Twelve-month outcomes following midurethral sling procedures for stress incontinence: impact of obesity

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1 DOI: / Urogynaecology Twelve-month outcomes following midurethral sling procedures for stress incontinence: impact of obesity EA Brennand, a S Tang, a T Williamson, b C Birch, a M Murphy, a M Robert, a S Ross, a for the Calgary Women s Pelvic Health Research Group a Division of Urogynaecology, Department of Obstetrics and Gynaecology, University of Calgary, Calgary, AB, Canada, b Department of Community Health Sciences, University of Calgary, Calgary, AB, Canada Correspondence: EA Brennand, Division of Urogynaecology, Department of Obstetrics and Gynaecology, University of Calgary, 4th Floor, North Tower, Foothills Medical Centre, th Street, Calgary, AB T2N 2T9, Canada. erin.brennand@albertahealthservices.ca Accepted 1 September Published Online 15 October Objective To evaluate impact of body mass index (BMI) 30 on objective and subjective cure rates 12 months after midurethral sling surgery. Design Secondary analysis. Setting Three hospitals in Calgary, Canada, Population A total of 182 women enrolled in a randomised control trial of tension-free vaginal tape versus transobturator tape. Methods Women were classified as obese or nonobese from height and weight on day of surgery. Women underwent postoperative standardised pad tests, self-reporting of urinary incontinence, and quality of life scores. Categorical data compared with chi-square or Fisher s exact, continuous data compared with Mann Whitney U test. Main outcome measures Primary outcome was objective cure, defined as <1 g urine loss on postoperative 1-hour pad test. Secondary outcomes were subjective cure of incontinence (no stress incontinence in previous 7 days), presence of urinary urgency in previous 7 days, Urogenital Distress Inventory (UDI-6) scores, Incontinence Impact Questionnaire (IIQ-7) scores, and surgical complication rates. Results Objective cure differed, with 85.6% of nonobese women leaking <1 g on 1-hour pad test, versus 67.8% of obese women (P = 0.006, risk difference [RD] 17.8%, 95% confidence interval [95% CI] %). Subjective cure was 85.8% for nonobese women versus 70.7% for obese women (P = 0.016, RD 15.1%, 95% CI %). For both groups, improvement was seen for postoperative UDI-6 (median 33.3 [ 44.4 to 22.2] and 27.2 [ 44.4 to 16.7]) and IIQ-7 scores (median 26.2 [ 45.2 to 14.3] and 23.8 [ 42.9 to 14.3]). No differences existed in rates of operative complications between the two groups. Conclusion Twelve months after midurethral sling surgery, obese women experience lower rates of cure than those who are nonobese. Keywords Midurethral sling, obesity, tension-free vaginal tape, transobturator tape, urinary incontinence. Please cite this paper as: Brennand EA, Tang S, Williamson T, Birch C, Murphy M, Robert M, Ross S, for the Calgary Women s Pelvic Health Research Group. Twelve-month outcomes following midurethral sling procedures for stress incontinence: impact of obesity. BJOG 2015;122: Introduction Midurethral slings (MUS), such as retropubic tension-free vaginal tape (TVT) and transobturator tape (TOT) procedures, are the standard treatment for female stress urinary incontinence (SUI). 1 The two procedures have been shown in the literature to have comparable outcomes 2 4 with selfreported cure rates of 85%. Although this rate is admittedly high, failures are frustrating to both patient and physician. A number of studies have looked for factors that may predict or raise odds of failure. Knowledge of these factors would allow physicians to provide better preoperative counselling to those likely to have suboptimal outcomes. One patient characteristic that may predict surgical outcome is body mass index (BMI). 5 9 It is thought that obesity may place additional pressure on the pelvic floor and sling causing the procedure to function less optimally. 8 A number of studies, of various methodologies including mail-out surveys, retrospective chart reviews and a prospective cohort, have reported no relationship between BMI and clinical outcome after midurethral slings All of ª 2014 Royal College of Obstetricians and Gynaecologists 1705

