Clinical

Assessing babies and pre-school children for Dr Simon Barnard, Ellis Johnson and Alex Levit present an overview of strabismus or heterotropia, also known in the UK as a squint. It is the condition in which only one of the visual axes is directed towards the fixation object

his article will discuss Table 1 some of the methods that Vision assessment methods recommended for use based on age optometrists use to detect the presence of strabismus Age Vision test during case finding 6-12 months Forced choice preferential looking examinations or screening. (Keeler gratings) ManagementT protocols for strabismus 12-18 months Cardiff acuity cards (preferential looking) will not be discussed but, needless to say, within the profession there is a 18-24 months Kay pictures range of skill sets. Some optometrists 2-3 years Keeler crowded cards might refer all strabismus patients for ophthalmological management. Others, Crowded Kay pictures who have the clinical knowledge 3+ years LogMAR tests and experience to treat those types of strabismus that are likely to respond to the therapeutic interventions available to optometrists, will manage the patients themselves by prescribing for refractive errors, and carrying out occlusion therapy and orthoptic exercises on suitable patients. Why is strabismus important? Strabismus is a common condition with a prevalence of 3.9 per cent in pre-school children and is a risk factor for amblyopia.1 Amblyopia is defined as a visual loss resulting from an impediment or disturbance to the normal development of vision.2 Amblyopia is generally known by lay persons as ‘lazy eye’, Figure 1 Infant with no manifest strabismus, symmetrical corneal reflexes and positive angle a term which can belie the depth lambda in each eye of visual loss. The visual loss is not regarding the strabismus characteristics associated with any recognisable present, it is unlikely to be noticed (which eye, deviation direction, pathological cause but can be attributed either by the parent or a non-eye care constant or intermittent). Crucially to an amblyogenic (amblyopia-causing) professional. Optometrists have an here, parents may fail to offer up factor, most commonly strabismus and/ important role in detecting strabismus. information regarding their child, as or anisometropia.3 Conventional wisdom is that early they may not feel they are relevant, The prevalence of amblyopia is 3-4 detection of strabismus is important so detailed and exact questioning per cent.4,5,6 In the population under as early treatment and management is is vital from the practitioner. For the age of 20 years, amblyopia is 10 more likely to lead to a more successful example, any birth problems, forceps times more common a cause of visual outcome. or prematurity. Equally important loss than all others taken together, is knowledge of family history of whether caused by trauma or disease.7 Investigation of babies and strabismus or amblyopia. Throughout It is difficult to define the respective pre-school children the history, the clinician must be prevalence of amblyopia secondary As with any thorough , attentive to patient information and to strabismus and to anisometropia it is essential that a detailed history be aware that presenting signs may as these two conditions often occur is taken and documented. Important not always be manifest to the parent. together. components include the age of onset It is not uncommon for children to Unless a large angle squint is of any signs or symptoms, details present with the sensory adaptation of

