This is distracting and encourages eye movements. Small vertical or lateral movements with the occluder are all that is required.

8.Diagnosing as a heterotropia the immediate loss and recovery of fixation of an eye when the other eye is covered. Because of a heterophoria movement of the covered eye behind the cover, the fixing eye may move

from its original accurate foveal position due to Hering’s law of equal innervation and then refixate due to the fixation reflex (Fig. 5.3).

9.Not watching for an alternating tropia when switching from covering/uncovering left eye to covering/uncovering right eye.

10.Allowing binocular fixation during the alternating cover test due to poor technique.

5.6HIRSCHBERG, KRIMSKY AND BRUCKNER TESTS

The Hirschberg test compares the position of the corneal reflexes (the first Purkinje images) of the two eyes that are formed by a pentorch. Using prisms from the trial case, the Hirschberg test can be extended to provide more accurate measures of the angle of heterotropic deviation. When carried out in this fashion, the test is known as the Krimsky test. It is important to realise that, in most patients, the corneal reflections will be displaced slightly nasally relative to the pupil centres. This displacement arises because a separation exists between the pupillary and visual axes. The angle between these axes is referred to as the angle kappa, which is given a positive value if the reflexes are nasally displaced. Video clips of these tests being used are available on the website .

5.6.1 Objective assessment of heterotropia

In very young children, who may be unable to maintain fixation for long enough to allow the cover test to be performed, an objective assessment of oculomotor status in the primary position can provide useful information to indicate the presence or absence of heterotropia.

5.6.2 Advantages and disadvantages

The Hirschberg test is quick and easy to perform, and requires little cooperation on the part of the patient. However, it can really only be performed at near, the penlight target provides a poor stimulus to accommodation and it is relatively inaccurate. Choi & Kushner (1998) found that even experienced practitioners can obtain results that differ by up to 10 prism dioptres.

5.6.3 Procedure

1.Keep the room fully illuminated. Additional use of localised lighting is recommended so that the patient’s eyes can be easily seen without shadows.

2.Remove any spectacles that the patient may be wearing. However, if it is felt that the refractive correction will alter the result (e.g. in cases of significant hyperopia), the test should also be performed through the correction.

3.Hold a penlight horizontally 40 to 50 cm from the patient with the light aimed at the bridge of the patient’s nose. The back of the penlight should be very close to the tip of your nose.

4.Ask the patient to look at the light with both eyes open. Young children will automatically tend to look toward the bright light but may need a little encouragement.

5.Note the location of the corneal reflex in each eye individually. In order to do this you should briefly cover each eye in turn; you can do this with the palm of your hand. Remember that the reflex is frequently decentred about 0.5 mm nasally with respect to the centre of the pupil because angle kappa is normally positive.

6.Now compare the location of the corneal reflexes as the patient views habitually (i.e. without any occlusion). The eye that has the same angle kappa as in the monocular test is the fixing eye. The location of that reflex should be considered the reference position.

7.If there is a heterotropia present, the corneal reflex of the other eye will have shifted in a direction opposite to that of the ocular deviation. For example, in the case of an ESOtropia, the corneal reflex will be displaced temporally on the patient’s cornea relative to the position of the reflex in the fellow eye.

8.The magnitude of the deviation can be estimated from the displacement of the reflex in millimetres (mm) relative to the reference

position using the approximation of 1 mm 22 .

9.Instead of estimating the angle of deviation as in step 8, a more reliable estimate of the magnitude of deviation can be achieved using measuring prisms. This is known as the Krimsky test and it is a simple extension of the Hirschberg test. In the Krimsky test the practitioner varies prism power in front of the fixating eye in order to place the corneal reflex in the deviated eye in the same relative position as the corneal reflex in the fixating eye. As with the Hirschberg test, the penlight is held stationary in front of the patient’s eyes. The practitioner needs to be seated directly in front of the deviating eye in order to avoid problems of parallax. Measures of the angle of heterotropia obtained using the Krimsky test do rely upon the assumption that the deviating eye fixates centrally rather than eccentrically. While this assumption may not be valid in many instances, the error it introduces is likely to be small in relation to the overall size of the deviation. Furthermore, the same criticism can be applied to heterotropia measures obtained using the cover test in combination with a prism bar.

5.6.4 Recording

Record the eye that deviates, along with the direction of the deviation (see section 5.5.4 for more details). In your recording, make it clear that the observation was made using the Hirschberg or Krimsky technique. Equal nasal displacement of the corneal reflexes in each eye indicates a non-stra- bismic patient. For example:

5.6.5 Interpretation

A reflex located nasally to the reference point suggests an exotropia; a reflex located temporally to the reference point suggests an esotropia. Superior displacement of the reflex suggests a hypotropia, and inferior displacement suggests a hypertropia. It is important to remember that a large angle kappa may result in the misdiagnosis of heterotropia or failure to detect a heterotropia. For example, a large positive angle kappa will simulate the presence of an exotropia. Similarly, an existing exotropia will appear larger than it actually is and small esotropias may escape detection.

5.6.6 Most common errors

1.Basing your decision upon the absolute position of a single reflex relative to the pupil centre rather than on a comparison of the relative locations of the corneal reflexes in the two pupils.

2.Not viewing directly behind the penlight for the Hirschberg test or from directly in front of the deviating eye in the case of the Krimsky test.

3.Placing too much emphasis on the accuracy of the estimates provided by the Hirschberg and Krimsky tests. These tests are less accurate than the cover test in combination with prism bar for estimating the magnitude of heterotropic deviations (Choi & Kushner 1998).

