Ophthalmology is an exciting and rapidly advancing field. Our understanding of ocular anatomy, physiology and pathology has increased considerably over the last few decades. The treatment of eye disease has developed enormously with significant innovations in microsurgery, pharmacology and laser treatment. With these developments have come advances in the techniques we use to examine the eye, the orbit and the structural and functional integrity of the visual pathway.

Despite this rapid progress it remains as important as ever to understand how to use both old and new examination techniques to arrive at an accurate diagnosis with the minimum of inconvenience to the patient and the greatest degree of accuracy. It is also important to learn how to communicate with other specialists who may be involved in the investigation of ophthalmic disease, to appreciate what information they require to tailor their work. A complete understanding of the shortcomings of the equipment or test procedures is also vital if the results are to be interpreted correctly and given the correct weight in arriving at a diagnosis or developing a treatment plan.

Some of the newer diagnostic instruments described in this book rely on complex technology, produce a vast array of data and often beautiful and sometimes beguiling pictures. Although these may add greatly to our ability to detect, diagnose and treat ophthalmic disease, we must be sure that we understand something of the basis on which these machines work. Their limitations and sometimes the

inevitable uncertainty as to where they currently lie in the diagnostic armoury must also be considered. Those working in the field of ophthalmology are fortunate in being able to visualise most of the eye directly. A detailed anatomical and functional examination of the eye, its movements and surrounding structures, can be made with relatively little complex equipment. We still remain dependent on the skilful use of the slit lamp and other simple techniques for arriving at most of our diagnoses. This book describes both old and new examination and investigative techniques that those involved in eye care will use or may encounter in their work. The authors of each chapter not only describe how to undertake each test or examination but also outline the problems and pitfalls that will be encountered in their performance and interpretation.

No book can hope to replace a skilled teacher well versed in the examination of the eye. Practical teaching must remain the cornerstone of learning how to examine the ophthalmic patient. Similarly, time spent discussing the most appropriate test to perform with colleagues working in associated areas, for example optometrists, orthoptists, ophthalmologists, neurophysiologists, neurologists or radiologists, is never wasted. We hope that this book will provide a sound framework for this practical teaching and discussion.

Bruce James

Larry Benjamin

Introduction

The ophthalmic examination requires detection of abnormal structure and function. Vision, the ability to see, the fundamental function of the eyes, can be assessed in various ways:

•Visual acuity is a measure of the visual system’s ability to discriminate two high-contrast points in space.

•Contrast sensitivity is a measure of the minimum contrast required for the visual system to discriminate these points from the background.

•Colour vision is the ability of the visual system to differentiate light of different wavelengths.

•Dark adaptation is the ability of the visual system to adapt to reduced luminance.

•Motion detection is the ability to detect movement (see Ch. 7).

•Visual field is the area over which the visual system is able to detect light (see Ch. 7).

Distance visual acuity

The Snellen chart

Measurement of visual acuity is commonly performed in adults with the Snellen chart (Fig. 1.1). The chart uses standard letters of different size with approximately equal legibility, and measures high-contrast visual acuity. The letters are constructed on a 5 × 5 or 5 × 4 grid such that, when viewed at the intended distance, each limb subtends 1 min arc at the eye, and the whole letter 5 min arc (Fig. 1.2a). Snellen acuity is written as a fraction:

Distance at which the chart is viewed

Distance at which the letter subtends 5 min arc

Normal vision is taken to be 6/6 (although 6/4 is not unusual), so a letter on the 6/6 line subtends 5 min arc at the eye at 6 m. Vision of 6/60 means that, at

6 m from the chart, the subject can just read a letter which someone with normal vision would be able to see from 60 m away.

Practically, care must be taken to ensure that the chart is well illuminated and that the patient is at the correct distance from the chart. Distance glasses should be worn for the test. A pinhole will overcome moderate degrees of refractive error but will also compensate for some media opacities such as cataract. Each eye is tested separately. If only some of the letters on a line are seen, the acuity is recorded as the completed line minus the number of letters missed, or plus the number of letters seen on the next line, whichever is smaller.

The Landolt C test is similar in principle to the Snellen test and attempted to overcome the problem that some of Snellen’s optotypes were more recognisable than others (Box 1.1). It used a broken ring or C in different orientations (Fig. 1.2b). It has been used more in research than in clinical practice.

Box 1.1

Problems with the Snellen acuity chart

■Unequal number of letters per row. Fewer large letters on the chart results in reduced accuracy when measuring patients with poor vision. Also, one letter on its own is easier to read than one in a row (known as the crowding effect or contour interaction)

■The progression of letter size is not regular, nor is the spacing between letters

■Accurate recording is difficult as not all letters on a line may be seen correctly, some letters are easier to see than others, and a different selection is used on different charts

The Assessment of Vision

Fig. 1.1 A Snellen acuity chart.

Fig. 1.2 (a) A 6/6 Snellen’s letter; (b) a Landolt C 6/6 symbol.

For illiterate patients a Tumbling E chart is available with the letter E rotated in various positions. The examiner points to a letter and the patient uses a cut-out E to match its position. The conventional Snellen chart can also be used with patients matching the indicated letter to one on a sheet in front of them, similar to the Sheridan–Gardiner test for children, described below.

logMAR charts

To overcome some of the problems with the Snellen chart, a number of alternatives have been proposed. Those based on the logMAR scale have clear advantages and are increasingly used in clinical work, while being mandatory for research.

