Chapter 72 The clinical approach to strabismus

Anthony J. Vivian

Chapter contents

INTRODUCTION

THE CLINICAL SETTING

INTERACTION WITH THE CHILD AND PARENTS

EXAMINATION

DOCUMENTATION OF STRABISMUS FINDINGS

REFERENCES

Introduction

Strabismus surgery is a sub-specialty in its own right. The provision of high-quality care must be planned and executed strategically. This requires service delivery from primary care through to outcome and satisfaction audits at the end of the management. Although rates of strabismus surgery are declining, the management of strabismus and amblyopia is the majority of the workload of most pediatric ophthalmologists. I will give a few ideas about laying out and managing an efficient and effective strabismus practice. I will stress a friendly, non-threatening environment (both structural and emotional) and the fundamental principles of accurate documentation and audit to ensure clinically effective management.

The clinical setting

Frequently, the structural layout of our clinic space is constrained by immovable walls, doors, and windows, often not ideally placed. Occasionally there is an opportunity to start with a blank piece of paper, giving rise to some fundamental questions. How long should an examination room be? Do all the rooms have to be the same size? How can the most be made of a restricted floor area?

Room layout

The dimensions of an examination room for assessing children with strabismus have been dictated by the need to measure visual acuity at 6 m (20 ft) using Snellen acuity charts. However, tests based on the logarithmic scale of the minimum angle of resolution (logMAR) are more accurate methods for measuring acuity. Many of these tests can be projected or displayed on computer screens and so testing distances are more flexible.

Other factors determining the length of the examination room are the minimum distance required to prevent accommodation affecting the basic deviation measurement and other distance tests such as distance stereopsis and fusion tests. The maximum amount of accommodation when viewing an accommodative target at 6 m is about an eighth of a diopter, which is clinically insignificant. At 4 m accommodation is not completely suspended and could influence the basic deviation measurement (angle of deviation in the primary position for distance with accommodation suspended). At least some rooms should be 6 m in length to best perform accurate motility testing. However, these rooms do not need to be rectangular. To make the best use of limited floor area the rooms can “interlock” so that there is a wide end for the examiner and patient and a thin end for the fixation and acuity targets (Fig. 72.1).

Fig. 72.1 Design for two interlocking 6 m orthoptic rooms making maximum use of available floor space.

Distance fixation and acuity targets

Obtaining accurate results for the basic deviation measurement in children can be challenging. Failure to achieve an accurate measurement for distance is a common cause of surgical error, as many children with strabismus have a disparity between distance and near deviation. Operating on the near angle (which is easier to measure) may result in an over- or undercorrection for distance. Interesting distance fixation targets are essential in the management of childhood strabismus. The letters of a Snellen acuity chart will not hold the child’s attention for long. A variety of fixation targets are necessary, some requiring greater resolution than others. Cartoon characters and soft toys arranged at 6 m provide more interest for the child. DVDs can be displayed on a TV screen and there are many computer based fixation and acuity programs available (including Apps for iPads and mobile phones) (Fig. 72.2).

Fig. 72.2 By using a television playing age-appropriate videos, distance fixation is enhanced, allowing better distance measurements.

Tools of the trade

When assessing strabismus, there are a number of useful items (Fig. 72.3). The list is not exhaustive but emphasizes that if the equipment is not at hand, the test will not be done.

1. Occluder for performing cover tests: a transparent (Spielmann) occluder allows visualization of the occluded eye.

2. Single prisms (Berens’ square prisms are the most accurate) for performing prism cover tests: single prisms have a number of advantages over prism bars. The larger prism area allows better visualization of the fixation targets by the patient, and allows better visualization of the eye by the examiner. Both horizontal and vertical deviations can be corrected simultaneously in front of the same eye (Fig. 72.4).

3. Near acuity target for determining the near angle as accommodative demand is increased (particularly useful in assessing children with convergence excess esotropia).

4. Near stereoacuity tests (Lang, Frisby, TNO, Wirt).

5. Maddox rods or Bagolini lenses for measuring torsion.

6. Fixation light.

Fig. 72.3 A selection of useful instruments for strabismus assessment.

Fig. 72.4 Using single prisms held in one hand, vertical and horizontal deviations can be assessed simultaneously.

Interaction with the child and parents

First meeting

Children can find their first eye clinic appointment intimidating. They may be too young to understand what is going on and have to wait for a long time to be seen, absorbing the anxiety of their parents. They need to have dilating drops. Make the first visit a positive experience for both the child and the parents; if it has not been too traumatic, subsequent visits will yield more information.

The waiting area should be child-friendly. Play assistants make a significant difference to the overall mood of the waiting area, relieving some of the stress of the parents, which reflects in the behavior of the child.

Several methods for getting the most information out of the first visit can be employed:

1. Observation: this can be performed while talking to the parents or while the child plays. Look at the visual behavior and look for an abnormal head posture or obvious strabismus.

2. Lang stereotest: this can be made into a game, which can be played first by a parent or sibling. It is a good initial test because it is not intimidating (it does not require special glasses to be worn).

3. Distance fixation using DVDs: children’s DVDs playing at the far end of the examination room are a useful distance fixation target to enable cover testing and determining fixation preference.

4. Cycloplegic drops: cyclopentolate 1% instilled 30 minutes before refraction is adequate for most children. Best results are obtained if someone other than the examiner, in a separate room, instills them. Topical anesthetic (proxymetacaine 0.5%) reduces the discomfort of the cycloplegic drops considerably and should be considered. Atropine 1% can be used if cycloplegia with cyclopentolate cannot be obtained. If the child finds instillation of drops in the clinic very traumatic, atropine drops or ointment can be instilled at home by the parents prior to the clinic appointment.

