Uniocular and binocular fields of rotation measures: Octopus versus Goldmann

Transcription

1 DOI /s NEURO-OPHTHALMOLOGY Uniocular and binocular fields of rotation measures: Octopus versus Goldmann Fiona J. Rowe & Sahira Hanif Received: 15 June 2010 /Revised: 8 October 2010 /Accepted: 2 December 2010 # Springer-Verlag 2011 Abstract Purpose To compare the range of ocular rotations measured by Octopus versus Goldmann perimetry. Methods Forty subjects (20 controls and 20 patients with impaired ocular movements) were prospectively recruited, age range years. Range of uniocular rotations was measured in six vectors corresponding to extraocular muscle actions: 0, 67, 141, 180, 216, 293. Fields of binocular single vision were assessed at 30 intervals. Vector measurements were utilised to calculate an area score for the field of uniocular rotations or binocular field of single vision. Two test speeds were used for Octopus testing: 3 / and 10 /second. Results Test duration was two thirds quicker for Octopus 10 /second than for 3 /second stimulus speed, and slightly quicker for Goldmann. Mean area for control subjects for uniocular field was degrees 2 for Goldmann, for Octopus 3 /second and for Octopus 10 /second. Mean area for patient subjects of right uniocular field was degrees 2 for Goldmann, F. J. Rowe (*) Directorate of Orthoptics and Vision Science, University of Liverpool, Thompson Yates Building, Liverpool L69 3GB, UK rowef@liverpool.ac.uk S. Hanif Department of Ophthalmology, Warrington and Halton Hospitals NHS Foundation Trust, Warrington, UK for Octopus 3 /second and for Octopus 10 /second. Mean area for left uniocular field was degrees 2 for Goldmann, for Octopus 3 /second and for Octopus 10 /second. Range of measured rotation was significantly larger for Octopus 10 /second speed. Conclusions Our results suggest that the Octopus perimeter is an acceptable alternative method of assessment for uniocular ductions and binocular field of single vision. Speed of stimulus significantly alters test duration for Octopus perimetry. Comparisons of results from both perimeters show that quantitative measurements differ, although qualitatively the results are similar. s per mean vectors were less than 5 (within clinically accepted variances) for both controls and patients when comparing Goldmann to Octopus 10 /second speed. However, differences were almost 10 for the patient group when comparing Goldmann to Octopus 3 /second speed. Thus, speed of stimulus must be considered if wishing to use these perimeters interchangeably. Keywords Uniocular ductions. Field of binocular single vision. Octopus perimetry. Stimulus speed Introduction Reliable assessment of uniocular ductions is important in the diagnosis and management of ocular motility disorders [1]. Uniocular ductions and binocular fields of single vision are typically measured in patients with restrictive ocular motility due to conditions such as

2 Graves' ophthalmopathy, mytochondrial cytopathies, and orbital fractures [1]. Accurate measurement is essential in following these patients, in order to detect improvement or deterioration in eye movements. There are many objective methods for measuring ocular ductions which measure eye position accurately, including electro-oculogram, infrared eye tracking, videobased techniques, and sclera search coil techniques [1]. However, these methods are often not routinely available for clinical examination in out-patient eye clinics. Quantitative assessment measures that have been utilized in out-patient eye clinics include Kestenbaum limbus displacement [2], synoptophore [3], Aimark and Lister perimeters [4 6], Goldmann perimeter [7, 8] and cervical rangeofmotiondevice[9]. Many devices are no longer in manufacture, such as the Goldmann and Aimark perimeters. It is important to evaluate new equipment to establish an evidence base for their potential and expected results in clinical practice. Haggerty and colleagues [7] proposed an alternative method of assessment of uniocular ocular ductions in order to improve ease of assessment for patients and thus increase accuracy. Measurements were taken in six cardinal directions that reflect the primary field of action of each extra ocular muscle. They found the technique