to keep as large a range of clear vision as possible (Table 4.9). If possible, do not change the near refractive correction by more than 0.50 D to ease spectacle adaptation, although this may be less of an issue with progressive addition lenses.

4.23.6 Most common errors

1.Under-plussing a near correction (Hrynchak 2006). This can particularly occur with patients undergoing nuclear cataract induced refractive changes in the distance prescription that may require a significantly increased reading addition.

2.Giving extra plus when it provides no change and thus prescribing too high an addition.

3.Not determining the patient’s near vision needs and subsequently prescribing an addition that gives an inadequate range of clear near vision for those needs.

4.Estimating instead of measuring the near point distances with a tape measure.

4.24 PRESCRIBING

This section provides various points regarding prescribing refractive corrections. Note that they are generalities and must not be used as hard and fast rules. In particular, they are unlikely to be valid when prescribing refractive corrections for young children or low vision patients. At the end of a primary care examination, you will commonly have to answer one of the following three questions regarding prescribing a refractive correction.

1.What prescription (Rx) should be given?

You may consider that deciding upon the final refractive correction given to the patient is easy and that it is the result found after subjective refraction. This is not always correct.

2.Does a patient with a small refractive correction need spectacles or contact lenses?

3.Is a small change in refractive correction necessary? Does the patient need to update their lenses?

4.24.1 Demonstrate changes to the patient

The improvements in vision provided by a first refractive correction can be shown by alternately showing the patient the vision with their optimal refractive correction and without. The patient should be asked to look at a distance or near chart (whichever is appropriate) or look out of the practice window into the far distance and using trial case lenses with a trial frame. The effect of any refractive correction changes can be shown to the patient by alternately showing the patient the vision (distance and/or near) obtained with their optimal refractive correction in a trial frame compared to their current spectacles. This can be awkward and it can be easier, if there are negligible cylindrical changes (which is relatively common), to place appropriate spherical trial cases lenses over the top of their current spectacles to allow a comparison.

4.24.2 What refractive correction should you prescribe?

It would be easy to assume that the power of any new spectacles should be the subjective refraction result. This is true in a lot of cases, but is not always correct. Note that the subjective refraction result is not a perfectly repeatable measurement and both the spherical and cylindrical components can vary from test to retest by up to 0.50 D (Goss & Grosvenor 1996). Here are a few points that indicate when the Rx given should be different from the subjective refraction result. Note that these points are generalities and not strict rules. The patient’s input is the best source of information when deciding what refractive correction to prescribe.

1.‘IF IT AIN’T BROKE DON’T FIX IT’. A very important rule for all age groups. If a patient is happy with their Rx, but would like a new frame, the only change you can make by changing the Rx (particular cylinder power or axis) is to make them unhappy. Remember that the subjective refraction result is not a perfectly repeatable measurement and can vary up to 0.50 D from test to retest (Goss & Grosvenor 1996).

The exception is if you can make a significant improvement in their VA, and even then it should be remembered that a patient may want slightly blurred vision (for example, the presbyopic myope may prefer to be slightly under-corrected at distance so that they can read in their distance spectacles). Ignoring this rule is one of the major causes of patients being dissatisfied with their spectacles.

2.Compare against the patient’s spectacles: Changes in cylindrical correction are often minimal, so that the major change in refractive correction is spherical. It can be useful to place appropriate spherical trial case lenses over the patient’s spectacles and ask the patient whether they like the change in vision (or not!)

3.Non-progressive myopes: Do not always ‘push the plus’ and give ‘maximum plus’ for non-progressive myopes. Remember that if you are refracting at 6 m or 20 ft, this is not infinity, so that patients are likely to be over-plussed by 0.17 D with a 6 m

(20 ft) refractive correction. In addition, some low myopes tend to wear their Rx only for driving and especially at night.

Here ‘night myopia’ is an additional problem. Therefore, for non-progressive myopes, it is often better to err on the side of over-minus when prescribing, rather than over-plus. In particular, be extremely careful of reducing a myopic Rx in older non-progressive myopes, especially if there are no symptoms. Over-plussing

the distance correction has been reported as the most common reason for failure of spectacle lens acceptance (Hrynchak 2006).

