THE COVER TEST is there anything more to learn ?, Exercises of Literature

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THE COVER TEST

is there anything more to learn?

Simon Barnard PhD, FCOptom, FAAO, DipCL, DipClinOptom

Table of Contents

• Table of Contents
• Introduction
• The “cover test” and the “alternate cover test”
• The cover test – a review of the literature
  • Method
    • Eye movements during the cover test
    • Optimum duration for occlusion
    • Effect of target characteristics........................................................................
    • Accuracy
• An Investigation of Eye Movement Characteristics during the Cover Test............
  • Equipment
  • Method
  • Subjects
  • Eye movement characteristics during the cover test
    • Parameters analysed
  • A summary of some of the results
    • Relationship between right and left eye phoria amplitudes
    • What happens to the eye under the cover…
• How long should a practitioner occlude an eye during the cover test? - How long does it take an eye to reach the 10 s position?
  • Mean times for each group were
    • What about recovery eye movements?
      • Saccade or vergence?
      • Latency.....................................................................................................
      • How many eye movements for recovery?
      • Recovery time
      • Does recovery time relate to associated phoria?
      • Do large phorias take longer to recover?
      • Relationship between recovery time and number of movements
  • Conclusions
  • References

(1) the size of the initial deviation, (2) the duration of the occlusion, and (3) the strength of vergence adaptation.

Earlier researchers have distinguished between fast and slow fusional vergences by the time course for ‘relaxation’ of convergence following occlusion of one eye. Ludvigh et al (1964) found that when convergence is stimulated for a short period of time (5 sec) fast fusional vergence relaxes within 10-15 seconds after one eye is occluded. The time course of relaxation has been described as a "decaying exponential " (Krishnan & Stark, 1977; Toates, 1974).

Cooper (1992) proposed that the increase in the angle during alternate occlusion is due to the rapid decay of fast fusional vergence by occlusion followed by a longer delay of slow fusional response. Stated another way, a measurement which increases with repeated alternate occlusion represents an initial elimination of fast fusional vergence followed by a subsequent elimination of slow fusional vergence. Conversely, removal of an occluder during a unilateral cover test permits fusion to reoccur. This results in stimulation of the fast fusional vergence system that feeds into the slow fusional vergence system. Repeated occlusion with unilateral cover testing results in the elimination of fast fusional vergence signals with minimal effect on slow fusional vergence signals because slow fusional vergence has a long time constant. Therefore, the deviation measured with the alternate cover test is usually larger than the amount measured with a unilateral cover test. The former is the result of the sum of the fast and slow fusional vergence system whereas the latter is a measure of the fast fusion system.

The cover test – a review of the literature

Method

The patient is asked to fixate a target at a known distance. The cover is introduced before one eye and the other eye is observed to detect any movement suggesting the presence of a heterotropia. In the presence of heterophoria, the occluded eye will deviate to what is assumed to be a “physiological position of rest”. The occluded eye is then uncovered whilst the practitioner checks for any heterophoric recovery movement in that eye. The direction of the recovery movement determines whether the phoria is an esophoria, exophoria or hyperphoria. With practise, an observer can estimate the amplitude of the phoria and will make subjective judgements concerning the speed and quality of the recovery movement or movements. Alternatively the movement can be neutralised with prisms in order to assess the amplitude (Franklin, 1997) but this requires repetitive occlusion and there is the possibility of introducing prism adaptation artefacts. The procedure is then repeated for the

other eye. The measure of amplitude is generally assumed to be the same for each eye.

