CET Continuing education Routine eye examination Part 3 – Binocular assessment In the third part of our series on the eye examination, Andrew Franklin and Bill Harvey look at the assessment and interpretation of binocular status. Module C8290, one general CET point, suitable for optometrists and DOs he assessment of binocular previous question. Leading questions function is often one of the should be avoided, especially when weaker areas of a routine, dealing with children. If they are out if observation of candidates of line ask the patient ‘Which one is out in the professional qualifica- of line with the X?’ Ttions examination is any guide. Tests are You should know before you start the done for no clearly logical reason, often test which line is seen by which eye. because they always have been, and in If you cannot remember it from last an order which defeats the object of the time, simply look at the target through testing. Binocular vision seems to be one the visor yourself (Figure 1). Even if of those areas that practitioners shy away you think you can remember, check from, and students often take an instant anyway, as it is possible for the polarisa- dislike to. Many retests and subsequent tion of the visor not to match that of the remakes of spectacles are the result of a Mallett Unit at distance or near or both, practitioner overlooking the effects of a Figure 1 A distance fixation disparity target especially if the visor is a replacement. change of prescription on the binocular If both eyes can see both bars, nobody status of the patient. before the assessment of fixation dispar- will have a fixation disparity. Under no It would be useful to define what it ity. This is particularly important if the circumstances cover one of the patient’s is we are trying to determine: patient has been dissociated recently, as eyes and ask ‘Which line can you see ● Does the patient have a squint or a they always will be during an examina- now?’ This is dissociation. phoria? tion. Taking monocular acuities, retin- Following this, ask ‘Is it to the left ● If they have a phoria, is it oscopy, ophthalmoscopy and so on will or to the right?’ or ‘Is it towards me compensated? all disrupt the binocular status. It should or away from me?’ With children, the ● If they have a binocular problem, is it be remembered that the polarised visor latter question may be more sensible, going to need referral or management? used significantly reduces light levels as they tend to confuse their lefts and Tests of motor function can be and illumination should be adjusted rights on occasion. divided into ‘binocular’ tests which accordingly. The minimum prism (to the nearest maintain fusion (for example, fixation Before applying the visor, the patient ½∆) or alteration in the spherical disparity) and those which dissociate should be asked: ‘Are the two red (green) element of the Rx which aligns the the eyes (such as cover test and the lines exactly in line with each other or polarised lines should be noted. Maddox rod). As a general principle, are they out of line?’ It is surprising how When assessing fixation dispar- binocular tests should always precede often a patient will report displacement ity for near, don’t be restricted to one dissociation tests. In practice, this is of the lines even without the visor. Are target position. Many patients read or often reversed and frequently patients they trustworthy observers? Do you look at computer screens in a variety of are tested for fixation disparity having want to prescribe on their subjective positions. Many patients have ‘A’ and ‘V’ been thoroughly dissociated before, and response? patterns and it doesn’t take long to check in some cases, during the test. The odd Having established that the patient is if the patient is still compensated when false positive might be expected in these not likely to lead you astray, repeat the the eyes are elevated or depressed. cases. Until you have done a cover test, you won’t necessarily know what the Fixation disparity PRODUCT LIST fixation disparity findings mean. No slip Method These are some of the UK suppliers of found on a patient with a phoria would The fixation disparity test is often instruments needed for binocular assessment indicate that the phoria is compensated, performed in such a way as to render at least for the moment, but the same it worthless. Even the wording of the ● Haag Streit (Clement Clarke) finding on a strabismic subject would instructions and questions to the patient ● Birmingham Optical Group (Nidek) indicate harmonious anomalous retinal can affect the chances of finding a ● Buchmann UK correspondence. Small degrees of slip fixation disparity, particularly in ● Grafton Optical usually point towards an uncompen- children and the more literal minded ● Institute of Optometry sated phoria, especially if there are adult. This test should ideally precede ● Keeler symptoms, but a larger slip may be any