Visual Assessment

Visual acuity (VA), contrast sensitivity and visual field tests should be performed with both eyes open and using the patient’s habitual correction, so that their current performance is measured. The effect of lighting, altered contrast and tints can be demonstrated at this stage.

Refraction should be performed, including the demonstration of high power adds.

Determining Refractive Correction

At this stage, it is not usually productive to measure vision without spectacles as it will almost invariably be less than the optimal acuity, and thus gives the patient a negative impression. Instead, if a small prescription is found, the difference with and without glasses can be demonstrated at the end of the refraction when deciding whether to prescribe spectacles.

The subjective routine should begin with an approximately correct prescription in place whenever possible: a reliable retinoscopy result or (if this cannot be achieved) the result from neutralising previous spectacles. There can be risks in taking a prescription from old spectacles: one of the lenses may, for example, be a balance lens: a spherical lens of the same average power as its partner prescribed to the eye with poorer acuity. Alternatively, the patient may have become confused about their old spectacles and, as lenses appear to have little effect on vision, now be wearing reading glasses for distance tasks. It is therefore particularly important to assess the refractive error objectively if possible, as subjective responses may be less reliable. If the reflex is dim, so-called radical retinoscopy can be performed by getting very close to the patient to brighten the reflex. This requires a change to the correction made for the retinoscopy working distance. Keratometry or keratoscopy may give an indication of the cylindrical correction, particularly in corneal disease. Optometrists know it is important to refract along the visual axis, because errors in refraction can be induced if not. This is because the astigmatic error changes in an unpredictable way when measured in the periphery ( ). It has been suggested that this could contribute to the optimum prescription not being correctly determined in patients with eccentric viewing. For example, when carrying out retinoscopy on the right eye of a patient, the gaze would be controlled by the left eye. On subjective refraction, the right eye would be fixating. The practitioner should be aware that this phenomenon may cause unexpected changes in prescription, or differences between retinoscopy and subjective measurements. Clinical judgement should be used to decide which prescription is more appropriate, but it has been suggested that optimising the prescription for the eccentric gaze direction does improve acuity (perhaps by about 0.2 logMAR) ( ) so care should be taken to ensure that eccentric viewing is adopted consistently during refraction.

For subjective confirmation, the patient will need to view a letter chart. Due to the Snellen chart’s limitations (see Chapter 3 ), a logMAR chart is generally preferred. Computerised test charts can be useful as long as the screen and viewing distance are sufficient to allow large letters—of at least 1.3 logMAR—to be displayed, preferably with crowding bars. It is also important that the screen has high contrast and luminance, which may not be the case with older systems. Methods which allow lines of letters also allow the patient’s localisation of visual targets to be judged: can they find the third line down, or the second letter?

If the acuity is not known, starting vision testing at 1 m is a good idea. It is then easy to retreat to 3 m if the letters viewed from 1 m are obviously well within the patient’s capabilities, or to move to 50 cm (or even 25 cm) if needed. The aim should be to encourage the patient, always giving them the impression that they are doing better than expected. The patient should be given longer to make judgements and encouraged to view eccentrically if this helps: reassure the patient that this is not ‘cheating’. You need to objectively measure the acuity of each eye as accurately as possible. If no optotypes can be read from 25 cm, then hand movements and light perception should be measured. Finger counting should not be used as it is not repeatable, has variable size and contrast (depending on the examiner’s hand size and skin colour, and the colour of their shirt), and is often disliked by the patient. Someone who can count low-contrast fingers at 50 cm would also be able to see letters at the same distance. A number of ‘ultra low vision’ tests are available, such as the Berkeley Rudimentary Vision Test, which can measure acuity down to 2.9 logMAR (6/4800) ( ; see Chapter 3 ).

The acuity of the poorer eye should also be measured precisely, and not simply categorised as, for example, ‘<6/60’: any future change in vision which might be a sign of active pathology must be detected, and this eye may end up with better vision in some circumstances. Neither should you simply accept the patient’s assurance that they ‘can’t see anything’ with one eye. There are considerable differences between patients in what they mean by such a statement, and a long-standing ‘lazy’ eye can be surprisingly successful when vision deteriorates in the previously dominant eye. A phrase like ‘can you see movement with this eye?’ or ‘could you tell if a light is off or on?’ may gently encourage the patient to allow acuity measurement in their poorer eye.

