Subjective Refraction: Principles and Techniques for the Correction of Spherical Ametropia

$Subjective Refraction: Principles and Techniques for the Correction of Spherical Ametropia$ Subjective Refraction: Principles and Techniques CHAPTER for the Correction of Spherical Ametropia 10 Andrew Franklin Introduction The signs of uncorrected myopia may include: There are two methods of evaluating the refractive poor distance vision on a letter chart error of an eye: • • good near vision on a near test chart. 1. A subjective refraction where the result depends on the patient’s ability to discern The symptoms of uncorrected hypermetropia may changes in clarity. This process relies on the include: cooperation of the patient. • eyestrain, especially for close work, caused by 2. An objective refraction (usually retinoscopy) the accommodative effort to form a clear where the result depends purely on the image examiner’s judgement to determine the optimum • blurred vision with medium-to-high amounts of optical correction. Retinoscopy has been covered hypermetropia and in advancing age (blurred vision in detail in Chapter 8. An autorefractor (see is not usually a problem with low amounts of Chapter 16) can also be used to obtain an hypermetropia). objective refraction. The signs of uncorrected hypermetropia may Subjective refraction include: Subjective refraction consists of three distinct phases. The fi rst is designed to correct the spherical • usually no signs in low hypermetropia – screwing element of the refractive error in such a way as to up the eyes and wrinkling of the brow may be a facilitate the accurate determination of any astigmatic sign of high amounts of uncorrected element present. It should be remembered that, hypermetropia although astigmatism is often present, a refractive • a nasalward deviation (esotropia) of one eye in error may be entirely spherical. The second phase high amounts of uncorrected hypermetropia. is the determination of the astigmatic error (see Chapter11) and the third phase involves the Objective refraction Objective refraction (retinoscopy) is often used to balancing and/or modifi cation of the refractive determine the initial spherical element of refraction. correction to ensure optimal visual performance and However, when a patient has a recent correction patient comfort (see Chapter 12). that is providing good acuity, say 6/9 or better, this As always, the patient’s history and symptoms are can be taken as the starting point. important and can be used to help predict a The purpose of the fi rst phase of a subjective refractive error. Remember that a symptom is a refraction is to determine the best vision sphere patient’s complaint whereas a sign is a practitioner’s (BVS). This can be defi ned as the most positive (or observation. The symptoms of uncorrected myopia least negative) spherical lens that provides best visual may include: acuity. During a subjective refraction accommodation • blurred distance vision must be not be allowed to fl uctuate randomly. The • headaches from screwing up the eyes to try to eye should be as relaxed as possible so that changes obtain clearer vision by the pinhole effect in the accommodative state do not infl uence the • clear near vision. end-result. As the accuracy of any subjective test or 101 T1 CCh010-H8889.inddh010-H8889.indd 110101 66/15/2007/15/2007 66:38:24:38:24 PPMM Optics of the Eye, Ametropia and its Correction routine relies on the individual patient’s ability to in general, the simpler the task, the more likely it discriminate and communicate accurately, the will be performed well. To determine the BVS a potential for error must be kept to a minimum. suitable letter chart (logMAR or Snellen), trial frame The ability to discriminate and communicate will and trial case of lenses (or a refractor head) are of course vary widely from person to person but, required. Chapter content • Determination of the best vision sphere • The use of the pinhole disc • The duochrome test • Patients with poor visual acuity (VA) • The Scheiner disc Determination of the best vision sphere The following discussion takes place in two Figure 10.1 A trial frame shown with the left eye parts: the fi rst assumes that retinoscopy has not occluded. been performed whereas, in the second, the determination of the BVS follows retinoscopy. If a good retinoscopy has been performed, the technique of fi nding the BVS in isolation should, When using the best sphere technique in theory, be redundant. However, it can prove without the aid of retinoscopy, the practitioner useful when retinoscopy is diffi cult (small must fi nd the maximum amount of positive pupils or media opacities). It is usual to start power or the minimum amount of negative with the right eye, the left being occluded power that can