while the patient wears polaroid glasses. This provides a check on suppression because the patient will only be able to read half of the text if suppression is present.

5.19.4 Recording

Record the number of cycles/minute for each eye and then for the binocular viewing condition. One cycle consists of clearing both the plus and the minus lenses.

5.20 WORTH 4-DOT TEST

This test checks for suppression by asking the patient to report the number and colour of dots they can see when looking through red–green goggles at four lights or dots of different colours. Two of the lights are green in colour and there is one red and one white light. The transmission characteristics of the red–green goggles are such that the eye wearing the red filter (usually the right eye) views the red lights and the eye viewing through the green filter (normally the left eye) will see the two green lights. Both eyes see the white light.

5.19.5 Interpretation

The normative data reported in the literature are variable, possibly because data were gathered across a range of ages but reported as a grand average or because they were collected from unselected samples (e.g. samples may have included patients with symptoms and accommodative or vergence dysfunctions). For these reasons, published normative data may be somewhat unreliable (Wick et al. 2002) and practitioners are encouraged to gather their own normative data for a range of age groups. Suggested ‘clinical pass’ criteria in young adults are 11 cycles/minute (monocular). The task becomes more difficult with the polaroid system, so that a clinical pass binocularly is 8 cycles/minute (Zellers et al. 1984). For children aged between 8 and 12 years, ‘clinical pass’ criteria are 7 cycles/minute (monocular) and 5 cycles/minute (binocular polaroids) (Scheiman et al. 1988).

5.19.6 Most common errors

1.Holding the flippers so that the patient cannot see the page.

2.Not allowing the patient to practise before starting the test.

3.Not turning the flippers fast enough so that the cycles/minute reflect the hand-speed of the examiner instead of a measure of accommodative facility.

4.Attempting to perform the test on a presbyopic patient.

5.20.1 Assessment of suppression

A properly functioning motor system is a requirement for binocular vision, but it does not guarantee that binocular vision exists. Suppression testing provides an indication of whether the patient is capable of fusing the images from the right and left eyes. When the retinal images differ in size as in aniseikonia, or in clarity as in uncorrected anisometropia, amblyopia or unilateral eye disease, it is possible that the image from the two eyes are not fused because one eye is suppressed. An inability to appreciate diplopia in some of the motor system assessments, such as the near point of convergence (section 5.15), may already have suggested suppression. Simple assessments of suppression are also available on the Mallett unit (section 5.13) and with some stereopsis tests (sections 5.22 & 5.23). Binocular refraction techniques, such as the Turville infinity balance, can also provide an assessment of whether gross suppression exists (section 4.18).

5.20.2 Advantages and disadvantages

The Worth 4-dot test is widely available, relatively cheap, easy to use and can be used to assess fusion at distance and near. It provides a rather coarse indication of suppression in the sense that other tests may reveal the presence of suppression when the 4- dot test suggests that none is present. This is particularly true for near 4-dot testing because of the relatively large angular size of the lights when viewed at near compared to distance viewing. Conversely, the rivalry produced by the red–green

goggles may lead to dissociation even in a patient with useful or normal binocular vision so that the test can suggest the existence of suppression when none is present under habitual viewing conditions. The major disadvantage of the test is that luminance of the red and green targets can vary widely between tests, as can the transmission characteristics of the red and green goggles, with the result that the test outcome can vary depending on whether the goggles are used in the standard format (red goggle in front of the right eye) or reversed (Simons & Elhatton 1994). Another disadvantage of the test is that a patient with constant strabismus and abnormal retinal correspondence may achieve a normal result. A positive test result does not therefore guarantee the presence of normal binocular vision.

Fig. 5.11 Diagram illustrating the possible patient responses to the Worth 4-dot test.

5.20.3 Procedure

1.Explain the test to the patient: ‘This test checks whether you are using both eyes at the same time to see.’

