4.1.1 Symptoms

Distance vision blur or difficulties with certain distance tasks such as reading from the whiteboard, watching TV or driving, but with good near vision suggests myopia. Near vision blur and/or reading difficulties with good distance vision suggests hyperopia or presbyopia depending on the patient’s age. Significant astigmatism in combination with myopia or hyperopia can cause blur at all distances. Headaches and asthenopia can accompany uncorrected hyperopia and presbyopia. Myopes who squint to see can develop frontal headaches and uncorrected astigmatism can often lead to complaints of asthenopia. If these symptoms are due to ametropia, they usually have a gradual onset.

4.1.2 The patient’s ocular history

The natural progression of the type of ametropia given the patient’s age can indicate what change in refractive correction to suspect. For example, a childhood-onset myope who obtained their first spectacles at age 12 and is now 16 is likely to attend an examination complaining of increased myopia given the typical progression of myopia. Any mention of cataracts in the case history should lead to a careful investigation for increased myopia (nuclear cataract) or astigmatic change (cortical cataract).¹

4.1.3 Family ocular history

When examining children who do not wear spectacles, it is useful to ask whether any of the patient’s family wear glasses or contact lenses. Mutti and colleagues reported that juvenile onset myopia was evident in 33% of the offspring of two myopic parents, compared with only 6% of the children of two non-myopic parents.² A family history of hyperopia highlights possible amblyopia.

4.1.4 General health

Diabetes, either undiagnosed or poorly controlled, can lead to wide fluctuations in refractive error, with either hyperopic or myopic shifts. In addition, a variety of systemic medications can lead to refractive error shifts.³

4.1.5 Distance vision and habitual VA

For low myopic refractive errors and hyperopic changes in absolute presbyopes, a degradation of one line of vision (on a logMAR chart) corresponds to approximately −0.25 D of refractive error (although very approximate and dependent on pupil size and patient blur tolerance).⁴ For example:

4.1.6 Far point estimation

An estimate of myopia can be determined from the patient’s far point. Ask the patient to remove any spectacles, occlude one eye and bring in the near VA card until they can just see it. The far point provides an estimate of the mean sphere refractive correction. For example, patients with far points of 33 cm, 25 cm and 20 cm have mean sphere refractive corrections of approximately −3.00 DS, −4.00 DS, and −5.00 DS respectively.

4.2.1 Spectacle lens identification

Focimeters measure the vertex power, axis direction, prism and optical centres of ophthalmic lenses. They do not provide information about all the important features of spectacle lenses, however, and it is important to consider that changes in other spectacle characteristics could cause patients problems and need to be checked. These include base curve and lens form, segment style, height, size and inset, centre or edge thickness, optical and surface quality and the presence of a lens tint and/or surface coating (antireflection, anti-scratch).

4.2.2 Procedure

See online video 4.1.

4.2.3 Recording

Record the sphero-cylinder correction in minus cylinder form for both eyes and the reading addition power if a multifocal. Also record any prism, the type of lens, any tints or coatings, etc. Use ‘x’ rather than the word ‘axis’. Record the spherical and cylindrical power to the nearest 0.25 D, and the cylinder axis to the nearest 2.5°. The axis should be between 2.5° and 180°. Use 180 rather than 0 degrees.

For example:

4.2.4 Interpretation

One of the most common errors in focimetry is an axis reading incorrect by 90°.⁶ Given that the cylindrical axes in the two eyes are often mirror images of each other (for example, both axes 90° or both axes 180°; right axis 175°, left 5°; right 20°, left 160°; right 45°, left 135° etc.), if axes are 90° different to this (for example, 180° and 90°; 175° and 95°; 20° and 50°; both axes 45°; both axes 135°) then recheck the two cylindrical axes.^5,7 Reading additions are typically the same in both eyes, so that if they are read as different, they should be rechecked.

4.2.5 Most common errors

(a) To place the optical centre of the phoropter/trial frame lenses in front of the patient’s visual axes to control prism and avoid aberrations.

(b) So that the optical centre of spectacle lenses can be placed in front of the patient’s visual axis to avoid unwanted prism and aberrations or deliberately placed elsewhere to produce desired prism.

4.3.1 Anatomical PD

Anatomical PD measurement is quick and convenient to use during an eye examination and it requires no instrumentation other than a simple millimetre ruler. The repeatability of anatomical binocular PD measurements is similar to that for a pupillometer.8,9 A pupillometer could be considered when refracting or dispensing a patient with a large amount of ametropia, where slight discrepancies in PD could lead to induced prism, and for monocular measurements when dispensing progressive addition lenses.

4.3.2 Procedure

4.3.3 Recording

The values are normally recorded as distance PD/near PD (in mm). For example, PD: 63/60.

4.3.4 Interpretation

For women the distance PD is most commonly in the range of 55 to 65 mm, and for men, 60 to 70 mm.¹⁰ Young children may have PDs as low as 45 mm. The distance PD value is usually 3 or 4 mm greater than the near PD at 40 cm.¹⁰ Inaccuracies in anatomical PD can occur due to parallax error when there is a large difference between your PD and patient’s PD. However, the error is slight, with an 8 mm difference in the examiner and patient’s PDs leading to a 0.5 mm error in the measured patient PD.¹¹ The repeatability of anatomical PDs taken by an experienced practitioner is approximately ± 1–2 mm.^8,9 Repeatability between practitioners is slightly poorer at about ±1.5–2 mm.⁹

4.3.5 Most common errors

4.3.6 Alternative procedure: Corneal reflection pupillometer

Pupillometers allow monocular PDs to be measured more accurately than an anatomical measurement.⁹ This is beneficial when ordering spectacles for high refractive errors or for progressive addition lenses where precise centration of each lens along the patient’s visual axes is necessary. In addition, the procedure is quick and simple and could be performed by a clinical assistant and the examiner does not need to be binocular. The PD measured with a corneal reflection pupillometer will typically be 0.5–1 mm smaller than the anatomical PD.^8,9 This is because pupillometers measure the ‘physiological PD’, the distance between the two principle corneal reflexes, and locate the visual axes, whereas the anatomical PD locates the lines of sight or optical axes. Note that many pupillometers use a correction for the parallax error mentioned in the anatomical PD section.¹¹ Inaccuracies can occur if the pupillometer sits higher or (usually) lower on the bridge than the intended spectacle frame and the nose is not straight, so that the monocular PDs can be shifted to one side.

4.4.1 Advantages of a phoropter

See online video 4.2.

The use of a phoropter (Figure 4.4) is the preferred technique for distance vision refraction of the majority of patients. The main advantages of phoropters are:

4.4.2 Advantages of a trial frame

See online video 4.3.

