It has been estimated that up to 10

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1 It has been estimated that up to 10 per cent of primary care patients attend an eye examination complaining of some form of binocular vision anomaly. With the ever-increasing use of computers in the workplace and home, it is essential that some form of basic binocular vision assessment is conducted on every patient seen in practice, particularly young children or those presenting with suspicious symptoms indicative of active pathology. Practitioners often consider binocular vision as a challenging part of general practice requiring sophisticated techniques and equipment only used during undergraduate training. In reality, the basic tests available are inexpensive, easily accessible and yield a significant amount of information in a short space of time. The average full-term newborn possesses all the basic elements required to achieve binocular single vision (BSV) in the form of two frontally located eyes that generate monocular images of similar size and clarity, and normal visual pathways to process and integrate this information. BSV, however, is not present at birth but develops in a visually stimulated process between three and five months of age. This refinement of innate processes and cortical connections continues in a process called synaptogenesis during a two-stage critical period for development primarily up to two years of age and to a lesser degree up to eight years at which 80 per cent of cortical neurons are expected to respond to binocular stimulation. Any anomaly during this developmental process that prevents the fovea receiving a clear image or causes the eye to suppress can result in amblyopia defined as the reduction in acuity in the absence of pathology. An eye turn properly termed strabismus but colloquially known as a squint is one of the major causes of amblyopia alongside significant refractive error. Surprisingly, it is held accountable for the loss of vision in more people under 45 years of age than all other ocular diseases and traumatic incidents combined, prevalent in an estimated 2-3 per cent of the general population. It is a largely avoidable cause of partial sight, highlighting the importance of early detection and management of any obstruction to the development of binocularity such as an anomaly of the extraocular muscles or their nerve supply. The first three tests of basic binocular function are the cover test, ocular motility 24 Optician Binocular vision Part 1 The cover test Priya Dabasia begins a new series looking at the basic skills used in practice to assess binocular vision status. She begins with the cover test. Module C14710, one general CET point for OOs and DOs (a) Figure 1 Visual axes in (a) heterotropia and (b) heterophoria (dotted line represents axis without cover in position) and convergence. The selection of any supplementary tests such as stereopsis, accommodative facility and tests for retinal correspondence are dependent on the knowledge of previous ocular history, refraction and the outcome of these preliminary investigations. These tests will be considered in a series of four articles discussing their indications for use, procedures and interpretation of results. This article will consider the cover test (CT) one of the simplest objective tests conducted during the course of an eye examination in which the examiner observes the behaviour of the eyes covered and uncovered in turn while fixated at a given distance. It yields precise information of eye alignment as well as the presence, amplitude, frequency and direction of an ocular deviation. The test can be undertaken grossly once a child can fixate from three months of age using an appropriately brightly coloured near target. A distance cover test can be performed from two years of age and a more detailed examination at near using an accommodative target from 3.5 years of age. Uses of the cover test The CT is broadly used to compare and interpret eye movements at near and distance and in different positions of gaze when used in conjunction with ocular motility assessment. Aside from (b) detecting the presence of a deviation it definitively distinguishes the two sub-types: Heterotropia (otherwise known as a manifest strabismus or squint ) where the visual axes do not coincide at the object of interest or at infinity for distance viewing (Figure 1a). It can be unilateral, alternate between the eyes or only present with tiredness or at a particular fixation distance. Heterotropia is most commonly primary in aetiology but can be described as secondary when it occurs as a result of pathology Heterophoria (otherwise known as a latent deviation) where the visual axes do not intersect at the object of intersection on dissociation in which the eyes are covered alternately to prevent sensory fusion (Figure 1b). The eye deviates under a cover but moves to take up fixation and regain BSV on removal of the occluder. The test is thereby conducted in two stages: Cover-uncover or unilateral CT determines the presence of any ocular deviation and distinguishes a heterophoria from a heterotropia. It detects the habitual angle of deviation through minimal dissociation of the eyes Alternating CT completely dissociates binocular fusion to elicit the maximum size of a deviation or increases the amplitude to allow small