was reproducible to within 4 degrees ( ) for healthy subjects undergoing assessment by a single observer, and intraobserver results showed repeatability of 7.9. The maximal variability of the technique for patients with Graves ophthalmopathy was 7.8. Thus, the authors concluded that for clinical purposes, only a change of 8 or more could be assumed to be significant, which was larger than the allowance of 5 which was taken to represent significant change in previous studies. A recent unpublished survey of UK orthoptic units has shown that the technique described by Haggerty et al. has been employed clinically in many units [10]. However, the ability to conduct this assessment using the Goldmann perimeter will eventually cease with the demise of the Goldmann perimeter [11]. In recent years, the Octopus 900 perimeter has been developed and marketed as a combined static and kinetic perimeter that directly replaces the decommissioned Goldmann perimeter (Haag Streit, Switzerland). This perimeter may be used in the examination, diagnosis and documentation of the differential light sensitivity and functional aspects of the eye [12]. Thus, it is used for kinetic evaluation of the visual field, static visual field perimetry, but may also be used as a kinetic assessment of the binocular field of single vision and uniocular fields of ocular rotation. Vectors for assessment of visual field or rotations may be preset to run automatically or selected and run real-time, depending on the assessment required and patient ability. The purpose of this study is to compare the range of uniocular ductions and fields of binocular single vision, measured by Octopus perimetry versus Goldmann perimetry, to determine the level of agreement between these perimeters. Methods and materials This is a prospective cross-section study undertaken with local research ethical approval and in accordance with the Tenets of the Declaration of Helsinki. Part 1 of this study was undertaken to evaluate ocular rotations in healthy controls with full ocular rotations. The subjects had clinically full ocular rotations confirmed by qualitative evaluation of smooth pursuit eye movements, and manifest strabismus was not evident on cover test. Part 2 of this study evaluated ocular rotations in a group of patients recruited because of impaired ocular rotation relating to neurological or restrictive ocular conditions such as cranial nerve palsy or orbital wall fracture. Each subject underwent assessment of ocular rotations on both Goldmann and Octopus perimetry within the same testing session. The order of testing was randomised for perimeter type, right/left eye, speed of stimulus on Octopus perimetry and binocular/uniocular measurements. For uniocular cardinal axes, the following were used: (right eye) lateral rectus 0, superior rectus 67, inferior oblique 141, medial rectus 180, superior oblique 216 and inferior rectus 293. The axes for the left eye mirrored those for the right eye. For the binocular field of single vision, the target was moved from central fixation outwards at 30 degree intervals, starting with direct elevation. The head was stabilised with chin and head rests on each perimeter. The foveal light threshold was established as the smallest, dimmest light visible to each subject, and this target was then used to measure the rotations. The use of a foveal light threshold had the advantage of aiding discrimination of the end point of movement in that, when the patient could no longer move his eyes to follow the target, the target moved off the fovea and thus disappeared from the patient s view.when assessing the range of uniocular ductions, the patient followed the target from central fixation outwards along each axis until the target disappeared from view. When assessing fields of binocular single vision, the patient followed the target from central fixation outwards along each axis until the target disappeared from view or was seen as double, at which point the subject pressed the perimeter s response button. The same instructions were provided for both Goldmann and Octopus perimetry. Movement of the target on the Octopus perimeter was set at 3 / or 10 /sec. The subjects did not undergo a practice of the test prior to the study.