2 Brennand et al. these papers have significant limitations including potential confounders such as concomitant prolapse surgery at time of midurethral sling, 9 very short term and variable followup periods, 10,11,13 16 use of BMI cut-offs that did not correspond with World Health Organization definitions of obesity 14,16 and very small sample sizes. 13,17 To date, five studies a retrospective cohort, two secondary analyses of the E-TOT trial, and two mail out questionnaires have shown a relationship between BMI and cure rates, suggesting rates of cure that are 10 30% lower than those seen in nonobese women. 5 9 These papers also have limitations including use of a nonvalidated research survey as the primary outcome, 7,9 variable follow-up periods, 9 and short-term follow up of only 6 months. 6 It is important to understand if a difference in cure rates exists for obese women so that they can be informed of lower success rates as part of the informed consent process. If a difference does exist in cure rate, consideration of weight loss as an alternative or complimentary therapy could be important for the obese women. The literature cannot presently guide us regarding the impact of preoperative obesity on outcomes following midurethral sling procedures. Between 2005 and 2007, our institution conducted a randomised controlled trial (RCT) evaluating the effectiveness of the MUS and TOT procedures with follow up at 12 months. 18 A total of 199 women were recruited into the study, including a significant proportion of obese women. Due to the high proportion of obese women in our RCT, a secondary analysis comparing outcomes of nonobese and obese women was possible. Our null hypothesis was that obese group would have similar cure rates than the nonobese group of women. Methods The original RCT was conducted by seven surgeons (urogynaecologists and urologists) at one tertiary-care hospital and two community hospitals in Calgary, Canada between 2005 and The aim of the study was to compare the effectiveness of MUS and TOT procedures with 12-month follow up. All participating surgeons were experienced in incontinence surgery, including MUS and TOT procedures. Both the original study and this secondary analysis were approved by the University of Calgary Conjoint Health Research Ethics Board (Ethics ID 18421). Women were eligible for inclusion in the RCT if they had demonstrable SUI, elected surgical management, and were suitable candidates for both MUS and TOT approaches. Women were excluded from participation if they had previous incontinence surgery, had overactive bladder, postvoid residuals of >100 ml, desired future childbearing or had major medical comorbidities such as neurological illness or immunocompromised states. Women were randomised to receive either procedure. Boston Scientific (Natick, MA) devices were used for all procedures: the outside-in Obtryx Halo for transobturator procedures, and the Advantage system for MUS procedures. Midurethral sling procedures were performed in isolation, no concomitant prolapse surgeries were conducted. Surgical details, height and weight at time of surgery, intraoperative and postoperative complications were extracted from the hospital and clinic charts. For the RCT, women completed demographic questionnaires at baseline, which included self-reported severity of stress and urge urinary incontinence (UUI) symptoms over the previous 7 days. The Urogenital Distress Inventory (six-item) and Incontinence Impact Questionnaire (seven-item) were administered to serve as baseline status of incontinence-related quality of life. 19,20 Other patient characteristics and surgical details were extracted from hospital charts. Patient follow up was conducted 12 months after surgery. Women were invited to attend the clinic for review and to carry out a standardised pad test for objective cure of SUI and to repeat the subjective outcome questionnaires for symptom experience and incontinencerelated quality of life questionnaires. Women who were unable to attend the pad test appointment were invited to complete and mail the questionnaires. The complete details of the study can be found in our previous publication. 18 Secondary analysis of the RCT data involved comparing the outcomes for the obese and nonobese cohorts. For this analysis obesity was defined using weight and height on the date of surgery recorded from the woman s hospital chart. The BMI was calculated using the standard formula of kg/ m 2. The current World Health Organization International definitions for obesity (BMI 30), as well as the three subclassifications of obesity, were used. 21 Objective measurement of SUI cure at 12 months following surgery was by standardised 1-hour pad test. Women undertaking this test had their bladders emptied by catheterisation for postvoid residual volume, and were then retrofilled with 300 ml of sterile water. Women wore preweighed pads while undertaking physical activities recommended by the International Continence Society. 22 Women who leaked <1 g on pad testing were considered to be objectively cured. Subjective cure was measured at 12 months by questionnaire. Women were asked if they lost or leaked urine when you coughed, laughed, sneezed, lifted, exercised, etc. over the previous 7 days. If women did experience stress leakage, they were asked to qualify if it was no problem, a small problem or a big problem. For this secondary analysis, subjective cure was defined as no leakage. Patients were also asked to complete the Urogenital Distress Inventory (six-item) and Incontinence Impact Questionnaire (seven-item) questionnaires ª 2014 Royal College of Obstetricians and Gynaecologists