2 Figure 2 The translucent Speilman occluder movements to be appreciated more easily. In general, the cover-uncover test is primarily used to evaluate heterotropia, while the alternate CT is used more in the detection of heterophoria or to investigate a latent element to a small angle heterotropia. Apparatus The CT requires minimal equipment comprising simply an occluder and a suitable fixation target (Figure 2). A standard black paddle occluder is the most widely used by practitioners although the translucent Speilman type (Figure 2) affords the practitioner the advantage of observing the approximate position of the eye behind the cover, while its frosted surface reduces any form perception. A measure of visual acuity is required for each eye to allow a suitable fixation target to be selected accordingly. Use of single letter optotypes without crowding bars should be avoided wherever possible as it can significantly underestimate amblyopia. The recommended fixation target at 26 Optician Table 1 A summary of the possible observations, diagnoses and recording of results of the cover/uncover test Observation of uncovered eye Diagnosis ecording to take up fixation No movement No tropia Orthotropic Divergent horizontal movement Convergent squint Esotropia/SOT Convergent horizontal Divergent squint Exotropia/XOT movement Upward vertical movement Vertical squint Hypotropia /HypoT Downward vertical movement Vertical squint Hypertropia /HyperT Both eyes diverge to take up fixation Both eyes converge to take up fixation Small angle movement of less than 10 dioptres far distance is a letter or equivalently sized picture on one line above that resolved by the eye with the poorest acuity. This serves to stimulate and stabilise accommodation while still ensuring accurate fixation with each eye. If the vision at far distance in one eye is sufficiently poor, some authors recommend the use of a spotlight target, while others prefer to direct the patient to a specific feature of a larger letter (eg the centre of the X letter on the 6/60 line). Near acuity using N scoring can be estimated by dividing the denominator of the distance measure in standard Snellen notation by three (for example 6/18 acuity in the poorest eye at distance equates to N6 at near) and re-checked with a reading chart for each eye in turn. Direct the patient to a single letter on the reduced Snellen chart of the Budgie stick accordingly and check that each eye can discern the target before commencing the CT. Figure 3 Practitioner and patient set-up Alternating convergent squint Alternating divergent squint Microtropia Alternating esotropia /Alt SOT Alternating exotropia /Alt XOT MicroT Procedure Once your patient is set up comfortably in the chair, observe for any abnormal head postures such as a marked face turn/tilt, any facial asymmetry or obvious eye turns. Adjust the room lights or supplementary lamps to ensure that the patient s face is adequately illuminated to make subtle deviations easier to detect. You should be seated facing the patient and slightly to their side to prevent obscuring the fixation target at distance (Figure 3). The test is usually conducted first without any spectacle correction. Step 1: Direct the patient to the appropriate fixation target and ask them to concentrate on the detail. For young children, it is often helpful to ask them to describe the colour or details of the image particularly at near to maximally stimulate accommodation. Step 2: Introduce the occluder in front of the eye with better vision for 2-3 seconds while observing the uncovered eye. Take care as subtle deviations can easily be missed with inadequate dissociation if the test is performed too quickly; some authors recommend a significantly longer occlusion of up to 10 seconds to reveal the full deviation. Any movement of the uncovered eye to take up fixation indicates a heterotropia. Note the speed of movement to take up fixation as it provides a measure of the level of vision in this uncovered eye as well as the direction of the movement (Table 1 shows a summary of how to interpret and record your observations of the cover/uncover test). NB Ensure the occluder is held accurately to fully cover the eye and prevent any peripheral fusion that can