2.Place the red–green spectacles on the patient (over their spectacles if worn for that particular test distance). The eye with the red filter in front of it (usually the right eye) will see the red light and the eye with the green filter in front of it (usually the left eye) will see the green lights. Do not allow the patient to see the torch before putting the red–green spectacles on.

a)For testing at 6 m: Ensure that the patient is wearing their distance spectacles/contact lenses.

b)For testing at 40 cm: Hold the Worth 4-dot torch/flashlight at the patient’s reading position, so that the patient looks slightly downward at it. In the case of presbyopic patients, ensure that the patient wears appropriate refractive correction for the near test distance.

he torch is usually held with the red light at the top and white light at the bottom (Fig. 5.11).

3.Keeping the room lights on, now turn on the Worth 4-dot instrument.

4.Ask the patient: ‘How many dots do you see?’

5.There are four possible responses (Fig. 5.11):

a)‘4 dots seen’: This generally indicates that the patient has normal flat fusion. The response can be checked by asking ‘How many red dots do you see? How many green ones?’ Normally, patients will see one red, two green and one yellow dot. The white dot may appear yellow, or alternate between red and green due to retinal rivalry.

b)‘2 dots seen’. These will be the red and white, seen by the patient as two red dots. This indicates suppression of the eye with the green filter in front of it (usually the left). To detect alternating and/or intermittent suppression ask: ‘Are the number of dots changing as you look at them?’ If the number of dots seen is constant, check to see if fusion can be achieved by briefly occluding the nonsuppressed eye.

c)‘3 dots seen’. These will be the two green dots and one white dot, seen by the patient as three green dots. This indicates suppression of the eye with the red filter in front of it (usually the right). To detect alternating and/or intermittent suppression ask: ‘Are the number of dots changing as you look at them?’ If the number of dots seen is constant, check to see if fusion can be achieved by briefly occluding the nonsuppressed eye.

d)‘5 dots seen’: This indicates diplopia. The right eye (usually with the red filter) will see two red dots. The left eye (with the green filter) will see three green dots. Ask the patient to indicate where the red dots are in relation to the green ones. If the red dots (usually seen by the right eye) are to the right of the green dots, this indicates uncrossed diplopia and an eso deviation. If the red dots are to the left of the green dots, this indicates crossed diplopia and an exo deviation. If the red dots are below the green dots, this indicates a R/L deviation. If the red dots are above the green dots, this indicates a L/R deviation.

6.If suppression or diplopia is found, repeat the testing with the room lights off.

7.If suppression is found at distance but not at near, measure the extent of the suppression scotoma by moving the near target away from the patient and asking them to report when suppression occurs.

8.In patients who show suppression, it can be useful to repeat the test with the red–green goggles reversed to ensure an accurate assessment (Simons & Elhatton 1994).

Note: Children who cannot respond verbally can be asked to touch the dots to indicate the number seen, and ‘touching four’ indicates normal flat fusion. There is some evidence to indicate that although the test will reliably detect suppression in this way, it is unlikely to differentiate between normal fusion and alternating suppression (Lueder & Arnoldi 1996).

5.20.4 Recording

Record the normal perception of four dots at 6 m and 40 cm as:

‘W 4-dot: fusion @ DV & NV’ or similar.

If suppression is found, indicate which eye was being suppressed. Indicate whether suppression was found at both distance and/or near in both the light and dark. Indicate whether the condition was intermittent or constant.

If diplopia is found, indicate the direction of deviation suggested. Indicate whether diplopia was found at both distance and/or near in both the light and dark.

