Intermittent exotropia is the most common form of exodeviation and is controlled intermittently by fusional mechanisms. The misalignment is often more significant at distance than at near and usually occurs during periods of visual inattention, stress, fatigue, or illness where fusional control may have declined. Severity and deterioration of intermittent exotropia primarily rely on the evaluation of the angle of exodeviation, the ability to control the exodeviation, and the degree of stereoacuity. Treatment modalities include orthoptics, monocular patching, base-in prism, over-minus spectacles, and surgical approaches.1,2 Although there are many treatment options available for intermittent exotropia, surgery is still generally considered as the treatment mainstay. However, the indication and the optimal timing for surgical intervention have long been controversial.
Worsening of exotropia control may be an indicator of early signs of deterioration, and its detection is essential for a timely surgical intervention. Therefore, control assessment is vital to monitoring disease stability or progression in patients with intermittent exotropia.1 Hatt et al3 defined control as an estimate of the proportion of time the deviation is manifest and the ease of reestablishing fusion after dissociation. Various control scales4–7 aimed at quantifying the control of the exodeviation have been developed, some of which are currently used in clinical practice. However, quantifying the control of intermittent exotropia and determining the real change in control is still a matter of debate. Recent studies on control variability3,8,9 showed that an assessment of control at one point in time should not be used as a basis for clinical decision making; because control has been shown to vary throughout the day, it might not be representative of the severity of the condition. Instead, an average of three control measures over an examination is recommended, which more closely represents the control for the entire day, as a standard for control assessment. The Look And Cover, then Ten seconds of Observation Scale for Exotropia (LACTOSE) is a tool for measuring control in patients with intermittent exotropia that was designed to allow ophthalmologists to assess the control efficiently on their own in a typical office setting in a standardized and reproducible fashion.10
The purpose of this study was to determine the extent of change in control over multiple measurements during a single clinic visit using the LACTOSE scoring system in patients with intermittent exotropia. This study also determined the relationship between the degree of variability in control and the specific clinical features of intermittent exotropia, such as age, stereoacuity, the amount of angle, and the degree of control.
Patients and Methods
This study was performed following the tenets of the Declaration of Helsinki. The approval of the institutional review board of Asan Medical Center was obtained for this study. All study participants provided written informed consent prior to their participation in this study.
Patients aged 3 years and older with basic type intermittent exotropia of 14 prism diopters (PD) or greater at both distance and near were prospectively enrolled. Excluded were patients with coexisting ocular or neurologic pathologies, developmental delay, history of prematurity, previous intraocular or strabismus surgery, and monocular or binocular amblyopia having a corrected visual acuity of worse than 20/40. Demographic and ophthalmologic data were collected from all participants.
Measurement of Control
Measurement of the control of each patient was performed using the LACTOSE scoring system, a reliable and valid 5-point control scale scored separately for distance (6 m) and near (33 cm) fixation.7 Scores for both distance and near were summed, yielding a total score ranging from 0 to 8 (5-point scale: 0 to 4 in each). Control assessment using the LACTOSE, which can be accomplished in 1 minute, is essentially a “LOOK, COVER, and OBSERVE” procedure with the examiner initially “looking” for the presence of exotropia for 10 seconds while having a patient “look” at an accommodative target. The examiner covers both eyes of the patient if exotropia is present or performs an alternate cover test if the eyes remain aligned, and then finally “observes” the patient's eyes for the presence of exodeviation while having the patient look at the target. A score of “4” was given if exotropia was noted throughout the assessment; “3” if the eyes initially remained aligned but manifested exodeviation for more than 10 seconds after the dissociation of fusion (alternate cover test); “2” if the exodeviation manifested only after alternate cover testing but realigned in 3 to 10 seconds; “1” if the exodeviation manifested only after alternate cover testing but realigned in less than 3 seconds; and “0” when the exotropic eye immediately realigned after the alternate cover test. Control assessment was never conducted under cycloplegic conditions. A more in-depth discussion of the LACTOSE scoring system for assessing control has been previously published elsewhere.6,10
Hour-to-Hour Variability of Control
Control measurements were taken two or three times on separate occasions during a single clinic visit with the same LACTOSE scoring system. Whether the control would be assessed two or three times was determined based on the patient's willingness to participate in further serial measurements. A senior investigator (HTL) measured the control throughout the study repeatedly, with a minimum interval of 1 hour between measurements. For each patient, distance and near control scores taken from the initial measurement were compared with the corresponding scores from the subsequent assessments. The percentage of patients showing different control scores across measurements was calculated, and the size and distribution of differences in control scores were analyzed to determine the extent of change in control from moment to moment.
Comparison of Clinical Features of Patients With Consistent and Variable Control
To compare the difference between groups with consistent control and groups with variable control, pertinent patient demographics and clinical findings such as age, gender, stereoacuity, amount of angle deviation for distance, amount of angle deviation for near, LACTOSE control scores for distance, and LACTOSE control scores for near were also collected.
The statistical analyses in this study were conducted using SPSS software version 20.0 (SPSS, Inc). The relationship between the initial control scores for distance fixation and the variability or consistency of the control scores was analyzed using the Pearson chi-square test. The same statistical analysis was used to determine the relationship between the initial control scores for near fixation and the variability or consistency of the control score.