In the routine refraction of presbyopic patients, the trial frame (Figure 4.5) is preferred for the final determination of the near addition, as the test can be performed at the patient’s preferred working distance and position, and the range of clear vision can be easily measured and compared to the near vision requirements of the patient. A trial frame is also useful to illustrate the improvement in distance vision in the ‘real world’ that a pair of spectacles could provide. For example, the new refractive correction can be placed into the trial frame and the patient shown the improvement of their vision while looking through the window of the practice. This can be particularly useful when partially prescribing (section 4.15).

A trial frame is required for refractions during home (domiciliary) visits and is preferred when refracting:

4.5.1 Comparison of tests

Retinoscopy provides a more accurate result of refractive error in a greater array of patients than autorefraction, although autorefraction is a useful and reliable alternative in many ‘standard’ adult patients and can be particularly accurate at determining astigmatism.13–15 Autorefractors should not be used with young children without cycloplegia because of proximal accommodation errors producing significantly more minus results than subjective refraction, particularly in young hyperopes.^13,16 Retinoscopy also provides a sensitive assessment of the ocular media (e.g. early detection of cataracts, keratoconus), can be used to determine refractive error at distance and near, identify accommodative dysfunction, and is portable, less expensive and less likely to break down.⁶

Retinoscopy’s major disadvantage is that it requires several years of training to become proficient at using it. When a subjective refraction is not possible, limited or unreliable, it is preferable to have more than one assessment of objective refractive correction.

There appears to be no research literature that compares the accuracy of streak or spot retinoscopes or refractions using negative or positive cylinders. The procedure will be described for streak retinoscopy, but spot retinoscopy appears an acceptable alternative. Positive cylinders have the advantage of making retinoscopy easier to learn, as ‘with’ movement is typically easier to see than ‘against’ movement. However, negative cylinders are preferred as they are standard in phoropters. In addition, there is the possibility of stimulating accommodation during subjective refraction when removing a plus cylinder from a trial frame to replace it with one of another power. For these reasons the procedure will be described using negative cylinders.

4.5.2 Procedure: Retinoscopy (Summary in Box 4.1)

See online videos 4.4 to 4.7.

4.5.3 Adaptations to the standard procedure

4.5.4 Adaptations for older patients

Older patients will often have small pupils and some will have media opacities/cataract and you will see a dim reflex as a reduced amount of light reaches the retina and even less returns to your retinoscope. Increasing the retinoscope light intensity may just reduce the pupil size further and a medium intensity is usually best. Often an autorefractor result is not possible for these patients,¹³ but retinoscopy might provide a useful result if used with these three adaptations:

4.5.5 Recording

Record your retinoscopy results as the sphero-cylindrical correction that neutralised the patient’s refractive error after removing your working distance lenses. Do not use a degree sign as ° can look like a 0 and make an axis of 15° look like 150 degrees. Use ‘x’ rather than the word ‘axis’. Record the spherical and cylindrical power to the nearest 0.25 D, and the cylinder axis to the nearest 2.5 degrees. The axis should be between 2.5 degrees and 180 degrees. Use 180 rather than 0 degrees. Also record the monocular visual acuity with the retinoscopy result.

For example:

4.5.6 Interpretation

On average, retinoscopy provides a refractive result slightly more positive than subjective refraction in young patients.¹⁹ This decreases with age, so that retinoscopy and subjective results are similar in presbyopic patients. As the stimulus to accommodation is greater in subjective refraction than in retinoscopy, the retinoscopy result in young hyperopes can be much more positive than accepted in subjective refraction. Errors can occur in retinoscopy if it is performed off-axis (Figure 4.6b), which will induce spherical and astigmatic errors, or if it is performed at an incorrect working distance, which will induce a spherical error.¹⁷ The most common working distance error is to work too close, particularly when the reflex is dim. Note that cylinder axes in the two eyes are often mirror images of each other.^5,7 For example, right axis 175°, left 5°; right axis 20°, left 160°; right axis 45°, left 135°, etc.

4.5.7 Most common errors

4.6.1 Procedure

See online video 4.8.

4.6.2 Recording

Record the refractive correction using the same format described for retinoscopy (section 4.5.5). Record the monocular VAs. If pinhole VA is measured and reveals no improvement in VA, record PHNI (‘pinhole no improvement’); otherwise record the VA with the pinhole. For refractive corrections above 5.00 D equivalent sphere, record the vertex distance. Make sure that the prescription details that you provide to patients are clearly legible. Illegible prescription forms have been reported as a surprisingly common error in optometric practice.⁶

Examples of recording:

4.6.3 Interpretation

The subjective results should be compatible with the retinoscopy results in most cases, although young patients may provide a more positive (less minus) correction than retinoscopy.¹⁹ Inconsistent results may be due to technique error or the patient may be an unreliable observer for behavioural or visual reasons. A subjective result that is significantly less positive (more negative) than the retinoscopy result or a subjective result more minus than suggested by unaided visual acuity could indicate latent hyperopia or pseudomyopia and a cycloplegic refraction may be required (section 4.13). A patient with reduced VA (typically in both eyes) and a retinoscope result that indicates emmetropia or slight hyperopia may have non-organic visual loss (section 4.12.6). The difference between the patient’s own spectacles and the subjective refraction should be compatible with the difference between the habitual (with own spectacles) and optimal VAs (section 4.12.6).

4.7.1 Procedure

4.7.2 Adaptation for older patients

Processing speed slows significantly with age, so provide a longer presentation time for each lens than you would normally do for younger patients.²⁰ Note that you are more likely to over-plus than over-minus older patients (section 4.7.3).

4.7.3 Interpretation

The MPMVA approach is designed to take advantage of a patient’s depth of focus to provide the maximum range of clear vision.²¹ For example, after refraction, the retinal image should be conjugate with the distance VA chart at 6 m (20 ft). However, this does not take advantage of the depth of focus. For example, if the depth of focus was +0.50 D and the retinal image was conjugate with the distance VA chart so that 0.25 D of the depth of field was in front of the VA chart and 0.25 D behind it, the chart would be clear from 2.4 m (8 ft) to ‘beyond’ infinity. Using the MPMVA technique places the distal edge of the depth of focus conjugate with the VA chart.²¹ Therefore, if the depth of focus is +0.50 D, use of the MPMVA technique ensures that the range of clear vision is from 1.5 m to 6 m (5 to 20 ft). However, using this technique does mean that patients are slightly under-minused or over-plussed by 0.16 D as the distance VA chart is at 6 m (20 ft) and not infinity. This can be offset in young patients due to a lead of accommodation (+0.25 DS) during distance refraction, but this does not occur in older patients who have lost accommodation.²¹ This effect can be aggravated if a truncated VA chart is used (i.e. only reducing plus to obtain a VA ‘bottom line’ of 20/20 or 6/6) and/or if the patient has a large depth of focus (such as older patients with small pupils) as there will be very slight retinal defocus over the entire range except at the precise point of conjugacy.²¹ Over-plussed/under-minused refractive corrections are more commonly found in older patients than under-plussed/over-minused ones.²² An indication that this has occurred is that the measured addition is lower than expected in the presbyope.