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suppression, which is not apparent to Figure 2 Schematic the parent and only revealed through representation of the optometric examination. components of angle lambda.10 AO is the Vision assessment distance from the front Ideally, both vision and visual acuity surface of the to the should be assessed monocularly. This Pupillary axis A o entrance , measured is far from always being possible. λ along the pupillary axis; the x For those children who are very pupillary axis is the line uncooperative, an attempt to obtain perpendicular to the cornea binocular vision and visual acuity which passes through the should be made. For babies and infants, Line of sight centre of the entrance a meaningful assessment of vision pupil; the distance along may sometimes be achieved, but the the line of sight from the clinician will invariably rely on results cornea to the entrance of tests of eye position and refraction to pupil is XO; angle AOX is determine risk factors. angle lambda This article is not intended to review methods of assessing vision but some image. Cycloplegic drugs should not be corneal reflex of most fixating eyes is examples of appropriate methods of instilled until other tests including that not normally positioned in the centre visual assessment should be chosen of eye position have been first carried of the pupil. Generally, the fovea is according to patients’ age and reading out. temporal to the posterior pole, so that ability (Table 1). The importance of the reflex is nasal to the corneal apex, the use of crowded test types is stressed Refraction producing a positive angle lambda. here, as these are more sensitive It is not the remit of this article to Angle lambda is the angle subtended at to amblyopia detection due to the discuss refraction methods in young the centre of the entrance pupil of the crowding phenomenon. children. However, the importance of eye by the intersection of the pupillary retinoscopy under cycloplegia should axis and the line of sight. Figure 1 Ocular examination not be underestimated. shows an infant with no manifest The assessment of pupillary reflexes strabismus and a positive angle lambda ocular adnexa (including eyelid Eye alignment in each eye. position) and examination of the An early test for checking eye position Although routinely measured ocular media and fundi is vital and and alignment was initially described clinically, the angle is commonly is especially important if strabismus to by Hirschberg in 1881 with further wrongly designated as angle kappa. is present. All patients diagnosed case studies published in 1885.8,9 Angle kappa is actually the angle with strabismus should undergo an between the visual and pupillary axes examination of the ocular media and Hirschberg test measured at the nodal point, a point fundi under the mydriasis induced by The corneal reflexes (first Purkinje not clinically accessible.10 It is angle the cycloplegic drug used for refraction. image) are observed from a 50cm lambda (Figure 2) that is measured Indirect ophthalmoscopy will provide working distance using a centrally clinically.11 the adequate field of view required. located pen torch. For a subject whose If a deviation is present, corneal The author has found the Optomap visual axes are aligned, no asymmetry reflexes may no longer be viewed as laser scanning ophthalmoscope to be will be apparent. being symmetrically placed with regard invaluable for capturing a wide field It should be understood that the to the pupil centre. Hirschberg found

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opticianonline.net 27.09.13 | Optician | 31 Clinical

that each 1mm of decentration of the corneal reflection corresponded to 7° of deviation of the visual axis. Thus if the corneal reflection in the deviated eye is found to be at the limbus (equal to a displacement of approximately 6mm), the deviation is estimated at about 45°.12 Various subsequent studies re-evaluated the conversion factor and a 1mm displacement of corneal reflex is generally accepted as translating to about 22Δ deviation.13,14 Figure 3 shows a right of an Figure 3 Child with an estimated 20Δ right esotropia estimated 20Δ amplitude. Note that the left corneal reflex is slightly nasally further distraction. Covering the eye it is likely that only a near cover test displaced compared to that of the right with the thumb while resting the will be possible. An initial alternating eye. fingers on the head is sometimes more cover test is performed to estimate It should be noted that angle lambda successful. However, when a baby is angle and direction of deviation. The is usually large in infants, giving the uncooperative the clinician may be direction of the prism base employed appearance of but is often forced