5.6.7 Alternative test: Bruckner test

The Bruckner test is an alternative method for detecting the presence of strabismus in infants. In a dimly lit room, direct the light from a direct ophthalmoscope from about 1 m away onto the child’s face and compare the colour and brightness of the fundus reflexes. In the presence of a strabismus the reflex will be brighter and whiter in the deviating eye as compared to the reflex from the fixing eye. It is suggested that this is because the fundal reflections from a deviating eye are thought to be greater than from the darkly pigmented macular area of a

normally fixating eye. Some controversy exists regarding the usefulness of this test but it appears that it can help in the diagnosis of small angle strabismus (Miller et al. 1995). When interpreting the results of the Bruckner test, it is important to remember that differences in brightness can be caused by factors other than strabismus including anisometropia and media opacities.

5.7 MODIFIED THORINGTON

TEST

The modified Thorington test is used to measure the amount of heterophoria at distance and/or near. As the name suggests, the technique is a modification of the Thorington method, which used a vertical prism to produce dissociation. In the modified technique, the Maddox rod (section 5.8) is used to produce dissociation.

5.7.1 Subjective assessment of heterophoria

The cover test is objective in that it requires no direct response from the patient (although subjective responses regarding phi movement should be obtained when possible), but all other tests for heterophoria are subjective. Because the cover test can

be difficult to perform when using a phoropter or reduced aperture trial case lenses, subjective procedures may need to be used to assess any heterophoria with the optimal refractive correction. Also remember that the objective cover test is insensitive to small eye movements below about 2–3 (Fogt et al. 2000) and a subjective cover test or other subjective test should be used to check for small vertical heterophorias that may be clinically significant. To investigate any heterophoria, fusion must be suspended. This can be achieved by occluding one eye (cover test), distorting the image seen by one eye (Maddox rod) or by vertical separation of the images seen by the two eyes (von Graefe).

5.7.2 Advantages and disadvantages

The modified Thorington technique is a very simple and quick technique that can be used in a phoropter, trial frame or free space. It produces the most repeatable results of the most commonly used techniques (Casillas & Rosenfield 2006, Rainey et al. 1998b). The principal problem of the Maddox rod technique is the lack of control of accommodation (section 5.8.2). The modified Thorington overcomes this problem by using a target of small letters or numbers (Fig. 5.4). It is principally used at near, but Thorington cards are available for both distance and near. In view of its many advantages

it is somewhat surprising that it is not widely used at present. Like other subjective heterophoria tests, the modified Thorington is more repeatable with a trial frame than with a phoropter (Casillas & Rosenfield 2006).

5.7.3 Procedure

The test is typically used after subjective refraction to determine the heterophoria with the optimal correction. In pre-presbyopic patients, near phorias should be measured immediately after the distance heterophoria measurements. In presbyopes, they should be measured after inclusion of the required reading addition.

1.Inform the patient about the test: ‘This test is to check how your eye muscles work together with the new spectacle prescription’.

Horizontal near heterophoria

2.Ensure the patient is wearing their optimal near refractive correction and adjust the phoropter/trial frame to the near centration distance.

3.Place the Maddox rod in front of one eye making sure that the ‘grooves’ are absolutely horizontal. Note that it is conventional to place the Maddox rod before the right eye. Dim the room lights.

4.Shine the light from a penlight through the central aperture of the Thorington near card.

5.Direct the patient to look at the letters and keep them clear. Ask them to then look at the spotlight, and tell you whether the vertical red line is seen to the right, left or straight through the spotlight.

6.Some patients have difficulty seeing the red line initially. If they cannot see the red line, cover each eye in turn to demonstrate that one eye sees the spotlight and the other sees the red line. Once they are aware of the test format they are often able to see the red line and spotlight simultaneously. Placing a green filter before the eye viewing the spotlight can also help the patient to perform the test. If difficulty is still experienced place the Maddox rod in front of the left eye and try again. If the

spotlight and red line cannot be seen together then suppression may be present and followup tests should be performed.

7.With the Maddox rod in front of the right eye the following responses may be given:

a)If the line is seen to pass through the spotlight the patient has no horizontal phoria.

b)If the line is to the left of the spotlight (crossed images) the patient has an exophoria. If the line is to the right of the spotlight (uncrossed images) the patient has an esophoria. Determine the size of the deviation by asking the patient which number on the horizontal series of letters on the Thorington card the line passes through. There is no need to insert prism to produce an overlap of the line and spot because the scale on the card is a tangent scale; i.e. the number read off the scale corresponds to the number of prism dioptres of horizontal heterophoria.

8.Compare the horizontal heterophoria found with the cover test result performed with the patient’s spectacles. If there is no significant change in refractive error, the heterophoria should be similar to that found with the cover test. Any change in the heterophoria should be consistent with the change in refractive correction (see section 5.7.5).

Vertical near heterophoria

1.Rotate the Maddox rod so that the ‘grooves’ are oriented vertically.

2.Ask the patient if the red line is seen above, below or straight through the spot.

3.With the Maddox rod in front of the right eye the following responses may occur:

a)If the line is seen to pass through the spotlight the patient has no vertical phoria.

b)If the line is above the spotlight the patient has a right hypophoria. It is possible to specify vertical heterophorias with respect to the right or left eye. Thus, a right hypophoria can also be called a left hyperphoria. As above, the size of the deviation is determined by asking the