4.7.4 Recording

The results of MPMVA are not recorded as the technique is just part of the subjective refraction.

4.7.5 Most common errors

4.8.1 Procedure

4.8.2 Adaptations to the standard procedure

4.8.3 Adaptations for older patients

Older patients, particularly those with reduced VA, may be unable to tell any difference with ±0.25 DS, so that you should use ±0.50 DS or even larger steps. Some older patients also prefer to have two options to choose from rather than three (see 4.8.2).

4.8.4 Recording

The results of the best vision sphere are not recorded as the technique is just part of the subjective refraction.

4.8.5 Most common errors

4.9.1 Procedure

See online video 4.8.

Note: Some practitioners prefer to add +0.50 DS or +0.75 DS to the spherical correction so that the bichromatic test is initially ‘on the red’ and then reduce the plus power (or increase the minus power) in 0.25 DS steps until the targets on the red and green look equally black and clear.

4.9.2 Recording

The result of the duochrome test when used prior to the JCC is typically not recorded. However, some practitioners record the result of the duochrome at the end of the monocular refraction.

For example:

4.9.3 Interpretation

The duochrome is a check test and you should not use the test as a final arbiter of the best vision sphere. Some patients give poor results with the duochrome and always prefer one colour, regardless of the changes you make to the spherical refraction, and the test gives unreliable results with blur of more than ~1.00 D. Older patients, due to small pupils and the increased absorption of low wavelength light by the lens (particularly in nuclear cataract) tend to give more unreliable duochrome results. Colour defective patients can use the test, although the red side of the test will appear duller to protans. In addition, not all charts provide light of appropriate red and green wavelengths.

4.9.4 Most common errors

4.10.1 Comparison of tests

There has been little comparison of the various tests to determine astigmatic power and axis in the research literature and the study by Johnson and colleagues provides limited information and further study is needed (section 1.1.2).²⁷ Many practitioners use the Jackson cross-cylinder (JCC) test as it is simple and easy to use and is designed to fine tune the cylinder found in retinoscopy or autorefraction. The method described for fan-shaped tests often assumes that no retinoscopy result is available and even if it is available, the retinoscopy cylinder is removed and you start from scratch. However, it is important to be able to use another subjective test for astigmatism in case a patient responds poorly to the demands of the JCC. Fan-shaped tests have an advantage over JCC in that accommodation is well controlled as the patient is fogged prior to the use of the procedure. They also do not require the patient to be able to memorise two pictures presented sequentially and compare them. They can be used to quickly determine if any astigmatism is present after the best vision sphere procedure by asking patients whether any of the lines look clearer (the block and fan looks like a clown’s face if set up as in Figure 4.9 and can be used with children in this way: Ask ‘Do any of the hairs on the clown’s head look clearer than the others?’). However, fan-shaped tests should only be used if they include an arrow or dial to refine the precision of the axis estimation, otherwise they are significantly limited in sensitivity.²⁸ A very simple test to estimate the astigmatic axis is axis rotation, which involves asking the patient to view the smallest line of VA they can see and rotating the correcting cylinder axis first clockwise and then anti-clockwise until the patient reports that the letters start to blur. The cylinder axis indicated by the technique is the mid-point between the two blur points so that if the two blur points are at 25° and 55°, the indicated cylinder axis is 40°. In patients where the subjective assessment of astigmatism is poor, it is advisable to consider multiple objective measures of astigmatism from retinoscopy, autorefraction, and (to a lesser degree and if the cylinder is not lens-induced) keratometry. The astigmatism present in the patient’s old spectacles should also be considered.

4.10.2 The Jackson cross-cylinder

The test only works if the circle of least confusion is on the retina so that it must follow a best vision sphere assessment. During the test you present two lenses to the patient, one after the other: when the correcting cylinder is not correct, one lens should increase the interval of Sturm and slightly blur vision and the other should decrease the interval of Sturm and slightly improve vision. The zero mean power of the cross-cylinder ensures that the circle of least confusion remains on the retina for both presentations. The effective axis shift is greater if the power of the correcting cylinder is low: A ±0.25 JCC will shift the effective axis by ±22.5° when combined with a −0.50 DC, but will only shift the effective axis by about 7° when combined with a 2.00 DC.²³ Therefore when making changes based on patient responses to the JCC, the amount of rotation of the correcting cylinder should consider the power of that cylinder (Table 4.1).

4.10.3 Procedure

See online videos 4.9 to 4.14.

4.10.4 Poor JCC technique: ‘Nudge, nudge, same’

This is a common technique used by students and means ‘nudging’ the cylinder axis by 10–15 degrees or so (essentially regardless of the cylinder power and not really knowing the exact value of the axis you have moved it to or from) in the direction indicated by the JCC until the patient first indicates that both views look the same. The cylinder axis position that first receives a ‘same’ response is taken as the true cylinder axis. This technique is inefficient and will lead to an incorrect result in some cases. A ‘same’ response from the patient may not indicate the correct cylinder axis for the following reasons (also see online video 4.11):

4.10.5 Examples of efficient JCC procedures for cylinder axis

Two simple examples are provided here, but there are a larger number of examples for JCC cylinder axis and power determination on the website. (see online videos 4.9 to 4.14).

4.10.6 Adaptations to the standard procedure

If a patient has provided poor responses in other parts of the eye examination, it can be useful to repeat the same JCC task to the patient at the start of the procedure until they start providing the same response each time. If accurate responses are never obtained, you may need to use a ±0.50 JCC. A hand-held cross cylinder may be held over the phoropter in these cases. If a patient still cannot provide reliable answers, then you may need to use an alternative assessment of astigmatism.

4.10.7 Adaptations for older patients

4.10.8 Recording

The results of the JCC are not recorded as the technique is just part of the subjective refraction (section 4.6).

4.10.9 Interpretation

Solsona retrospectively analysed 51,000 patients with astigmatic corrections greater than or equal to 0.75 D and found that 67% had mirror symmetry within 10°.⁷ This means that the two axes should add up to approximately 180°: Both axes could be 90° or both axes 180° (i.e. 0° and 180°); one axis 175°, the other 5°; one axis 20°, the other 160°; one axis 45°, the other 135°, etc. You may wish to recheck astigmatic axes that do not follow this pattern, particularly if one axis has changed significantly from a previous examination or is significantly different from the retinoscopy result.

