Fig. 18.33C). e Downward movement indicates right hypertropia and upward movement right hypotropia.

fThe test is repeated on the opposite eye.

2Uncover test detects heterophoria. It should be performed both for near (using an accommodative target) and for distance as follows:

aThe patient fixates a straight-ahead distant target.

bThe examiner covers the right eye and after 2–3 seconds removes the cover.

cNo movement indicates orthophoria (Fig. 18.34A); a keen observer will frequently detect a very slight latent deviation in most normal individuals, as very few people are truly orthophoric, particularly on near fixation.

d If the right eye had deviated while under cover, a re-fixation movement (recovery to BSV) is observed on being uncovered. e Adduction (nasal recovery) of the right eye indicated exophoria (Fig. 18.34B) and abduction esophoria (Fig. 18.34C).

fUpward or downward movement indicates a vertical phoria.

gAfter the cover is removed, the examiner notes the speed and smoothness of recovery as evidence of the strength of motor fusion.

h The test is repeated for the opposite eye.

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Fig. 18.33 Possible results of the cover test

Fig. 18.34 Possible results of the uncover test

Most examiners perform the cover test and the uncover test sequentially, hence the term cover–uncover test.

Alternate cover test

The alternate cover test is a dissociation test which reveals the total deviation when fusion is suspended. It should be performed after the cover–uncover test.

aThe right eye is covered for several seconds.

bThe occluder is quickly shifted to the opposite eye for 2 seconds, then back and forth several times. After the cover is removed, the examiner notes the speed and smoothness of recovery as the eyes return to their pre-dissociated state.

cA patient with a well compensated heterophoria will have straight eyes before and after the test has been performed whereas a patient with poor control may decompensate to a manifest deviation.

Prism cover test

The prism cover test measures the angle of deviation on near or distance fixation and in any gaze position. It combines the alternate cover test with prisms and is performed as follows:

aThe alternate cover test is first performed.

bPrisms of increasing strength are placed in front of one eye with the base opposite the direction of the deviation (i.e. point the apex of the prism in the direction of the deviation). For example, in a convergent strabismus the prism is held base-out, and in a right hypertropia, base down before the right eye.

cThe alternate cover test is continuously performed (Fig. 18.35). As stronger prisms are brought in, the amplitude of the re-fixation movement gradually decreases.

dThe end-point is approached when no movement is seen; to ensure the maximum angle is found, the prism strength is increased further until a movement is observed in the opposite direction (point of reversal) and then reduced again to find the neutral value; the angle of deviation then equals the strength of the prism.

Fig. 18.35 Prismcover test

Maddox wing

The Maddox wing dissociates the eyes for near fixation (1/3 m) and measures heterophoria. The instrument is constructed in such a way

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that the right eye sees only a white vertical arrow and a red horizontal arrow, whereas the left eye sees only horizontal and vertical rows of numbers (Fig. 18.36). Measurements are made as follows:

a The horizontal deviation is measured by asking the patient to which number the white arrow points.

bThe vertical deviation is measured by asking the patient which number the red arrow intersects.

cThe amount of cyclophoria is determined by asking the patient to move the red arrow so that it is parallel with the horizontal row of numbers.

Fig. 18.36 Maddox wing

Maddox rod

The Maddox rod consists of a series of fused cylindrical red glass rods which convert the appearance of a white spot of light into a red streak. The optical properties of the rods cause the streak of light to be at an angle of 90° with the long axis of the rods; when the glass rods are held horizontally, the streak will be vertical and vice versa. The test is performed as follows:

aThe rod is placed in front of the right eye (Fig. 18.37A). This dissociates the two eyes because the red streak seen by the right eye cannot be fused with the unaltered white spot of light seen by the left eye (Fig. 18.37B).

bThe amount of dissociation (Fig. 18.37C) is measured by the superimposition of the two images using prisms. The base of the prism is placed in the position opposite to the direction of the deviation.

c Both vertical and horizontal deviations can be measured in this way but the test cannot differentiate phoria from tropia.

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Fig. 18.37 (A) Maddox rod test; (B) appearance of a point of light through Maddox rods; (C) possible results

Motility tests

Ocular movements

Examination of ocular movements involves assessment of smooth pursuit movements followed by that of saccadic movements.

