Characteristics of participants
A total of 583 participants with asthenopia and 97 with non-asthenopia were included. There were 186 asthenopia participants [median(Q1, Q3) age: 29(25,33) years, 36% males] in phase 1, 199 asthenopia participants [median(Q1, Q3) age: 30(24,39) years, 38.2% males] in phase 2, 198 asthenopia participants [median(Q1, Q3) age: 28(25,33) years, 44.4% males] and 97 non-asthenopia normal control [median(Q1, Q3) age: 29(24,33)years, 52.6% males] in phase 3 and 4. There was no statistically age and gender difference between the 198 asthenopia and 97 normal controls (Table 1).
Phase 1 Assessment of the original ASQ-19
There were no misfitting item in the ASQ-19 (both Infit and Outfit MnSq values of all items were 0.75~1.49, Appendix 2 Table 1), indicating that the items measured single construct. This was further supported by the principal component analysis of the residuals, which showed that the variances explained were 48.5%, 52.7%, 47.9% in dimension A (ocular organic symptoms), B (visual functional symptoms) and C (systemic symptoms) respectively. The eigenvalue and unexplained variances explained in the first contrast in dimension A, B, C were 1.60, 1.40, 1.90 and 12.70%, 12.30%, 20.10% respectively. ASQ-19 also had good person separation index and person separation reliability values (2.18 and 0.83 respectively).
The responses for 0, 1, 2, 3, 4, 6 and 9 scores were 102, 502, 1030, 890, 645, 313 and 52 respectively (more than 10 responses per rating scale category). Option sorting analysis (Figure 1) showed the curve peak of 1-3 scores covered with each other, and 4,6 scores covered each other. Sores with curve peak covered each other should be combined into one option. So the scoring mode should be reconsidered.
Project suitability analysis (Appendix 3 Figure 1) showed mean person location was -2.16 logit, it means the average scores of items were lower than the average scores of individuals, suggesting that the items were more difficult than individuals’ ability. In clinical practice we still found a few patients had difficulty in distinguishing the 18th item "Did eye discomfort make you feel anxious?" from the 19th item "Did eye discomfort make you feel depressed?". Their average item location on Wright map (Appendix 3 Figure 1) were close, indicating the similar difficulty.
Consequently, we adjusted the item scoring mode to "Intensity" scoring, a four-point Likert response rating scale from “never” (0), "mild"(1), "moderate"(2) to "severe"(3). The 18th and 19th item were combined into a new item, so a new 18-item Asthenopia Survey Questionnaire (ASQ-18) was obtained.
Phase 2 Assessment of the ASQ-18
There was one misfitting item in the ASQ-18(Infit and Outfit MnSq values of 11th item was 1.55 and 1.60 respectively, rest seventeen items were 0.80~1.27, Appendix 2 Table 3), indicating the 11th item didn’t fit the construct well. The 11th item “Did you feel blur or ghosting when looking far?” was eliminated.
The responses for 0, 1, 2, 3 scores were 1426, 1323, 623 and 210 respectively (more than 10 responses per rating scale category). Option sorting analysis (Figure 1) showed the curve peak of 0,1,2,3 scores were separated from each other and in the same order, indicating the four options were reasonable.
The mean person location was -1.12 logit, which was better than ASQ-18, but still without the standard value (Appendix 3 Figure 1).
The eigenvalue and unexplained variance explained in the first contrast and PSI/PSR were similar with ASQ-19(Table 2), indicating the items were measuring single construct and good reliability.
Consequently, we deleted the 11th item, so another new 17-item Asthenopia Survey Questionnaire (ASQ-17) was obtained.
Phase 3 Assessment of the ASQ-17
There were no misfitting item in the ASQ-17(both Infit and Outfit MnSq values of all items were 0.67~1.48, with the standard value 0.5~1.50, Appendix 2 Table 3).
The mean person location was -0.80 logits (Appendix 3 Figure 1, meet the standard less than 1.0 logit), which is better than ASQ-18, indicating ASQ-17 has good targeting between the difficulty of items and individual ability.
The variance explained by the principal component and unexplained variance explained by the first contrast in overall 17 items were 42.7% and 3.60(more than standad value 2.00), indicating ASQ-17 has more than two dimensions. While the variance explained by the principal component was 59.4%, 54.7%, 53.9% in dimension A, B and C respectively, which were closer to 60% and better than ASQ-18. The eigenvalue and unexplained variance in the first contrast in dimension A (ocular organic symptoms), B (visual functional symptoms) and C (systemic symptoms) were 1.70, 1.50, 1.80 and 10.00%, 11.10%, 12.30% (were closer to 10.00% than ASQ-19 and ASQ-18) respectively, indicating three dimensions setting was reasonable and enough for ASQ-17 and unnecessary to be divied into more dimensions.
The responses for 0, 1, 2, 3 scores were 453, 1846, 864 and 203 respectively (more than 10 responses per rating scale category). The results of option sorting analysis (Figure 1) and PSI/PSR (Table 2) were within the standard and similar with ASQ-18.
DIF analysis (Appendix 2 Table 4) showed the functional differences of each item by gender were -0.38~0.51logit, by age were -0.36~0.62 logit, indicating ASQ-17 has no significant difference between gender and age, and was suitable for different gender and age populations.
Consequently, the new ASQ-17 was adopted as the final version.
Phase 4 The predictive ability of the ASQ-17
Figure 2 showed the mean total scores of dimension A, B, and C and all three dimensions in asthenopia participants were significantly higher than non-asthenopia participants, which were 9.5±4.1 vs 3.5±3.2, 7.3±3.3 vs 2.5± 2.7, 4.3±2.2 vs 1.4±2.0, and 21.1±8.1 vs 7.4±7.0 respectively (p values <0.001).
Figure 3 showed the AUC of ASQ-17 in two groups was 0.899, indicating ASQ-17 has good diagnostic ability (p value<0.001). Youden’s index was up to the maximum value 0.784 when the cut-off value was 12.5, and the specificity and sensitivity index were 78.4% ,100% respectively. So, we suggested that asthenopia can be diagnosed if the total score was higher than 12.5 and treatment is recommended.
The top four mean score items were the 8th (the brightness of the screen caused eye discomfort when using mobile phones or computers), 1st (discomfort around eyes),2nd (eye dryness) and 4th (eye soreness), ranged from 1.51 to 1.53(Figure 4).