5. If a fundus view is not achieved, another attempt should be made at a later date. It is occasionally necessary to physically restrain a child for fundus examination if there is suspicion of fundus pathology.

6. Examination of the optic nerve should include a magnified view. This can be achieved with a direct ophthalmoscope or by slit-lamp biomicroscopy, which gives a much better view. It is important to recheck the fundus and disc appearance in children with amblyopia refractory to treatment to exclude other causes of poor vision such as optic nerve hypoplasia.

7. Hand-held slit-lamp: it is possible to look at children with an adult slit-lamp, but examining small children is easier with a hand-held slit-lamp.

Role of the orthoptist

The orthoptist plays a very important role in the strabismus clinic. However, it is important that the strabismus surgeons are competent at performing a complete examination themselves. Only by examining the patient themselves can they get a feel for the subtleties of strabismus. Strabismus management should involve a multidisciplinary approach. The doctor should be able to perform a sensory and motor examination, and increasingly orthoptists are expanding their role to include parts of the examination traditionally the territory of the ophthalmologist, such as refraction.

Practical history taking: extracting clinically relevant details

A focused assessment of symptoms and past history is mandatory. The history taking does not have to be extensive but should be careful and specific. It is important to find out about:

1. The parents’ observations about the strabismus: whether the eyes turn in or out, which eye is most commonly affected, whether the strabismus is intermittent or constant, and changes in the strabismus when the child is tired or unwell.

2. Other ophthalmic symptoms: whether there are any associated abnormalities such as closing one eye in bright light, whether double vision is experienced, and whether the parents have noticed a vertical element to the strabismus or an association with changes in lid height.

3. Past ophthalmic history: whether there is any history of ophthalmic care (including glasses or patching), trauma, or previous strabismus surgery.

4. Past medical history: previous history of systemic or neurologic disease; the pregnancy, delivery and developmental history; and the drug and allergy history.

5. Family history: whether there is a family history of glasses wear in childhood, lazy eyes, or strabismus surgery; and whether there is a family history of either ophthalmic or systemic inherited disease.

Examination

Examination of the strabismus patient should include:

• A sensory examination (acuity, fixation preference, binocular vision, fusion, accommodation, and convergence).

• Ocular motility testing.

• Ocular examination (slit-lamp examination, media and fundus examination).

• Refraction (cycloplegic in children).

Further specific examination including a search for dysmorphic or neurologic signs may be necessary.

Sensory examination

Acuity

There are many tests to measure visual acuity in preverbal and verbal children, some more successful than others. They rely on experience in both performing and interpreting the tests. A careful measurement of acuity is essential at each visit. Both near and distance acuity should be measured. In order to maintain reproducible results, acuity measurement techniques and recording should be standardized with specific acuity tests for children at each level of developmental ability. There is a very large range of different acuity tests, each with its own merits. It is important to have a hierarchy of tests that give reproducible results at various ages.

Preferential looking techniques

Preferential looking is a behavioral method of assessing visual acuity, which can be performed from a very young age. The principle is that the child will be more interested in looking at a target with a resolvable pattern on it than looking at a target with no obvious pattern. The resolution of the target can be reduced until the child is unable to make a choice. The point at which the child is unable to make a choice gives an estimate of the visual acuity. For infants and children to the age of about 2 years, grating acuity cards such as Keeler or Teller can be used, and for children between 2 and 3 years of age vanishing optotypes such as Cardiff cards are more appropriate.

Kay pictures

Picture chart acuity tests such as Kay pictures are useful but they tend to overestimate the acuity. They are useful for comparing the acuity of each eye and logMAR versions are available.

logMAR-based matching tests

At about to 4 years most children will be able to perform a logMAR-based matching test, which can be presented on cards¹ or on a screen.² These give accurate and reproducible acuity results.

logMAR

Once the child knows the alphabet, a full logMAR acuity is possible using a logMAR chart (such as a Bailey-Lovie chart). It is important to measure the worst eye first because children have excellent memories.

Fixation preference

Fixation preference is a simple and effective test for detecting amblyopia and can be performed in infants. It is most accurate with strabismus of 10 prism diopters or more. Fixation preference is analyzed by assessing the centrality, steadiness, and maintenance of fixation (CSM method). “Central” fixation requires that the corneal reflex is central in the pupil. “Steady” fixation requires no evidence of nystagmus or oscillatory eye movements. “Maintained” fixation refers to the ability of the non-preferred eye to maintain fixation after occlusion is removed from the preferred eye. This can be graded by determining whether the fixation is maintained through a blink or during a smooth pursuit movement, or by observing the time period through which fixation is maintained by the non-preferred eye when occlusion is removed from the preferred eye.³

In orthotropia fixation preference can be assessed by creating a deviation using a 10- or 12-diopter prism either base-up or base-down in front of one eye. The prism is used vertically because vertical fusion is much smaller than horizontal fusion. Fixation should alternate if acuity is equal. If a fixation preference is detected, it can be graded in a similar fashion to strabismic patients.

Binocular single vision, fusion and stereopsis

Amongst the most difficult areas to understand are binocular single vision, fusion, and stereopsis. This is partly the terminology and partly our limited understanding of how the eyes work together. The terminology used by physiologists and ophthalmologists is different, which has contributed to our confusion. Although concepts such as “the horopter” and “Panum’s fusional area” are useful for the physiologist, they confound the ophthalmologist who decides the sensory side of strabismus is too much to cope with and leaves it to the orthoptist and the theorist. This is a pity because it is useful and affects management. We must be very clear about our definitions. What follows is a list of definitions, which simplifies the sensory assessment of strabismus patients, and an account of how we use this information in management.

Retinal correspondence

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