3 Impact of obesity Rates of postoperative complications (blood loss >200 ml or bladder perforation), intermittent or indwelling catheter at time of discharge, or postoperative sling revision (lysis for obstruction or excision for erosion) were also compared between obese and nonobese cohorts. Given the sample size of 121 nonobese and 61 obese women who completed the 12-month follow up in the RCT, and assuming an estimated cure rate of 85% for nonobese women, 5 7,10 15 our sample had 80% power to detect a difference of 20% (85% for nonobese versus 65% for obese) in cure rate, at the 5% level of significance. Original data entry and management were carried out using ACCESS (Microsoft, Redmond, WA) and statistical analysis for this study was carried out using SPSS 20.0 (IBM, Armonk, NY) and SAS 9.3 (SAS Institute Inc., Cary, NC). Descriptive statistics (medians, interquartile ranges, proportions) were calculated for the baseline data. Baseline patient characteristics of age, ethnicity, nulliparity, smoking, menopausal status, presence of UUI symptoms before surgery and quality of life scores were compared between the women in each BMI category to determine whether differences existed that would need to be adjusted for in the analysis of the primary outcome. We tested for baseline differences between the nonobese and obese groups by chi-square test for categorical variables. Age, Urogenital Distress Inventory and Incontinence Impact Questionnaire scores were not normally distributed and the Mann Whitney U test was used to test for differences between the two BMI cohorts. The analysis for objective outcome compared the proportion of women who demonstrated cure on the 1-hour pad test using the chi-square test; risk difference (RD) and 95% confidence interval (95% CI) are also reported. A logbinomial model was used to examine whether objective cure between the obese and nonobese groups was affected by surgical procedure (tension-free vaginal or transobturator tape). The obese patient population was broken down into the three classes of obesity, and objective and subjective cure rates were compared by Fisher s exact test. Analysis of subjective outcomes, the presence of postoperative SUI symptoms and UUI were carried out by chisquare test for binomial outcomes. Urogenital Distress Inventory and Incontinence Impact Questionnaire scores, as well as overall change from baseline scores, were compared by Mann Whitney U test. The STROBE Statement guidelines for reporting observational cohort research were followed. 23 Results Our original study included 199 women, of whom n = 61 (30.7%) were defined as obese and n = 136 (68.3%) as nonobese using the World Health Organization classification. 18,21 The number of individuals in each stage of the study are shown in Figure 1. Objective outcome (pad test result) was available for 111 nonobese women and 59 obese women, and subjective outcomes (questionnaire responses) for 121 nonobese women and 61 obese women. Baseline data are presented in Table 1: no statistically significant differences were found between the baseline variables in the obese and nonobese cohorts. Primary and secondary outcomes at 12 months postoperatively are reported in Table 2. Of the women in the nonobese group, 95 (85.6%) were objectively cured by 1- hour pad test, compared to 40 (67.8%) in the obese group (P = 0.006, RD 17.8%, 95% CI %). Subjective cure differed between the two cohorts with 103 (85.5%) of the nonobese women being cured compared with 41 (70.7%) of the obese (P = 0.016, RD 15.1%, 95% CI %). A difference in postoperative UUI symptoms also existed, with 37.1% of nonobese women and 61.0% of obese women reporting presence of these symptoms (P = 0.003, RD 24.0%, 95% CI %). For nonobese women, 22.4% reported bothersome urgency compared with 45.8% of obese women (P = 0.001, RD 23.4%, 95% CI %). No statistical differences existed between the two groups in terms of complications (blood loss >200 ml or bladder perforation, 2.9%, n = 4 nonobese versus 4.9%, n = 3 obese), intermittent or indwelling catheter at time of discharge (65.4%, n = 89 nonobese versus 62.3%, n = 23 Non-obese n = 136 in RCT Number of individuals providing follow up at 12 months* n = 121 (response = 89.0%, 121/136) Primary outcome Pad test at 12 months * Pad test done n = 111 (91.7%) Pad test not done n =10 (8.3%) Secondary outcomes Questionnaire completed at 12 months Questionnaires done n = 120 (99.2%) Questionnaires not done n = 1 (0.8%) Women in original study n = 199 Obese n = 61 in RCT Baseline BMI not available n = 2 Number of individuals providing follow up at 12 months* n = 61 (response = 100%, 61/61) Primary outcome Pad test at 12 months * Pad test done n = 59 (96.7%) Test not done n = 2 (3.3%) Secondary outcomes Questionnaire completed at 12 months Questionnaires done n = 58 (95.1%) Questionnaires not done n = 3 (4.9%) Figure 1. Number of women in each stage of the study. Note *Some women provided partial follow up, the pad test or questionnaire, but not both at 12 months. ª 2014 Royal College of Obstetricians and Gynaecologists 1707

4 Brennand et al. Table 1. Baseline characteristics of women included in this analysis Characteristic BMI <30 (n = 121) BMI 30 (n = 61) P value Median age (interquartile range) 50 (45 57) 50 (44 58) 0.992** White ethnic group* 104 (93.7%) 54 (96.4%) 0.460*** Current smoker* 22 (18.8%) 8 (13.1%) 0.336*** Nulliparous * 3 (2.5%) 4 (6.7%) 0.228*** No vaginal delivery* 10 (8.4%) 8 (13.3%) 0.301*** Postmenopausal status* 51 (44.7%) 28 (50.0%) 0.518*** Hormone replacement use* 22 (18.2%) 10 (16.3%) 0.710*** Questionnaire findings Median UDI-6 score* (interquartile range) 38.9 ( ) 47.2 ( ) 0.272** Median IIQ-7 score* (interquartile range) 33.3 ( ) 33.3 ( ) 0.331** SUI symptoms in past 7 days* None 4 (3.5%) 1 (1.8%) >0.999*** Yes, symptoms present 111 (96.5%) 56 (98.2%) Bothersome SUI in past 7 days* None or no bother 5 (4.3%) 1 (1.8%) 0.665*** Small or large problem 110 (95.7%) 56 (98.2%) UUI symptoms in past 7 days* None 39 (34.2%) 13 (22.8%) 0.127*** Yes, symptoms present 75 (65.8%) 44 (77.2%) Bothersome UUI in past 7 days* None or no bother 41 (36.0%) 13 (22.8%) 0.081*** Small or large problem 73 (64.0%) 44 (77.2%) Midurethral sling approach Transobturator tape (TOT) 56 (46.3%) 29 (47.5%) 0.872*** Tension-free vaginal tape (TVT) 65 (53.7%) 32 (52.5%) *Excludes missing values. For some comparisons, up to 11 observations were excluded. Statistical test: **Mann Whitney U test; *** chi-square test. obese), or postoperative sling revision (lysis for obstruction or excision for erosion, 3.7%, n = 5 nonobese versus 1.6%, n = 1 obese). Statistically significant differences existed between nonobese and obese women for the median Urogenital Distress Inventory (5.6, 95% CI versus 11.1, 95% CI , P = 0.044) and Incontinence Impact Questionnaire scores (0.0, 95% CI versus 0.0, 95% CI , P = 0.033) at 12 months. The log-binomial model found that the probability of objective cure remained significantly higher for nonobese women than obese women (P = 0.018, relative risk [RR] 1.26, 95% CI ), after controlling for surgery type (Table 3). Surgery type was not significantly associated with cure (P = 0.492, RR 0.95, 95% CI ). Objective cure rates differed across the three classes of obesity. Figure 2 depicts a decline in percentage of women objectively cured as the severity of obesity increased. In the Class I group (BMI kg/m 2 ), 75.6% (n = 28/37) of women were objectively cured. In the Class II group (BMI kg/m 2 ), 66.7% were objectively cured (n = 10/15). For the Class III group (BMI 40 kg/m 2 ), 28.6% (n = 2/7) were objectively cured. Objective cure rates were compared by Fisher s exact test (P = 0.049). Subjective cure rates also differed across the classes of obesity. In the Class I group, 77.8% (n = 28/36) of women were objectively cured. In the Class II group, 80.0% were subjectively cured (n = 12/15). For the Class III group, 14.3% (n = 1/7) were subjectively cured. Subjective cure rates were compared by Fisher s exact (P = 0.004). Given that three pairwise comparisons were being made, a Bonferroni-adjusted significance level of was used to test for pairwise differences between obesity classes. None of the pairwise comparisons in the objective outcome analysis were considered significant with the Bonferroni correction. For the subjective outcomes, both the Class I versus Class III and Class II versus Class III pairwise analyses were significant with the Bonferroni correction (P = and P = 0.007). Discussion Main findings Our findings suggest that preoperative obesity impacts the cure rate of midurethral sling at 12 months. In this study, 1708 ª 2014 Royal College of Obstetricians and Gynaecologists

