Ординатура / Офтальмология / Английские материалы / Binocular Vision and Orthoptics Investigation and Management_Doshi, Evans_2001

This book started life as a compilation of a series of 16 articles that were published in the journal Optician between 1998 and 2000. Nine leading experts in the field of binocular vision and orthoptics were invited to write these articles. The series was acclaimed a success. and led to the idea for this venture. Each author was invited to update their article to form the chapters of this book. It is due to the willingness and enthusiasm of these individuals that this book has been possible. and we thank them for this.

The original articles were designed to meet the needs of eye-care professionals who encounter binocular vision anomalies; hence the reader will find that this book is biased towards the commonest clinical binocular vision problems. Several chapters concentrate on the very common conditions of heterophoria and vergence

anomalies. particularly convergence insufficiency. Different authors have been invited to cover these important topics from slightly different approaches. and it is our belief that this diversity of expert opinion strengthens the book. Indeed, whilst this book is cohesive in its content, it is hoped that the presentation of different authors' approaches will help readers to gain a balanced perspective.

More than 90 per cent of NHS contacts are said to occur in primary care. With its focus aimed at practitioners working in this sector, Binocular Vision and Orthoptics has been designed to stress investigations and treatments that are appropriate in this area of patient care. Although a great many orthoptic problems can be treated in this environment, some can only be treated medically or surgically, and these topics are also covered in this book.

Some of the topics in Binocular Vision and Orthoptics are dealt with in more detail in another Butterworth-Heinemann book.

Pickwell's Binocular Vision Anomalies, written by Dr Bruce Evans. For the keen reader this book provides an interesting, additional source of information.

Some practitioners confess to perceiving binocular vision and orthoptics as intimidating subjects. We believe that, once attacked with an inquisitive and logical approach, these topics are far from daunting. Indeed, we feel that they are amongst the most fascinating and rewarding challenges that eye-care practitioners can encounter. We hope that this book will help practitioners to share this enthusiasm, which can only be of benefit to their patients.

Bruce Evans

SandipDoshi

The routine eye examination should be structured to detect all abnormalities of the eye and refraction. To that end, some form of evaluation of the binocular system should be undertaken on every patient with two eyes. A working knowledge of binocular vision anomalies is thus essential to enable practitioners to rule out. treat or refer appropriate binocular vision cases.

Prevalence of binocular problems

Research shows that up to 8 per cent of children have significant refractive errors, although the definition of 'significant' varies (lngran et al., 1986). According to many authorities (Vaughan and Asbury,

1977; Simpson et al., 1984; KasmannKellner et al., 1998; Abrahamsson et al., 1999). 2-4 per cent have strabismus and up to 5 per cent have amblyopia. depending on how amblyopia is defined (Preslan and Novak. 1998). In optometric practice. the child patient is likely to be self-selecting or referred if the practice has a special interest in binocular vision or in children. The result of this is that most optometric practices will have a very large incidence of patients who have significant refractive errors and binocular problems. Children will become adults. and these adults will be very aware of the problems they had when young and seek help or reassurance. Added to this is the large number of patients who have common low-grade binocular problems such as convergence insuffi-

ciency and accommodative dysfunctions. There is little in the literature to establish reliably the prevalence of general binocular problems. One study of second-year optometry students (Porcar and MartinezPalomera, 1997) found that 32 per cent suffered from such problems, and Hokoda (Hokoda, 1983) found a prevalence of 21 per cent with general binocular dysfunctions.

History

Taking a good history is the cornerstone of a good binocular routine. This will help to establish a differential diagnosis and aid investigation, which should begin with determining the most likely cause.

Z Billocular Visioll alld Orthoptics

This applies to patients requiring investigation for binocular vision anomalies and to patients who have come for routine eye examination.