Evans (1997) suggested that when undertaking the cover-uncover test it is useful to hold the occluder a few centimetres from the eye so that the observer can ‘peep’ round the edge and see the covered eye. However, Evans also acknowledged the importance of occluding "most of the visual field". Stidwill (1990) also suggested observing the eye under the cover by placing oneself in the monocular temporal field of the covered eye and stresses that patients must not be allowed to use their binocular field during the cover period. However it should be noted that even very brief periods of peripheral stimulation of the binocular visual field can instigate a fast fusional response which will prevent the complete decay of slow fusional vergence (McCormack et al, 1991; McCormack & Fisher, 1996; Larson, 1992). Cridland (1964) observed that, during the uncovering period of the cover test, the occluded eye often moves to take up fixation long before the image of the fixation object can have fallen upon its retina. This was particularly apparent when the dominant eye was being uncovered. Cridland postulated that binocular cues gained from peripheral visual field may be sufficient to initiate the recovery movement.

There have been suggestions that the nature of recovery eye movements following the cover test may depend upon factors such as the size of the heterophoria, eye dominance, visual acuities and the presence of suppression (Pickwell, 1973). Stidwill (1990) stated that a rapid recovery movement indicates a compensated heterophoria. Lyle & Wybar (1967) observed that recovery may be rapid when the occluder is removed from one eye and slow when removed from the other eye. However, these suggestions are mostly anecdotal or observational in nature.

Eye movements during the cover test

Both latency (reaction time) and velocities of version and vergence eye movements have been studied extensively (Westheimer 1954; Rashbass & Westheimer 1961; Krishnan et al,1973Bahill et al,1975a; Bahill et al, 1975b; Bahill & Stark 1979; Enright, 1984; Semmlow et al, 1986; Erkelens, 1987; Erkelens et al, 1989a; Erkelens et al, 1989b; Semmlow et al, 1993; Hung et al, 1994; Tam & Ono,1994; Collewijn, et al 1995). One aim of this present study was to quantitatively measure both latencies and time for recovery following the cover test to investigate some of the anecdotal claims mentioned above.

It is often assumed that the eye that is not occluded retains fixation during the cover-uncover procedure, i.e. there is an asymmetric vergence or version movement. However, it has been noted that the “fixing” eye often moves, particularly during the recovery phase, that the amplitude of this movement is usually about half that of the movement of the eye being uncovered and is greater when the dominant eye is covered (Pickwell, 1973). Peli & McCormack

hand there will be diplopia for a brief space of time, and the covered eye will have to move, in order to fuse the two images - in, if there was latent divergence, and out, if convergence.”

Percival (1928) suggested an even longer period of occlusion, recommending holding a card over the eye "for at least a minute".

As will be seen in due course, Clarke’s twenty seconds or even Percival’s minute, is likely to provide a more ‘useful’ result than the 1 - 2 seconds that is recommended by clinicians of more modern times.

Calvin et al (1996) described a study in which the cover test was compared to the von Graefe test. The duration of occlusion used in the cover-uncover test was “about 1 second”. Other descriptions of the cover test technique (Hugonnier & Clayette-Hugonnier, 1969; Pigassou-Albouy & Jones, 1978; Mallett, 1988) make no mention of the time period of occlusion.

Rosenfield et al (1997), using a dissociation test combined with subjective responses, suggested that a more accurate assessment of phoria amplitude may be obtained by maintaining dissociation for 25 minutes. If this is the case, then the normal practise of occluding for about 2 seconds during the conventional cover test may be providing the practitioner with an underestimation of the ‘true’ amplitude of phoria. This has been alluded to already by Barnard & Thomson (1995a).

Effect of target characteristics

The type of fixation target used during the cover test varies from practitioner to practitioner. It is important that the patient fixates and accommodates as accurately as possible throughout. Even when fixating, the eyes are not completely stationary. The effect of target contrast on these involuntary eye movements occurring during fixation has been investigated (Caifa & Hebbard, 1967). Using a target subtending 11.45’ visual angle, they reported that as the contrast of the fixation target was decreased below 50 per cent there was an increase in the standard deviation of the eye position during fixation and a highly significant increase in the mean amplitudes of the involuntary saccades. For target contrasts between 50 and 100 per cent, the mean amplitude of the saccades and mean standard deviation of eye position did not change with contrast. It is therefore advisable for the clinician to use a high contrast target as a fixation target.