Trial lenses placed in a trial frame should be used in preference to a phoropter or refractor head, because it is important that you can see the patient’s head posture and eye position, and that the patient should be able to change these at will. Full-aperture trial lenses can allow a better view of the patient’s eyes. If the current spectacle prescription is >±10.00 DS it may be more appropriate to conduct an over-refraction, placing any additional trial lenses in a Halberg Clip. This ensures that prescription changes are genuine and not simply the result of a difference in the vertex distance between the trial frame and the spectacle frame. With the final prescription present, the resultant combined power of trial lenses and spectacles can be measured using a focimeter.

Subjective testing should begin with steps of at least ±2.00 DS (but sometimes up to ±6.00 DS) in order to produce a response from the patient. The lens power used is usually determined by experience but can be approximated by dividing the denominator of the Snellen fraction at 6 m by 60: thus, 6/60 acuity would suggest 60/60 = ±1.00 DS steps; 2/60 when converted to 6 m would be 6/180, giving 180/60 = ±3.00 DS steps ( ). An alternative method using logMAR acuity is ‘MAR/10’: find the MAR by calculating 10 ^logMAR and then divide by 10. For example, for logMAR 0.7, 10 ^0.7 ≈ 5, so the step change required would be ±0.50 DS. Theoretically, when using a cross-cylinder, the one selected could have a power equal to half of this calculated value, as the power change from ‘position 1’ to ‘position 2’ is doubled. However, clinical experience suggests using a cross-cylinder with powers equal to the sphere steps calculated (if available) is most effective.

The test chart should be placed at a reduced distance so that the patient can read at least three to four lines of letters as this will give a better basis for comparison. All lens choices offered should be successive comparisons for a defined visual target within a short space of time (‘is the second line of letters clearer with lens 1 or lens 2?’, ‘Is the letter T more distinct with this lens or without it?’) because alterations in the patient’s gaze angle may produce spontaneous changes in acuity which do not relate to the lenses used. All results should be checked several times, and the size of the changes in lens power can be reduced if the patient is giving confident and repeatable responses. Nonetheless, it may only be possible to ‘bracket’ the final prescription: to determine, for example, that with a +4.00 lens in place the patient reports that an extra −1.00 improves vision, but with +3.00 in place they prefer an extra +1.00, leading to a final prescription of +3.50. Any attempt to use smaller steps of power change to confirm this (e.g. ±0.50) can cause the patient to become unsure.

To check the cylindrical component, an appropriate cross-cylinder can be used to optimise the clarity of a circular letter of a size two lines above the lowest read. Alternatively, if the objectively determined cylinder is ≥1.00 DC, simply increase the axis by 20 degrees from its current position, and ask the patient to compare the shape and clarity of the target in the two positions; that is, at the original axis, or 20 degrees from that. Repeat by rotating the cylinder between its original position and one with an axis decreased by 20°. If the patient is confident in their responses, the rotation can be reduced to 10 degrees. When the axis has been fixed by bracketing, power increases and decreases at that axis can be tried.

The patient should be given more opportunities to make each subjective judgement, allowing them to compare two alternatives repeatedly. Reassurance should be given that they are ‘making sense’ and not contradicting themselves.

Other refractive techniques for checking cylindrical correction are less successful. The difficulty in obtaining a handheld version of the fan and block target usually makes this technique impractical. It is also possible to obtain unreliable results when irregular refraction in the media causes randomly oriented target lines to appear clear. A stenopaeic slit may produce much better acuity than can be obtained with lenses alone: it may occlude sections of the ocular media which are distorted or hazy and prevent them causing deterioration of the retinal image. The refractive result obtained may not then offer such good acuity, as the distorted media is now scattering light. For the same reason, the use of a pinhole aperture to determine the effect of correcting any refractive error may not be reliable. This test relies on cutting down the size of the blur circle from the out-of-focus image on the retina (indicating what level of vision could be achieved by focusing the retinal image) but there may be an additional effect in which rays of light passing through irregular or partially opaque media are occluded. The improvement achieved in this case would not be replicated with lenses alone. Nonetheless, the pinhole may have diagnostic use, in determining whether symptoms which the patient complains of are due to scattering by the media. If, for example, the patient complains of seeing multiple images (polyopia) with an eye which suffers from cataract, and only one image when seen through the pinhole, this suggests that the multiple images are produced by light scatter from the opacity: cataract extraction, reversed contrast text, tinted spectacles, or a clear pupil painted contact lens may help.