Since amblyopia and strabismus frequently present at typical ages, it is important to elicit details of onset and, where possible, the course of the condition. This is useful in determining the likely prognosis and if referral is appropriate. Vital information required to make a proper evaluation includes:

•Age of onset of symptoms (if any)

•Frequency of symptoms

•Time of occurrence of symptoms

•Speed of onset of symptoms

•Constancy of symptoms

•General health of the patient at the time the symptoms were first noticed

•Any previous ocular investigations or treatment.

Age of onset of symptoms

Understanding the age of onset of the binocular problems is vital in order to enable a prognosis to be made. Most authorities suggest that unless there is early intervention for amblyopia and strabismus, the prognosis is poor. Griffin and Grisham (1995) feel that any strabismus manifest before the age 4 months can be classified as congenital. After this period there has been significant development of the accommodative system, and the description 'early acquired' could be used. They use the term 'late acquired' for problems occurring up to the age of 2 years but after 4 months.

Determining the exact month of onset will help to determine how long there has been normal cortical stimulation prior to strabismus setting in. This will help the practitioner to decide on the urgency of the case, especially if surgery is a consideration.

Frequency of symptoms

Establishing the frequency ofthe symptoms will help to establish the severity of the condition and the prognosis. Severe, annoying symptoms usually suggest recent onset. and in these cases binocular evaluation needs to rule out incomitancy due to systemic conditions such as diabetes or Graves' disease. Other causes include a decompensating heterophoria. Previous history is obviously a factor here, and it is sometimes useful to contact the patient's

previous practitioner if the patient has not been seen previously.

Factors in decompensating heterophoria will depend upon the binocular status of the patient, but could be caused by general illness, fatigue, stress or an increase in work load, as is frequently the case in students coming towards the completion of a course that culminates in examinations.

In these cases there is usually a decompensating heterophoria, commonly exophoria, but on occasions there is a tendency towards accommodative excess as a direct consequence of attempting to utilize accommodative convergence for a mild convergence insufficiency.

Time of occurrence of symptoms

Careful evaluation of when the symptoms occur helps to complete the picture. For example, in a young child symptoms often occur after close work, suggesting particular attention be paid to the near visual performance during the consultation. In divergence excess, for example, intermittent asthenopia and photophobia associated with television viewing or driving is a useful clue. In these cases it is prudent to evaluate distance ocular motor balance outside the confines of modern examination rooms. Viewing distances at the full 6 m or more will often enable the problem to be found, using a cover test, that will otherwise be missed.

Speed of onset of symptoms

Understanding when the symptoms first started allows the practitioner to pinpoint the aetiology. Diplopia with sudden recent onset deserves very careful evaluation and assessment. Symptoms should always be evaluated in association with other signs. Vascular accidents, neurological problems or space-occupying lesions could cause sudden onset diplopia. Raised intracranial pressure is another cause of sudden onset strabismus often associated with reduced vision. These patients often have recent history of headache and nausea, and spending an appropriate time on interviewing the patient is invaluable. Practitioners should not be slow to reach for an ophthalmoscope or in arranging urgent visual field assessment in this category of patient.

Constancy of symptoms

Constant strabismus rarely gives rise to symptoms if it has been present for some time. Almost by definition, it will be oflong

duration and less problematical for the patient. Intermittent binocular problems can be more difficult to assess because they may not be present at the initial consultation. Where binocular evaluation is unable to elicit any problems commensurate with the symptoms, it is always advisable to ask the patient to return at a time when there are most likely to be obvious difficulties. IT this is not possible, repetitive testing may fatigue patients enough to break down their ability to maintain compensated binocular vision.

General health of the patient

It is useful to establish the state of health of the patient at the time that symptoms first presented. For example, there is the oftenquoted onset of hypermetropia and convergent strabismus associated with measles. It seems unlikely that the measles would cause either of these conditions, but it is much more likely that the patient will become decompensated and unable to cope with the effort to maintain stable binocular vision during a febrile illness such as this.

Family ocularhistory

It is well known that there is a strong association between those patients who have a family history of strabismus and those that arc hypermetropic. The incidence of strabismus, for example, is between 2 and 4 per cent in the general population. In primary care, nearly 10 per cent of patients have some sort of binocular problem.