Typically, younger patients will have ‘with-the-rule’ astigmatism, with a steeper vertical meridian (minus cylinder axis between 160–20), likely due to pressure from the eyelids. This lid tension decreases slowly with age, so that with-the-rule astigmatism slowly disappears and older patients typically have ‘against-the-rule’ astigmatism (minus cylinder axis between 70–110).⁵ Note that this change with age is slow and any significant refractive correction changes between eye examinations 1–3 years apart are likely to be largely spherical in nature. Significant changes in astigmatism over a 1–3 year period are likely to be due to refraction error at test or retest or possibly due to ocular pathology such as keratoconus, cortical cataract, chalazion, etc., causing significant astigmatic changes.³

4.10.10 Most common errors

4.10.11 Alternative techniques: Fan and block, sunburst, Raubitschek arrow, etc.

There are a variety of fan-shaped tests but all use a similar methodology. Those tests that do not include axis refinement using a rotating arrow or dial are not described as they provide poor estimates of cylinder axis and power.²⁸

4.11.1 Comparison of binocular balancing tests

There has been little comparison of the various binocular balancing techniques in the research literature and the study by West and Somers provides limited information and further study is needed (section 1.1.2).³⁰ Some tests directly compare the vision in the two eyes (prism-dissociation balance, alternate occlusion) and are directed towards balancing accommodation while others are based on binocular refraction techniques (e.g., polaroid, monocular fogging, Humphriss) and determine the spherical correction in conditions close to the patient’s normal viewing situation.

The alternate occlusion test continues to use an occluder, acts like a cover test and does not allow binocularity, so that its usefulness over monocular refraction seems limited. The prism dissociated binocular balance is also fully dissociated, so that fusional vergence is not present and accommodation may not be at the same level when binocular.²⁹ Both these tests need the patient to have equal VA in the two eyes to be able to provide accommodative balance. Other tests, such as the Polarisation balance, monocular fogging and Humphriss immediate contrast test, are minimally dissociated and aim to determine the spherical correction in conditions similar to the patient’s normal viewing situation so that vergence and pupil size are in their normal binocular state. Polaroid tests on computer-based systems are best if they include three lines of VA with a fusion lock line that is seen by both eyes. With only two lines, one seen by the right eye and one by the left, the lines may float freely and cause confusion. Fogging of one eye by a small amount in the monocular fogging technique has several advantages in that it relaxes accommodation and suppresses central vision whilst maintaining peripheral fusion. Both the polaroid dissociated (or prism-dissociated) balance tests can be used with the duochrome (section 4.9), where balance is achieved by gaining the same endpoint in both eyes (i.e. red = green in both eyes, or ‘just on the red’ in both eyes).²⁹ The duochrome and monocular fogging techniques can balance accommodation in patients with unequal monocular VA. The Turville Infinity Balance, which is not described here, appears to be rarely used nowadays, although it has the advantage that the measurement procedure inherently includes a screening test for decompensated heterophoria and suppression. However, it requires physical movement of a septum on a mirror, which makes it somewhat cumbersome.

4.11.2 Procedure: Polaroid binocular balance of accommodation

Polaroid balance tests typically include several VA lines with one (or more, dependent on the programme) line seen by the right, left and both eyes, respectively. The line seen by both eyes provides a fusional lock.

4.11.3 Procedure: Monocular fogging balance (modified Humphriss)

See online video 4.15.

4.11.4 Procedure: Humphriss Immediate Contrast

The following procedure is based on the technique described by Humphriss.³¹

4.11.5 Procedure: Prism-dissociated blur balance of accommodation

See online video 4.16.

4.11.6 Recording

The monocular subjective refraction result and the correction after prism-dissociated balance can both be recorded with the accompanying VA results. Alternatively, the change in spherical power made with the binocular balance can be recorded. The binocular VA is also measured after the prism balance. For example:

4.11.7 Interpretation

Typically, binocular balance tests either find no change in refractive correction from the monocular refraction result or find a small amount of additional positive lens power in one eye, or more rarely both. However, with latent hyperopia there can be a significant increase in the amount of plus accepted with this technique. This is because monocular subjective refraction can lead to possible over-minusing or under-plussing, particularly in patients with hyperopia, pseudomyopia and antimetropia, as the occluder can stimulate accommodation in the non-tested eye and thus an equivalent increase in accommodation in the eye being tested.

4.11.8 Most common errors

4.12.1 Procedure: Monocular fogging

4.12.2 Procedure: Polaroid binocular refraction

4.12.3 Check tests after refraction

4.12.4 Adaptations to the standard procedure

4.12.5 Recording

For example:

Make sure that the prescription details that you provide to patients are clearly legible. Illegible prescription forms have been reported as a surprisingly common error in optometric practice.⁶

4.12.6 Interpretation

A subjective result that is significantly less positive (more negative) than the retinoscopy result or a subjective result more minus than suggested by unaided visual acuity could indicate latent hyperopia or pseudomyopia and a cycloplegic refraction may be required (section 4.13).

The difference between the patient’s own spectacles and the subjective refraction should be compatible with the difference between the habitual (with own spectacles) and optimal VAs. For example, if the patient has a visual acuity of 6/12 in their spectacles and 6/4.5 after subjective refraction, you could expect the subjective refraction to be 1.00 DS more myopic than the spectacle correction (−0.25 ≈ 1 line of logMAR visual acuity). Thus, a 6/12 VA with a spectacle correction of −1.00/-0.50 × 180 would suggest a refractive correction of −2.00/−0.50 × 180. If the subjective refraction was −2.50/−0.50 × 180 or even −3.00/−0.5 × 180, this could suggest you have over-minused the subjective refraction. Changes in hyperopic refractive errors are more difficult to explain in this way, as they are dependent on the amount of accommodation the patient has (and therefore their age). Changes in astigmatism that are not due to pathology are usually small.⁵ Patients with refractive error change that is a result of cataract or other eye disease will not typically follow the same rules for improvement in visual acuity (e.g., nuclear cataract can cause a 1.00 D myopic shift with only a 1–2 line improvement in visual acuity).

