The static position of an eye around its visual axis (Fick′s Y axis) is called anatomic torsion or ocular torsion. Measurement of ocular torsion is valuable (1) in the management of cyclovertical muscle dysfunction, (2) in the management of torsional diplopia induced by oblique muscle abnormalities or by macular translocation surgery, (3) to anticipate the anatomical location of the recti muscles before strabismus surgery, and (4) to differentiate primary inferior oblique overaction from dissociated vertical divergence. Observations on ocular torsion in various positions of gaze and under monocular versus binocular conditions have advanced the basic understanding of cyclofusional reflexes and mechanisms of torsion-related diplopia. Although the presence of significant ocular torsion can be a sign of overriding clinical importance, most ophthalmologists are not familiar with methods to look for an abnormal torsion of the fundus. This is partly due to the disorientation caused by the inversion and reversal of the image on indirect ophthalmoscopy.
Excessive ocular intorsion was first reported by von Graefe in 1856 as a vertical macular displacement in a patient with incyclotropia. Locke described shifting of the blind spot in the visual field of subjects with A and V patterns as well as cyclovertical muscle imbalances.1 Levine and Zahoruk using fundus photography studied the disc-fovea relationship in subjects with vertical muscle pareses.2 Since then, numerous objective and subjective techniques to measure ocular torsion are described.3 The drawbacks of subjective techniques are (1) sensory adaptations make them inconsistent and less accurate (Guyton DL, von Noorden GK. Sensory adaptations to cyclodeviations In: Reinecke RD, ed. Strabismus; Proceedings of the Third Meeting of the International Strabismological Association, May 10-12, 1978, Kyoto, Japan, New York: Grune & Straton, 1978;399-403).
(2) The subjective nature of the tests make them not useful in children, uncooperative adults, in subjects with large angle deviations, and in subjects who cannot appreciate diplopia. The gold standard test for objective measurement of ocular torsion is the fundus photographic technique.3 With this technique the optic nerve head (ONH)-foveal angle can be accurately measured from a still photograph using a caliper (geometric analysis method) or a protractor.3
Spierer described an objective method of measuring ocular torsion by projecting a horizontal light beam from the slitlamp biomicroscope to the retina, connecting the fovea and the optic nerve.4 Although he reported a good correlation of this technique with the double Maddox rod test (DMRT) and indirect ophthalmoscopic technique, no data were provided. Additionally, the correlation between two objective tests was not reported.
Our study reports on a comparison of the fundus photographic technique of torsion measurement, and the slitlamp biomicroscopic technique. Comparison of two objective techniques of ocular torsion measurements is not previously reported in the literature.
Materials and Methods
This prospective masked study was performed on 36 consecutive subjects presenting in the department of Paediatric Ophthalmology and Strabismus at a teaching eye institute in India from June to July 2003.
The inclusion criteria were
- Cooperation of subjects
- Age 6-45 years
- Clear view of disc and macula
- Normal fundus
- Normal ocular motility
The exclusion criteria were
- Horizontal or vertical strabismus
- Nystagmus or dissociated vertical divergence
- Lack of patient cooperation
- Pathological lesion in the fundus
A qualified strabismologist (MK) examined all the subjects. Informed consent was obtained from the subjects and/or from the parents when appropriate. An ophthalmic photographer (GV) - taking care that the subject′s head was well aligned - using the side marks and chin rest as a guide - took wide-field (50°) fundus photographs using a TRC- 50IX (Topcon, Japan) fundus camera. Photographs were taken through the dilated pupil after instilling tropicamide 1% eye drops. The internal fixation device within the camera was not used as it made the torsion measurements difficult. The disc-foveal angle (DFA) was calculated from a well focused single still photograph using IMAGEnet software (Topcon, Japan) and a protractor (Camlin Mumbai, India). A horizontal reference line was drawn through the fovea and a line joining the fovea and the centre of the disc Figure 1 by the second observer (GV). The second observer was masked to the torsion measurements of the first investigator (MK). We did not average multiple photographs as this has been reported as less useful.5 Ocular torsion was measured using the slitlamp biomicroscopic technique (SL-8Z, Topcon, Japan) described by Spierer.4 A well focused horizontal beam of light was projected through a 90 diopter aspheric lens on the fovea and the patient was asked to fixate on the center of the beam (Figure 2a). The thinnest possible width and lowest possible magnification were used to view the light beam through the slitlamp. While the patient fixated on the center of the slit beam, the slit housing was rotated to make the light beam pass through the center of the optic disc (Figure 2b). Measurement of torsion was directly noted from the protractor scale (Figure 2c) present on the slit housing. The protractor in the slitlamp was provided with the gradation in the unit of 5° (Figure 2c). The angle was read to the nearest degree and recorded in an Excel spread sheet. Both eyes of the patient remained open during the measurements.
A sign convention was used wherein the measured value was considered positive when the position of the fovea was below the center of the optic disc and negative if the fovea was located above the center of the optic disc. Clinical agreement between the two techniques was calculated. The two-tailed student′s t-test was used to determine the statistical significance of the mean ocular torsion between the two techniques. Two-sided P values of P < 0.05 were considered significant. Pearson′s correlation coefficient was also used to evaluate the relationship between the two techniques.