1Versions towards the eight eccentric positions of gaze are tested by asking the patient to follow a target, usually a pen or pen-torch (the latter offers the advantage of corneal light reflections to aid assessment). A quick cover test is performed in each position of gaze to confirm whether a phoria has become a tropia or the angle has increased and the patient is questioned regarding diplopia. Versions may also be elicited involuntarily in response to a noise or by the doll's head manoeuvre in uncooperative patients.

2Ductions are assessed if reduced ocular motility is noted in either or both eyes. A pen-torch should be used with careful attention to the position of the corneal reflexes. The fellow eye is occluded and the patient asked to follow the torch into various positions of gaze. A simple numeric system may be employed using 0 to denote full movement, and −1 to −4 to denote increasing degrees of underaction (Fig. 18.38).

Fig. 18.38 Grading of right lateral rectus underaction

Near point of convergence

The near point of convergence (NPC) is the nearest point on which the eyes can maintain binocular fixation. It can be measured with the RAF rule which rests on the patient's cheeks (Fig. 18.39A). A target (Fig. 18.39B) is slowly moved along the rule towards the patient's eyes until one eye loses fixation and drifts laterally (objective NPC). The subjective NPC is the point at which the patient reports diplopia. Normal NPC should be nearer than 10 cm without undue effort.

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Fig. 18.39 (A) RAFrule; (B) convergence target

Near point of accommodation

The near point of accommodation (NPA) is the nearest point on which the eyes can maintain clear focus. It can also be measured with the RAF rule. The patient fixates a line of print, which is then slowly moved towards the patient until it becomes blurred. The distance at which this is first reported is read off the rule and denotes the NPA. The NPA recedes with age; when sufficiently far away to render reading difficult without optical correction, presbyopia is present. At the age of 20 years the NPA is 8 cm and by the age of 50 years it has receded to approximately 46 cm. The amplitude of accommodation can also be assessed using concave lenses in 0.5 DS steps whilst fixating the 6/6 Snellen line and reporting when the vision blurs.

Fusional amplitudes

Fusional amplitudes measure the efficacy of vergence movements. They may be tested with prisms bars or the synoptophore. An increasingly strong prism is placed in front of one eye, which will then abduct or adduct (depending on whether the prism is base-in or baseout), in order to maintain bifoveal fixation. When a prism greater than the fusional amplitude is reached, diplopia is reported or one eye drifts the other way, indicating the limit of vergence ability.

Postoperative diplopia test

This simple test is mandatory prior to strabismus surgery in all non-binocular patients over 7–8 years of age to assess the risk of diplopia after surgery.

•Corrective prisms are placed in front of one eye (usually the deviating eye) and the patient asked to fixate a straight-ahead target with both eyes open. The prisms are slowly increased until the angle has been significantly overcorrected and the patient reports if diplopia occurs.

•If suppression persists throughout there is little risk of diplopia following surgery; in a consecutive exotropia of 35 Δ, diplopia may be reported from 30 and persist as the prism correction mimics an esotropia.

•Diplopia may be intermittent or constant but in either case constitutes an indication to perform a diagnostic botulinum toxin test (see below).

•Diplopia is not restricted to patients with good visual acuity in the deviating eye.

•Intractable diplopia is a difficult condition to treat.

Investigation of diplopia

The Hess screen and the Lees screen are two similar tests that plot the dissociated ocular position as a function of extraocular muscle action and enable differentiation of paretic strabismus caused by neurological pathology from restrictive myopathy such as in thyroid eye disease or a blow-out fracture of the orbit, and recent onset paresis from long-standing. They also allow quantitative monitoring of progress in a range of conditions.

Electronic Hess test

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The screen contains a tangent pattern (2D projection of a spherical surface) printed onto a dark grey background. Red lights that can be individually illuminated by a control panel indicate the cardinal positions of gaze within a central field (15° from primary position) and a peripheral field (30°); each square represents 5° of ocular rotation.

aThe patient is seated 50 cm from the screen and wears red-green goggles (red lens in front of the right eye) and holds a green pointer.

bThe examiner illuminates each point in turn which is used as the point of fixation. This can now be seen only with the right eye, which therefore becomes the fixating eye.

cThe patient is asked to superimpose their green light on the red light, so plotting the relative position of the left eye. All the points are plotted in turn.

d In orthophoria the two lights should be more or less superimposed in all nine positions of gaze.

eThe goggles are then reversed (red filter in front of the left eye) and the procedure is repeated.

fThe relative positions are marked by the examiner on a Hess chart and connected with straight lines.