A patient with reduced VA (typically in both eyes) and a retinoscope result that indicates emmetropia or slight hyperopia may have non-organic visual loss (also called functional or psychogenic visual loss).35,36 It may be accompanied by visual field defects, which are often tubular.³⁶ In young children, this is often because the child wishes to wear glasses (perhaps their best friend or a parent wears glasses), but can be due to social problems at home or school and can include sexual abuse.^35,36 In adults, non-organic visual loss is linked with trauma (typically head trauma), chronic pain conditions including migraine and underlying minor psychiatric problems that included feelings of stress, anxiety and depression.³⁶ A useful test, particularly with children, is to perform a subjective refraction using a variety of lenses that have no effect on VA (such as a combination of +0.25 DS and -0.25 DS in a trail frame) and encourage the patient to read further down the VA chart. In many cases, the patient can be encouraged to read normal or near normal VA in this way and if ocular health appears normal, a diagnosis of non-organic visual loss can be made and reassurance given to the child and parent (section 2.4.2).

4.12.7 Most common errors

4.13.1 When is a cycloplegic refraction necessary?

A cycloplegic refraction may be necessary if there are any indications of excessive or fluctuating accommodation during the refraction. Accommodative fluctuations can lead to wholly incorrect results of objective and subjective refraction. In addition, excessive accommodation, particularly during subjective refraction, can lead to a very over-minused (or under-plussed) refractive correction. Indeed, a myopic refractive correction can be found in a hyperopic patient due to excessive accommodation during refraction and such patients are defined as pseudomyopes.

The following can indicate the need for a cycloplegic refraction:

Of course, cycloplegic refractions are only used with patients who have accommodation, and usually with those who have the most accommodation: children. The least toxic drug and the lowest dosage (concentration and number of drops) that will produce sufficient cycloplegia should be used. Other assessments of the accommodative system are required (sections 6.9 to 6.11) if the cycloplegic refraction result is unrevealing.

4.13.2 Adverse effects

The key disadvantages of cycloplegia are the temporary symptoms of blurred vision and photophobia experienced by patients. The degradation of vision is caused by the abolition of the accommodation response and increase in ocular aberrations as a result of dilated pupils. Adverse effects and allergic reactions to cyclopentolate are rare and the severe reactions such as psychosis, hallucinations, ataxia and incoherent speech have only been reported for the 2% concentration (or multiple drops of 1%), which should be avoided.³⁷ In all cases, choose the drug with the least possible adverse effects and the lowest concentration that will allow you to efficiently attain the cycloplegia that you require. For example, research has suggested that cyclopentolate 1% is sufficient to produce good cycloplegia, with an effect similar to atropine 1%, in patients with accommodative esotropia and that tropicamide 1% is as effective as cyclopentolate 1% for the measurement of refractive error in most healthy, non-strabismic infants.^38,39

4.13.3 Procedure

4.13.4 Recording

Record the cycloplegic used and the time of instillation. Then record the refraction results (retinoscopy and subjective if both used) in the standard manner, with the approximate time of the refraction. As an alternative to noting the times of instillation and refraction, you could note the period of time the refraction was performed after instillation of the cycloplegic.

For example:

4.13.5 Interpretation

Clearly there are limitations to many of the measurements made in ‘dry’ (non-cycloplegic) refractions of patients with excessive or fluctuating accommodation, but there are also limitations in ‘wet’ (cycloplegic) refractions and all must be considered when prescribing and deciding on the best patient management. During wet retinoscopy it is vital to concentrate on the reflex in the central 3–4 mm and ignore the reflex in the periphery that is influenced by peripheral aberrations. For this reason it can be particularly useful to obtain two objective measures of cycloplegic refractive and use both retinoscopy and autorefraction. Autorefraction provides a particularly accurate assessment of refractive correction under cycloplegia in children.15,16 Also note that the cycloplegic VA is likely to be slightly reduced compared to the VA after a dry refraction due to the peripheral aberrations.

In some patients, the VA after dry retinoscopy can provide useful information about the VA the patient will likely obtain in new spectacles if the full cycloplegic refraction is prescribed. During retinoscopy there is little stimulus for patient accommodation, unlike during subjective refraction, so that the retinoscopy result is typically significantly more hyperopic (or less myopic) than subjective. Therefore, the VA measured after dry retinoscopy is often reduced. However, note that this is because the patient cannot appropriately relax their accommodation during the subjective refraction and the refractive correction found with retinoscopy is likely to be more accurate. It is important to record both the retinoscopy result and the subsequent VA as it can give an indication of the likely distance VA with any new spectacles. Here is an example:

The 12-year-old patient complains of distance vision blur and frontal headaches after a lot of close work.

This large reduction in hyperopia from retinoscopy to dry subjective is indicative of active accommodation during the subjective refraction. In this particular case the subject has become a slight pseudomyope. The difference between a pseudomyope and latent hyperope is small; both are due to accommodative spasm, but the latent hyperope is hyperopic in the dry subjective and the pseudomyope is myopic. Due to proximal accommodation effects, these types of patients provide autorefractor results more similar to the dry subjective results than to retinoscopy. Clearly a cycloplegic refraction is required:

Grade IV (shadow test), 2 drops cyclopentolate 0.5% (2.30 pm):

The cycloplegic results confirm that the dry subjective and dry autorefractor results were incorrect and due to the patient’s inability to relax accommodation. What should you prescribe? The dry retinoscopy results indicate that if you prescribed the full cycloplegic result, the patient would initially see about 6/18 in each eye. Without the cycloplegic, the patient would be unable to relax accommodation and would see very poorly in the distance. This would improve over time as the hyperopic spectacles helped the patient to relax accommodation, but the blur could be a disincentive to initially wearing the spectacles and this should be considered. A compromise prescription may work:

This is likely to initially blur distance VA to about 6/9+, which the patient should be able to cope with, yet will sufficiently relax the accommodation to improve the patient’s symptoms. The patient should be seen within about a month and the prescription could be increased at that point if required. The effect on symptoms, visual acuity and binocular vision should be considered when deciding on the amount of hyperopia to correct in a young person with accommodation.

Even more than for routine refraction results, you should not automatically prescribe the results found in cycloplegic refraction to the patient, although this may be required for patients with accommodative esotropia. A reduced amount of plus power is usually prescribed to compensate for ciliary muscle tonus. For infants and young children, a full hyperopic correction may not be given to ensure emmetropisation is not hindered,⁴⁰ and the fact that hyperopia is normal for infants.

4.13.6 Most common error

Neutralising the retinoscopy reflex seen for the whole of the pupil. The periphery of the reflex should be ignored, and you should concentrate on the centre of the pupil when interpreting retinoscopy reflex movements.