A high contrast letter at the end of the line seen by the eye with the poorest vision or visual acuity on the Snellen chart is often used for fixation during the distance cover test. If the acuity of the poorest eye is below 6/18 a spotlight may be used but whenever possible a target stimulating accommodation is preferable (Franklin, 1997). For the near cover test, the target

is usually a fine high contrast letter on a hand held near chart. A penlight should never be used as a fixation object (von Noorden, 1990) as this is an inadequate stimulus for accommodation. For both distance and near the surround and target will provide a stimulus for binocular lock when the eyes are uncovered.

For younger children, Hugonnier & Clayette Hugonnier (1969) suggest that the target should be a luminous spot. It may be argued however that this is not as good a stimulus to accommodation as a fine letter target. Other targets may be used for children and it is with some incredulity that this author noted that Pigassou-Albouy and Jones (1978) described a fixation target in the form of a doll smoking a cigarette!

Accuracy

Ludvigh (1949) and Romano & von Noorden (1971) studied the minimum amplitude of eye movement that could be detected objectively. Ludvigh’s subjects were experienced observers and he found that under optimal conditions of illumination and with co-operative patients, the smallest observable change in eye position is approximately 2Δ. Romano & Von Noorden (1971) found similar results.

An Investigation of Eye Movement Characteristics during the

Cover Test

Equipment

Eye movements were recorded during a distance (340 cm) and near (40 cm) cover test. High contrast black targets (++ 00 ++ )) were used to provide both a fixation target (00) and paracentral “binocular lock”. Each symbol subtended 0. degrees and were separated by 0.6 degrees. The distant target was mounted on uniform white background subtending visual angle 40 x 40 degrees and there was an average luminance 65 cdm -2^. The near target was identical but displayed on a high resolution monitor.

Binocular eye movements recorded using an infra-red limbal-tracking system interfaced to a PC (Dell 486P/50).

Eye movement signals digitised using a 10 bit A/D converter sampling at 200 Hz Calibration via recessed LEDs for distance and small crosses displayed on screen for near produced a linear response up to approximately +/- 10 degrees.

Eye position during subsequent recording was ascertained by means of linear interpolation between calibration points. Resolution of the system was approximately 0.1 degrees and with the use of dental bite to restrain head

• The time at which the amplitude measured at 10 seconds was reached
• The latency, following uncovering the eye, from when the occluder theoretically passed across the visual axis, to the commencement of a recovery movement (latency for recovery)
• The time taken for recovery of the occluded eye
• The number and type of eye movements during the recovery process
• The size of each of these recovery movements

The data was recorded in Microsoft Excel for analysis using Excel and Unistat software.

A summary of some of the results

A larger number of conclusions were made from this study. A small number are discussed below.

Relationship between right and left eye phoria amplitudes

• There was a strong correlation between each eye whether measured at 2 seconds, 10 seconds or as the maximum phoria during 10 seconds of occlusion for both distance and near cover test.

What happens to the eye under the cover …

…does it vary between subjects?

• The eye movement profiles under the cover are varied. In some subjects the eye moves rapidly to a position of rest and the amplitude remain stable until the cover was removed at 10 seconds. In other cases the eye would gradually drift with the amplitude increasing at a slower rate.

… and how about the right and left eye of each subject…?

• … there was no linear relationship between the right and left eyes of subjects for the time taken to reach the phoria amplitude measured at 10 seconds for both distance (R 2 = 6%; n = 78 subjects) and near (R 2 = 6%; n = 82 subjects)

How long should a practitioner occlude an eye during the cover test****?

Is one or two seconds long enough? A univariate analysis of variance for repeated measures of amplitude of phoria at 2 and 10 seconds show a significant statistical difference (p < 0.001) between the phoria amplitude at 2 and 10 seconds for individuals for both the distance and near cover test.

How long does it take an eye to reach the 10 s position?