4.Hyperopes: Only prescribe the full hyperopic Rx if the patient is presbyopic (or nearing presbyopia), esotropic or has esophoria (particularly convergence excess). Otherwise consider prescribing a partial Rx. All you need to do is prescribe a hyperopic Rx that

is sufficient to remove any symptoms. The amount will depend upon the patient’s symptoms, age, manifest and latent hyperopia, e.g. if fully manifest, then

prescribe 1/2 to 3/4 of the Rx. The older the patient, the more likely you will prescribe3/4 to full Rx. The more pronounced the symptoms, the more likely you are to prescribe more of the hyperopia. Overplussing the distance correction has been reported as the most common reason for failure of spectacle lens acceptance (Hrynchak 2006).

5.Latent hyperopes: With a large latent component, you are likely to perform a cycloplegic refraction. Prior to cycloplegia it is important to assess the effect of extra plus over the manifest dry Rx to determine the effect of giving extra plus on distance visual acuity. This will indicate how much extra plus the patient is likely to be able to tolerate before distance blur becomes too great (section 4.20.5).

6.Heterophoria in younger patients: You must consider any significant heterophoria when prescribing in young patients (Dwyer & Wick 1995).

7.Older patients: It is easier to make big changes in Rx in younger patients. In patients over 25 years old, be wary of making changes over 0.75 D.

8.Presbyopes: Be very wary of increasing a distance or near Rx (other than the first presbyopic addition) by 0.75 D. This is a good general rule for the majority of patients with simple increasing age-related hyperopia and/or presbyopia. Large increases in Rx in older patients tend not to be tolerated and can even lead to an increase in falls in frail elderly patients (Cumming et al. 2007).

9.Presbyopes: The near correction can often be under-plussed in a patient undergoing a minus shift in the distance refractive correction due to nuclear cataract (Hrynchak 2006). For example, consider a patient with spectacles of 1.00 DS OU with an addition of 1.50. With a minus shift making the distance correction plano in both eyes, the patient will need a 2.50 addition to retain the same near correction.

10.Presbyopes: It is vital that you know what the patient wants to see with the near vision Rx and determine their point of best focus and range of clarity. The two must overlap (section 4.23).

11.Cylinder changes:

a)Many practitioners do not prescribe

0.25cylinders, particularly when spheres are relatively large and/or when the astigmatism was not seen on retinoscopy. 0.25 cylinders should be prescribed when the spherical Rx is low and/or if the patient has a detailoriented personality and responses during the subjective determination of astigmatism were precise and repeatable.

b)When cylinder changes are moderate to large, generally make partial changes in cylinder power and axis and no more than 0.50 DC. Changes in power are more tolerable if the axes are not oblique. Changes in axis should never be large for large cylinders. Carefully look at the change in VA made by the cylinder change, and whether the astigmatic change may be the cause of any of the patient’s symptoms. If there are significant VA changes and symptoms, you would be more likely to give more of the cylinder. Allow the patient to participate in the decision if possible. It can be useful to trial frame the partial Rx you are going to prescribe.

c)Remember, if you partially prescribe a change in cylinder power, an appropriate change in sphere should be made (to give the same mean sphere). It can be useful to trial frame the partial Rx you are going to prescribe.

12.Poor adaptors: If a patient has a record of poor adaptation to new spectacles (this should always be recorded), then make small changes subsequently.

