Continuing education CET

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

24 | Optician | 24.09.10 opticianonline.net Continuing education CET

Table 1 A summary of the possible observations, diagnoses and recording of results of the cover/uncover test Observation of uncovered eye Diagnosis Recording 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 Alternating convergent Alternating esotropia fixation squint /Alt SOT Both eyes converge to take up Alternating divergent Alternating exotropia Figure 2 The translucent Speilman occluder fixation squint /Alt XOT Small angle movement of less Microtropia MicroT movements to be appreciated more than 10 dioptres easily. In general, the cover-uncover test is primarily used to evaluate heterotropia, while the alternate CT is used more far distance is a letter or equivalently Procedure in the detection of heterophoria or to sized picture on one line above that Once your patient is set up comfortably investigate a latent element to a small resolved by the eye with the poorest in the chair, observe for any abnormal angle heterotropia. acuity. This serves to stimulate and head postures such as a marked face stabilise accommodation while still turn/tilt, any facial asymmetry or Apparatus ensuring accurate fixation with each obvious eye turns. Adjust the room The CT requires minimal equipment eye. If the vision at far distance in one lights or supplementary lamps to ensure comprising simply an occluder and a eye is sufficiently poor, some authors that the patient’s face is adequately suitable fixation target (Figure 2). recommend the use of a spotlight illuminated to make subtle deviations A standard black ‘paddle’ occluder is target, while others prefer to direct the easier to detect. You should be seated the most widely used by practitioners patient to a specific feature of a larger facing the patient and slightly to their although the translucent ‘Speilman’ letter (eg the centre of the ‘X’ letter on side to prevent obscuring the fixation type (Figure 2) affords the practitioner the 6/60 line). target at distance (Figure 3). The test the advantage of observing the Near acuity using ‘N’ scoring can be is usually conducted first without any approximate position of the eye behind estimated by dividing the denominator spectacle correction. the cover, while its frosted surface of the distance measure in standard ● Step 1: Direct the patient to the reduces any form perception. Snellen notation by three (for example appropriate fixation target and ask them A measure of visual acuity is required 6/18 acuity in the poorest eye at to concentrate on the detail. For young for each eye to allow a suitable fixation distance equates to N6 at near) and children, it is often helpful to ask them target to be selected accordingly. Use re-checked with a reading chart for to describe the colour or details of the of single letter optotypes without each eye in turn. Direct the patient to image particularly at near to maximally crowding bars should be avoided a single letter on the reduced Snellen stimulate accommodation. wherever possible as it can significantly chart of the ‘Budgie stick’ accordingly ● Step 2: Introduce the occluder in underestimate amblyopia. and check that each eye can discern the front of the eye with better vision The recommended fixation target at target before commencing the CT. 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 Figure 3 cover/uncover test). Practitioner NB Ensure the occluder is held and patient accurately to fully cover the eye and set-up prevent any peripheral fusion that can

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

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R L R L

R L R L

R L R L

R L R L

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

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Multiple-choice questions – take part at opticianonline.net

Which of the following does NOT describe a binocular vision You wish to conduct a prism cover test to measure the size 1anomaly where the visual axes deviate at the object of 4of a moderate R hypertropia with large esotropia. In which interest? directions would you orientate the prism bar before this deviating A Heterotropia eye? B Latent deviation A Base out and down C Squint B Base up and out D Manifest deviation C Base down and in D Base in and up Which of the following statements is TRUE? 2 Which of the following statements is FALSE in consideration of A The alternate CT minimally dissociates binocular fusion 5heterophoria? B The cover-uncover CT is primarily used to elicit the maximum size of A A large angle heterophoria can break down to a heterotropia a deviation B A slow recovery indicates moderate control of the deviation C The alternate CT can be used to investigate the manifest element of C A vertical deviation is most likely the result of an accommodative a small-angle heterotropia anomaly D The alternate CT is predominantly used to evaluate latent deviations D Deviations are usually equal in each eye

You observe a nasal movement of the left eye as the fellow eye In the measurement of ocular deviation, which of the following 3is covered. Which of the following is the most likely diagnosis? 6is CORRECT? A R SOT A Hirchberg’s method uses prisms to correct the position of the corneal B L XOT reflexes C L XOP B Krimsky’s method is based on visual estimation of the corneal D R XOP reflections C PRT requires the prism to be placed in front of the fixing eye D PRT 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 21 2010 Greener glazing A new filtration unit makes waste removal more efficient and significantly reduces water use

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