3.1.1 Symptoms

Symptoms of blurred vision or an inability to see well enough for certain tasks (reading the whiteboard, schoolbooks or a newspaper, watching TV, driving, etc.) all suggest reduced vision due to ametropia or ocular disease. These symptoms should be explained by reductions in visual acuity and/or contrast sensitivity. If a patient has symptoms of poor vision, but the visual acuity is normal, contrast sensitivity and visual fields should be measured. Symptoms including unexplained headaches, unexplained visual acuity or contrast sensitivity loss, positive or scintillating scotoma, bumping into things on one side and symptoms consistent with neurological disease, such as cluster headache, acquired migraine, dizziness, tingling of limbs all suggest the need for visual field testing. Obviously if a patient complains of colour vision changes, their colour vision should be assessed.

3.1.2 Ocular, family and medical history

The ocular history may indicate an ocular condition that requires monitoring using visual acuity, contrast sensitivity, colour vision and/or visual fields, such as cataract or glaucoma. The family history may indicate a hereditary condition, such as open-angle glaucoma, that should be carefully checked using visual field assessment. The medical history could indicate a systemic condition that requires particular assessment, such as diabetes, or systemic medication that could affect some aspect of vision, such as chloroquine.

3.2.1 When should distance visual acuity be measured?

Visual acuity (VA) is a measure of the patient’s ability to resolve fine detail. It is the most commonly used measurement of visual function you will make. Distance VA is used to assess the adequacy of spectacle corrections and as a key indicator of ocular health. VA is also used to assess a person’s fitness to drive or enter into some professions such as the police force and to enable registration as a partially sighted or blind person. The need to measure VA accurately is obvious.

There are three principal measures of VA:

3.2.2 LogMAR versus Snellen

LogMAR visual acuity (VA) charts (Figure 3.1) use the design principles suggested by Bailey and Lovie, including 0.1 logMAR progression of letter size from −0.3 to 1.0 logMAR (equivalent to 6/3 to 6/60 or 20/10 to 20/200), five letters per line, letters of similar legibility and per-letter scoring.¹ Snellen charts were devised by the German ophthalmologist Hermann Snellen in 1862 and have been widely used ever since. There is not a standard Snellen chart and the letter size sequences, number of letters and/or numbers, varieties of letters, etc., vary from manufacturer to manufacturer. They typically contain one letter at a VA level of 6/60 (20/200; 0.1 logMAR) and increasing number of letters at smaller letter sizes with a typical bottom line of about 6/5 or 20/15 (~ −0.1 logMAR).

The majority of Snellen charts have only one 6/60 or 20/200 letter, two 6/36 or 20/125 letters and three 6/24 or 20/80 letters. LogMAR charts typically have five letters on each of these lines and additional lines of letters at 6/30 or 20/100 (0.70 logMAR) and 6/48 or 20/160 (0.90 logMAR). Most logMAR charts have a ‘bottom line’ of −0.3 logMAR (6/3 or 20/10), whereas many Snellen charts have a bottom line of 6/5 or 20/15 and thus provide truncated data given that the average visual acuity of a young adult is about 6/4 (Table 3.1).²

LogMAR charts are widely recognised as providing the most reliable and discriminative VA measurements and are standard for clinical research or clinical trials of ophthalmic devices or drugs.3,4 Visual acuity measurements using a logMAR chart have been shown to be twice as repeatable as those from a Snellen chart and over three times more sensitive to inter-ocular differences in VA and therefore substantially more sensitive to amblyopic changes for example.^3,5 The Bailey–Lovie or ETDRS charts are the most commonly used logMAR charts for adults and the Glasgow Acuity Cards (commercially available as the Keeler logMAR crowded charts) have been designed specifically for children.^4,5

Many Snellen charts do not contain lines of small letters and are truncated to 6/4, 6/4.5, 6/5 or even 6/6. For some patients, this takes the approach of measuring ‘distance vision adequacy’ (i.e. determining whether distance VA is adequate for a patient’s daily needs, similar to the approach used for near VA) rather than distance VA (a threshold measurement). This makes the detection of slightly reduced VA due to eye disease or uncorrected refractive error in patients with good VA impossible. For example, if your chart is truncated to 6/5 or 20/15, you will not be able to detect a VA loss from 6/3 (or 6/4) to 6/5 or from 20/10 to 20/15.