4.13.7 Alternative procedure: Mohindra near retinoscopy

Mohindra near retinoscopy was developed as an alternative to cycloplegic refraction in children and infants.⁴¹ It may be used when a cycloplegic refraction is contra-indicated or when it is extremely difficult to instil the drops. A dim retinoscope light is used as a fixation target and seen in complete darkness it provides little stimulus to accommodation, so that patients assume their resting focus due to tonic accommodation. This is typically +0.75 DS, so that when working at 50 cm, −1.25 DS rather than the standard −2.00 DS (i.e. the working distance lens) is added to the final retinoscopy result.⁴¹ You need to dim the retinoscope light as low as possible while ensuring it still provides you with an easily visible retinoscopy reflex. Turn off all room lights and perform retinoscopy using a lens rack or individual trial case lenses. An infant will only hold a steady gaze for a short period of time, so that you need to perform the test quickly. The standard technique is to occlude the non-fixating eye using your hand or the parent’s hand. However, this can cause infants to become agitated and even begin to cry, so that performing the test binocularly may be preferable in some cases.³⁹ Some studies that have compared Mohindra near retinoscopy with cycloplegic retinoscopy have indicated that the test is variable and should not be used while others have suggested that the test is comparable to cycloplegic retinoscopy.^39,42 It would appear that to become competent with the technique, it needs to be performed regularly and this is a disadvantage for the primary care optometrist who only occasionally examines an infant.³⁹ Saunders and Westall recommend adding −0.75 DS for infants (<2 years) and −1.00 DS for children (over 2 years) rather than the original −1.25 DS.⁴²

4.14.1 Presbyopia and the reading addition

About the age of 40–45 years of age (earlier for some ethnic groups, people with short arms or working distances and hyperopes, later for people with long arms/working distances and myopes) most people become presbyopic.43,44 This means that they do not have enough accommodation to be able to read or perform other near work clearly and comfortably. These patients require a positive lens addition to the distance refractive correction. This is called the reading or near addition. With increasing age and further losses in accommodation, the power of the reading addition needs to be increased. At 55–60 years of age, accommodation is essentially zero (what can be measured clinically is probably depth of focus).⁴⁵ Although the average reading addition continues to increase after age 60 years, it does so at a slower rate (Figure 4.14) and is likely due to the increases in add needed by some older subjects due to a reduced working distance that is required to improve vision to compensate for reduced visual acuity.^43,44,46,47

4.14.2 Comparison of tentative addition techniques

A determination of the reading addition begins with a tentative addition being determined prior to refinement. This is similar to using an objective measurement of refractive correction (retinoscopy, autorefraction) as a starting point for subjective refraction of the distance correction.

In a very useful study, Hanlon et al. determined the required reading addition of 37 dissatisfied patients who returned to a university clinic due to improper add power.⁴⁸ From the case history information in the review (recheck) examination, it was determined whether the improper addition was too low or too high. For each patient, their reading addition was then determined using four methods (age, ½ amplitude of accommodation, NRA/PRA balance and binocular cross-cylinder). The percentage of additions for each test that gave the same result as the improper addition or worse (higher than an improper addition determined too high or lower than an improper addition determined as too low) was determined. They reported that the simplest and quickest test, asking the patient their age, accounted for the fewest errors (14%). The other techniques gave errors in 61% (binocular cross-cylinder), 46% (NRA/PRA) and 30% (½ amplitude) of cases. This suggests that for most patients the tentative addition should simply be based on age. Over the age of about 55 years, the patient’s working distance appears to determine their addition as accommodation is zero, so this can be a useful additional measurement in patients of this age.⁴⁷

The tentative add estimates based on accommodation tests provide an estimate for both eyes and unequal estimates of the tentative addition in the two eyes can indicate that the distance refractive correction has not been adequately balanced and needs to be rechecked. This ‘double checking’ of the spherical powers of the distance refraction in presbyopes may be an advantage to inexperienced refractionists. Note that for patients over 60, these tests are measuring depth of focus and not accommodation, which is zero.

Another alternative for experienced clinicians, would be to use the patient’s symptoms with their old near correction, which is similar to the test used by Hanlon and colleagues⁴⁸ to determine if additions were low or high; i.e. their gold standard test.

4.14.3 Comparison of techniques for determining the final reading add

An incorrect reading addition is a common cause of patients’ unhappiness with their new spectacles, although progressive addition lenses may have decreased the number of problems in recent years as they are more forgiving of over-plussed near corrections.22,48,49 To help to avoid such complaints, it is important to determine the range of clear near vision required by the patient and prescribe spectacles that fulfil those requirements. It is difficult to determine appropriate near working distances and ranges in a phoropter and an addition determination using a trial frame with trial case lenses is recommended.

4.14.4 Procedure: Tentative addition by age and working distance

Estimate the tentative reading addition by age and working distance.

4.14.5 Procedure: Tentative addition as a proportion of the amplitude of accommodation

This is most useful for presbyopes less than 55 years of age. In older patients, their accommodation is essentially zero and age-related pupillary miosis means that any measurement of amplitude of accommodation essentially measures their depth of focus, and the rules become less useful.

4.14.6 Procedure: Binocular or fused cross-cylinder

4.14.7 Procedure: Balancing negative and positive relative accommodation (NRA/PRA)

4.14.8 Procedure: Tentative addition using the patient’s symptoms and habitual correction

This technique is best described by using some examples.

Example 1: 50-year-old patient, wearing bifocal spectacles:

Students must remember that patients read through their near vision correction (distance refractive correction + reading addition) and NOT their reading addition. The refractive correction for near is RE: +2.25 DS, LE +2.25 DS. Given that the average change in distance spherical refractive correction with age in presbyopes who do not develop nuclear cataract is a hyperopic shift,⁵⁰ a common change for the patient in the example above is for the distance correction to change to:

With the new distance correction of +1.50 DS, the tentative addition would be estimated as shown in Table 4.4.

Note that for the most common case (namely, a patient finding difficulty reading and easier if near work held further away), the most appropriate tentative add is the same as the habitual add. Because the distance correction has increased by +0.50 DS, the near vision refractive correction has increased to +2.75 DS and would likely alleviate the symptoms.

Example 2: Some patients develop nuclear cataract with age and this can lead to a myopic shift to the refractive correction.¹ For example, 72-year-old patient; present spectacles and habitual VA:

(The mean sphere equivalent of the near correction is therefore +2.25 DS.)

If the distance refractive correction undergoes a small myopic shift and the new distance correction is:

The mean sphere equivalents of the distance refractive correction is −0.50 DS, so that appropriate tentative additions would be estimated as shown in Table 4.5.

Note that an increased tentative add (compared to their habitual add) is suggested if the patient has no symptoms to counteract the minus shift in the distance correction. This situation is not uncommon because while these patients are under-minused in their habitual spectacles (at both distance and near) they adapt by using a slightly closer reading distance, which provides a little magnification to help counteract reduced visual acuity due to the cataract.