Confirmatory refractive tests such as the duochrome test or the +1.00 DS blur test are also not usually helpful, as the vision is not usually good enough to see the required targets.It is important that both eyes are carefully refracted. In a patient with a progressive disease, the better eye may change: in some cases, an amblyopic eye, previously prescribed a balance lens, may become the better eye and require full spectacle correction. When the optimum refractive correction has been determined, the monocular acuity of each eye can be recorded, along with any comments on performance (e.g. ‘using eccentric viewing to the right,’ ‘slowly,’ or ‘better at the start of the line’).

If the refraction has been carried out with the test chart closer than 6 m, the final spherical component of the refractive error will be over-plussed. Testing at 2 m gives a refractive result +0.50 DS stronger than for distance, and at 1 m the refraction will be +1.00 more than at optical infinity. This may need to be modified for distance spectacles, although the over-plussed ‘intermediate’ prescription may be more appropriate for some tasks such as watching TV.

For near vision, the acuity should be determined for the patient’s preferred working distance, using any reading addition which is required for the patient to focus at that distance. The closer the patient can be persuaded hold the reading material, the larger the retinal image, but increasing the reading add to +4.00 DS (at 25 cm) is considered to be the maximum ‘normal’ reading addition for presbyopes. The acuity with this addition, in conjunction with appropriate lighting, is determined. The reading chart should contain words of various sizes, and these may be in the form of paragraphs or isolated words (see Chapter 3 ). A word chart should be used rather than letters, because the spacing of the letters within a word is closer, and thus can be more difficult. If words in meaningful sentences are used, this gives the additional clue of context to the reader, which is absent in isolated word texts.

This procedure should also be followed for young, pre-presbyopic patients. Children and teenagers may use relative distance magnification successfully to read small print, but the close working distance required may not be sustainable for long durations. With the patient holding the print at their required distance, extra plus lenses are introduced binocularly to relax the accommodation, until the point where the patient finds the print has now blurred, and or they need to bring the print closer to focus it again.

If the near word acuity achieved does not comfortably reach the required print size, then magnification will be required (see Chapter 5 ). This magnification may be in the form of a spectacle-mounted high addition (see Chapter 7 ), if the patient is willing to hold the reading material closer.

Contrast sensitivity will give an indication of task performance, in particular for reading (see Chapter 3 ). Interpreting the results in terms of the contrast reserve can give an indication of whether the patient will be able to achieve spot reading or survival reading, or whether sensory substitution might be necessary. It also allows some of the patient’s difficulties to be explained (e.g. reading coloured text on coloured backgrounds; seeing steps and kerbs); and for lighting recommendations (e.g. task lighting; illuminated magnifiers) to be targeted. When VA is equal in each eye, monocular contrast sensitivity testing can reveal which is the better eye, to guide which eye to use with monocular low vision aids.

Binocular Vision

Having determined the best acuity in each eye, it is now possible to consider whether an assessment of binocular vision is required. A possible incomitant deviation, suggested by a report of diplopia which varies with gaze direction, should be investigated using motility and cover tests. Gross binocular vision can be demonstrated in people with vision impairment, and sometimes difficulties in reading can be caused by binocular vision problems rather than the visual impairment itself ( ). More subtle binocular vision problems such as convergence insufficiency or decompensated phorias should be considered if the acuity in the two eyes differs by a factor of two or less, and binocular correction is being considered.

Binocular VA should be measured, as this will reflect visual performance under real-world conditions. People with low vision can still show binocular summation of VA (so the binocular VA is better than the VA of the better eye), even when each eye has a different level of vision ( ). However, contrast sensitivity is often better monocularly than with both eyes open ( ).