5 Impact of obesity Table 2. Patient outcomes at 1 year after surgery 12-month follow up BMI <30 (n = 121) BMI 30 (n = 61) P value RR (95% CI) RD (95% CI) Pad test <1 g (cure) 95 (85.6%) 40 (67.8%) 0.006** 1.26 ( ) 17.8 ( ) 1 g or more 16 (14.4%) 19 (32.2%) SUI symptoms in past 7 days None 103 (85.8%) 41 (70.7%) 0.016** 1.21 ( ) 15.1 ( ) Yes, symptoms present 17 (14.2%) 17 (29.3%) Bothersome SUI in past 7 days None or no bother 108 (90.0%) 45 (77.6%) 0.025** 1.16 ( ) 12.4 ( ) Small or large problem 12 (10.0%) 13 (22.4%) UUI symptoms in past 7 days None 73 (62.9%) 23 (39.0%) 0.003** 1.61 ( ) 24.0 ( ) Yes, symptoms present 43 (37.1%) 36 (61.0%) Bothersome UUI in past 7 days None or no bother 90 (77.6%) 32 (54.2%) 0.001** 1.43 ( ) 23.4 ( ) Small or large problem 26 (22.4%) 27 (45.8%) Median UDI-6 score (IQR) 5.6 ( ) 11.1 ( ) 0.044* n/a Median change in UDI-6 score (IQR) 33.3 ( 44.4 to 22.2) 27.2 ( 44.4 to 16.7) 0.294* n/a Median IIQ-7 score (IQR) 0.0 ( ) 0.0 ( ) 0.033* n/a Median change in IIQ-7 score (IQR) 26.2 ( 45.2 to 14.3) 23.8 ( 42.9 to 14.3) 0.814* n/a Excludes missing values. For some comparisons, up to 11 observations were excluded. Statistical test: *Mann Whitney U test; **chi-square test. Table 3. Log binomial: pad test at 1 year after surgery Independent variable P value Relative risk (95% CI) BMI <30 vs ( ) Surgical procedure (TOT vs TVT) ( ) both the objective cure rate defined by pad-testing and subjective cure rates were statistically significantly lower for obese women. Rates of postoperative urgency were higher for obese women after surgery, and incontinence related quality of life scores (UDI-6 and IIQ-7) were lower. A trend was observed that suggests that cure rates decline as class of obesity increases. Strengths and limitations The strength of our study is that unlike previous publications, the SUI procedures were performed in isolation without any concomitant hysterectomies or prolapse repairs. Therefore, patient-reported success was purely related to patient satisfaction with the incontinence procedure. Additionally, our primary outcome of 1-hour pad testing was standardised and validated and uniformly applied at the 12-month postoperative time point. Our study was limited by the size of our obese study population. With the size of this data set, we only had power to detect extremely large differences in cure rates between MUS and TOT in the obese patient group. No large differences appeared to exist between the two procedures for obese women, but we cannot say whether one procedure is better for obese women as this data set cannot evaluate whether smaller differences in cure rate exist for the two procedures. Recent publication suggests the two procedures are equivalent for obese women. 24 Also, because of the clustered distribution of BMIs in our data set we were not able to explore the impact of BMI as a continuous variable with regression analysis. To evaluate if women with severe obesity have poorer outcomes than those with milder obesity, the objective and subjective outcomes of obese women were further grouped by the three classifications defined by the World Health Organization. An overall trend of declining cure rates was seen for both objective and subjective cure as obesity class increased. Only subjective cure was shown to be statistically lower for Class III (BMI 40 kg/m 2 ) when compared with Class I and Class II. Although this analysis suggests that the higher an individual s BMI, the lower the chances of cure, caution must be taken when interpreting these data because of the small number of women in the Class III obesity group. ª 2014 Royal College of Obstetricians and Gynaecologists 1709

6 Brennand et al. Figure 2. Objective cure rates by BMI category. Interpretation Our findings agree with the previously published papers that suggest obese women have lower rates of cure. 5 9 This study adds to the literature, as we are the first to report differences for both objective and patient-reported subjective cure rates of 12-month outcomes. Additionally, our data suggest that obese women may experience higher rates of postoperative urgency. In this secondary analysis, the obese women started out with a nonstatistically significant lower rate of urge symptoms at baseline enrolment, but ended up with a postoperative rate of urge symptoms that was 24% higher than nonobese women. Only one previous study has concluded that obese women are at a higher risk