Bailey–Lovie chart

Two Australian optometrists, Ian Bailey and Jan Lovie-Kitchen, developed the first commercially available logMAR chart. Designed for use at 6 m, it has five letters per line, each letter based on a 5 × 4 grid.

logMAR is short for log10 minimum angle of resolution. If normal vision is the ability to see a letter 5 min high where each limb is one-fifth of the height, then the minimum angle of resolution is 1 min. The log10 of 1 is zero, so the logMAR equivalent of 6/6 is 0.00. Similarly, the logMAR equivalent of 6/60 is 1.00. For acuities less than this, the testing distance can be halved, and the score simply doubled, with 3/60 being logMAR 2.00. One slight disadvantage is that acuity better than 6/6 has a negative logMAR score, with 6/3 being logMAR –0.30.

This minor disadvantage is outweighed by several key advantages: a 0.1 logMAR progression in letter size, an equal number of letters on each line, the spacing between the letters is equal to the letter width, and the spacing between lines is equal to the

height of the line below. Thus each line presents a task of equal difficulty with letter size being the only variable. This has the further advantage of allowing accurate scoring of incompletely seen lines. Since each line represents a 0.1 step, and there are five letters per line, each letter seen scores 0.02. Thus, if all the 6/6 equivalent line is seen, the score is logMAR 0.00, but three letters more would be –0.06, and three letters less would be 0.06. Similarly, if the 0.2 line is seen, but only two letters on the next line, the score is 0.2 – 0.04, giving a logMAR of 0.16.

Early Treatment in Diabetic Retinopathy Study (ETDRS) chart

The ETDRS chart was developed from the Bailey– Lovie chart (Fig. 1.3). It uses letters based on a 5 × 5 grid, chosen from the Sloan character set where each letter has similar legibility. A further revision ensured that the letters were mixed in such a way as to ensure equal legibility for each line. Three charts were used in the ETDRS study to reduce the learning effect: one each for right and left eyes, and one for refraction. It is scored in a similar way to the Bailey–Lovie chart. Alternatively the number of letters read can be

Fig. 1.3 An Early Treatment in Diabetic Retinopathy Study (ETDRS) chart.

recorded and this is particularly useful in patients with poor vision where acuity can be measured with greater accuracy than is possible with a Snellen chart. Increasingly used clinically, this test is the gold standard for acuity testing in vision research. Frontand back-lit versions of the chart are available. They are designed to be used at 4 m; versions designed for use at shorter distances are also available.

Near vision

This is assessed with a reading test-type book (Fig. 1.4). The patient should wear reading glasses if necessary and position the book a comfortable distance from the eyes. Various test-type books are available, including the Faculty of Ophthalmologists’ near-acuity book. This is the commonest chart used in the UK. It is based on a printer’s block where N72 comprises a lower-case letter 13.5 mm high. The Jaeger near-acuity book is rarely used now.

It is important to ensure that the book is adequately illuminated. The size of the test type seen with either eye, and then both eyes simultaneously, is recorded, together with the distance at which it is held.

The Assessment of Vision

Fig. 1.4 Near-vision test type.

Children’s vision

The assessment of visual acuity in children requires special methods until they are at least able to match pictures or letters on a chart. Until a child is 6 months old, vision is assessed by the ability to fixate and follow an object. As the child reaches 6 months the ability to detect hundreds and thousands cake decoration shapes is a traditional test. Similarly the size of fixation objects can be varied and the ability of the child to maintain fixation assessed.

The first formal test between 6 months and 3 years is performed with a preferential looking test, for example, Cardiff cards (Fig. 1.5) or Keeler acuity cards. These are based on the premise that children prefer to look at a picture or pattern rather than a plain card. Cardiff cards use a variety of targets consisting of a simple picture drawn by a white band bordered by two black bands, on a grey background. The mean brightness of the picture is equal to that of the background so that as the width of the bands decreases, the object becomes harder to see because it merges into the background. The card is presented either with an object on the top or on the bottom. The direction of the child’s gaze is noted. When the examiner is unable to detect the position of the object from the direction of the child’s gaze the child is assumed not to have seen the object.

From the age of 2 years matching tests can be performed with children matching a picture at 6 m to one in a book held close to them by the examiner.

a

b

Fig. 1.5 (a, b) Cardiff cards. Note the difference in the width of the outline between the two cards.

This provides a more repeatable measure of visual acuity. Initially charts with single pictures of differing sizes are used. Kay pictures and the Sheridan– Gardiner letter tests are examples of this type of test (Fig. 1.6). Kay pictures can be presented in a crowded form to reproduce the crowding phenomenon mentioned in the description of Snellen visual acuity.

The Keeler logMAR crowded test, also known as the Glasgow acuity test (Fig. 1.7), is a lettermatching test that can also test single letters or reproduce the crowding phenomenon. The progression of letter size is similar to the Bailey–Lovie chart. It is usually measured at a distance of 3 m. If both tests are performed the crowding ratio can be calculated as the uncrowded acuity divided by the crowded acuity.