Leaving the reading addition the same at +2.50 DS, under the misapprehension that this would provide the same near correction as in the old spectacles, would leave the patient under-plussed by +0.50 DS. An under-plussed near correction, particularly in patients with nuclear cataract, has been shown to be a cause of patients rejecting new spectacles.²²

4.14.9 Procedure: Final reading addition and range of clear vision (Summary in Box 4.2)

See online video 4.17.

4.14.10 Recording

Record the tentative addition and the method(s) of determination (when an undergraduate student). This is not usually necessary when experienced unless you are using a variety of tentative addition tests and want to keep a track of which tests was used for each individual patient.

Also record the final addition(s), as well as the acuity attained and range of clarity for the addition.

Examples:

4.14.11 Interpretation

Most additions are equal for the two eyes. Unequal additions require further testing: either a retest of the near addition endpoints used for each eye or a recheck of the distance binocular balance. The prescribing of unequal additions between the eyes is the exception and is rarely satisfactory. Assuming no accommodative insufficiency, the power of the addition usually increases with age in patients above 40–50 years. Patients in poor general health can ask for a higher addition than is normal for their age and working distance. In some cases a reading addition that is low for a patient’s age and working distance can indicate that the distance refraction has been over-plussed/under-minused.²² Practitioners rarely give additions greater than +3.00 D in patients with normal visual acuity. It is prudent to keep the addition as weak as possible to keep a large range of clear vision (Table 4.6).

4.14.12 Most common errors: Reading addition

4.15.1 Why not prescribe the subjective refraction result?

4.15.2 Clinical maxims/pearls

4.15.3 Prescribing maxims for elderly patients at risk of falls

To help elderly patients in high risk groups avoid the potentially devastating consequences of a fall, the following prescribing maxims should be followed.⁵⁹ Patients at high risk of falling include those over 75 years of age, with a history of falling, using more than three medications, taking antidepressants with systemic conditions that reduce mobility and patients who may be more dependent on their vision for balance control (patients with somatosensory system dysfunction such as diabetes and/or peripheral neuropathy; or those with vestibular system dysfunction, such as Ménière’s disease).

4.15.4 Should you prescribe a small Rx?

Should you prescribe a small Rx, such as 0.50 D of hyperopia or hyperopic astigmatism? This can be a very difficult question. Here are some points to consider:

4.15.5 Should you make small changes to the refractive correction?

1. Pesudovs, K, Elliott, DB. Refractive error changes in cortical, nuclear and posterior subcapsular cataract. Br J Ophthalmol. 2003;87:964–967.

2. Mutti, DO, Mitchell, GL, Moeschberger, ML, et al. Parental myopia, near work, school achievement, and children’s refractive error. Invest Ophthalmol Vis Sci. 2002;43:3633–3640.

3. Locke, LC. Induced refractive and visual changes. In: In: Diagnosis and management in vision care (JF Amos, ed.). Boston: Butterworths; 1987:pp. 313–67.

4. Smith, G. Relation between spherical refractive error and visual acuity. Optom Vision Sci. 1991;68:591–598.

5. Leung, TW, Lam, AK, Deng, L, Kee, CS. Characteristics of astigmatism as a function of age in a Hong Kong clinical population. Optom Vision Sci. 2012;89:984–992.

6. Steele, CF, Rubin, G, Fraser, S. Error classification in community optometric practice – a pilot study. Ophthalmic Physiol Opt. 2006;26:106–110.

7. Solsona, F. Astigmatism as a congenital bilateral and symmetrical entity. (Observations based on the study of 51,000 patients). Br J Physiol Opt. 1975;30:119–127.

8. Osuobeni EP, al-Fahdi M. Differences between anatomical and physiological interpupillary distance. J Am Optom Assoc 1994;65:265-71.

9. Holland BJ, Siderov J. Repeatability of measurements of interpupillary distance. Ophthalmic Physiol Opt 1999;19:74-8.

10. Pointer, JS. The interpupillary distance in adult Caucasian subjects, with reference to ‘readymade’ reading spectacle centration. Ophthalmic Physiol Opt. 2012;32:324–331.

11. Brown WL. Interpupillary distance. In: Clinical procedures in optometry (JD Eskridge, JF Amos, JD Bartlett, eds.) Philadelphia: JB Lippincott, 1991, pp. 39-52.

12. Casillas, E, Rosenfield, M. Comparison of subjective heterophoria testing with a phoropter and trial frame. Optom Vision Sci. 2006;83:237–241.

13. Elliott DB, Wilkes RD. A clinical evlautaion of the Topcon RM-6000 autorefractor. Clin Exp Optom 1989;72:150-3.

14. McCaghrey, GE, Matthews, FE. Clinical evaluation of a range of autorefractors. Ophthalmic Physiol Opt. 1993;13:129–137.

15. Walline, JJ, Kinney, KA, Zadnik, K, Mutti, DO. Repeatability and validity of astigmatism measurements. J Refract Surg. 1999;15:23–31.

16. Zhao, J, Mao, J, Luo, R, et al. Accuracy of noncyclopegic autorefraction in school-age children in China. Optom Vision Sci. 2004;81:49–55.

17. Jackson, DW, Paysse, EA, Wilhelmus, KR, et al. The effect of off-the-visual-axis retinoscopy on objective refractive measurement. Am J Ophthalmol. 2004;137:1101–1104.

18. Roorda, A, Bobier, WR. Retinoscopic reflexes: theoretical basis and effects of monochromatic aberrations. J Am Optom Assoc. 1996;67:610–618.

19. Millodot, M, O’Leary, D. The discrepancy between retinoscopic and subjective measurements: Effect of age. Am J Optom Physiol Opt. 1978;55:309–316.

20. Albinet, CT, Boucard, G, Bouquet, CA, Audiffren, M. Processing speed and executive functions in cognitive aging: how to disentangle their mutual relationship? Brain Cogn. 2012;79:1–11.

21. Wang, B, Ciuffreda, KJ. Depth-of-focus of the human eye: Theory and clinical implications. Surv Ophthalmol. 2006;51:75–85.

22. Hrynchak, P. Prescribing spectacles: reason for failure of spectacle acceptance. Ophthalmic Physiol Opt. 2006;26:111–115.

23. Bennett AG, Rabbetts RB. Clinical Visual Optics. 3rd edition. Oxford: Butterworth-Heinemann; 1998.

24. Elliott, DB, Cox, MJ. A clinical assessment of the +1.00 blur test. Optom in Pract. 2004;5:189–193.

25. Rosenfield, M, Aggarwala, KR, Raul, C, Ciuffreda, KJ. Do changes in pupil size and ambient illumination affect the duochrome test? J Am Optom Assoc. 1995;66:87–90.