of developing postsurgical UUI. 15 Although both nonobese and obese women had significant improvements in their quality of life scores after surgery, obese women had slightly higher Urogenital Distress Inventory and Incontinence Impact Questionnaire scores at 1 year post surgery. The simplest explanation for this is that the obese cohort had more failed surgeries, resulting in residual stress incontinence and distress from these symptoms. It is also possible that the higher rates of postoperative urgency symptoms led to poorer quality of life scores for obese women. Conclusion Our data suggest that 1 year after midurethral sling, obese women have lower rates of subjective and objective cure, lower incontinence-related quality of life scores, and higher rates of urinary urgency. Some may argue that the risk of surgical failure is only slightly elevated for obese women (absolute risk increase 17.8%; relative risk 1.26, 95% CI ), and that this small difference should not impact clinical decision making. But given that female obesity rates in developed countries are 20 35% 25,26 and rising, this small difference in cure will add up to many surgical failures in a busy surgical practice. Therefore, to improve our obese women s rate of cure for stress incontinence, physicians may want to delay surgery until reasonable attempts at preoperative weight loss are made. Although there are no data proving that women who achieve weight loss before surgery will improve postoperative outcomes, it is reasonable to believe that this approach would be beneficial based on research that has shown only 5 10% total body weight needs to be lost to significantly improve SUI symptoms. 27 For morbidly obese women who may not be successful with a diet and exercise approach, consideration could even be given to bariatric surgery as studies have shown that those able to lose large amounts of weight through surgical intervention can achieve improvement in SUI symptoms comparable to midurethral sling results. 28,29 By emphasising the importance of achieving a healthy weight as part of the initial treatment for SUI in obese women, we have the opportunity to improve the health of our patients in a broader sense. Further research is required to see if the differences in cure rates exist at long term follow up, but based on the data that exist today, surgeons should be routinely informing obese women who are considering midurethral sling surgery that they are at higher risk of surgical failure. Disclosure of interest Erin Brennand has received an unrestricted educational grant from Cook Medical, grant-in-aid research funding from American Medical Systems as well as speakers fees 1710 ª 2014 Royal College of Obstetricians and Gynaecologists

7 Impact of obesity from Novo Nordisk and Astellas. Sue Ross and Magali Robert have received grant-in-aid research funding from Boston Scientific, Johnson & Johnson and Cook Medical. Magali Robert is on the Advisory Committee for Cook Myosite. Magnus Murphy and Colin Birch have received praeceptorship compensation for teaching from Cook Medical. Selphee Tang and Tyler Williamson have no disclosures. Contribution to authorship EB contributed to project development, chart review, data analysis and manuscript writing; ST and TW contributed to data analysis and manuscript writing; CB and MM contributed to project development and manuscript writing; and SR and MR contributed to project development, data analysis and manuscript writing. Details of ethics approval Both this secondary analysis and the original study were approved by the University of Calgary Conjoint Health Research Ethics Board (Ethics ID 18421). Funding Sue Ross, Magali Robert and other members of the Calgary Women s Pelvic Health Research Group received peerreviewed funding for the original randomised clinical trial from the Alberta Heritage Foundation for Medical Research, and grant-in-aid funding from Boston Scientific. Neither of the funders were involved in study design, conduct, analysis or writing. Acknowledgements This work was presented at the International Urogynecology Association s 38th Annual Meeting in Dublin, Ireland. Members of the Calgary Women s Pelvic Health Research Group: Richard Baverstock, Colin Birch, Penny Brasher, Erin Brennand, Kevin Carlson, Dave Cenaiko, Lorel Dederer, Philip Jacobs, Doug Lier, Tom Mainprize, Magnus Murphy, Magali Robert, Sue Ross, Cheryl Swaby, Selphee Tang and Tyler Williamson. & References 1 Lee E, Nitti VW, Brucker BM. Midurethral slings for all stress incontinence: a urology perspective. 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