Detecting Strabismus by Dr Nadia Northway PhD BA DBO Created on behalf of NHS NES as supplement to workshops on binocular vision and additional techniques. Introduction Most of us use our binocular vision without thinking about it. It is not until symptoms occur that problems with binocular vision grab our attention. Assessing patients for binocular vision anomalies is easy to carry out and requires little equipment. The techniques that we describe can be used to objectively assess if a binocular vision anomaly is present, even in patients who are unable to provide subjective confirmation such as young children and those with learning difficulties. Done well, the tests can determine if a strabismus is present and if it is the probable cause of symptoms. Additional information regarding visual acuity when observing fixation speed for example can be gained from some simple techniques. Experienced clinicians will be able to determine if a deviation is concomitant or incomitant and if motility disorders are neurogenic or mechanical in origin. How to Detect Latent and Manifest Strabismus Using Cover Test Binocular vision anomalies are frequently the cause of symptoms such as asthenopia, headaches, difficulty reading and diplopia Also 3‐4% of the population develops childhood strabismus resulting in either the loss or degradation of binocular singular vision (BSV). The best way to detect strabismus is using the COVER TEST. This simple test can detect the presence of latent and manifest strabismus and is the first step to detecting binocular vision anomalies. A latent strabismus or heterophoria is the tendency for the eyes to deviate from a straight position when the eye is covered. A manifest strabismus or heteroptropia occurs when binocular fixation is not present under normal seeing conditions, for example when one eye is deviated intermittently or constantly when both eyes are open. The test requires little equipment and be carried out on children and adults. It is an objective test and relies on accurate observations of the examiner. You will need an occluder, pen torch and near and distance fixation target. Interesting fixation targets such as pictures or toys will maintain a child’s interest which is essential to carry out the cover test accurately. Some patients have binocular vision but look as if they are squinting due to anatomical factors (pseudostrabismus) Comment [WL1]: Am I right in remembering that these images were just examples and not ones we have the copyright to use? Image 1: pseudostrabismus Use of Corneal reflections to detect Strabismus Comment [NN2R1]: Yes I would prefer if you used other images Αsk the patient to fixate a pen torch at near and distance • if the reflected images from the cornea appear central and symmetrical assume the eyes (visual axes) are aligned and that no manifest strabismus is present Comment [WL3]: Does this apply to all images? I take it you don't have them yourself? I have a contact at Moorfields to try and source them, just need to know which I'm asking for. Image 2‐ normal corneal reflections • If corneal reflections seem asymmetrical then the approximate size of manifest strabismus can be calculated using the following guidelines. 1mm of displacement of corneal reflection corresponds to 7° of deviation of the visual axis (Hirschberg) Comment [WL4]: Diagrams from Nadia, and OK to use 15 ° 20/25 ° 45 ° Image 3‐ displaced corneal reflections • If the corneal reflection is displaced temporally = eso deviation • If the corneal reflection is displaced nasally = exo deviation Corneal reflections can provide a good introduction before carrying out a cover test. However an abnormal angle kappa may make this mode of assessment less accurate. An angle kappa is defined as the angle between the visual line (which connects the point of fixation with the nodal points and the fovea) and the pupillary axis (which is a line through the center of the pupil perpendicular to the cornea). A positive angle kappa (displacement toward the nose) of up at 5 degrees is physiologic. A positive angle kappa may hide a small angle esotropia or cause pseudoexotropia, whereas a negative angle may simulate esotropia or hide exotropia. How to Carry Out Cover test Cover Test (CT) is an objective dissociation test that detects the presence of horizontal and vertical forms of strabismus. It is possible to carry out the cover test in different ways in order to elicit different forms of strabismus: These steps should be followed systematically • first establish if there is a heterotropia (manifest strabismus) using cover –uncover test • if there is no heterotropia proceed to look for a heterophoria (latent deviation) using cover / uncover test • elicit the maximum angle of deviation with complete dissociation i.e. alternate cover test. This method is the easiest method to detect small amounts of vertical misalignment The following steps describe how to carry out cover test in detail: • Patient seated comfortably, head erect opposite fixation target. Note should be made of any abnormal head posture and cover test should be carried out with and without the head posture • Observe corneal reflections • Carry out at near and distance fixation (also far distance in exotropia) • For near use a spotlight and an accommodative target • Perform test with and without glasses The cover test is carried out in the primary position. Step 1 detect a manifest deviation 1. one eye is covered and the UNCOVERED eye is observed for any movement 2. if the uncovered eye makes a movement a HETEROTROPIA has been revealed 3. the test is repeated covering the opposite eye (allowing binocular viewing in between occlusion) 