In large studies of children (Blum et a!" 1959; Waters, 1984) levels of visual difficulties have been found to be between 1 'i per cent and 22 per cent, with some studies finding as many as 38 per cent of patients with abnormalities (Kasmann-Kellner et al., 1998). Most of these are non-strabismic and relatively easy to treat.

For patients with a parent who has strabismus, there is a four-fold increase in the incidence. Where there is strabismus in a parent and over 2 D of hypermetropia, the risk factor for strabismus is increased by four to six times (Ingram and Walker, 1979; Abrahamsson et al., 1999). For those patients with a strong family history and high hyperopia there is a 50 per cent chance of developing strabismus (Abrahamsson et al., 1999).

The group of patients most likely to visit an optometrist is self-selecting (those who are particularly aware of potential visual problems will be those who have family

members with significant visual histories). Optometrists should therefore expect to see a higher than average number of patients presenting with binocular problems.

The patient's family history, including that of parents, uncles, cousins, siblings, grandparents and children, should be sought. Details of ocular deviations, amblyopia and refractive difficulties should be recorded.

General personalhistory

General personal history should include pregnancy and birth delivery, including details of pre-term, forceps or Caesarean deliveries. which could cause birth trauma. and the health of the baby after birth.

Developmental history and information about illness. regular medication. systemic disease. accidental or non-accidental head trauma and emotional crises must be recorded.

Previous ocular history and treatment This may be confused. since the patient has often been told that there are epicanthal folds or has sought advice from inappropriate sources. Where there has been intervention from an eye-care professional, it is always best to ensure that the details are sought from that practitioner rather than relying on the history from the patient.

Symptoms

Patients suffering from decompensated heterophoria often complain of a variety of symptoms that could disappear if strabismus develops along with sensory adaptations (suppression or abnormal retinal correspondence). These symptoms may be difflcult to elicit from small children since, even if they are able to express themselves. children often assume that everyone suffers from the same problems and consider them to be normal.

Symptoms could include intermittent blur for distance or reading; letters or words doubling or moving; closing one eye when reading; confusing, omitting or repeating words; and lack of concentration on close work (Table 1.1). In addition, the patient might report loss of place during reading. burning. itching. tearing and sensitivity to light in the absence of anterior segment pathology. Some patients report that they have difficulty with activities requiring good eye-hand co-ordination,

Figure 1.2

Lo,qMAR charts:letter sizes are equalas they are based on a logarithmic scaleof the minimum angleof resolution

Another possible cause of difficulty in amblyopia is that localization and fixation skills tend to be poor. Griffin and Grisham ( 1995) suggest that in a complex environment, amblyopic spatial uncertainty and unsteady fixation can lead to an increased number of fixation errors. A restricted field with fewer errors is less distracting and therefore each letter can be fixated more easily.

Bailey-Lovie charts are a distinct improvement over Snellen charts and steps between the letter sizes are equal. being based on a logarithmic scale of the minimum angle of resolution. These are known as LogMar charts. and are excellent for binocular vision assessment (Figure

1.2).

When testing children there are several alternative tests available, including the Kay picture test. Lea symbols, Ffooks cube, Ilhterute-E and Sheridan-Gardiner. Others are available to deal with crowding, such as the Cambridge crowding cards (Figure 1.3), Keeler LogMar crowding charts, Lea crowding cards and the Kay crowding picture cards. Preferential looking (PL) tests such as the Cardiff acuity cards are exceptionally helpful and easy to use for very small children unable to letter match or name letters/pictures.

As an additional diagnostic aid, a 2 log unit neutral density filter placed in front of the amblyopic eye will distinguish between refractive, strabismic and organic amblyopia. An eye with strabismic amblyopia will perform at least as well (or even better) through the filter once dark adapted, while the eye with organic amblyopia will show a reduction in acuity that can be quite marked.