The major disadvantage of logMAR charts is that although they are available in printed, and projector chart form, they are not as widely available as Snellen charts. However, this is slowly changing. In particular, logMAR VA measurements are being promoted on computer-based visual assessment systems. The new generation of flat panel displays appear especially useful as they are light (easy to wall mount), have excellent resolution, luminance and contrast and are flicker-free. VA measurements on these systems have the advantage of allowing randomisation of letters, calculation of VA scores and conversion of VA into different notations. One practical disadvantage of printed panel logMAR charts is that patients may not be able to see the ‘bottom line’ of the chart (−0.3 logMAR, 6/3, 20/10). If they have been used to having their VA measured on a truncated VA chart and being able to see the bottom line, this may upset some patients. You should explain that the new chart includes lines with smaller text than the old one (‘this new bottom line is just for superman/superwoman’) and indicate, or even highlight, the 6/6 (20/20) line.

Snellen charts have a major advantage at present in that they are widely available and Snellen notation of VA is universally understood. From a practical viewpoint, these charts can generally be produced in a smaller format as considerably fewer large letters are presented and this allows easier display on projector systems and the easy addition of other targets such as a duochrome and spotlight, etc., to wall charts. The charts provide a good target for refraction of patients with good VA as the number of lines between approximately 20/15 to 20/40 (6/5 to 6/12) is similar to logMAR charts and these lines typically have as many, if not more letters than logMAR charts. Although the truncation of Snellen charts (to say 6/5 or 20/15 when some patients can see 6/3 or 20/10) can be a disadvantage, it can speed up the measurement and allows most patients to read your ‘bottom line’.

3.2.3 Procedure

See online video 3.1.

3.2.4 Alternative procedures to assess VA in children

Amblyopia can be missed if single letters are used rather than a letter chart because of the lack of contour interaction. Ideally logMAR-based charts with contour bars at the end of lines, such as the logMAR crowded charts, should be used when measuring VA in children (Figure 3.2).⁸ The logMAR crowded charts have been shown to be over three times more sensitive to inter-ocular differences in VA than single letter Snellen charts and therefore substantially more sensitive to amblyopic changes.⁵ Crowded and standard and logMAR charts can even be used in children who do not know their letters by providing them with a key card that includes a selection of the letters from the chart. You then point to a letter on the chart and ask the child to identify the letter on their key card. For children that are unable to use a key card, charts are now available in logMAR format that include pictures rather than letters (Figure 3.2), such as the Kay crowded picture test, which has been shown to provide comparable results to the logMAR crowded charts.⁹

3.2.5 Adaptation of VA measurement with older patients

A hyperopic shift is common in older patients and some varifocal wearers adapt to this change by habitually raising their chin to improve distance vision by viewing through the additional plus power in the intermediate section of the lens. This can be seen when assessing habitual VA as these varifocal wearers will raise their chin during measurements to improve VA. VAs should be measured through the distance portion of the lens. You should make a mental note that these patients will likely require additional plus (or less minus) power in their distance correction.

3.2.6 Recording for logMAR and Snellen charts

VA measurements can be scored in logMAR notation, using the visual acuity rating (VAR) score or converting scores to an equivalent Snellen value. Computer-based systems will typically convert VA values for you. LogMAR or VAR could be used on your own record cards. However, equivalent Snellen values should generally be provided when writing referral letters and reports, as Snellen notation is universally understood, whereas logMAR is not at present. In all cases, it is preferable to score using a by-letter system rather than measuring the lowest line at which the majority of letters were correctly read as it provides more repeatable and discriminative measurements.¹⁰ Comparisons of various logMAR scores with Snellen and other recording notations are shown in Table 3.2.