26. Clementi, M, Angi, M, Forabosco, P, et al. Inheritance of astigmatism: evidence for a major autosomal dominant locus. Am J Hum Genet. 1998;63:825–830.

27. Johnson BL, Edwards JS, Goss DA, et al. A comparison of three subjective tests for astigmatism and their interexaminer reliabilities. J Am Optom Assoc 1996;67:590-8.

28. Borish, IL, Benjamin, WJ. Monocular and binocular subjective refraction. In: In: Borish’s Clinical Refraction (WJ Benjamin, ed.). Philadelphia: WB Saunders; 2006.

29. Rosenfield, M. Subjective refraction. In: In: Optometry: Science, techniques and clinical management (M Rosenfield, N Logan, eds.). Edinburgh: Elsevier; 2009:209–228.

30. West, D, Somers, WW. Binocular balance validity: A comparison of five different subjective techniques. Ophthalmic Physiol Opt. 1984;4:155–159.

31. Humphriss, D. Binocular refraction. In: In: Optometry (K Edwards, R Llewellyn, eds.). Oxford: Butterworths; 1988.

32. Rosenfield, M, Ciuffreda, KJ, Hung, GK, Gilmartin, B. Tonic accommodation: a review II. Accommodative adaptation and clinical aspects. Ophthalmic Physiol Opt. 1994;14:265–277.

33. Charman, WN, Jennings, JA, Whitefoot, H. The refraction of the eye in the relation to spherical aberration and pupil size. Br J Physiol Opt. 1978;32:78–93.

34. Martin, J, Vasudevan, B, Himebaugh, N, et al. Unbiased estimation of refractive state of aberrated eyes. Vision Res. 2011;51:1932–1940.

35. Muñoz-Hernández, AM, Santos-Bueso, E, Sáenz-Francés, F, et al. Nonorganic visual loss and associated psychopathology in children. Eur J Ophthalmol. 2012;22:269–273.

36. Lim, SA, Siatkowski, RM, Farris, BK. Functional visual loss in adults and children patient characteristics, management, and outcomes. Ophthalmology. 2005;112:1821–1828.

37. Jones, LWJ, Hodes, DT. Possible allergic reactions to cyclopentolate hydrochloride – case reports with literature review of uses and adverse reactions. Ophthalmic Physiol Opt. 1991;11:16–21.

38. Celebi, S, Aykan, U. The comparison of cyclopentolate and atropine in patients with refractive accommodative esotropia by means of retinoscopy, autorefractometry and biometric lens thickness. Acta Ophthalmol. 1999;77:426–429.

39. Twelker, JD, Mutti, DO. Retinoscopy in infants using a near noncycloplegic technique, cycloplegia with tropicamide 1%, and cycloplegia with cyclopentolate 1%. Optom Vision Sci. 2001;78:215–222.

40. Leat, SJ, Mittelstaedt, A, McIntosh, S, et al. Prescribing for hyperopia in childhood and teenage by academic optometrists. Optom Vis Sci. 2011;88:1333–1342.

41. Mohindra, I. A non-cycloplegic refraction technique for infants and young children. J Am Optom Assoc. 1977;48:518–523.

42. Saunders, KJ, Westall, CA. Comparison between near retinoscopy and cycloplegic retinoscopy in the refraction of infants and children. Optom Vision Sci. 1992;69:615–622.

43. Millodot, M, Millodot, S. Presbyopia correction and the accommodation in reserve. Ophthalmic Physiol Opt. 1989;9:126–132.

44. Pointer, JS. The presbyopic Add I, II and III. Ophthalmic Physiol Opt. 1995;15:235–254.

45. Charman, WN. The path to presbyopia: straight or crooked? Ophthalmic Physiol Opt. 1989;9:424–430.

46. Blystone, PA. Relationship between age and presbyopic addition using a sample of 3,645 examinations from a single private practice. J Am Optom Assoc. 1999;70:505–508.

47. MacMillan, ES, Elliott, DB, Patel, B, Cox, M. Loss of visual acuity is the main reason why reading addition increases after the age of sixty. Optom Vision Sci. 2001;78:381–385.

48. Hanlon, SD, Nakabayashi, J, Shigezawa, G. A critical view of presbyopic add determination. J Am Optom Assoc. 1987;58:468–472.

49. Freeman, CE, Evans, BJ. Investigation of the causes of non-tolerance to optometric prescriptions for spectacles. Ophthalmic Physiol Opt. 2010;30:1–11.

50. Guzowski, M, Wang, JJ, Rochtchina, E, et al. Five-year refractive changes in an older population: the Blue Mountains Eye Study. Ophthalmology. 2003;110:1364–1370.

51. Howell-Duffy, C, Scally, AJ, Elliott, DB. Spectacle prescribing II: practitioner experience is linked to the likelihood of suggesting a partial prescription. Ophthalmic Physiol Opt. 2011;31:155–167.

52. Hrynchak, PK, Mittelstaedt, AM, Harris, J, et al. Modifications made to the refractive result when prescribing spectacles. Optom Vision Sci. 2012;89:155–160.

53. Werner, DL, Press, LJ. Clinical Pearls in Refractive Care. Boston: Butterworth-Heinemann; 2002.

54. Milder, B, Rubin, ML. The fine art of prescribing glasses without making a spectacle of yourself. 2nd edition, Gainesville: Triad Pub; 2004.

55. Leat, SJ. To prescribe or not to prescribe? Guidelines for spectacle prescribing in infants and children. Clin Exp Optom. 2011;94:514–527.

56. Goss, DA, Grosvenor, T. Reliability of refraction – a literature review. J Am Optom Assoc. 1996;67:619–630.

57. MacKenzie, GE. Reproducibility of sphero-cylindrical prescriptions. Ophthalmic Physiol Opt. 2008;28:143–150.

58. Howell-Duffy, C, Hrynchak, PK, Irving, EL, et al. Evaluation of the clinical maxim: ‘If it ain’t broke, don’t fix it’. Optom Vision Sci. 2012;89:105–111.

59. Elliott, DB. Falls and vision impairment: guidance for the optometrist. Optom in Pract. 2012;13:65–76.

60. Lord SR, Dayhew J, Howland A. Multifocal glasses impair edge-contrast sensitivity and depth perception and increase the risk of falls in older people. J Am Geriatr Soc 2002;50:1760-6.

61. Dwyer, P, Wick, B. The influence of refractive correction upon disorders of vergence and accommodation. Optom Vision Sci. 1995;72:224–232.