Refraction

Any binocular assessment would be incomplete without a thorough refractive evaluation. This is not surprising given the relationships between accommodation and convergence as well as the influence on accuracy of these factors in relation to acuity. Furthermore the patient may develop strategies to deal with minor binocular problems, which can easily be remedied by prescribing accurate and appropriate refractive compensations.

Many practitioners are surprised by the effect of even quite low-powered prescriptions on relatively minor vergence or accommodative difficulties. Dwyer and Wick (1994) reported improvement in binocular function after 1 or more months of wearing initial spectacle corrections in 143 nonstrabismic patients who had a refractive error and a vergence anomaly, an accommodative anomaly, or a combined

vergence and accommodative anomaly. Recovery of the normal vergence or accommodative function depended on the type of refractive error. They reported that 79 per cent of hyperopic astigmats recovered and 20 per cent of myopes recovered. Of the astigmats, 67 per cent of those with against- the-rule improved, as did 45 per cent of those with against-the-rule astigmatism. Patients were more likely to benefit from this approach under the age of 12 years (63 per cent), and only 41 per cent of older patients benefited. Those with vergence disorders were more likely to be successful (67 per cent), but only 38 per cent of patients with basic exophoria were successfully treated in this manner. Whilst this study shows the benefit of dealing with the refractive element, caution and careful follow-up is advised.

Cover-uncover test

This is an essential test in all binocular vision routines, and one with which all practitioners will be familiar. In fact. while it is the cornerstone of binocular assessment (after all, it is the only test that will distinguish between heterophoria and strabismus), many practitioners are so familiar with it that they pay it scant regard. In recent years students have been encouraged to estimate the degree of the movement, and hence the amount of

heterophoria present. Checking the approximate amount of movement is easy. At 6 m. the 6/12 Snellen line is approximately 12 em long. and looking from one end to the other will give the approximate movementof 2~.

While estimations are useful, practitioners should beware. since it has been shown that cover tests give underestimations of results - especially at near - by as much as I I A of exophoria and 13~ of esophoria (Calvin et al., 1996) using the US standard method of measuring heterophorlas.

Cover testing is used to determine if there is a heterophoria or strabismus present, to elicit the presence of A or V patterns. and to allow estimates of the size. direction and rate of recovery of the deviation. It is also used to establish the preferred eye and to allow a rough evaluation of the patient's ability to maintain fixation. Whilst this book will not attempt to explain the basic method of cover test procedures, a few useful points are discussed. The author's 10 tips on the cover-uncover test are given below.

Clinical pearls for the coveruncover test:

•Make sure that the patient looks at a letter size slightly larger than the letter representing the threshold acuity to ensure that fixation Is easily maintained.

•Remove the cover vertically; this will avoid the observer doing a beautiful cover test on hlrn- or herself.

•Hold the cover In place long enough to get a good result - some authorities suggest 1 s, some 2 s (Evans, 1997). Recent research suggests that 4-5 s may be the period of choice (Barnard. 1447). Barnard has suggested that

6 Binocular Vision and Orthoptics

either plus or minus lenses may be added. The results are not, however, always the same for both the plus and minus gradients. The reason for this difference is often to be found in the results of the accommodative facility tests. It cannot necessarily be assumed that the patient will accommodate accurately through the plus or minus lenses, and thus all the results should be treated with a certain amount of caution. Nevertheless, the AC/A ratio can be usefully employed to assess the effect of the refractive correction on the ocular motor balance whilst considering management options in binocular vision management.

Convergence

Testing for convergence can be done immediately after the cover test. If a repeated cover test has been necessary to ensure that accurate information is recorded, care should be taken to ensure that there is sufficient time to allow binocular vision to be re-establish cd. This may involve asking the patient to read a few lines of text. It is as well to start off by assessing how well fixation is maintained and then to bring the target in closer to the patient. The target used is either a vertical line, which can more easily be noticed by the patient if it becomes double, or a letter of 6/9 equivalent, which has become the clinical standard (Griffin and Grisham, 1995). A picture may be better for small children. These targets will allow a normal amount of accommodation to be used. For this reason it is not really a test of pure convergence, and it may be better to use a dim pen-torch light if it is desirable to measure convergence free from accommodation.