The fact that logMAR VAs better than 6/6 or 20/20 are negative is counterintuitive. The VAR score provides a simpler method for scoring logMAR charts. VAR = 100 − 50 logMAR. Therefore 0.00 logMAR = 100 VAR and each letter has a score of 1. For example if a patient reads all the letters down to the 100 row and gets one letter wrong on this row, their score is 99, two letters wrong 98, etc. If they read all of the letters on the 100 row and one letter on the row below, their score is 101, two letters on the line below 102, etc. A disadvantage of the VAR score is that it suggests that 100 (6/6) is normal VA, which is far from true for many patients with healthy eyes.²

The Snellen fraction is defined as:

Vision or visual acuity is recorded as the smallest line in which the majority of the letters are seen, irrespective of subjective blur. Errors are recorded by appending a minus one, two or three to the Snellen fraction or decimal notation. If additional letters are seen on the following line, the Snellen fraction or decimal notation can be appended by a plus (usually up to no more than 3). If the patient could not see the 6/60 letter at 6 m, but could at 2 m, record 2/60. Similarly if the patient could not see the 20/200 letter at 20 feet, but could see the 20/120 letter at 5 feet, record 5/120.

Vision or ‘VA s Rx’ or VAsc or Vsc all mean visual acuity measured without a correction.

VA c Rx or VAcc or Vcc all mean visual acuity measured with a correction and generally refer to the VA with the patient’s spectacles. VA measured with the patient’s contact lenses would typically be recorded as VA c CLs or similar.

VAs are recorded for the right eye (RE or OD), left eye (LE or OS) and binocular (BE or OU).

Examples (logMAR charts):

Examples (Snellen charts):

3.2.7 Interpretation

Any deviation from normal age-matched results, as shown in Table 3.1, should be noted. Note that the average VA for patients less than 50 is about −0.14 logMAR (6/4.5, 20/15) and that 6/6 or 20/20 represents reduced VA for the vast majority of patients. 6/6 or 20/20 only becomes the average VA for patients over 70 years of age with healthy eyes. You must also check whether there has been any change in VA from the previous examination. In patients with normal or near normal VA, a significant change in VA is more than 0.1 logMAR or one line.^5,11 Also note any inter-ocular asymmetry of a line or more, or a binocular result that is worse than the monocular response.⁵

By comparing vision and optimal VA or habitual and optimal VA and using the one line of VA is equivalent to −0.25DS rule, an estimate of the mean spherical correction can be gained (section 4.1.5). If this estimate is widely different from the actual subjective refraction result, an error may be suspected and the subjective (and/or spectacle power) rechecked.

3.2.8 Most common errors

3.3.1 Comparison of different near vision charts

Near vision cards typically present sentences or paragraphs of words rather than isolated letters and can incorporate examples of near vision tasks such as sheet music, technical drawings and telephone directories (Figure 3.3). They are also now available on e-tablet and e-phones. There are five main types of notations used to measure near VA: logMAR, N-point, M-scale, equivalent Snellen, and Jaeger. LogMAR near charts have all the advantages of distance logMAR charts (section 3.2.2) and should be used whenever an accurate, non-truncated measurement of near VA is needed and particularly when distance VA and near VA may differ, such as with multifocal implants/contact lenses and patients with subcapsular cataract. Some near logMAR charts use isolated letters rather than words, but word charts are preferred, particularly in patients with conditions such as age-related macular degeneration as it relates better to reading performance and is typically worse than near letter VA.¹² N-point uses the Times New Roman font and is the standard test in the UK and Australia. It is based on the ‘point size’ used by printers and word processing packages on modern computers, in which 1 point = 1/72 inch (~0.353 mm). This can be a useful aid when indicating to a patient the level of vision that would be provided for computer use by new lenses. N8 is approximately equal to 1.0 M and this eight times conversion holds for all print sizes. M-units are widely used in North America and indicate the distance in metres at which the height of a lower case ‘x’ subtends 5 minutes of arc. A 1.0 M letter ‘x’ is therefore 1.45 mm high. Equivalent Snellen notation is a confusing notation, especially when near VAs are not measured at the standard 16′: What does 20/20 near VA at 8′ mean? In addition, near vision adequacy should indicate what patients can see at their own near working distance, not an arbitrary standard of 16′ (working distances for reading in patients with normal vision range from 10′ to 20′¹³). Cards using Jaeger notation should not be used as there is no standardisation of what J1 or J5, etc., means and different charts can give totally different sizes of print with the same J-value.