13.Anisometropia: Symptoms of aniseikonia are mainly asthenopia and headaches and very few complain of spatial distortions, etc.

a)Generally, less than 1 D of anisometropia does not cause problems.

b)In young patients, the first step is to prescribe the full Rx. Young patients will adapt to surprisingly large amounts of anisometropia.

c)The best mode of refractive correction for anisometropia is often contact lenses. They remove any prismatic problems as the contact lens moves with the eye. They also may remove problems due to aniseikonia.

d)With anisometropia ( 4D) and amblyopia of long standing (age 10, VA 6/36), then a balance lens may be appropriate for the amblyopic eye. Tell the patient that the good eye will not deteriorate because of strain.

e)If a patient has alternating vision (e.g. RE(OD): 3.00, LE(OS): plano so that right eye is used for near work and left eye for distance) and no symptoms, then spectacles may not be necessary.

f)Anisometropia in presbyopes: This is most commonly found with cataractinduced myopia and astigmatism and after monocular cataract surgery. Generally

use a partial Rx of the more myopic eye (reduce by 1/3 of anisometropia). Large cylinders, especially oblique, may require partial Rx of power. Partial cylinder axis changes may also be made to reduce meridional aniseikonic effects. Make sure that using a partial correction does not reduce the visual acuity below the standard required for driving.

g)If aniseikonia is still a problem and contact lenses cannot be fitted, then try:

i)keeping the spectacle vertex distance as small as possible by appropriate selection of frames.

ii)reducing the thickness of the more hyperopic Rx.

iii)changing the blank size.

iv)changing the base curve. You may consider prescribing equal base curves in the two lenses.

v)using size/isogonal lenses.

vi)using a bicentric grind to eliminate the dynamic anisophoria in the vertical meridian.

4.24.3 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:

1.If there are no symptoms related to the use of the eyes, then a first Rx should not be prescribed.

2.Always consider other ocular causes of the symptoms, which might not be related to the small refractive error and include inadequate convergence, accommodative facility or vergence facility and decompensated heterophoria. Also consider non-ocular causes of headaches, including tension, migraine, nasal sinusitis and hypertension. Unfortunately, tension headaches, which are a common headache, can be difficult to differentiate from ocular headaches as they are often frontal or occipital, get worse towards the end of the day and are better over the weekend.

3.If a patient has symptoms that are related to detailed vision tasks, you are more likely to prescribe a small Rx if the patient does a lot of detailed work and/or if the patient has a personality that is detail-oriented, precise or intense.

4.The relative certainty of responses should help your decision of whether to prescribe a small Rx. If glasses are to be of any value, the responses during subjective refraction should be very certain, appropriate and repeatable.

5.Usually small Rxs make little change to the VA (particularly if a truncated Snellen chart is used) and so basing decisions on VA improvements is usually not helpful.

6.The effect of the Rx on binocular vision tests can be helpful (Dwyer & Wick 1995). For example, if binocular vision tests suggest that a heterophoria is decompensated with no refractive correction and compensated

with it, then the spectacles are likely to help and should be prescribed (Dwyer & Wick 1995).

7.You can view prescribing glasses as a diagnostic tool. Often the only way to be certain whether the symptoms are due to the uncorrected refractive error is to prescribe it and see if the symptoms disappear. You could offer the patient a pair of basic loan spectacles to determine whether the refractive correction will relieve the symptoms. This approach is often used in medicine. However, be aware that spectacles can provide a placebo effect and relieve the symptoms for a short period before they return.

4.24.4 Should you make small changes to the refractive correction?

1.If there are no symptoms and a small change to the refractive correction and the patient wants a new frame, it may be better to stick with their old correction unless a significant improvement in VA over their old correction can be obtained (‘IF IT AIN’T BROKE DON’T FIX IT’).

2.Consider the points in section 4.24.3; in particular, if a patient has symptoms which are related to detailed vision tasks, you are more likely to prescribe a small change in Rx if the patient does a lot of detailed work and/or if the patient has a personality which is detail-oriented, precise or intense. Consider the relative certainty and repeatability of responses during the subjective refraction.

3.Even if there is no change in refractive correction, a patient should always be asked if they want a new pair of glasses. They may want a change of frame or their old lenses may be scratched and need replacing.

4.24.5 Prescribing for elderly patients at risk of falling

Although falls are multifactorial, it is clear that visual impairment is linked with an increased risk of falling (Buckley & Elliott 2006). The following are points to consider when prescribing for a patient who is at high risk of falling (over 75 years of age, history of falling, using more than three