In practice it is best to demonstrate vision on real-world tasks with both eyes open and with the poorer eye covered. An Amsler chart can also be used to determine whether the vision is clearer with both eyes open or with one closed. If tasks are significantly easier with one eye covered then a frosted or occluded lens may be useful. When looking at a reading test at a close distance, some patients will use their dominant eye, or the eye corresponding to their dominant hand, rather than using the eye with better vision. On questioning, patients may sometimes be aware that they have to close or cover one eye in order to see more clearly and should be encouraged to try reading with the better eye.

Binocular vision tests which rely on good VA—such as many stereopsis tests—will not be appropriate for people with low vision. More gross tests of binocularity, such as a bar reading test are more likely to produce relevant results. To carry out this test, a bar (e.g. a pen) is placed midway between the patient (viewing binocularly) and the reading task. If lines of text can be read without any interruption, then both eyes are contributing: if only one eye is being used, then the pen will cover part of the text and a full line cannot be read. The Bagolini test can also be used successfully in people with low vision.

Additional Tests

The use of additional tests of vision at this stage is for functional rather than diagnostic purposes, and their use will be dictated by the particular circumstances of the patient. The different strategies for testing different visual functions in a patient with visual impairment are discussed in Chapter 3 .

The most common would be tests of the absolute dimensions of the peripheral visual field using a manual confrontation technique, a large white target on a perimeter, or a binocular Esterman test. Testing for a central scotoma using an Amsler chart or California Central Visual Field Test may be indicated in other cases.

Testing colour vision may be informative, particularly in people with retinal disease. Standard colour vision tests may be less accurate when acuity drops to lower than about 0.8 logMAR ( ). Larger versions of some colour vision tests are available.

There are some patients who appear to present with visual problems, who in fact have disturbed visual perception yet may have relatively normal acuity or visual fields to conventional testing. Additional tests to confirm the nature of the defect should be used if the patient has a history of neurological disease or cortical lesion and has symptoms which do not correlate with the visual performance measured (see Chapter 3 ).

Spectacle Dispensing

Under the Opticians Act 1989 , spectacle dispensing for people who are registered as sight impaired or severely sight impaired can only be performed by a General Optical Council registered optometrist or dispensing optician. It is important for the person dispensing spectacles to have an understanding of the special lens and frame requirements for someone with vision impairment.

Although it is important that the patient is happy with the appearance of any glasses, larger spectacles are often better for people with nystagmus, especially if they use a compensatory head posture, and for those who need to use eccentric viewing to see.

When dispensing high power reading spectacles, the patient should be reminded about the close and specific working distance at each stage: prescribing, dispensing and collection. It is common for patients to complain that these glasses ‘don’t work’ when they are merely holding the page too far away. If binocular correction at near is dispensed for any patient, the correct near centration must be provided: extra decentration/inset or base in prism may be required (see Chapter 7 ). A single vision near correction will also be preferred if the patient may use it with a magnifier: it is very difficult to combine positioning the magnifier close to the eye to get the required field of view, and looking down to use the reading area of the lens.

Bifocal spectacles with high reading adds can be very useful under other circumstances: it is possible to obtain bifocals with adds of up to +25 DS (see Chapter 7 ). These glasses can be suitable for someone who wants to, for example, watch television and read a book at the same time. However, the bifocal is also very useful for a patient who only requires a reading correction: it avoids the extreme blur experienced when looking up, and is a much thinner and lighter lens. It is wise to ensure that the patient also has another pair of spectacles—either single vision distance glasses, or some with a lower powered add—for everyday use. Progressive addition lenses (PALs or ‘varifocals’) are not ideal for patients with visual impairment, although they are very commonly used. Reasons for this include the variation in power across the pupil compromising image focus; difficulty in using the correct part of the lens when using unusual head or eye positions/movements; limitation in add power available (or excessive aberrations when such adds are possible through freeform surfacing). Despite this, some people (particularly teenagers and young adults) prefer the cosmetic appearance of a multifocal lens and sometimes refuse bifocal lenses.