be tolerated by the eye, without (Figure 10.1). This is called a monocular refrac- causing blurring of the retinal image. After tion. The procedure is repeated on the left eye occlusion, the fi rst task is to measure the unaided with the right occluded. However, it is possible vision. This is useful because unaided vision and often preferable to refract under binocular can give a reasonable estimate of the magnitude conditions. Binocular refraction is discussed in of any uncorrected myopia or manifest hyper- Chapter 12. metropia (Table 10.1). It is important to note In both binocular and monocular refraction, that the estimates in Table 10.1 are of no use it is important to control accommodation in the if the patient is accommodating to ‘correct’ person with pre-presbyopia, so a ‘fogging’ tech- any hypermetropia. nique is employed whereby the spherical The questioning technique is very important element is deliberately over-plussed and then throughout subjective refraction because the reduced to fi nd the fi nal spherical power. Tra- use of appropriately phrased questions can ditionally, the right eye is usually refracted fi rst, make the difference between a quick, precise because it is the nearest one to the practitioner refraction and a long-winded, potentially inac- in most consulting rooms. However, when the curate refraction. left eye has signifi cantly worse acuity as a result The patient’s attention is directed towards of amblyopia or pathology, or if the right eye is the letter chart. Whenever a positive lens is held markedly dominant, the left eye should be before the eye, the question to the patient should T1 102 refracted fi rst. take the following format: CCh010-H8889.inddh010-H8889.indd 110202 66/15/2007/15/2007 66:38:26:38:26 PPMM Subjective Refraction: Principles and Techniques for the Correction of Spherical Ametropia Table 10.1 Expected vision for any uncorrected mean Should the target appear darker but not clearer, sphere (myopia or manifest hypermetropia) extra minus power should not be added because Vision Equivalent sphere (myopia or this just stimulates accommodation. Also, extra manifest hypermetropia) minus power should not be added if the target appears smaller but not clearer. A negative lens 6/5 Plano should be added to the trial frame only if the 6/6 0.25–0.50 DS patient can resolve a greater number of letters on 6/9 0.50–0.75 DS the letter chart. The results are often rechecked 6/12 0.75–1.00 DS 6/18 1.00–1.25 DS and confi rmed throughout the test using the 6/24 1.25–1.75 DS same or a different technique, e.g. best sphere 6/36 1.75–2.25 DS and duochrome because patient’s answers are frequently inconsistent! When the BVS has been reached the point focus (in the case of spherical ametropia) or disc of least confusion (in the case of astigmatic ametropia) should be on or very Is the target better with or without this close to the retina. The distance vision with this lens, or is there no real difference? correction should be measured and recorded because it is useful for estimating the magnitude Positive lenses either blur the retinal image, of any uncorrected astigmatic error. indicating that the maximum amount of posi- The use of ±0.25 DS ‘twirls’ (see below) helps tive power is already in place, or relax accom- in fi ne-tuning the BVS. Remember that the end- modation (where it is in use). Therefore an point is the maximum plus or minimum minus answer of ‘no difference’ indicates the need that the patient will tolerate without causing to add positive power to the trial frame combi- blurring of the retinal image. nation of lenses until no more can be tolerated. The initial plus lens may be in the region of +1.00 DS. Later in the procedure, a +0.50 DS Summary of the procedure to fi nd the may be used. When the total power of the BVS without the use of retinoscopy trial lenses in the trial frame is close to the end- 1. Occlude the left eye. point; the practitioner should add spherical lens 2. Measure the unaided vision (V). power in ±0.25 DS steps. The lens must be held 3. If possible, estimate the ametropia. This in the plane of the trial frame and along is particularly helpful in the case of the visual axis to avoid inducing off-axis aber- uncorrected myopia. Also in myopia, the rations. It should also be moved quickly and position of the true far point can be used precisely, allowing enough time in each posi- to estimate the refractive error (see Table tion for the patient to make a decision. Some 8.6, Chapter 8), e.g. a person with −8.00 D patients require longer than others. Practitio- myopia sees clearly if a target is placed ners usually need to repeat this process a at approximately 12.5 cm from the eye. number of times to confi rm the result and con- 4.

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