There is often debate in optometric circles regarding the best target to be used, with some suggesting that a fine pencil point or pen is sufficient and others insisting that an 'accommodative' target is required. This is generally believed to be a small letter. This author believes that an accommodative target can be either of these, since the main criterion in this respect is the ability of the patient to pay and maintain attention to the target, which then results in accurate accommodation.

Many patients are unable accurately to report diplopia, sometimes confusing it with blur, while others are so slow that the results would be totally inaccurate if

recorded. Objective assessment is therefore essential.

Failure values of over 8 em have been suggested (Griffin and Grisham, 1995), although Jones (1997) emphasized that it is better to be able to maintain convergence control accurately in the 15-20 cm range. There is a lack of research that has attempted to control all the variables in testing of convergence. Hayes et al. (1998) suggested that a cut-off value of 6 em would identify the worst 15 per cent of cases in children, and that 73 per cent of these were likely to be symptomatic.

Three attempts should be made at the test before any conclusion is reached. As with most things practice makes perfect. and an improvement is expected on each successive attempt. If the results become repeatedly worse, there is a suggestion of illsustained convergence. This can be the result of poor convergence, reduced fusional reserves, poor accommodation, or simply difficulty in maintaining attention.

The patient should be asked when discomfort is noticed, as this may be more relevant than when diplopia occurs. A note of this distance should be made, as well as the more usual blur, break and recovery points. When blur or diplopia is noted, cover each eye in turn and ask the patient if the print becomes clear or is still blurred. This will allow differentiation between an accommodative and a convergence problem.

Assuming that the motility test reveals no abnormalities of the medial recti muscles, then there should theoretically be no problem in converging almost all the way to the nose for most individuals (Griffin and Grisham, 1995).

Jump convergence

This is a measure of the ability to make vergence movements from one distance to another and to maintain fixation without being distracted. The targets used are often pencil tips or small letters on tongue depres- sor-type boards, held at 20 em and approximately 35 ern, or sometimes 50 em, The patient is asked to fixate from one ofthe targets to the other and back again on command. The quality of movements is recorded. It can often be observed that while convergence is normal. the patient has a problem with maintaining fixation. The optometrist needs to decide if there is a

primary problem with attention, fixation, convergence or visual figure ground.

Occasionally convergence can be so fragile and physiological diplopia such a surprise that patients are unable to maintain concentration on the target because they are distracted by the physiological diplopia or the background against which the target is held. This can cause confusion, and it is as well to ask patients to explain what they can see. Sometimes the convergence can be better controlled if the background is more homogeneous. Occasionally the brightly coloured shirt and tie or clothing that the examiner is wearing can be the cause of an apparent convergence problem. Where this is the case, it could be assumed that any increase in background 'noise' from a visual point of view could cause a breakdown in binocular function.

The 20b. dioptre fusion test

This is often used with children as a screening test to evaluate the ability of patients to make a vergence movement to fuse a detailed near vision target. It is an excellent test for young children where other tests prove difficult. It is quick, easy and requires no special equipment.

A 20b. prism is placed with the base along the horizontal line and orientated base outwards. This will displace the image towards the nasal position and if single binocular vision is to be maintained, will result in the immediate initiation of a fusional vergence movement inwards to re-establish fixation. Observation of the fusional movement is made. In addition, the number of movements, smoothness and speed of re-taking fixation is assessed to gain an idea of the quality of fusion. The test should be performed once with the prism before the right eye and again, after a pause for fusion to stabilize, with the prism before the left eye.

The 4b. base-out suppression test

This test is used to attempt to ascertain if a suppression area is present. If this is the case during a cover test, it implies there is a micro-strabismus.

The patient fixes an accommodative target and the clinician introduces a 4b. base-out prism in front of the right eye and then in front of the left eye. If there is no suppression zone, there will be no movement of the eye after the prism is added. This is because the apparent