3 Test duration was timed automatically by the Octopus perimeter, but was manually timed using a stopwatch for the Goldmann assessment. All ocular rotation measurements were taken in degrees of rotation on both Octopus and Goldmann perimetry. An area calculation is calculated automatically by the Octopus by joining all end points of the six ocular rotation vectors per eye. However, an area calculation is not made by the Goldmann perimeter. Thus, a manual calculation of area was undertaken for Goldmann assessment by the following method: ðvector end point summation=6þ 2 31:42ðpiÞ: In order to compare Goldmann area to Octopus area, a manual calculation (using the above formula) was also undertaken for Octopus perimetry. To establish test reproducibility, ten controls underwent testing on a second occasion, with a direct comparison made of area and individual vector rotations between tests, A direct comparison of each rotation was made for Goldmann and Octopus perimetry results using the statistical package SPSS version 15. Vector rotations and areas of field of rotation were assessed for goodness of fit (Smirnov Kolmogorov test) and for skewness. Data with standard distribution was compared using paired t-tests, with Bonferroni adjustment for multiple comparisons. Bland Altman strategy was used to compare the differences between two independent measurements versus the average of both. When measuring agreement between two tests, a plot of difference against the mean allows determination of relationship between the measurement error and the true value. The latter is taken as the mean of the two measurements as a best estimate of true value. The further the mean bias is away from a mean of 0, the more this indicates that the two tests are producing different results. This difference must be evaluated as to its clinical significance. Variability of comparisons may be seen across the range of results, or may change indicating a trend such as a larger or smaller difference as the average increases. Results Twenty healthy controls and 20 patients were recruited to this study. Part 1 involved healthy controls who were recruited from hospital staff and medical students. There were 12 females and eight males, with a mean age of 36.5 years (SD 10.3). When the test durations and rotation area were plotted for right and left eyes, there were no significant differences, and data were therefore combined for analysis. Test duration: uniocular rotations Mean test duration was slightly quicker for Goldmann assessment (1.52 minutes, SD 0.33) than Octopus 3 /sec assessment (1.68 minutes, SD 0.29), p=0.01, and three times as quick for Octopus 10 /sec stimulus (0.52 minutes, SD 0.15) versus Octopus 3 /sec stimulus speed, p= Rotation area: uniocular rotations Mean ocular rotation area was slightly larger for Goldmann assessment ( degrees 2, SD ) compared to Octopus 3 /sec stimulus speed ( degrees 2, SD ), p= There was no significant difference in area when comparing Goldmann to Octopus 10 /sec ( degrees 2, SD ) stimulus speed. Bland Altman analysis showed a mean difference for Goldmann versus Octopus 3 /sec of degrees 2 (Fig. 1a) and versus Octopus 10 /sec of (Fig. 1b), with the Goldmann area being larger for both comparisons. Using these mean difference values, the difference per mean vector between Goldmann and Octopus 3 /sec perimetry was calculated (using reverse area calculation) as 2.79, and between Goldmann and Octopus 10 /sec as Extra ocular muscle vector measurements The means and standard deviations for individual vector rotations are given in Tables 1 and 2. The smallest rotation was for the vector corresponding to superior rectus function, whilst the largest rotation was for inferior rectus function. The mean vector rotation for Goldmann was 50.18, for Octopus 10 /sec and for Octopus 3 /sec. Thus, a maximum clinical difference of 2.87 degrees was present (within a clinical acceptable variation of 5 degrees). However, there was considerable variation in measurements from Goldmann and Octopus assessments for individual vectors. The results were compared further by Bland Altman analysis. Bias differences are shown in Tables 1 and 2. It was found that the mean bias fell within 5 degrees when comparing Goldmann to Octopus 3 /sec, but the mean bias increased to within 14 degrees on comparison to Octopus 10 /sec and to within 18 degrees for comparison of both Octopus stimulus speeds. Greatest differences were noted for vectors corresponding to the superior oblique, inferior rectus and superior rectus extra ocular muscles. As a further evaluation, a sum of opposing vectors was made to allow for possible head movement during assessment, i.e., sum of rotation measurements for lateral rectus and medial rectus, superior rectus and superior oblique, inferior rectus and inferior oblique (Table 3). Less variation was seen by this method, but with a significant difference still present for superior rectus and superior oblique vectors, which were larger in measurement for Goldmann than Octopus assessments, p=