Depending on the data distribution, either an independent two-sample t test or the Mann-Whitney U test was used to compare the difference in clinical features such as age (in years), angle of deviation (in PD), median control score for distance, median control score for near, and stereopsis (in seconds of arc) between patients with variable control and patients with consistent control. The same statistical analysis was used to compare the difference in the clinical features and to determine the difference in score consistency between patients with a total LACTOSE score of 7 or 8 and patients with a total LACTOSE score of 6 or less.
Ninety-five eligible patients were included in the study, of whom 55 (57.9%) were female and 40 (42.1%) were male. The mean age was 7.16 ± 6.72 years (range: 3 to 63 years). The mean angle of deviation for distance and near was 25.54 ± 6.83 PD (range: 14 to 40 PD) and 25.68 ± 7.81 PD (range: 14 to 40 PD), respectively. Mean stereoacuity was 117.94 ± 125.72 seconds of arc (range: 40 to 400 seconds of arc).
The criterion for the change of control was having at least a 1-point change on the scale. The change in the control score for either distance or near fixation was noted in 45 (47.4%) of the 95 patients (Figure 1). The remaining 50 (52.6%) patients showed consistency in the control scores for both distance and near fixation. Among those having variable control, 27 (60.0%) presented with only 1-point changes in the control score, whereas the remaining 18 (40.0%) patients showed 2-point changes. None of the patients with variable control showed more than 2-point changes in the control score between measurements.
The magnitude of changes in control for both distance and near fixation on multiple measurements. The majority of the patients with variable control showed a 1-point change (n = 27, 60%) in control using the Look And Cover, then Ten seconds of Observation Scale for Exotropia (LACTOSE). A 2-point change in control was noted in 14 (31.1%) patients. Another 4 (8.9%) patients showed either a 1-point change at distance and 2-point change at near or a 1-point change at near and 2-point change at distance.
Of the 95 patients included in this study, 66 (69.5%) were assessed two times (Figure 2) and 29 (30.5%) were assessed three times for their control (Figure 3). The mean interval between the first and last assessments was 1.64 hours (range: 1.17 to 2.50 hours) for patients assessed two times and 4.36 hours (range: 3.17 to 5.50 hours) for patients assessed three times. Distance and near control scores were measured separately, giving a total of 95 measurements for distance and 95 measurements for near. There was no difference in the control scores across multiple assessments in 63 (66.3%) patients for distance and 70 (73.7%) patients for near. On the other hand, there was a noted change in control in 32 (33.7%) patients for distance and 25 (26.3%) patients for near.
Consistency versus variability of control for patients assessed two times (n = 66). Control scores were mostly consistent for distance (n = 47, 71.2%) and near (n = 51, 77.3%) fixation. The remaining 19 (28.8%) and 15 (22.7%) patients showed variability in control for distance and near fixation, respectively.
Consistency versus variability of control for assessed three times (n = 29). Control scores were mostly consistent for distance (n = 16, 55.2%) and near (n = 19, 65.5%) fixation. Variable control in the Look And Cover, then Ten seconds of Observation Scale for Exotropia (LACTOSE) score was noted in 44.8% (n = 13) for distance fixation and 34.5% (n = 10) for near fixation.
Variability in the control was noted in 27 (40.91%) of the 66 patients who were assessed two times and 18 (62.1%) of the 29 patients who were assessed three times, but the difference was not statistically significant (Pearson chi-square test; P = .09). For both groups, the majority of the patients presented with only a 1-point change (n = 27, 60.0%) in control, and most of them manifested the variability in control for distance fixation only (n = 20, 44.4%).
In our LACTOSE control measurements, the control score was not different at distance and near in 29 (30.5%) of the 95 patients. The score was higher (worse control) at distance than at near in 65 (68.4%) patients, whereas the score was higher at near than at distance in only 1 patient (0.01%). There was no significant difference in the variability of control between those having the same scores at distance and near and those having different scores.
Analysis using the Pearson chi-square test showed a significant association between initial LACTOSE control score and the control score consistency for distance and near fixation, with patients having an initial control score of 1 or 4 showing a higher propensity to be more consistent in control than patients with an initial LACTOSE score of 2 or 3 (Table 1).
Relationship Between Initial Control Scores and Score Consistency for Distance and Near Fixation
Among the clinical variables such as age, stereoacuity, angle of deviation, and control score for distance and near fixation, only the median control score for near was significantly different between patients with consistent and variable control (Table 2). A better control score at near (LACTOSE score of less than 2) was more likely to be associated with consistent control across multiple measurements.
Comparison of Clinical Variables Between Patients With Consistent and Variable Control
Comparison of the clinical variables between patients with poor control (total LACTOSE score of 7 or 8) and patients with fair to good control (total LACTOSE score of 6 or less) showed a significant difference between the two groups with regard to the angle of deviation and the stereoacuity. The angle of deviation was significantly larger, and the stereoacuity was significantly worse in patients with total LACTOSE scores of 7 or 8, compared with those with scores of 6 or less (Table 3). Also, a significant difference (P < .001) was identified with regard to the consistency of the control scores between patients with a total LACTOSE score of 6 or less and a total LACTOSE score of 7 or 8. The control of the latter patients was more consistent than that of the former patients.