4. if there is no movement of either eye and the corneal reflections appear symmetrical, this suggests that BSV is present 5. concentrate only on the movement of the uncovered eye at this stage Possible findings on Cover test : Esotropia (convergent squint) • the eye is deviated inwards i.e. convergent • the uncovered eye will move outwards (temporally) to take up fixation when the left eye is covered • on removal of the cover the eye will move inward again The picture below image 4 shows a right esoptropia Exotropia (divergent squint) • the eye is deviated outwards i.e divergent • the uncovered eye will move inwards (nasally) to take up fixation • on removal of the cover the eye will move outward again The picture below shows a right exotropia Image 5: A right convergent strabismus Image 6: Divergent strabismus in right eye Hypertropia (vertical squint) • the eye is deviated upwards i.e. elevated • the uncovered eye will move downwards to take up fixation • on removal of the cover the eye will move upward again The picture below shows a right hypertropia Image 7‐ right hypertropia Hypotropia (vertical squint) • the eye is deviated downwards i.e. depressed • the uncovered eye will move upwards to take up fixation • on removal of the cover the eye will move downward again The picture below shows a left hypotropia Image 8 left hypotropia Step 2 detect a latent deviation If no manifest deviation was noted then cover test should continue to detect latent strabismus: 1. one eye is covered and as the cover is slowly removed. The examiner should observe the eye which was COVERED as the cover is removed 2. if the eye which was covered makes a movement as you take the cover away a HETEROPHORIA has been revealed 3. the other eye will also make these movements on dissociation 4. The rule is to think of where the eye has been. If you see it move out, then it was in under the cover and this is an esophoria or convergent deviation. The following deviations will be detected if the following is found: Esophoria (latent convergent squint) • the eye moves inwards under the cover • on removal of the cover it moves outwards to refixate • binocular vision is regained Exophoria(latent divergent squint) • the eye moves outwards under the cover • on removal of the cover it moves inwards to refixate • binocular vision is regained Hyperphoria (latent vertical squint) • the eye moves upwards under the cover • on removal of the cover it moves downwards to refixate • binocular vision is regained Hypophoria(latent vertical squint) • the eye moves downwards under the cover • on removal of the cover it moves upwards to refixate • binocular vision is regained Step 3 alternate cover test 1. cover one eye for a couple of seconds 2. transfer occluder directly to the opposite eye 3. DO NOT allow both eyes to view target simultaneously 4. repeat several times 5. observe movement of previously covered eye 6. finally remove cover and allow eyes to view binocularly It is easier to see small deviations using alternate cover test. It is also easier to detect small vertical deviations using this method especially when combined with a larger horizontal strabismus. Summary of the Cover Test The cover test when carried out well detects all manifest and latent strabismus. In addition it • indicates size and variation in deviation with distance and the effect of any refractive error • gives an indication of the level of visual acuity in either eye ‐ fixation • indicates if a latent deviation is controlled (well compensated) ‐ rate of recovery Advantages of the cover test • Quick and easy to carry out • No verbal response required so can be carried out babies and those with speech and language difficulties • Minimal co‐operation required • Gives idea of presence of amblyopia or poor vision Disadvantages • Poor fixation may make results inaccurate • Small deviations may not be recognized • Difficult to see strabismus is associated with nystagmus • Corneal reflections may not give accurate guide to size of deviation Cover Test and Motility The cover test is the first step in identifying abnormal eye movements. If there is an imbalance in one or more of the extraocular muscles then a deviation or strabismus will be evident in the primary position. A horizontal deviation may suggest weakness of the horizontal muscles and a vertical deviation may indicate a weakness of one of the vertical muscles. These rules apply in acquired strabismus only. The table below shows the type of deviation that will occur in the presence of a weak extra ocular muscle. Muscle Primary action Secondary action Lateral Rectus Medial Rectus Superior Rectus abduction adduction elevation adduction Tertiary action intorsion Inferior Rectus depression adduction extorsion Superior Oblique depression abduction intorsion Inferior Oblique elevation abduction extorsion Deviation Esophoria/tropia Exophoria/tropia Hypophoria/tropia with exo Hyperphoria/tropia with exo Hyperphoria/tropia with eso Hypophoria/tropia with eso Key Points • Cover test is the first step in detecting strabismus • An occluder and fixation target are required. • A manifest deviation may be detected by observing corneal reflections • If a manifest strabismus is present the squinting eye will move when the other eye is covered in order to take up fixation • If a latent deviation is present movement will be seen of the occluded eye as the occluder is removed from the eye • If a convergent deviation is present the eye will be seen to move out to take up fixation • If a divergent strabismus is present the eye will move inwards to take up fixation • During cover test it is possible to detect nystagmus and pupillary abnormalities