There was no significant difference between groups for distance and near and between “normals” and “referred subjects”

Mean times for each group were

Normals distance = 4.36 s (SD = 3.27 s) Normals near = 4.96 s (SD = 3.48 s) Referred distance = 4.27 s (SD = 3.39 s) Referred near = 4.50 s (SD = 3.07 s)

What about recovery eye movements?

There are two main phases during recovery following the cover test:

1. Latency period before eyes commence recovery movement(s)
2. Followed by recovery movement(s) which may be saccades and/ or vergence movements

Saccade or vergence?

63.8% of all esophoric eyes (distance or near) commenced recovery with a saccade whereas 63.7% of all exophoric eyes (distance or near) commenced with a vergence movement. There was a significant difference between exophores and esophores (Chi squared p = 0.006).

Latency

Of the 400 possible measurements, 187 latencies were obtained from the recordings (72 for distance & 115 for near). The mean latency = 0.29 s (SD 0. s) with an apparent minimum = 0.07 s and a maximum = 6.78 s. There was no significant difference between distance and near (p = 0.58).

Esophoric versus exophoric latency

For distance there was a significant difference between latencies in the esophoric and exophoric groups (p = 0.04). For near there was no significant difference (p = 0.55) .This should be viewed with caution because of the small number of near esophores.

How many eye movements for recovery?

For the normal group distance cover test the median number of movements was 2 (range = 1 to 5; mean = 1.7;SD = 0.8). For near it was also 2 (range =1 to 8; mean of 2.8; SD = 1.0).

Conclusions

• The age old assumption that one phoria measurement does for both eyes is probably correct.
• Practitioners should cover each eye for 4 to 5 seconds (not 1 -2 seconds)
• For near exophores, the highest symptom scores occur for “intermediate” recovery times.
• However, the number of recovery movements is not a good predictor of symptoms

References

1. Bahill AT, Clark MR, Stark L (1975a) The main sequence, a tool for studying human eye movements. Math Biosci, 24, 191- 204

2. Bahill AT, Adler D, Stark L (1975b) Most naturally occurring human saccades have magnitudes of 15 degrees or less. Invest Ophthalmolol 14:468 - 469 3. Bahill AT, Stark L, (1979) The trajectories of saccadic eye movements. Sci Am 240,1, 85- 4. Barnard NAS, Thomson WD (1995a) A quantitative analysis of eye movements during the cover test - a preliminary report. Ophthal Phys Opt, 15, 5, 413- 5. Caifa RP, Hebbard FW (1967) Involuntary eye movements occurring during fixation: effects of changes in target contrast. Am J Optom Arch Am Acad Optom, 44, 73- 6. Calvin H, Rupnow P, Grosvenor T (1996) How Good is the Estimated Cover Test at Predicting von Graefe Phoria Measurement? Optom Vis Sci, 73,11, 701-
3. Clarke E (1893) Eyestrain, pp 103-126, Churchill, London
4. Collewijn H, Erkelens CJ, Steinman RM (1995) Voluntary binocular gaze-shifts in the plane of regard: dynamics of version and vergence. Vision Res, 35, 3335- 9. Cooper J (1992) Clinical Implications of vergence adaptation. Optom Vis Sci, 69, 4, 300- 10. Cridland N (1964) The deviation behind the cover. Br Orthopt , 21, 63-
5. Enright JT (1984) Changes in vergence mediated by saccades, J Physiol, 350, 9-
6. Erkelens C J (1987) Adaptation of ocular vergence to stimulation with large disparities. Experimental Brain Research , 66 507-
7. Erkelens CJ, Collewijn H, Steinman RM (1989a) Asymmetrical adaptation of human saccades to anisometropic spectacles. Invest Ophthalmol Vis Sci, 30, 1132- 1145
8. Erkelens C J, Steinman RM, Collewijn H (1989b) Ocular vergence under natural conditions. II. Gaze shifts between real targets differing in distance and direction. Proceedings of the Royal Society of London B, 236, 441-
9. Evans BJW (1997) Pickwell’s Binocular Vision Anomalies. Investigation & Treatment. 3 rd^ Ed, Butterworth Heinemann, Oxford
10. Franklin A (1997) The Cover Test. CE Optometry, 1, 1, 8-
11. Hugonnier R, Clayette-Hugonnier S (1969) Strabismus, Heterophoria and Ocular Motor Paralysis 338-340, CV Mosby, St Louis 18. Hung GK, Ciuffreda KJ, Semmlow JL, Hornig JL (1994) Vergence eye movements under natural viewing conditions. Invest Ophthal Vis Sci, 35, 3486-