dissociation test. ● Norville associated with microtropia. Binocular vision should be stabilised, ● Sussex Vision Sometimes the slip seems to vary at by reading a line of letters or words random, and the markers may oscillate opticianonline.net 18.01.08 | Optician | 23 Continuing education CET across the neutral point. This is indica- tive of binocular instability. The condi- tion is associated with uncompensated heterophoria and the fusional reserves tend to be low. In general, if you keep adding small amounts of prism or sphere the slip will stabilise. Cover test The cover test is essential, being the only way to tell squints and phorias apart. It can also be used to estimate or measure the direction and size of the deviation and to indicate whether a phoria is compensated or not. Figure 2a Looking behind an opaque Figure 2b Transparent occluders help visualise eye There is a certain amount of confu- occulder is useful movements behind the cover sion about the cover test, partly arising because it tends to be taught in a module 6/18, a spotlight may be used. Note that Multiple positions of fixation may be labelled ‘binocular vision’ and often this is a non-accommodative target. The useful to investigate incomitancy. based on orthoptic practice rather than reason for using it is that the angular When examining myopic patients the needs of a routine eye examination. subtense of letters larger than 6/18 is without their spectacles it should be While there is considerable overlap, the large enough to significantly distort remembered that unaided distance two have different goals and require- the results, particularly when looking vision below 6/60 does not mean that ments. In a routine eye examination at small vertical elements. However, a non-accommodative target should be we are screening a normal population many practitioners find there is still used at near. Many myopes habitually for those individuals whose motor or more eso-deviation when a letter target read without spectacles at a non-stand- sensory status requires intervention. In is used. If in doubt, use both. ard distance, and most will be able to the hospital eye service, there is a greater The position of the target is impor- fixate an accommodative target for the need to classify and quantify so that the tant, particularly at near. The cover test near cover test. This applies to myopic effects of treatment may be recorded. should be performed with fixation at a presbyopes too. As this is an article about routine eye distance and position which is relevant examination, the approach will be to the patient and their habitual visual Occluder biased towards that. environment. There is occasionally a The occluder may be opaque or translu- tendency to perform the near cover test cent. Opaque occluders should be wide Target with the eyes level, in the same position enough to allow them to be angled so as The target should be accommodative that they were in when fixating a to allow observation of the eye under the (small enough for the hypermetrope distant target. This is fine if you want cover, while maintaining proper disso- to employ enough accommodation to to compare distance and near deviations ciation (Figure 2a). The latter allows us overcome their ametropia and hence while removing any variability that an to observe eye movements behind the reveal any eso-movement), so use a A or V pattern would induce. In other cover (Figure 2b) and is particularly letter slightly larger than the acuity of words, if you are trying to classify the helpful when trying to see dissociated the worst eye. If the target is too small, deviation accurately. However, we are vertical deviations. the maintenance of accurate fixation generally trying to find out what is The use of fingers, thumbs, hands is difficult. The patient may narrow happening while the patient goes about and so on should be avoided as adequate the palpebral fissure to see better, their daily business. dissociation may not be achieved. A making observation of the eyes diffi- When performing the near cover translucent or white occluder is an asset, cult. Occasionally ciliary spasm might test, it is a good idea to quickly check causing fewer pupil reactions which be triggered. if there are any significant variations can be distracting when looking at small If the acuity of the worst eye is below with elevation and depression of gaze. deviations. THE MOST COMFORTABLE DAILIES® CONTACT LENS EVER New © CIBA Vision (UK) Ltd, a Novartis Company, 2007 (UK) Ltd, a Novartis Company, © CIBA Vision The secret’s in the blink 24 |HFK018_QuarterPageAd.indd Optician | 18.01.08 1 opticianonline.net9/11/07 14:40:00 Continuing education CET correction that the patient is currently wearing is allowing the patient comfort- able vision, so checking both fixation disparity and cover test at any fixation distance relevant to the patient (includ- ing computer distance in many cases) would seem a good idea.