4 A Octopus 3 versus Goldmann B Octopus 10 versus Goldmann Octopus 3 stimulus speed (manual) versus Goldmann (Area) SD Mean bias Octopus 10 stimulus speed versus Goldmann (Area) +1.96SD 3000 Mean bias Fig. 1 Bland Altman comparison of Goldmann and Octopus areas. a Octopus area is smaller than Goldmann area, with mean bias of There is increasing variability of differences as the average increases, indicating greater discrepancies between measures. b Octopus area is smaller than Goldmann area with mean bias of , which is close to 0, indicating similar results between measures. There is variability in most measures within ±1.96SD Test retest reliability Means and standard deviations for area scores and individual vector rotations are given in Table 4. The mean difference in area from first to second test was degrees. The maximum mean difference in individual vector measurements was 3.3 degrees for the vector corresponding to the inferior oblique muscle. All repeat measurements were similar to that of the first test, with no significant difference found in any measure (paired t-test). Field of binocular single vision duration and area Test duration was halved using the 10 /sec stimulus speed, and Goldmann test time was quicker than the Octopus 3 /sec speed. Mean area of binocular field was degrees 2 (SD ) for Goldmann, degrees 2 (SD ) for Octopus 3 /sec and degrees 2 (SD ) for Octopus 10 /sec. in area for Goldmann versus Octopus 3 /sec assessment was significant, p= Bland Altman analysis (Fig. 2) showed a mean difference for Goldmann versus Octopus 3 /sec of degrees 2 and versus Octopus 10 /sec of degrees 2 (Goldmann area being larger for both comparisons). Using these mean difference values, the difference per mean vector between Goldmann and Octopus 3 /sec perimetry was calculated (using reverse area calculation) as 3.38, and between Goldmann and Octopus 10 /sec as Test retest reliability was undertaken in ten control subjects. The mean area calculation on first assessment was degrees 2 (SD ), and on retest assessment was degrees 2 (SD ). On Bland Altman Table 1 Individual vector rotations: control group Perimeter LR SR IO MR SO IR Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD Goldmann Octopus 3 /sec Octopus 10 /sec Vector rotations shown in mean degrees of movement. Goldmann area is greater than Octopus 3 /sec area. Octopus 10 /sec is consistently larger than Octopus 3 /sec and Goldmann in all vectors. The smallest vector rotation is that relating to the superior rectus muscle; the largest being the inferior rectus muscle. LR Lateral rectus MR Medial rectus SR Superior rectus SO Superior oblique IO Inferior oblique IR Inferior rectus

5 Table 2 Bland Altman analysis Perimeter LR SR IO MR SO IR Bias SD Bias SD Bias SD Bias SD Bias SD Bias SD Goldmann vs 3 /sec Octopus 3 vs 10 /sec Goldmann vs 10 /sec The bias (mean difference) and standard deviation are shown for comparison of Goldmann to Octopus 3 /sec and 10 /sec plus Octopus 3 /sec and 10 /sec. The mean difference is within 5 per vector for Goldmann compared to Octopus 3 /sec, within 14 for Goldmann compared to Octopus 10 /sec, and within 18 for Octopus 3 /sec compared to 10 /sec. LR Lateral rectus MR Medial rectus SR Superior rectus SO Superior oblique IO Inferior oblique IR Inferior rectus analysis, the mean bias was (SD ), and the differences between the first test and retest assessments were not significant, p=0.36 (paired t-test). Part 2 consisted of 12 females and eight males, with a mean age of years (SD 13.3). These patients were referred for assessment of ocular motility because of symptoms of diplopia. Participants were identified randomly, i.e., notes were taken consecutively from the list waiting for orthoptic assessment without prior knowledge of patient ability and cognition. Ocular rotations were reduced in the right and/or left eyes because of ocular motility conditions including cranial nerve palsy, orbital wall fracture, thyroid eye disease, iatrogenic and mechanical restrictions. There were no cases of myogenic involvement such as myasthenia gravis. Thus, data for ocular rotations are presented separately for right and left eyes because of differences of involvement of the two eyes. Test duration: uniocular rotations Mean test duration was slightly quicker for Goldmann assessment (right eye: 1.07, SD 0.58; left eye: 0.84 minutes, SD 0.31) than Octopus 3 /sec assessment (right eye: 1.62, SD 0.32; left eye: 1.60 minutes, SD 0.28), p=0.049 and p= Table 3 Combined vector rotations: control group Perimeter LR/MR SR/SO IR/IO Mean SD Mean SD Mean SD Goldmann Octopus 3 /sec Octopus 10 /sec Goldmann vectors are greater than Octopus 3 /sec area, as are