3 otherwise elicit unexpected deviations. It can be held from below or above but take care in the latter that your arm does not obscure your view by casting a shadow over the patient s eyes. Step 3: emove the occluder in a quick, smooth motion. Some practitioners prefer to do this vertically to ensure that subtle deviations are detected as the majority of deviations tend to be horizontal in nature. Step 4: epeat steps 2 and 3 by occluding the eye with the poorer vision for 2-3 seconds while again observing the uncovered eye. Small vertical deviations are difficult to observe but more likely to be symptomatic. These can be detected more easily by observing the upper lids for any flick movements. Step 5: epeat the cover/uncover test described in steps 2 to 4 for each eye in turn while observing the eye as the cover is removed. Any movement to take up fixation indicates a heterophoria as the eye was deviated under cover in the absence of a visual stimulus. Step 6: epeat steps 2 to 5 at least twice more to determine the presence, frequency and nature of any deviation. Observe carefully for the direction and amplitude of deviations. If observations of the cover-uncover test revealed the absence of a manifest deviation, a subtle deviation difficult to discern or a small-angled tropia with a suspected latent component, proceed to the alternate CT described in steps 7 to 8. Step 7: Cover the right eye for three seconds before swapping the occluder across to the left eye in a swift, smooth movement. Hold the occluder over this left eye for another three seconds before swapping back across to the fellow eye. Some practitioners prefer to hold the occluder from above to avoid inadvertently hitting the patient s nose as it is swapped between the eyes. Step 8: epeat this cyclic occlusion at least three times to maximise dissociation and ensure the target is always viewed monocularly to reveal the full angle of deviation. Make note of the amplitude and speed of recovery to binocular vision as this is indicative of the strength of the fusional reserves and the likelihood of a patient becoming symptomatic. A heterophoria is a binocular function and is therefore usually the same direction and amplitude in each eye, but be aware that the speed of recovery can vary as a consequence of ocular dominance. Step 9: If the deviation is too small to detect visually, ask the patient to subjectively report any jump of the Figure 4 Image of CT performed with prism in front of eye fixation target on alternate occlusion. A positive response is known as a Phi movement in which an image shift with the direction of the occluder indicates a divergent deviation. Step 10: epeat steps 1 to 9 at near fixation using your dominant hand to occlude the eyes and the non-dominant hand to hold the budgie stick target at the appropriate distance. Take care to hold the target just inferior to the midline, as setting it too high or low can induce A or V alphabet patterns (to be discussed further in the next articles of this series). You can double check accurate fixation by observing the patient s eyes while moving the fixation stick horizontally from and back to the primary position. In practice the testing distance for near assessment is indicated by the patient s habitual reading distance. In a more extensive BV assessment, the most useful information is elicited when the CT is repeated at 33cm, 6m and far distance. Step 11: epeat steps 1 to 9 wearing any spectacle correction as required and again with the head in a straightened position if the patient naturally adopts an abnormal head posture. Measuring the size of a deviation With experience the size of a deviation can be estimated visually to the nearest 2 dioptres (Δ). Using the definition of a dioptre as the movement of the eye of 1cm at a distance of 1 metre, this visual evaluation can be practised using a cross comprised of two bisecting lines 20cm in length, drawn and marked up at 1cm intervals. The target is positioned at eye level to a willing volunteer at a distance of 1 metre. Ask them to change fixation from the centre point to eg 10cm to the right while you observe the 10Δ amplitude movement of their eyes. Alternatively, a guide of deviation size is provided by observing the 2Δ movement as a subject moves their eyes between one end of the 6/12 line on the standard Snellen chart to the other. Arguably, the most accurate method of measurement requires the use of single prisms or a prism bar to neutralise the movement on CT (Figure 4). However, it can only be used on cooperative patients, able to fixate a target accurately with either eye. If a deviation has both a horizontal and vertical element, it is advisable to begin with the largest component first as follows. Step 1: For heterotropia, the prism is held in front of the deviating eye, selecting the appropriate strength and base direction to the initial observations on CT. As a general rule the apex is positioned in the direction of the deviation (eg a convergent deviation requires a base out while a hyper deviation requires a base down prism). Step 2: Use the occluder to cover/ uncover the fixing eye, adjusting the prism power until the movement of the deviated eye is neutralised. This