Lees screen

The apparatus consists of two opalescent glass screens at right-angles to each other, bisected by a two-sided plane mirror which dissociates the two eyes (Fig. 18.40). Each screen has a tangent pattern marked onto the back surface which is revealed only when the screen is illuminated.

1Procedure. The test is performed with each eye fixating in turn.

aThe patient faces the non-illuminated screen with his chin stabilized on a chin rest attached to the mirror support and fixates the dots in the mirror.

bThe examiner indicates the dot required for the patient to plot.

cThe patient positions the pointer on the non-illuminated screen at a position perceived to be on top of the dot indicated by the examiner.

dWhen all of the dots have been plotted on a Hess chart the patient is repositioned to face the other screen and the procedure is repeated. The results are charted as before.

2Interpretation

aThe two charts are compared (Fig. 18.41).

bThe smaller chart indicates the eye with the paretic muscle (right eye).

cThe larger chart indicates the eye with the overacting yoke muscle (left eye).

dThe smaller chart will show its greatest restriction in the main direction of action of the paretic muscle (right lateral rectus).

eThe larger chart will show its greatest expansion in the main direction of action of the yoke muscle (left medial rectus).

fThe degree of disparity between the plotted point and the template in any position of gaze gives an estimate of the angle of deviation (each square = 5°).

Fig. 18.40 Lees screen

Fig. 18.41 Hess chart of a recent right lateral rectus palsy

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Changes with time

Changes with time are very useful as a prognostic guide.

•For example, in right superior rectus palsy, the Hess chart will show underaction of the affected muscle with an overaction of its yoke muscle (left inferior oblique) (Fig. 18.42A). Because of the great incomitance of the two charts, the diagnosis is straightforward. If the paretic muscle recovers its function, both charts will revert to normal.

•Secondary contracture of the ipsilateral antagonist (right inferior rectus) will show up on the chart as an overaction which will lead to a secondary (inhibitional) palsy of the antagonist of the yoke muscle (left superior oblique), which will show up on the chart as an underaction (Fig. 18.42B). This could lead to the incorrect impression that the left superior oblique was the primary muscle at fault.

•With further passage of time, the two charts become more and more concomitant until it may be impossible to determine which was the primary paretic muscle (Fig. 18.42C).

Fig. 18.42 Hess chart showing changes with time of a right superior rectus palsy

Clinical examples

By analyzing the following examples familiarization is gained with ocular motor nerve palsies as discussed in Chapter 19.

1Left 3rd nerve palsy (Fig. 18.43).

•The left chart is much smaller than the right.

•Left exotropia – note that the fixation spots in the inner charts of both eyes are deviated laterally. The deviation is greater on the right chart (when the left eye is fixating), indicating that secondary deviation exceeds the primary, typical of a paretic squint.

•Left chart shows underaction of all muscles except the lateral rectus.

•Right chart shows overaction of all muscles except the medial rectus and inferior rectus, the ‘yokes’ of the spared muscles.

•The primary angle of deviation (fixing right eye – FR) in the primary position is −20° and R/L 10°.

•The secondary angle (fixing left eye – FL) is −28° and R/L 12°.

In inferior rectus palsy, the function of the superior oblique muscle can only be assessed by observing intorsion on attempted depression. This is best performed by observing a conjunctival landmark on the slit-lamp.

2Recently acquired right 4th nerve palsy (Fig. 18.44).

•Right chart is smaller than the left.

•Right chart shows underaction of the superior oblique and overaction of the inferior oblique.

•Left chart shows overaction of the inferior rectus and underaction (inhibitional palsy) of the superior rectus.

•The primary deviation (FL) is R/L 8°; the secondary deviation FR is R/L 17°.

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3Congenital right 4th nerve palsy (Fig. 18.45).

•No difference in overall chart size.

•Primary and secondary deviation R/L 4°.

•Right hypertropia – note that the fixation spot of the right inner chart is deviated upwards and the left is deviated downwards.

•Hypertropia increases on laevoversion and reduces on dextroversion.

•Right chart shows underaction of the superior oblique and overaction of the inferior oblique.