5.20.5 Interpretation

If a patient without strabismus sees all four dots on the test, they have normal ‘flat’ fusion. Flat fusion is binocular fusion that does not necessarily indicate stereopsis and is also called second-degree fusion. If a patient with strabismus sees four dots with the test, then this indicates that they have abnormal retinal correspondence (ARC). If the response is suppression of the right eye (i.e. the response is 2 green dots) or suppression of the left eye (i.e. the response is 3 red dots) then there is a suppression scotoma larger then the angular subtense of one of the four dots. The dots on the distance target have a smaller angular subtense than those on the near target. Because suppression is more common for targets imaged in central vision, suppression is therefore found more frequently for distance viewing than for near. The size of the suppression scotoma can be estimated by moving the near target further away from the patient than the standard 40 cm until they report suppression. The distance that the target is from the patient should be recorded. If the patient achieves fusion in the dark but not in the light, this indicates a shallower level of suppression as compared to the situation where suppression is present in both the dark and light room conditions.

5.20.6 Most common errors

1.Performing the test with the patient’s vision unaided, or when they have poor visual acuity through their spectacles.

2.Assuming that the absence of suppression confirms the presence of stereopsis.

5.21 4 BASE-OUT (BO) TEST

The 4 base-out test is used as a test of suppression in the specific case of a suspected microtropia. It is used in combination with tests of visual acuity,

refraction, eccentric fixation, abnormal retinal correspondence (ARC) and stereopsis to confirm a diagnosis of microtropia.

5.21.1 Assessment of microtropia

Microtropia may present as a primary condition or as a residual deviation after the treatment of a larger strabismus. It is characterised by a small strabismus that is not seen using the cover test, slight amblyopia (typically better than 6/12, 20/40), anisometropia, eccentric fixation, foveal suppression, ARC and reduced stereopsis. The lack of movement on the cover test is because the angle of the strabismus is equal, or similar, to the angle of eccentric fixation.

5.21.2 Advantages and disadvantages

The test requires little additional equipment and is quick and straightforward to perform. However, its repeatability is relatively poor and visually normal children can show atypical responses (Frantz et al. 1992).

5.Place the 4 BO prism over the eye with the better VA (Fig. 5.12b). The eye should make a swift movement inwards due to the prism. The fellow eye, which is likely to have slightly reduced VA (due to amblyopia and/or eccentric fixation if the patient has microtropia), should make a conjugate versional movement (i.e. in the same direction as the sound eye) due to Hering’s law. If the eye with reduced VA does not have suppression it will then show a fusional vergence or refixation movement to avoid diplopia (i.e. it will move back to where it started). If the eye with reduced VA has suppression, then no refixation will be seen. You should repeat this several times to confirm your result.

6.Now place the 4 BO prism over the eye with reduced VA (Fig. 5.12a). In a microtropia (which is generally of the esotropic type) the 4 BO prism will merely shift the retinal image within the suppression scotoma of the amblyopic eye. In such a case, neither eye will move. You should repeat this several times to confirm your result.

5.21.3 Procedure

1.Seat the patient comfortably. Keep the room lights on and, if necessary, use additional lighting so that the patient’s eyes can be easily seen without shadows. The test cannot be performed using a phoropter, and a trial frame with the optimal distance refractive correction (or the patient’s spectacles) should be used.

2.Explain the measurement to the patient:

‘I am going to perform a test that will help me diagnose the problem with your right/ left eye.’

3.Isolate a letter using the projector chart. The letter should be one line larger than the distance visual acuity of the amblyopic eye. Alternatively, use an isolated letter from a Sheridan–Gardner acuity test or similar. The target should be an isolated letter on a featureless background.

4.Ask the patient to keep looking at the letter, even if it appears to move.

5.21.4 Recording

Record ‘positive’ if there is no movement of the amblyopic eye with the 4 BO test. This indicates suppression. Record ‘negative’ if an appropriate eye movement was seen with the 4 BO test. For example, 4 BO test: positive LE (OS).

5.21.5 Interpretation

The 4 BO test is a useful test to help in the diagnosis of microtropia. However, it should be noted that visually normal children can show atypical responses (Frantz et al. 1992).

5.21.6 Most common errors

1.Making a decision on the test result on the basis of the first introduction of the prism.