to rely upon the Hirschberg test is dependent upon the direction of counteracted by epicanthal folds seen at despite its crudeness. deviation with base out interposed this age.15 In the presence of strabismus, on for eso deviations, base in for exo In very young children, covering the fixing eye, the deviated deviations, base down right for right uncooperative patients or for those eye should, if it has some degree of hyper and base down left for left hyper. with a markedly reduced visual vision, move to uptake fixation. When Using a prism bar, the strength of the acuity, this is often the only method the cover is removed, the behaviour prism is gradually increased during of assessing the angle of deviation. of the eye underneath the cover can alternating cover test until all eye Due to the gross nature of this test, it be examined. If the strabismus is movements are neutralised. To confirm is only suitable for moderate to large alternating, the patient will be able to that this is indeed the end point, the manifest deviations. Refinement using sustain fixation through the previously power of the prism is increased to the Krimsky technique,16 requires deviated eye through to the next blink. check the eye movement is reversed. a prism to be place over the fixing When fixation cannot be maintained When there is both a horizontal and eye of increasing strength until the and fixation reverts to original position, vertical component to the deviation, corneal reflexes are symmetrical. this highlights a fixation preference for the largest deviation should be Base out prism should be used for an the undeviated eye. The clinician will measured first and a second prism bar eso deviation whereas base in prism then record which eye manifests the used over the same eye to measure should be used for an exo deviation. deviation or whether the strabismus the remaining deviation. Needless to It has been argued that these methods alternates, the direction of deviation, say, when a patient is not cooperative of deviation angle evaluation are and an estimation of the amplitude (in for the cover-uncover or the alternate much less accurate than a prism cover prism dioptres or degrees). cover test, it is unlikely that the test, even when conducted by an Ideally a cover test with and without prism cover test can be meaningfully experienced clinician.17 refractive correction is advised to assess employed. It should be noted that a the effect of refractive correction on Another difficulty with this test disadvantage of using a pen torch to angle of deviation. occurs with infants because the produce the corneal reflex is that such The inter-observer repeatability practitioner will have to invite a gross target is unlikely to induce of the cover test in childhood assistance from the parent or assistant accommodation effectively. esotropia has been investigated and to hold a fixation target while the caution should be advised when clinician holds the cover with one hand Cover test monitoring change with this method. and the prism bar with the other. The cover test is the most commonly In children with esotropia with performed and arguably the most angles >20Δ, differences of 12Δ or Motor fusion important test relating to binocular more and for smaller strabismus vision and enables the clinician to angles between 10-20Δ, 6Δ changes Prism reflex test objectively detect the presence of are likely to be genuine. Anything For infants, motor fusion can be heterophoria or strabismus and to less than these differences, even for demonstrated by interposing a large measure the size of deviation.18,19 This experienced clinicians, are likely due to aperture prism in front of one eye is a vital component of any optometric measurement error although smaller and observing if a fusional movement examination. The patient should be change in angles can be indicative of occurs to restore binocular vision. A 5Δ directed towards a suitable fixation real change.20 or 10Δ base out is normally overcome target at 6m for distance and similarly by the cooperative infant of six for near. The target chosen should Prism cover test months.15 be selected to stimulate fixation and The prism cover is an objective way In young children this test is a accommodation. to measure the angle of the deviation, convenient objective method for use to The cover test on infants is not easy. either in the horizontal or vertical assess if there is a presence of binocular Keeping the infant’s attention on a plane. The patient should be directed to single vision and if motor fusion is distance target is rarely successful and the appropriate distance target at 6m or present. An accommodative near target the introduction of the cover causes 33cm for near. For very young children is chosen and a 20Δ base out prism is