Sample size calculation
Sample size was calculated using the formula6
Z1-α/2 = Standard normal variate corresponding to the level of significance = 1.96 for 5%
Z1-β = Standard normal variate corresponding to the power = -0.84 for 80%
Sample size = 2 (1.96+0.84)232/22 = 36
The mean age of subjects in the study was 13.7 years (range 6 to 44 years), and 15 (41.6%) were males. Refractive errors were determined for 25 (69.4%) subjects including 11 subjects with myopia and 4 with hyperopia. The average torsion using the slitlamp technique was 5.5 3.3° and 6.1 4.3° using the fundus using the slitlamp technique and fundus photographic technique respectively (P = 0.04). One patient had negative torsion (-1°, intorsion) in one eye and positive torsion (+4°, extorsion) in the other eye with both the techniques. Repeat examination of the same patient revealed -4° and -3° in the right and left eye respectively on both the techniques. Subjective response on double maddox rod was negative for torsion. The Pearson′s correlation coefficient (r2) was 0.5. The coefficient of variance was 60% and 70% with the slitlamp technique and fundus photographic technique respectively.
The mean disc-foveal-angle (DFA) was 5.5° 3.3° and 6.1° 4.3° using the slitlamp and fundus photographic techniques compared to 7.2° 2.5° reported by Bixenman and von Noorden7 in 100 eyes (50 subjects) and 4.8° in 10 eyes reported by Freedman et al8 using a photographic technique. Besides, mean inter-eye difference with both the techniques was significantly higher in our study compared to 1.6° 1.2° reported by von Noorden.7 The differences in these results can be due to small sample size, abnormal head positioning, poor resolution of protractor scale, differences in the measurements with monocular viewing conditions versus binocular viewing conditions, imprecision in slit rotation and large intra-test variability, or a true population variation.
Smaller Sample Size: Ocular torsion in our study ranges from -2.5° to 14.7° (95% CI). With a larger sample size of 100 eyes in Bixenman′s study the range was 2.2° to 12.2° (95%CI). Even those confidence limits were too large to make the normative data clinically useful. However, a larger sample size could produce tighter confidence limits. Guyton9 pointed out that a large range for torsion is seen in the normal population, so much so that a patient may develop a true cyclodeviation of as high as 18° between the two eyes, but still fall within the so-called “normal” torsion range.
Abnormal head positioning: Previous investigators have cautioned against the extreme sensitivity of photographic methods to the patient head positioning.3 Hence in the present study, the eye level marking and the chin rests were used to avoid the tilting of the head. Nevertheless, large coefficient of variance, large interocular differences and a poor correlation coefficient indicates imprecision of the test with current head positioning techniques. Use of braces, bite bar or a leveling device could improve the accuracy and precision of the test, finally improving its repeatability.
Poor resolution of protractor scale: The slitlamp protractor scale used in this study was graded for the resolution of 5°, which was then subjectively read to the nearest degree. Improving the protractor scale resolution by marking every degree would improve the accuracy of the slitlamp method. However, with the photographic method a large range of normal was present despite a 1° resolution scale, indicating that a large biological variation exists for ocular torsion in humans.
Measurements in monocular viewing conditions versus binocular viewing conditions: The measurements in this study were taken under the conditions of binocular viewing. The effect of monocular viewing conditions compared to binocular viewing conditions on static torsion of the eye remains controversial. Investigators in the past have reported differences in subjective torsion responses when Maddox rods are used in comparison with Bagolini glasses. This is probably due to the disruption of cyclofusion with the Maddox rod. It is believed that cyclofusion occurs more readily at a sensory level than at a motor level. Hence, objective measurements are less likely to be affected by the viewing conditions.
Imprecision in slit rotation and large intra-test variability: Morton et al5 did not report any difference in the amount of measured torsion by taking multiple photographs, but did not provide any data. Also, the imprecision associated with the rotation of the slit and its coupling with the protractor scale needs to be evaluated in further studies.
True anatomical differences in the Caucasian population compared to Asians: A biological plausibility of racial variation in disc-foveal relationship also exists. Jonas10 has pointed out the differences in the disc morphology and dimensions in Asians versus Caucasians. This needs to be further evaluated.
Presence of intorsion in one eye and extorsion in the other eye, was seen in one patient in this group. On a repeat examination this patient had normal ocular motility and there was bilateral intorsion.
Good correlation (r2 = 92) was previously reported between the modified Spierer′s technique and the double Maddox rod test in 35 eyes after macular translocation surgery.9 These investigators projected a slitlamp beam onto the retina using a 90D lens and then rotated the beam until the patient perceived that the light was horizontal. They were comparing two subjective horizontal senses. These techniques suffer from inaccuracies created by sensory adaptations. A previous report by Fricke et al11 found poor repeatability of the DMRT in a similar patient population. In the present study we found weak correlation between the two objective techniques - the slitlamp biomicroscopic technique and photographic technique. There appears to be increasing agreement between the two techniques for the higher degree of torsion. This may be related to the sensitivity of the current slitlamp protractor scale that provides gradation of 5.
In conclusion, fundus photographic technique is the current gold standard test for the measurement of ocular torsion. The slitlamp biomicroscopic method can be a useful technique in cooperative subjects when a fundus camera is unavailable. However, the correlation between these two techniques of ocular torsion measurement is weak but can be improved with 1) standardised head-positioning techniques, 2) better resolution of the protractor scale and 3) removing the imprecision of slit rotation. Further studies are also required to test the intratest variability of these tests.
The authors thank Prof. David L Guyton, Wilmer Eye Institute, Baltimore, Maryland, USA for his suggestions.
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Proprietary Interest: None