Octopus 10 /sec vectors except for combined inferior rectus/inferior oblique vectors. The smallest vector rotation is that relating to the superior rectus/superior oblique muscles, the largest being that relating to the inferior rectus/inferior oblique muscles.lr/mr Lateral rectus and medial rectus SR/SO Superior rectus and superior oblique IR/IO Inferior rectus and inferior oblique respectively) and slower than Octopus 10 /sec stimulus (right eye: 0.55, SD 0.19; left eye: 0.55 minutes, SD 0.22). Rotation area: uniocular rotations Ocular rotation area was significantly larger for Goldmann assessment for the right eye ( degrees 2, SD ), p=0.004, and for Octopus 10 /sec stimulus (right eye: , SD ; left eye: degrees 2,SD ) compared to Octopus 3 /sec stimulus speed (right eye: , SD ; left eye: degrees 2,SD ) for both right and left eyes, p=0.021, p= There was no significant difference in area for the left eye when comparing Goldmann ( degrees 2, SD ) to Octopus 3 /sec assessment. Bland Altman analysis (Fig. 3) showed a mean difference for Goldmann versus Octopus 3 /sec of degrees 2 and degrees 2 for right and left eyes respectively (Goldmann area being larger). The mean difference for Goldmann versus Octopus 10 /sec was degrees 2 and degrees 2 for right and left eyes respectively (Goldmann area was smaller than these faster Octopus speeds). Using these mean difference values, the difference per mean vector between Goldmann and Octopus 3 /sec perimetry was calculated (using reverse area calculation) as 4.85 (right eye) and 9.21 (left eye), and between Goldmann and Octopus 10 /sec perimetry as 1.47 (right eye) and 2.67 (left eye). Extra ocular muscle vector measurements The means and standard deviations for individual vector rotations are given in Tables 5 and 6. The smallest rotation was for the vector corresponding to superior rectus function, whilst the largest rotation was for inferior rectus function. There was considerable variation in measurements from Goldmann and Octopus assessments for individual vectors. The results were compared further by Bland Altman analysis. Bias differences are shown in Tables 5 and 6. It was found that the mean bias fell within 12 degrees

6 Table 4 Test retest results: control group Perimeter Area LR SR IO MR SO IR Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD Test Test Mean difference Vector rotations shown in mean degrees of movement. Similar measurements are obtained between tests, with no significant difference in any measure (paired t-test with 95% confidence) when comparing Goldmann to Octopus 3 /sec and when comparing both Octopus stimulus speeds, but the mean bias increased to within 18 degrees on comparison to Octopus 10 /sec. Greatest differences were noted for vectors corresponding to the superior oblique, inferior rectus, inferior oblique and superior rectus extra ocular muscles. As a further evaluation, a sum of opposing vectors was made to allow for possible head movement during assessment, i.e., sum of rotation measurements for lateral rectus and medial rectus, superior rectus and superior oblique, inferior rectus and inferior oblique (Table 7). Less variation was seen by this method, but with a significant difference still present for superior rectus and superior oblique vectors plus inferior rectus and inferior oblique vectors, p= Discussion The technique of assessing six cardinal positions for uniocular ocular rotations on Goldmann perimetry has been shown to be reproducible and repeatable: in addition, it is suitable for patients regardless of the origin of their ocular motility restriction [7]. Haggerty and colleagues reported means of rotation ranging from 42.7 degrees for the superior rectus muscle to 61.9 degrees for the inferior rectus muscle. We documented similar mean rotation values ranging from 42.5 degrees (superior rectus) to degrees (inferior rectus). Mean rotations for all extra ocular muscles are compared in Table 8, and show very similar measures. Haggerty et al. also reported that changes in head rotation and misalignment were potential sources of inaccuracy, but of low magnitude. They found improved reproducibility of paired scores compared to summed scores of antagonist muscles, and proposed that errors due to head position could be obviated by total movement scores which offset the gains and losses of rotation and could be calculated from the sum of the six ductions. Calculation of area score also has the same effect in negating the effects of head misalignment, but this was not undertaken by Haggerty et al. [7]. A Goldmann versus Octopus 3 B Goldmann versus Octopus 10 Goldmann versus Octopus 3 stimulus speed (area) SD Mean bias Goldmann versus Octopus 10 stimulus speed (area) Mean bias Fig. 2 Bland Altman comparison of Goldmann and Octopus binocular areas. a Octopus 3 /sec area is smaller than Goldmann area, with mean bias of There is increasing variability of differences as the average increases, indicating greater discrepancies between measures. b Goldmann area is larger than Octopus 10 /sec area, with mean bias of which is closer to 0 than other comparisons of binocular area, indicating similar results between measures. There is variability in most measures within ±1.96SD