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placed before each eye separately. The strabismus amplitude has been shown Blackwell Science, Oxford. prism shifts the image of the object to be poor 8 Hirschberg J. The Quantitative Analysis on the fovea so the eye must move to ● Prism fusion tests can be useful but of Diplopic Strabismus. Br Med J, Jan 1 maintain fixation. By definition of co-operation from the infant or young 1881;1(1044):5-9. Hering’s law, the other eye will move, child is required and a meaningful 9 Hirschberg J. Über die Messung abducting then adducting to maintain result may not always be attainable des Schielgrades und die Dosierung the image of the fovea of this eye. The ● The Hirschberg test is probably the des Schieloperation. Zentralbl Prakt converse of this sequence should be easiest test to employ but it is very Augenkeilkd. 1885; 9: 325. seen when the prism is removed. If crude and even if the practitioner 10 London R, Wick B. Changes in Angle these movements are not observed, this can determine accurately a 1mm Lambda during Growth: Theory and Clinical may indicate a lack of binocular single asymmetry this would detect a Apllications, Am J Optom Physiol Opt, 1982; vision and/or abnormal motor fusion. strabismus of over about 20Δ. Smaller 59, 7, 568-572. As with the cover test, the prism amplitudes of strabismus will not 11 Millidot M. Dictionary of Optometry and reflex test can be very difficult to carry be detected. It is likely that the Visual Science, 2009; Seventh Edition, out meaningfully in anything but a conventional Hirschberg test is not Butterworth Heinemann, Edinburgh. cooperative baby or young child. In very specific or sensitive for strabismus. 12 von Noorden GK, Campos EC. Binocular these circumstances the practitioner vision and ocular motility: Theory and may have to rely on the Hirschberg test. Recent developments Management of Strabismus, 2002; 6th Ed. A recent innovation is the IRISS Eye St. Louis: Mosby. Motility Check, a hand held device developed 13 Jones R, Eskridge JB. The Hirschberg test: To determine whether a strabismus by IRISS Medical Technologies which a re-evaluation. Am J Optom Arch Am Acad is concomitant or incomitant an enables a fully automated Hirschberg Optom, 1970; 47: 105-114. assessment of ocular motility should test to be carried out and the results 14 Brodie SE. Photographic calibration of be carried out to test the functions of displayed in real time within a few the Hirschberg test, Invest Ophthalmol Vis the extraocular muscles. The amplitude seconds. Initial results show a sensitivity Sci, 1987; 28, 736-742. of a concomitant strabismus will be of 95 per cent and specificity of 92 per 15 Jennings A (1996), Investigation of unchanged in different directions of cent for strabismus.22 Early indications Binocular Vision, Chapter 8 in Pediatric Eye gaze. suggest that it is able to detect Care. Eds Barnard, S. & Edgar, D. Oxford: differences as small as 0.04mm both in Blackwell Science Stereopsis terms of corneal reflex asymmetry and 16 Krimsky E (1943). The binocular The presence and quality of binocular pupil size. A further article discussing examination of the young child. Am J single vision can be established by use IRISS Medical Technologies Eye Check Ophthalmol. 26: 624. of stereotests. In the consulting room will be published in Optician next 17 Choi R.Y. Kushner B.J. (1998). The the Lang stereo test can be used with month ● accuracy of experienced strabismologists occasional success to show the presence using the Hirschberg and Krimsky tests. of stereopsis in infants from the age of References Ophthalmol, 105:1301-1306 about six months.21 1 The Vision in Preschoolers Study Group. Does 18 Barnard NAS & Thomson (1995) A Global stereotests use random dots assessing eye alignment along with refractive quantitative analysis of eye movements whereas local stereotests make use error or visual acuity increase sensitivity for during the cover test – a preliminary report. of contours. An example of a global detection of strabismus in preschool vision Ophthal Physiol Opt 15, 5, 413-419 stereotest for use on slightly older screening? Invest Ophthalmol Vis Sci, Jul 19 Franklin A (1997) The Cover Test. CE children is the TNO random dot test 2007;48(7):3115-3125. Optometry, 1, 1, 8-9 which depends on complex patterns 2 Evans BJW. Pickwell’s Binocular Vision 20 PEDIG (Pediatric eye disease investigator of displaced random dots to create Anomalies, 2007; 5th edition. Elsevier, group). Interobserver reliability of the prism disparity and therefore depth. The Oxford. and alternative cover test in children with Titmus fly is a local stereotest which 3 Evans BJW, Yu CS, Massa E, Mathews esotropia. Arch Ophthalmol, 2009; 127: uses displaced contoured targets to JE (2011) Randomised controlled trial of 59-65. create disparity and an impression of intermittent are sleeping go to sleep are 21 Stidwill D. Orthoptic Assessment and depth. As local stereotests are prone photic stimulation for treating amblyopia in Management, 1990; Blackwell Scientific to monocular clues, global stereotest older children and adults, Ophthal Physiol Publications, Oxford are considered more accurate as these Opt, 31, 56-68. 22 Dahlmann-Noor AH, Adams G, Maor R, are truly binocular and therefore can 4 Thompson JR, Woodruff G, Hiscox FA, Barnard S, Yashiv Y, Barnard S. Real-time identify strabismus and amblyopia. Strong N, Minshull C. The incidence and automatic strabismus screening using prevalence of amblyopia detected in digital image analysis techniques, Journal Conclusions childhood. Public Health, 1991; 105, 455- of AAPOS, 2013; Volume 17, Issue 1, Page Strabismus is a common condition and 462. e13, February. is a major risk factor for amblyopia. 5 Attebo K, Mitchell P, Cumming R, Smith Currently there are three commonly W, Jolly N, Sparkes R. Prevalence and ● Dr Simon Barnard works in private employed tests used to assess eye causes of amblyopia in an adult population. practice in north London and is associate alignment and diagnose the presence , 1998; 105: 455-462. professor, Department of Optometry & Visual of strabismus. All of these tests require 6 Williams C, Northstone K, Howard M, Science, Hadassah College, Jerusalem, Israel. considerable clinical skills. Harvey I, Harrad RA, Sparrow JM. Prevalence His PhD research was on eye movements ● The cover test is the most important and risk factors for common vision problems during the cover test. He is a director & chief as it enables diagnosis of the type of in children: data from the ALSPAC study. Br J medical officer of IRISS Medical Technologies. strabismus and its amplitude. However, Ophthalmology, 2008; 92: 959-964. Ellis Johnson is clinical research director even in skilled hands inter-practitioner 7 Grounds A. Amblyopia: in Pediatric Eye of IRISS Medical Technologies. Alex Levit reliability with regards to accuracy of Care; Eds: Simon Barnard & David Edgar, practises in north London

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