Many non-logMAR based reading charts are truncated as the smallest print sizes often provided are N5 (at 40 cm equivalent to ~6/9 distance VA) and 0.4 M (at 40 cm equivalent to ~20/20 distance VA) so that many patients could read sentences of text smaller than this if given the chance and a threshold is not measured. Thus ‘near vision adequacy’ is a more appropriate description of the measurements than near visual acuity. Near vision adequacy measurements have the added advantage that the measurement is quicker than a threshold measurement and most patients will be able to see the ‘bottom line’ of text. It is left to distance VA measurements, with its many advantages in this regard (fixed measurement distance, same letter format, letters of similar legibility, etc.) to provide an accurate measurement of a patient’s resolution.

3.3.2 Procedure: logMAR near VA and M or N-notation near vision adequacy

See online video 3.2.

3.3.3 Adaptation for older patients

You should have asked the patient whether they use additional lighting, such as an anglepoise or goose-neck light, to read in their home. If they do not and you subsequently find they cannot easily read N5 (or 0.4 M) at the end of the reading addition part of the refraction, they should be encouraged to obtain additional light for near tasks in their home. It is very useful to demonstrate how helpful such additional light can be. The main objective of people with vision impairment is to be able to read and a majority can be successfully helped in primary eye care using high reading additions, simple magnifiers and additional lighting.¹⁴ Note that although near VA levels can be associated with a range of near tasks (Table 3.3), the poorer near VAs are threshold measurements (measurements of N5, 0.4 M and 20/20 may be truncated and not thresholds as discussed earlier) so that patients would not be able to read print of that size comfortably for any length of time. For example, to allow somebody to read newspaper print comfortably requires a near VA better than N8 or 1.0 M which is the typical size of newspaper print (Table 3.3) as a ‘reading reserve’ of 2 : 1 is needed and a near VA of N4 or 0.5 M.¹⁵

3.3.4 Recording

Note the working distance and then record the smallest paragraph size seen by the right and left eyes and binocularly. For approximate equivalents to other notations see Table 3.3. Some clinicians do not note the working distance unless it is different from the ‘norm’. However, even patients with good vision present a wide range of normal working distances (22 cm to 50 cm for reading,¹³ and further away for computer use), and as stated earlier a reading acuity is meaningless without a working distance. It can also be useful to record the working distance(s) used to determine near VA with the patient’s own spectacles to allow a comparison with the one used to determine the reading add. This can also be useful for comparison if patients return to your practice dissatisfied with the near vision in any new spectacles. These cases are often due to problems with working distance determination rather than an incorrect refraction.

Near vision or ‘NVA s Rx’ or NVAsc or NVsc all mean near visual acuity measured without a correction. NVA c Rx or NVAcc or NVcc all mean near visual acuity measured with a correction and generally refer to the VA with the patient’s spectacles. Near VA measured with the patient’s contact lenses would typically be recorded as NVA c CLs or similar. Near VAs are recorded for the right eye (RE or OD), left eye (LE or OS) and binocular (BE or OU) and should include the near working distance in cm or inches. If the patient can only read a paragraph slowly or with difficulty, include this information.

Examples:

3.3.5 Interpretation

When determining a reading addition, do not assume you have the correct add just because a patient can see the smallest text on your near chart such as N5, 0.4 M or 20/20. At 40 cm 0.4 M is equivalent to about 20/20 or 6/6 distance VA and N5 is equivalent to 6/9 or 20/30. Therefore, a patient could have reduced distance VA and still read N5, 0.4 M or 20/20 at near. Apart from this truncation effect, near VA can be expected to be similar to distance VA in most cases provided that the eye is accommodating normally or that the reading addition is correct. Notable exceptions include patients with multifocal intra-ocular lenses (IOLs) or wearing multifocal contact lenses or patients with posterior sub-capsular cataract. Patients with some eye disorders, such as amblyopia, age-related macular degeneration (ARMD) and macular oedema, can have significantly worse reading VA than distance VA and isolated-letter near VA.¹²