•Left chart shows overaction of the inferior rectus and underaction (inhibitional palsy) of the superior rectus.

4Right 6th nerve palsy (Fig. 18.46).

•Right chart is smaller than the left.

•Right esotropia – note that the fixation spot of the right inner chart is deviated nasally.

•Right chart shows marked underaction of the lateral rectus and slight overaction of the medial rectus.

•Left chart shows marked overaction of the medial rectus.

•The primary angle FL is +15° and the secondary angle FR +20°.

•Inhibitional palsy of the left lateral rectus has not yet developed.

Fig. 18.43 Hess chart of a left 3rd nerve palsy

Fig. 18.44 Hess chart of a recently acquired right 4th nerve palsy

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Fig. 18.45 Hess chart of a congenital right 4th nerve palsy

Fig. 18.46 Hess chart of a right 6th nerve palsy

Refraction and fundoscopy

It should be emphasized that dilated fundoscopy is mandatory in the context of strabismus, to exclude any underlying ocular pathology such as macular scarring, optic disc hypoplasia or retinoblastoma. Strabismus is often secondary to refractive error and hypermetropia (hyperopia), astigmatism, anisometropia and myopia may all be associated.

Cycloplegia

The commonest refractive error causing strabismus is hypermetropia. Accurate measurements of hypermetropia necessitate effective paralysis of the ciliary muscle (cycloplegia), in order to neutralize the effect of accommodation, which masks the true degree of this refractive error.

1Cyclopentolate affords adequate cycloplegia in most children.

•The concentration employed is 0.5% under the age of 6 months and 1% thereafter. One drop, repeated after 5 minutes, usually results in maximal cycloplegia within 30 minutes, with recovery of accommodation within 2–3 hours and resolution of mydriasis within 24 hours.

•The adequacy of cycloplegia can be determined by comparing retinoscopy readings with the patient fixating for distance and then for near. If cycloplegia is adequate, there will be little or no difference.

•If cycloplegia is incomplete there will be a difference between the two readings and it may be necessary to wait another 15 minutes and to instil another drop.

•Topical anaesthesia with an agent such as proxymetacaine prior to instillation of cyclopentolate is useful in preventing ocular irritation and reflex tearing, thus affording better retention of the cyclopentolate in the conjunctival sac and effective cycloplegia.

2Atropine may be necessary in some children with either high hypermetropia or heavily pigmented irides, in whom cyclopentolate may be inadequate.

•Atropine may be used as drops or ointment. Drops are easier for an untrained person to instil, but there is less risk of

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overdose with ointment. The concentration is 0.5% under the age of 12 months and 1% thereafter. Maximal cycloplegia occurs at 3 hours; recovery of accommodation starts after about 3 days and is usually complete by 10 days.

•Atropine is instilled b.d. for 3 days before retinoscopy, but not on the day of examination. The parents should be warned to discontinue medication if there are signs of systemic toxicity, such as flushing, fever or restlessness, and to seek immediate medical attention.

Change of refraction

Because refraction changes with age, it is important to check this in patients with strabismus at least every year and more frequently in younger children and if acuity is reduced. At birth most babies are hypermetropic. After the age of 2 years there may be an increase in hypermetropia and a decrease in astigmatism. Hypermetropia may continue to increase until the age of about 6 years, levelling off between the ages of 6 and 8 and subsequently.

When to prescribe

Most children are mildly hypermetropic (1 to 3 D). There is some evidence that fully correcting hypermetropia in a normal child may reduce physiological emmetropization.

1Hypermetropia. In general up to 4 D of hypermetropia should not be corrected in a child without a squint unless they are having problems with near vision. With degrees of hypermetropia greater than this a two-thirds correction is usually given. However, in the presence of esotropia the full cycloplegic correction should be prescribed, even under the age of 2 years.

2Astigmatism. A cylinder of 1.50 D or more should be prescribed, especially in cases of anisometropia after the age of 18 months.

3Myopia. The necessity for correction depends on the age of the child. Under the age of 2 years, −5.00 D or more of myopia should be corrected; between the ages of 2 and 4 the amount is −3.00 D. Older children should have correction of even low myopia to allow clear distance vision.

4Anisometropia. After the age of 3 the full difference in refraction between the eyes should be prescribed if it is more than 1D. If there is no squint then any associated hypermetropic correction may be equally reduced for each eye.

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