‘3’ is a reference that can be used to compare the visibility of the two numbers on the page. A patient over 60 years of age fails the test if they make two or more errors, a patient less than 20 years of age fails the blue–yellow part of the test with two or more errors. The failure criteria for all other patients is one or more errors. Classifying an error based on a fail on plate 12 can be confusing because the figure can be missed by individuals with either a protan or tritan defect. In this case, errors on other plates should be considered. With other red–green errors, but not blue–yellow, classify the patient as a protan. With no other red–green errors, then the error should be classified as blue–yellow. With acquired defects, both red–green and blue–yellow errors can occur along with failing plate 12 and additional testing should be carried out with either the D-15 or TCU (sections 3.19 and 3.20).

3.22 BIBLIOGRAPHY AND

FURTHER READING

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Bailey, I.L. (2006) Visual acuity. In: Borish’s Clinical refraction, 2nd edn (ed. W.J. Benjamin). St Louis: Butterworth-Heinemann.

Birch, J. (2001) Diagnosis of defective colour vision, 2nd edn. Boston: Butterworth-Heinemann.

Bullimore, M.A. (1997) Visual acuity. In: The ocular examination: measurements and findings (ed.

K. Zadnik). Philadelphia: W.B. Saunders. Elliott, D.B. (1997) Supplementary clinical tests of

vision. In: The ocular examination: measurement and findings (ed. K. Zadnik). Philadelphia: W.B. Saunders.

Elliott, D.B. (2006) Contrast sensitivity and glare testing. In: Borish’s Clinical refraction, 2nd edn (ed. W.J. Benjamin). St Louis: ButterworthHeinemann.

Flanagan, J.G., Buys, Y. and Trope, G.E. (1996)

Automated perimetry: an interactive primer. Waterloo, Canada: Lifelearn Eyecare.

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4.1.Relevant case history information 83

4.2.Relevant visual acuity information 84

4.3.Keratometry 85

4.4.Focimetry 90

4.5.Anatomical interpupillary distance 93

4.6.Phoropter or trial frame? 95

4.7.Static retinoscopy 97

4.8.Autorefraction 103

4.9.Monocular subjective refraction 104

4.10.Maximum plus to maximum visual acuity (MPMVA) 107

4.11.The plus/minus technique for best vision sphere determination 108

4.12.Duochrome (or bichrome) test 111

4.13.The Jackson cross-cylinder 112

4.14.The fan and block test 117

4.15.Prism-dissociated blur balance of accommodation 119

4.16.Monocular fogging balance (modified humphriss) 121

4.17.Humphriss Immediate Contrast (HIC)

123

4.18.Turville Infinity Balance (TIB) 124

4.19.Binocular subjective refraction 125

4.20.Cycloplegic refraction 128

4.21.Tentative reading addition using calculations 132

4.22.Tentative reading addition using assessments of accommodation 137

4.23.Trial frame determination of a reading addition and range of clear vision

139

4.24.Prescribing 143

4.25.Counselling 147

4.26.Bibliography and Further reading 149

4.27.References 149

4.1 RELEVANT CASE HISTORY

INFORMATION

The case history can provide significant information about the need for a refractive correction or change in refractive correction and even the type of ametropia that might be present.

4.1.1 Symptoms

Symptoms of blurred vision or an inability to see well enough for certain tasks (reading the blackboard, schoolbooks or a newspaper, watching TV, driving, etc.) suggest undiagnosed ametropia or a change in ametropia in those already wearing spectacles. If these symptoms are due to ametropia, they usually have a gradual onset. Near vision blur with good distance vision suggests hyperopia or presbyopia depending on the patient’s age. Distance vision blur with good near vision suggests myopia and blur at all distances can indicate significant astigmatism. You should ask whether the blurred vision is in one eye or both. 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.

4.1.2 Ocular history

If a patient already wears spectacles or contact lenses, the ocular history can suggest the type of ametropia. Knowledge of the age at which spectacles were first worn can also help to indicate the ametropia. For example, patients who first wore spectacles at age 8–12 are likely to be childhoodonset myopes and those who first wore spectacles at age 18–22 are likely to be adult-onset myopes.

Adult-onset myopes (often 1.00–3.00 D) are typically less myopic than childhood-onset myopes (often 3.00–6.00 D) (e.g. Grosvenor & Scott 1991). Patients who first wore spectacles at age 45–55 are presbyopes and are likely to be emmetropic or slightly hyperopic at distance. The earlier they needed reading glasses, the more likely they are to be hyperopic. Finally, 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 have increased myopia given the typical progression of myopia after onset.

Any mention of cataracts in the case history should lead to a careful investigation for increased myopia (nuclear cataract) or astigmatic change (cortical cataract) (Pesudovs & Elliott 2003).

4.2.1 Distance vision vs. near vision (i.e. unaided measurements)

Reduced distance vision (unaided distance VA) with normal near vision (unaided near VA) indicates myopia, while normal distance vision with reduced near vision indicates moderate to severe hyperopia or presbyopia depending on the patient’s age. Young hyperopes typically have no vision loss because they can accommodate to see well at both distance and near. As patients get older, accommodation is lost, so that reduced near vision becomes more likely. Reduced vision at both distance and near can indicate astigmatism, which could be combined with either hyperopia or myopia. The vision measurements should reflect the symptoms presented in the case history.

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 (2002) 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.

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 (Locke 1987).

4.2 RELEVANT VISUAL ACUITY

INFORMATION

Visual acuity (VA) measurements can also provide significant information about the need for a refractive correction or change in refractive correction and the type of ametropia.

4.2.2 Distance vision (unaided distance VA)

Distance vision measurements can be used to predict the refractive corrections of myopes and older hyperopes (i.e. hyperopes over 60 years of age who do not have any accommodation). 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, e.g. a 1.00 D myope with an optimal VA of 6/4.5 (20/15; typical of a 20-year-old patient) should have vision loss of about four lines and distance vision of 6/12 or 20/40. Similarly, an older 1.00 D hyperope with optimal VA of 6/6 or 20/20 should have a vision loss of about four lines and distance vision of 6/15 or 20/50. Near horizontal and vertical astigmatic errors have a similar effect to the equivalent best mean sphere, so that a 1.00 DC would have a similar effect to a 0.50 DS, a twoline drop in vision. Cylinders at oblique axes tend to give a slightly greater degradation in vision.

4.2.3 Habitual distance VA

For low myopic refractive changes (and hyperopic changes in an absolute presbyope), a degradation of one line of visual acuity (VA) on a logMAR chart