method is otherwise known as the simultaneous prism cover test (PCT) as it does not completely dissociate the eyes. Step 3: Continue to increase the prism power until the deviation is seen to reverse direction. Step 4: ecord the size of the habitual deviation as the power required to neutralise the movement prior to reversal. Step 5: Steps 1 to 5 can be repeated using the alternate CT to measure the total angle of deviation with full dissociation. Step 6: The test can be repeated at near and far distance as required. For near measurements the patient is asked to hold the fixation target to leave both hands free for the examiner to hold the occluder and prism bar accordingly. epeat steps 1 to 4 to measure the size of a heterophoria using an alternate CT in step 2 with the prism held in front of either eye. If the test is being conducted on a young child or individual unable to fixate accurately, an adaptation to the PCT called the prism reflection test is indicated in which prisms are used to adjust the positions of the corneal reflections until symmetrical in both eyes. How to interpret and record results To determine whether an anomaly is long-standing or of recent onset, it is imperative that detailed notes are recorded at each patient visit. Horizontal deviations are recorded as eso or exo, 28 Optician

4 Figure 5 Observations of the Cover/ Uncover test in exotropia Figure 6 Observations of the alternate CT in esophoria with a / hyperphoria depending on whether the eye is convergent or divergent respectively. Vertical deviations are recorded as the higher eye described as a right or left hyper deviation accordingly. If the left eye moves down to take up fixation on covering the fellow eye, you have observed a left over right hypertropia recorded clinically as a / hypert. If a heterotropia is observed, your recordings should include details of the following: The eye affected while it is usually one or the other, movement of both eyes to take up fixation can be observed in an alternating heterotropia. In such cases, it is advisable to record the eye that the patient prefers to fixate with as it provides a strong indication of ocular dominance The type of deviation a tropia usually occurs alone, but in small-angle presentations it can occur with an associated latent element as indicated by a larger movement on alternate CT compared to the initial angle The direction of the deviation it can be purely horizontal, vertical, torsional (cyclo) or a combination (eg an esotropia with a hypertropia) The angle of deviation graded as minimal (less than 10Δ), moderate (25-35Δ) or marked (greater than 40Δ) The speed to take up fixation a rapid movement indicates a moderate to good level of vision while a wandering eye which is slow to move indicates poor vision with non-central fixation The frequency of the deviation it can be constant or intermittent in presentation with fatigue or fixation distance. An example of a right exotropia is shown diagrammatically in Figure 5 30 Optician with the likely observations of the coveruncover test at 6m. When it comes to recording your results, remember that abbreviations are used widely in general practice to ease the time constraints in the high street and reduce the length of recordings per patient episode. In this example a typical recording would be Dist CT 25Δ XOT. If a heterophoria is observed, your notes should specify the direction, angle and frequency of deviation as detailed above in heterotropia recordings as well as the following: The speed of recovery to binocular fixation a smooth and rapid motion indicates strong fusional reserves and good control of the deviation, while a slow and jerky movement warns the examiner of imminent symptoms if not present already Presence of Hering s movement occurs in large-angle phorias in which the uncovered as well as the covered eye exhibit a versional movement of half the amplitude of the total deviation on removal of the cover. arge-angle heterophorias can be intermittent in nature as they can decompensate to a heterotropia with increasing fatigue and dissociation. An example of a near esophoria with a / hyperphoria is shown in Figure 6 with the likely observations of the alternate CT. A suggested method of recording these observations is: Near CT 10 Δ SOP c 8 Δ / HyperP (moderate recovery). Once you have recorded your observations, the next step is to consider the primary aetiology and further tests that may be required to confirm your diagnosis. You may find the following guidelines useful: A deviation that varies is size between fixation distances such as an esotropia greater at near suggests the influence of an accommodative element A deviation that varies with direction of gaze is indicative of an incomitant element (to be considered further in the next article of this series) An alternating tropia indicates the absence of any binocular function A vertical deviation is usually the result of an