3.3.6 Most common errors

3.4.1 Comparison of tests

The speed and accuracy of contemporary fast threshold estimation strategies have made several of the traditional screening techniques somewhat redundant. Fast thresholding strategies can produce an estimation of visual field sensitivity in a time (2.5–4 minutes per eye) similar to single stimulus, suprathreshold screeners. All of the fast central field analysis techniques have the advantage over suprathreshold screening techniques in that they are better able to detect early visual field defects, and can give an idea of defect depth and area. They have the disadvantage of taking longer than some suprathreshold techniques. They are similar in sensitivity and specificity for glaucoma as frequency doubling perimetry, but much better at detecting other types of visual field defect.17,18 When compared to techniques for full central field analysis they are quicker but less precise and with worse test-retest characteristics.¹⁹ The Humphrey Field Analyser (HFA) II, SITAFast (Swedish Interactive Thresholding Strategy), Central 24-2 tests 58 locations over the central 25° in a 6° grid pattern that straddles the horizontal and vertical mid-lines, i.e. targets are located 3° either side of the mid-lines. In addition there are targets located on the nasal field between 25° and 30°. The SITAFast, 24-2 programme rarely takes more than 3.5 minutes in a normal patient, and can be as quick as 2.5 minutes. Most modern perimeters have similar fast thresholding central visual field programmes.

3.4.2 Procedure

3.4.3 Recording

If no test locations are highlighted on the Total and Pattern Deviation probability plots then record ‘SITA-Fast: WNL (within normal limits) R and L’. If there is a field defect, repeat the test, print and store both fields of both eyes and attach to the record card. SITA-Standard (or equivalent) could be used instead of SITA-Fast for the repeated field. The single field analysis of the repeated field should accompany any referral to the secondary eye care system. The single field analysis printout illustrates the data as an interpolated greyscale, raw data in decibels, and Total and Pattern deviation plots. It also summarises the field using the Glaucoma Hemifield Test, Global Indices, Reliability Indices, and Gaze Tracking plots (section 3.5.4).

3.4.4 Interpretation

These are interpreted in exactly the same way as the strategies for full central field analysis (section 3.5.4). It is important to be aware of the possible causes of artefact in cases where it would appear that a new defect has been detected, particularly in a patient with no previous experience of visual field screening or no history of field loss. A new defect is not a defect until it is repeated, ‘if in doubt always repeat’. The effect of learning and fatigue can be dramatic.²⁰ Note that when the Glaucoma Hemifield Test classifies the field as having a ‘general reduction of sensitivity’, the Mean Defect/Deviation is abnormal and/or the Total Deviation probability plot shows a majority of test locations as being outside of normal limits, care should be taken when interpreting the results. There is usually an obvious clinical reason, with the most likely association being with cataracts or small pupils.

3.4.5 Most common errors

• Significant risk factors for glaucoma (compare with section 3.4) including, but not limited to, combinations of IOP >21 mmHg, old age, family history, narrow angles, vertical elongation of the optic nerve head, notching of the neural rim tissue of the optic nerve head, disc haemorrhage, nerve fibre layer defect, exfoliative syndrome, pigment dispersion, optic nerve head asymmetry.

• Abnormal screening test (e.g. positive screening test, confrontation or Amsler).

• Symptoms consistent with neurological disease (for example, headache including migraine, dizziness, tingling of limbs) or neuro-ophthalmic disease.

• Symptoms consistent with central field loss, e.g. non-refractive reduced vision, positive scotoma, scintillating scotoma.

3.5.1 Comparison of tests

The instrument should be capable of monitoring fixation, providing full threshold fields in less than 8 minutes, providing reliability indices and analysing the results. A rapid threshold estimation algorithm, such as the HFA’s SITAStandard or the Octopus Dynamic Strategy is recommended. These strategies take approximately 7 to 9 minutes per eye, without compromising the accuracy or repeatability of the result.22,23 The use of faster, less repeatable, thresholding strategies (e.g. HFA SITAFast and Octopus TOPs; section 3.4) may be considered as an alternative for some patients with a demonstrated history of fatigue. There has been discussion that SITA should not be used in patients suspected of conditions other than glaucoma as they are optimised specifically for glaucoma. However, clinically the advantage of the reduced test time makes such a compromise worthy of consideration, and no evidence has been presented that suggests a reduction in diagnostic capability for non-glaucomatous defects.

3.5.2 Procedure

The procedure for central visual field analysis is the same as that for central visual field screening (section 3.4.2) except that:

3.5.3 Recording

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