19. Jenkins TCA, Pickwell LD, Yekta AA (1989) Criteria for decompensation in binocular vision. Ophthal Phys Opt, 9, 121- 20. Krishnan VV, Farazian F, Stark L (1973) An analysis of latencies and prediction in the fusional vergence system. Am J Optom Arch Am Acad Optom, 50, 933- 21. Krishnan VV, Stark L (1977) A heuristic model for the human vergence eye movement system. IEEE Trans Biomed Engin, BME-24, 1, 44-
20. Larson WL (1992) Prism adaptation without binocular vision is unlikely: a retraction of a claim to the contrary. Optom Vis Sci, 69, 336
21. Ludvigh E (1949) Amount of eye movement objectively perceptible to the unaided eye. Am J Ophthalmol, 32, 649-
22. Ludvigh E, McKinnon, P, Zaitzeff, L (1964) Temporal course of the relaxation of binocular duction (fusion) movements. Arch Ophthal, 71, 389-
23. Lyle TK, Wybar KC (1967) Practical Orthoptics in the Treatment of Squint. Lewis and Co, London. p. 194 26. McCormack G, Fisher SK, Wolf K (1991) Retinal eccentricity of fusion detail affects vergence adaptation. Optom Vis Sci, 68, 711-
24. McCormack G, Fisher SK (1996) The source of disparity vergence innervation determines prism adaptation. Ophthal Physiol Opt, 16, 73-
25. Mallett RFJ (1988) Techniques of investigation of binocular vision anomalies. In Optometry , Eds. Edwards K and Llewellyn R, Butterworths, London. p 238-
26. Peli E, McCormack G (1983) Dynamics of cover test eye movements Am J Optom & Physiol Opt, 60, 712-
27. Percival A (1928) The Prescribing of Spectacles, Bristol, England, J. Wright & Sons
28. Pickwell LD (1972) Hering’s law of equal innervation and the position of the binoculus. Vis Res, 12, 1499- 32. Pickwell LD (1973) Eye movements during the cover test. Br J Physiol Opt, 28, 23-
29. Pigassou-Albouy R, Jones ST (1978) The Cover Test. In Reinecke RD ed. Strabismus. Proceedings of the Third Meeting of the International Strabismological Association. May 10- 12, Kyoto, Japan. pp 61- 34. Rashbass C, Westheimer G (1961) Disjunctive eye movements. J Physiol, 159, 2, 339- 35. Romano PE, Noorden GK von (1971) Limitations of cover test in detecting strabismus. Am J Ophthalmol, 77, 10-
30. Rosenfield M, Chun TW, Fischer SE (1997) Effect of prolonged dissociation on the subjective measurement of near heterophoria. Ophthal Physiol Opt, 17, 6, 478-
31. Semmlow JL, Hung GK, Ciuffreda KJ (1986) Quantitative assessment of disparity vergence components. Invest Ophthalmol Vis Sci, 27, 558- 38. Semmlow JL, Hung GK Hornig J-L., Ciuffreda KJ (1993) Initial control component in disparity vergence eye movements. Ophthal. Physiol. Opt., 13, 1,48- 39. Stella SS (1968) The association of blinks and refusion in intermittent exotropia. Am J Optom Arch Am Acad Optom, 45, 465-
32. Stidwill (1990) Orthoptic Assessment and Management. Blackwell Scientific Publications, London
33. Tam WJ, Ono H (1994) Fixation disengagement and eye-movement latency. Perception & Psychophysics, 56, 251-