incomitancy or anomaly of the extraocular muscles, particularly when in conjunction with an abnormal head posture. Despite all its advantages, the CT is not suitable for use on all patients; for this reason, variations have been developed particularly for young patients or those incapable of cooperating for any length of time. Hirchberg s method uses the position of the corneal reflections from a pen torch directed at a distance of 33cm to estimate the size and direction of a heterotropia, using the knowledge that 1mm displacement equates to a movement of 20Δ. This is developed further in Krimsky s method which advocates the use of prisms to make the reflexes symmetrical between the eyes, placing the prism in front of the fixing eye unlike in the PT. It is important to note that while there are many subjective based tests available such as the Maddox od and Maddox Wing, their uses are limited by the very nature of the reliance on patient s responses, allowing the CT to continue to reign as the most accurate assessor for ocular deviation. Priya Dabasia is clinical optometrist at the Fight for Sight Optometry Clinic, City University and Moorfields Eye Hospital

5 Multiple-choice questions take part at Which of the following does NOT describe a binocular vision 1 anomaly where the visual axes deviate at the object of interest? A Heterotropia B atent deviation C Squint D Manifest deviation Which of the following statements is TUE? 2 A The alternate CT minimally dissociates binocular fusion B The cover-uncover CT is primarily used to elicit the maximum size of a deviation C The alternate CT can be used to investigate the manifest element of a small-angle heterotropia D The alternate CT is predominantly used to evaluate latent deviations You observe a nasal movement of the left eye as the fellow eye 3 is covered. Which of the following is the most likely diagnosis? A SOT B XOT C XOP D XOP You wish to conduct a prism cover test to measure the size 4 of a moderate hypertropia with large esotropia. In which directions would you orientate the prism bar before this deviating eye? A Base out and down B Base up and out C Base down and in D Base in and up Which of the following statements is FASE in consideration of 5 heterophoria? A A large angle heterophoria can break down to a heterotropia B A slow recovery indicates moderate control of the deviation C A vertical deviation is most likely the result of an accommodative anomaly D Deviations are usually equal in each eye In the measurement of ocular deviation, which of the following 6 is COECT? A Hirchberg s method uses prisms to correct the position of the corneal reflexes B Krimsky s method is based on visual estimation of the corneal reflections C PT requires the prism to be placed in front of the fixing eye D PT requires the prism to be placed in front of the non-fixing eye Successful participation in this module counts as one credit towards the GOC CET scheme administered by Vantage and one towards the Association of Optometrists Ireland s scheme. The deadline for responses is October Greener glazing A new filtration unit makes waste removal more efficient and significantly reduces water use Water is an increasingly valuable resource. This means that charges for water are on the increase and there are ever more rigorous rules regarding waste disposal. Glazing lenses is a very water costly process and the waste produced re-enters the water cycle and can prove costly to remove. Dramatically decreasing water use, the Nidek Fu-220 lens filtration system also ensures that waste created during lens production is collected as a solid block and is easily disposed of, so meeting new stringent waste regulations. This is achieved by a centrifugal action which means that any resultant liquid is contaminant free and operators never have to come in contact with any slurry. The unit has now been adopted by Vision Express throughout its labs. Paul Mothershaw, head of Vision Express Ophthalmic ens Production, said: We are committed 32 Optician uke Wyndham and Paul Mothershaw show off the new unit to enhancing our green credentials and our partnership with Nidek is helping us to achieve this. Hopefully other opticians will follow our lead by installing this innovative new equipment as we all push to ensure the optical sector is as green as possible. Considerable cost saving advantages can be achieved with the new filtration system, as uke Wyndham, divisional manager Nidek ens Edging, explained: A busy practice can save up to 1,400 on its water bill a year. The Nidek Fu-220 uses only litres of water for processing a lens, which is less than 1 per cent of that used by a conventional mains water supply system, he said. The Fu-220 automatically counts the number of lenses processed and an ED light and buzzer indicates when the unit needs emptying, making it simple and very easy to use, he added. eaders interested in the Fu-220 can contact Birmingham Optical Group on