7 A Right and left eyes: Goldmann versus Octopus Goldmann versus Octopus 3 stimulus speed (right eye; area) SD 0 - Mean bias Goldmann versus Octopus 3 stimulus speed (left eye; area) SD Mean bias B Right and left eyes: Goldmann versus Octopus Goldmann versus Octopus 10 stimulus speed (right eye; area) SD Mean bias Goldmann versus Octopus 10 stimulus speed (left eye; area) SD Mean bias Fig. 3 Bland Altman comparison of Goldmann and Octopus patient areas. a Octopus 3 /sec area is smaller than Goldmann area, with mean bias of (right eye) and (left eye). There is variability in most measures within ±1.96SD. b Goldmann area is smaller than Octopus 10 /sec area with mean bias of (right eye) and (left eye), which is closer to 0 than other comparisons of uniocular area, indicating similar results between measures. There is variability in most measures within ±1.96SD Using the Octopus perimeter, we were able to automatically calculate area score for the total sum of ocular rotations. We also manually calculated this for Goldmann perimetry. We had found variation in individual extraocular rotation measurements when comparing Goldmann results to Octopus results but variability was less when summed scores of antagonist muscles were compared. Thus, there may be benefit from generating an area score for total ocular rotation area when comparing these results over consecutive visits, to allow for potential errors from head movement. We stabilised head movement in our study using a forehead and chin rest. However, we acknowledge that use of a bite bar would provide further stabilisation of the head. This was not undertaken in this study, as clinically we do not use a bite bar with our patients and we wished to ascertain the clinical use of the Octopus perimeter.

8 Table 5 Individual vector rotations: patient group Perimeter LR SR IO MR SO IR Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD Right eye Goldmann Octopus 3 /sec Octopus 10 /sec Left eye Goldmann Octopus 3 /sec Octopus 10 /sec Vector rotations shown in mean degrees of movement. Goldmann area is greater than Octopus 3 /sec area. Octopus 10 /sec is typically larger than Octopus 3 /sec and Goldmann in most vectors. The smallest vector rotation is that relating to the superior rectus muscle; the largest being the inferior rectus muscle. LR Lateral rectus MR Medial rectus SR Superior rectus SO Superior oblique IO Inferior oblique IR Inferior rectus Bland Altman analysis was used to evaluate the results of Goldmann perimetry against those of different test speeds using Octopus perimetry [13]. Although differences were found between each perimeter programme, we did not find a bias for one meridian: the difference in area was similar across all vectors (Fig. 4). In part 1 of this study, greater discrepancies were seen when making comparisons with Goldmann perimetry and the Octopus 3 /sec stimulus speed. The mean difference related to a mean increase per vector of 2.79, which is within accepted clinical differences with a range of 0.55 to 4.53 degrees. The variability suggested that results are not interchangeable between these test strategies, particularly as increased variability was noted with an increasing average value, and not all comparisons fell within ±1.96SD. The closest comparison for uniocular rotations was between the Goldmann and Octopus 10 /sec stimulus speed when evaluating the control data, showing similar results and with a mean difference per vector of 0.79 and a range of 0.18 to degrees. This trend was also seen for binocular comparisons, although more variability was seen for binocular compared to uniocular results. These comparisons were closer to the mean of 0, indicating that both tests were producing similar results. For our healthy control subjects, area scores obtained using Goldmann perimetry were significantly greater in size than those obtained with Octopus perimetry using slower moving stimuli, whereas there was no significant difference comparing the area scores from Goldmann to the faster 10 /sec stimulus speed. Although the recommended speed when moving the Goldmann target is 2 3 degrees/sec, it is quite likely that there is an inherent bias effect where the target speed is faster than thought by examiners. There is no bias effect using the Octopus when preset stimulus speeds are set, and this may provide more repeatable and reliable measurements. Nevertheless, subjects report that the 10 /sec stimulus Table 6 Bland Altman analysis; Perimeter LR SR IO MR SO IR Bias SD Bias SD Bias SD Bias SD Bias SD Bias SD Right eye Goldmann vs 3 /sec Octopus 3 vs 10 /sec Goldmann vs 10 /sec Left eye Goldmann vs 3 /sec Octopus 3 vs 10 /sec Goldmann vs 10 /sec The bias (mean difference) and standard deviation are shown for Bland Altman comparison of Goldmann to Octopus 3 /sec and 10 /sec plus Octopus 3 /sec and 10 /sec. The mean difference is within 12 per vector for Goldmann compared to Octopus 3 /sec, within 18 for Goldmann compared to Octopus 10 /sec, and within 12 for Octopus 3 /sec compared to 10 /sec. LR Lateral rectus MR Medial rectus SR Superior rectus SO Superior oblique IO Inferior oblique IR Inferior rectus

