Continuing education CET Routine eye examination Part 6 – Objective refraction Andy Franklin and Bill Harvey look at objective refraction, the various ways of undertaking it and what may influence the outcome. Module C8872, two general point for optometrists and dispensing opticians bjective refraction Adjusting the trial frame includes retinoscopy and The optical centres of the trial frame the use of autorefractors, should be set to the distance PD. If which are appearing in there is marked facial asymmetry you optometric practice in may need to measure half-PDs and increasing numbers. In adjust the trial frame accordingly. Later, O general, objective methods are not when you move on to near vision tests required to give us a final prescription. the optical centres can be adjusted to They merely need to get us to a point equal the near PD, and dropped slightly from which subjective methods can by adjusting the nosepiece of the trial take us to the end point accurately and frame. However, this should only be quickly. With an alert and compliant done if the spectacles that you intend subject it is possible to get an accurate to prescribe are to be set for near vision result using subjective methods alone, only. On a pre-presbyopic patient the but it takes time. In general, excessively optical centres should stay set for prolonged refraction of normal patients distance throughout the refraction, as is usually indicative of poor technique the near ocular motor balance will be rather than ‘professionalism’. With affected by the centration of the lenses practice, and if a previous prescription (Table 1). is known, objective refraction should Figure 1 If and the position of the nasal limbus is ● Look at the trial frame and check take seconds rather than minutes. the frame is measured, ensuring that you have not that it is level, allowing for any facial There are patients who are unable not level, moved the rule meanwhile. asymmetries that may be present. If the to participate in a subjective refrac- the cylinder The pupillary distance for near may frame is not level, the cylinder axis that tion, because of limitations of under- axis you be measured by instructing the patient you find may be wrong, and vertical standing or communication. The very find may be to look at the bridge of your nose. The prismatic effects may cause artefacts on young, those with Alzheimer’s disease wrong zero is lined up using the left eye and the binocular tests (Figure 1). or a learning disability may require that PD read using the right eye as before. ● Check that the pantoscopic angle of a prescription is arrived at purely from Quite why you would wish to do this the frame is sensible (Figure 2). If the the objective findings. is another matter. The actual amount of frame is wrongly tilted, high powered We should have realistic expectations inset required will vary with both the prescriptions may throw up significant of retinoscopy. It has been found that PD and working distance of the patient, errors in both sphere and cylinder. there is only a 50 per cent probability so unless you know these and place your ● Make sure that the back vertex that two consecutive measurements of nose in precisely the right position the distance is sensible. If the power of the sphere power would be within 0.40D. measurement is of dubious value. The sphere you find is over +/-4.00DS you They found that cylinder axis was the actual inset required is shown in Table should measure the BVD and note it on most repeatable, followed by cylinder 1 below. the final Rx. power, then sphere power. And, unless you are fully ambidextrous, you are likely to be better with one eye than TABLE 1 the other. Of course, the skill of the Trial frame inset for near vision for a range of PDs examiner will influence both accuracy and repeatability. Binocular inset for near vision at; PD 33cm 40cm Measuring pupillary distance It is important that we set up the trial 74mm 5.5mm 4.5mm frame accurately, because failure to do 70mm 5.0mm 4.5mm so may introduce significant artefacts 66mm 5.0mm 4.0mm into the result obtained. Use a frame 62mm 4.5mm 4.0mm rule and instruct the patient to look 58mm 4.5mm 3.5mm first at your left eye. Line up the zero 56mm 4.0mm 3.5mm cursor with the temporal limbus. The patient now looks at your right eye, 20 | Optician | 11.04.08 opticianonline.net CET Continuing education (a) (b) Figure 2 If the frame is wrongly tilted (a), high powered prescriptions may throw up significant errors Adding lenses to the trial frame visible, though complete darkness can ● Place spheres in the back cells of the stimulate accommodation. It might also trial frame. Where you are using more be difficult to find the trial lenses. than one sphere in the trial frame the most powerful should be at the back Position to minimise the vertex distance effects. You must try to work within 5° of the However, if you are using an Oculus visual axis, both horizontally and verti- or similar trial frame, which incorpo- cally. Adjust the chair height for verti- rates a built in vertex distance scale, cal alignment, allowing for the fact that the most powerful sphere should be the test chart may be above the patient, placed in the first of the rear cells (the so the patient may be looking slightly back cell one nearest the front). This is upwards. Errors of the order of -0.50DC the cell to which the scale is referenced x 90 occur if 10 degrees off horizontally. (Figure 3). Unless you have reduced vision in one ● When you change spheres, try to eye, use your RE to test the patients RE, ensure that the patient is never grossly Figure 3 The sphere in the first of the rear cells and LE for the patients LE. If this is under-plussed when you change lenses. impossible, the Barrett method should It is best to add the next plus lens before there is some evidence that the rings be employed. For horizontal alignment, you remove the current one. It can be on the green block of the duochrome get the patient to look at the green of the tricky with modern trial frames but it might be the one that produces least duochrome, get your head in the way becomes easier with practice. accommodation. In the absence of any and ask the patient to tell you when ● It is essential to make sure that all contradictory evidence the rings on they can just see the green panel. Ask lenses are thoroughly clean throughout the green would be the recommended the patient to tell you if your head gets refraction. Experience suggests this is fixation target for retinoscopy. in the way. often not the case. Light conditions Working distance A note on phoropters A darkened room will cause pupil You should work at a distance that allows Increasingly, phoropter heads are dilation and make the reflex more you to change the lenses in the trial used instead of trial frames. Modern automated lens carriages allow fast lens insertion, greater accuracy in axis location, more rapid comparison and presentation of lenses, use of variable prism, and a variety of further options depending on the model, such as immediate comparison of new refrac- tive findings with previous results. They are not advisable for a few situa- tions, notably low vision assessment and over-refraction of multifocal contact lenses, where the reduced light levels may affect visual performance or pupil size respectively (Figure 4). RETINOSCOPY Target Ideally, we want a target that will promote accurate and steady fixation but Figure 4 no stimulus to accommodation. Various A modern targets are used and they probably make phoropter little difference to the end result, but head opticianonline.net 11.04.08 | Optician | 21 Continuing education CET frame without changing body position, TABLE 2 and for most people this means that the Working distance lenses for retinoscopy working distance will be less than the 2/3m which seems to be the expected Working distance (cms) Working lens allowance (D) norm. Only the tall can reach if they 50 2.00 work at 2/3m and for many 1/2m is more realistic. It doesn’t matter what the 57 1.75 distance is provided that you know how 66 1.50 much to allow for your working lens 80 1.25 and the distance is maintained through- 100 1.00 out the test. Measure your customary working distance so that you know how much spherical power to allow for it. Make sure that you can return to it by TABLE 3 measuring with your arm. Usually the Acuity associated with mean sphere values base of the fingers or the wrist is used as a reference point, as this allows you Vision Equivalent sphere (myopia/manifest to change lenses without moving your hyperopia) body position. Check your working 6/5 PLANO distance when you have moved from 6/6 0.25 - 0.50 DS it (eg to change a lens). If your working distance allowance is wrong, errors in 6/9 0.50 - 0.75 DS the power of the sphere (and usually the 6/12 0.75 - 1.00 DS cylinder too) will result. For example, 6/18 1.00 - 1.25 DS if you are 100mm out at 2/3m the 6/24 1.25 - 1.75 DS sphere will be approximately 0.25D in 6/36 1.75 - 2.25 DS error.