Comparison of the Clinical Variables Between Patients With a LACTOSE Sum Score of ≤ 6 and 7 or 8
Approximately half of the patients with intermittent exotropia in this study (n = 45, 47.4%) showed variability in the control score for either distance or near over multiple measurements during a single-day clinic visit. The overall change in control across measurements for both distance and near fixation showed a maximum difference of two levels on the LACTOSE scale, with the majority (n = 27, 60.0%) of patients showing a one-level difference in the LACTOSE control scores, indicating that the degree of variability in control may be less than what was previously reported.7
Variability in the control scores was noted in 40.91% of patients measured two times and 62.07% of patients measured three times, showing an increase in the frequency of variability in the control scores as the number of control measurements increased. Similar to previous studies,7–9,11 this study found that performing multiple assessments of control in patients with intermittent exotropia has a higher likelihood of demonstrating the presence and the degree of the change in control in patients with intermittent exotropia.
A significant association between the initial control score for distance and near fixation and consistency or variability of the control scores was shown in this study. For both distance and near fixation, a very poor or a very good initial LACTOSE control score had a significantly higher likelihood to have more consistent control than a fair to poor initial control score. Therefore, patients who initially have very poor control or very good control scores would most likely have the same level of control on subsequent examinations, and the initial control score could be representative of the overall control.
Comparison of the different clinical variables between groups with consistent and variable control scores showed that no significant difference existed between the two groups with regard to age, stereo-acuity, angle of deviation for distance fixation, angle of deviation for near fixation, and median control score for distance. The significant difference between the consistent and variable control groups noted for the median control score for near suggests that lower near control scores, or better control, is associated with a significantly higher likelihood of having consistent control on multiple measurements. This finding of better control at near fixation is in agreement with a recent study12 that reported an apparent worsening of the control in intermittent exotropia with increasing distance.
Patients with a total LACTOSE score of 7 or 8 or poor control of the intermittent exotropia showed a larger angle of deviation, worse stereoacuity, and more consistent control scores than those with a total LACTOSE score of 6 or less or fair to good control. The association between control of exotropia, stereoacuity, and angle of deviation has been investigated in a recent study,13 where worse exotropia control was associated with larger angles of deviation, although weak. In the current study, worse exotropia control (a total LACTOSE score of 7 or 8) was associated with significantly larger angles of exo-deviation and poorer stereoacuity compared with better exotropia control (a total LACTOSE score of 6 or less). The discovery of the total LACTOSE score of 7 or 8 (poor control) showing more consistent control scores concurs with the finding mentioned earlier of very poor control scores being less likely to be variable.
We found that the degree of control was more likely to be variable across multiple measurements in some of the patients with intermittent exotropia, especially in those who had a LACTOSE score of 2 or 3 on their initial measurements. Therefore, taking a single score of control measured at a single point in time as a true state of control may mislead clinicians into employing inappropriate diagnostic and therapeutic decisions. If the level of control is very good or very poor on initial measurement, it is unlikely that the measured control showed significant variability in the control score over the course of a single day.
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Relationship Between Initial Control Scores and Score Consistency for Distance and Near Fixation
|Fixation||Initial LACTOSE Control Scorea||Consistent||Variable||Pb|
|Distance||1||13 (72.2%)||5 (27.8%)||.049|
|2 or 3||27 (56.3%)||21 (43.8%)|
|4||24 (82.8%)||5 (17.2%)|
|Near||1||48 (90.6%)||5 (9.4%)||< .001|
|2 or 3||11 (44%)||14 (56.0%)|
|4||7 (77.8%)||2 (22.2%)|
Comparison of Clinical Variables Between Patients With Consistent and Variable Controla
|Characteristic||Consistent (n = 50, 52.6%)||Variable (n = 45, 47.4%)||Pb|
|Stereoacuity (seconds of arc)||128.85 ± 140.64||82.50 ± 39.92||.490|
|Age (years)||7.48 ± 7.89||6.48 ± 3.17||.892|
|Median control score for distance||2.88 ± 1.15||2.79 ± 0.99||.517|
|Median control score for near||1.53 ± 1.11||2.04 ± 1.15||.027|
|Angle of deviation for distance (PD)||25.52 ± 6.41||25.58 ± 7.75||.925|
|Angle of deviation for near (PD)||25.07 ± 6.34||26.84 ± 8.06||.328|
Comparison of the Clinical Variables Between Patients With a LACTOSE Sum Score of ≤ 6 and 7 or 8a
|Characteristic||LACTOSE ≤ 6 (n = 74)||LACTOSE 7 or 8 (n = 21)||Pb|
|Age||7.39 ± 7.11||6.33 ± 5.16||.082|
|Angle||23.82 ± 5.94||31.57 ± 6.43||< .001|
|Stereoacuity||113.21 ± 131.94||140.00 ± 92.64||.029|