9 Table 7 Combined vector rotations: patient group Goldmann vectors are greater than Octopus 3 /sec area, as are Octopus 10 /sec vectors except IR/IO. The smallest vector rotation is that relating to the superior rectus/superior oblique muscles; the largest being the inferior rectus/inferior oblique muscles. LR/MR Lateral rectus and medial rectus SR/SO Superior rectus and superior oblique IR/IO Inferior rectus and inferior oblique Perimeter LR/MR SR/SO IR/IO Mean SD Mean SD Mean SD Right eye Goldmann Octopus 3 /sec Octopus 10 /sec Left eye Goldmann Octopus 3 /sec Octopus 10 /sec speed was quite fast, and this may incur a degree of overestimation of rotation measurement because of delayed reactions from the point of discrimination of a diplopic or absent target to the action of pressing the response button. In part two of the study with the patient group, we found similar results to part one. Greater discrepancies were again seen when comparing Goldmann results to Octopus 3 /sec stimulus speed for the patient group. Closer comparisons were noted for comparisons to Octopus 10 /sec stimulus speeds, and the differences between Goldmann and Octopus 10 /sec areas were not significant. The mean difference per vector was 4.85 and 9.21 for right and left eyes when comparing Goldmann and Octopus 3 /sec perimetry (with a range of up to 12 ), which is outside accepted clinical differences. The mean differences per vector when Goldmann results were compared to Octopus 10 /sec ranged between 1.47 and 4.73, which fall within an accepted clinical difference of 5, but the range was up to 18, taking this outside accepted clinical differences. However, there was considerable variability, with wide ranges for ±1.96SD from the mean bias. Hence, it would appear that the results from Goldmann or Octopus perimetry, although more similar for 10 /sec, are not quite interchangeable. One limitation of this study was the small numbers of subjects recruited (40 in total). However, the results obtained using Goldmann perimetry were very similar to those in the reported literature. We undertook measurements with Octopus perimetry using different stimulus speed settings, and the results were consistent across groups as to the increase in area size and decrease in test duration for assessment, leading us to infer reliability from the use of Octopus perimetry for the assessment of ocular rotations. In particular, the comparison of differences of individual extraocular muscle vector mean rotations fell within 5 degrees for Goldmann and Octopus 10 /sec perimetry. Another limitation of this study is that test retest reliability was only assessed for measurements taken with the Octopus 3 /sec stimulus speed. These measurements showed very consistent results for overall area score as well as for individual vector rotations. The maximum mean difference was 3.3 degrees, corresponding to the vector for the inferior oblique muscle. The Octopus perimeter allows evaluation of subject reaction times, which has been shown to be useful in visual field assessment. Reaction time vectors may be set in addition to the test vectors, and an adjustment of response is made according to the reaction times of the patient. Thus, a small area or degree of rotation seen in patients with very slow reaction times may reveal a normal range of measure- Table 8 Comparison of extraocular muscle rotation measurements Haggerty et a.l [7] Current study: Goldmann perimetry Current study: Octopus 3 /sec Current study: Octopus 10 /sec Superior oblique Inferior rectus Inferior oblique Superior rectus Lateral rectus Medial rectus Mean Mean Mean Mean Mean Mean SD SD SD SD SD SD SD SD SD SD SD SD SD SD SD SD SD SD Mean values relate to vector rotations in degrees. SD represent standard deviations, and represent 95% confidence intervals for mean values

10 Fig. 4 Comparison of uniocular rotation measures: control subject. Results of rotation measures are shown for the right and left eyes for Goldmann assessment plus Octopus assessment with 3 and 10 degree/second stimulus speeds. s in area size vary across all vectors. Left eye Goldmann perimetry Right eye Octopus 3 degree/second perimetry Octopus 10 degree/second perimetry