Thirty percent of people aged 65 and over will fall at least once a year, as will half of those over 80 years of age. A fall can lead to pain, distress, loss of confidence, lost independence and even death: in around 5% of cases a fall leads to fracture and hospitalisation (Falls Consensus Statement). People with visual impairment are more likely to fall than those with good vision. Large changes in spectacle prescription (>0.75 DS) and wearing multifocal glasses (either bifocals or PALs) are associated with an increased fall risk ( ). Practitioners need to weigh up the risks of making large changes in the spectacle correction with the benefits of visual improvement. Patients need to be counselled that a sudden gain (or loss) of vision in one eye makes them vulnerable to misjudging distances and falling: found a twofold increase in falls after first-eye cataract surgery. suggests that active individuals spending time outdoors should have a separate distance correction: those who go out rarely can stay with multifocals if well adapted to them. Multifocal wearers with a minimal distance prescription should be recommended not to use them outdoors. Monovision corrections should also not be recommended to visually impaired patients because of an increased fall risk.

Tinted prescription lenses should be ordered using the colour and light transmission factor (LTF) (e.g. ‘20% LTF grey,’ or ‘50% LTF, 500 nm orange’). It is sensible to demonstrate tints outdoors before prescribing them, and to ensure that patients are aware of what colour the glasses will look (‘you realise that people will see that you are wearing yellow glasses?’). Wrap around frames or side shields will reduce glare from around the lenses. However, the patient does need to have colourless lenses available when lighting is low, or when reading, so may find an overspectacle more versatile (see Chapter 10 ).

Multilayer antireflection (MAR) coatings improve the cosmetic appearance of lenses, but do not have any effect on glare: in fact, as more light passes through, discomfort glare could be worse. They should under no circumstances be described as ‘antiglare’ coatings to the patient, who may be misled into thinking they have visual benefits.

Make sure that the patient is aware if you are using a balance lens in one eye, and the reasons for this being prescribed. If you choose to frost one lens, it is sometimes better to use a removable filter in case the patient dislikes this, or the vision changes. Blenderm or a Bangerter foil can be used: the foils have the advantage of being available with different degrees of fogging.

Referral and Signposting

During the assessment, it is likely you will have identified concerns which cannot be addressed solely by the low vision clinic: for example, problems with lighting, social isolation, or depression. Consider referral to other services where they are needed, such as local sensory teams, counsellors, the general medical practitioner, or sight loss charities. It is important to have information on other services available in a variety of formats including audio, large print and electronic versions.

If the vision has changed or there are any concerns with disease progression, or if the patient requires registration as sight impaired or severely sight impaired, then referral to a hospital ophthalmology clinic may be needed.

The Next Stage

A significant minority of patients (10%–20%) need only a good refraction, coupled perhaps with advice about lighting, in order to meet their requirements (e.g. see ). Although it could be argued that any improvement which can be offered will be worthwhile, experience suggests that an increase of around two lines of acuity will be needed in order for the patient to subjectively appreciate the improvement in everyday viewing.The initial assessment allowed the patient to identify their visual difficulties and to list the tasks for which their VA was inadequate. By considering this list, you can now determine if these are dealt with by the refractive correction just determined.

If the refractive correction does not improve visual performance sufficiently, then additional aids can be considered to prevent the impairment from becoming an activity limitation. Patient requirements can then be divided into three categories, and you must be quite honest with the patient about how you intend to approach each task:

• 1.
  

  No aid is suitable for the task. This may be because the amount of magnification required will be beyond a practical range, or because their VA does not meet a legal requirement for a task: for example, if the patient is registered as severely sight impaired and they want to drive a car. Some tasks are not amenable to low vision aids, such as face recognition or reading very low contrast subtitles on a television. In these cases, other strategies can be suggested such as letting their friends know that they may not recognise them and using television audio description.

  
  
• 2.
  

  The task may be tackled most effectively by nonvisual aids such as sensory substitution devices. For example, it may be easier to read a newspaper on a tablet computer with speech activated, or a talking thermostat may be the best solution.

  
  
• 3.
  

  The task can be approached by trying to overcome visual impairment using an optical or electronic low vision aid (LVA). The LVA may work in isolation, or the task may require a combination of strategies (e.g. an optical magnifier plus increased lighting, or with an eccentric viewing technique) and the patient must be fully trained in their use.