11 ments when the reaction time is taken into consideration. During this study, this operation mode was not used. Firstly, we wished to directly compare the Octopus results to Goldmann assessment, the latter not including any assessment of reaction time. Second, visual field assessment is a detection task of the presence (or absence) of a target of certain size and luminance [14]. Assessment of ocular rotation is a discrimination task in which the patient can already see the target but must decide whether it disappears from view or becomes double. This discrimination task is different from detection tasks. However, the potential influence of reaction time adjustment is an aspect worthy of further study to determine further the effect which reaction time measurements have on the perceived area of ocular rotation. Duration of assessment is important, as the longer the test takes the more inaccuracies may be incurred, as the patient is more likely to move and may experience fatigue. The Octopus 10 /sec speed adjustment on the Octopus perimeter is quicker than the Octopus 3 /sec speed. However, the mean duration for the Octopus 3 /sec speed is still below a 2-minute test duration per eye. A compromise is required between the speed of the stimulus and the ability of the patient to detect when the stimulus has disappeared or separated and then respond to this. Conclusions Speed of stimulus significantly alters test duration for Octopus assessment of ocular rotations. Octopus perimetry may overestimate field of rotations when utilising faster stimulus speeds, but equates better to area measurements obtained by Goldmann assessment: the latter is likely to incorporate a bias effect of altered test speed as a result of manual operation. Mean differences between Goldmann and Octopus perimetry have generally been within 5 per vector: only the patient group tested with the Octopus perimeter fell outside clinically accepted differences. The Octopus perimeter has been found to be an acceptable method of assessment of uniocular ductions and binocular fields of single vision, with the added benefit of preset consistent stimulus speeds and automatic area calculation, which is useful for comparisons over subsequent visits. Acknowledgement We acknowledge Haag Streit International for the research loan of an Octopus 900 perimetry used for the assessments in this study. Declaration This article is based on a study first reported in the Transactions of the 33rd European Strabismological Association, Belgrade, Serbia. 2009, in press. These transactions are not an open publication but only available to the members of ESA, and the scientific committee of ESA have been informed that publication in the transactions and submission to a medical journal do not constitute dual publication. Conflict of interest The authors do not have any commercial or proprietary interest in the Octopus 900 perimeter or Haag Streit International. Haag Streit has provided travel expenses for conference attendance for FJR. References 1. Hanif S, Rowe FJ, O Connor A (2009) The comparative analysis of assessment methods for uniocular fields of fixation. Br Ir Orthopt J 6: Kestenbaum A (1961) Clinical methods of neuro ophthalmic examinations, 2nd edn. Grune and Stratton, New York 3. Roper-Hall G (1975) Duction measurements in limited rotations. Br Orthopt J 32: Esser J, Melzer I (1986) Comparison of monocular excursion measurements in normals and in patients with motility disorders. In: Campos E (ed). Proceedings 5th International Strabismological Association. Athena Scientific Distributors, Modena, pp Steel DHW, Hoh HB, Potts MJ, Harrad RA (1995) Uniocular fields of fixation in Graves' orbitopathy. Eye 9: Arens B (1997) Estimation of duction ranges using visual acuity. In: Spiritus M. (ed). Transactions 23rd European Strabismological Association. Aeolus Press, Amsterdam, pp Haggerty H, Richardson S, Mitchell KW, Dickinson AJ (2005) A modified method for measuring uniocular fields of fixation. Arch Ophthalmol 123: Gerling J, Lieb B, Kommerell G ( ) Duction ranges in normal probands and patients with Grave s ophthalmopathy determined using the Goldmann perimeter. Int Ophthalmol 21: Kushner BJ () The usefulness of the cervical range of motion device in the ocular motility examination. Arch Ophthalmol 18: Hanif S (2008) UK Orthoptic survey of ocular rotation measures. (personal communication) 11. Rowe FJ, Hanif S (2009) Uniocular ocular rotation measures: Octopus versus Goldmann. Trans. 33rd European Strabismological Association. Belgrade, Serbia. Ed: Gomez de Liano R. pp Haag Streit International (2010) Octopus 900 Standard Operating procedures html Accessed 1 June Bland JM, Altman DG (1995) Comparing methods of measurement: why plotting difference against standard method is misleading. Lancet 346: Rowe FJ (2006) Visual fields via the visual pathway. Blackwell Science Publications, Oxford. ISBN