Characteristics and outcomes of patients who did not respond to a national spine surgery registry.

doi:10.21203/rs.3.rs-2011098/v1

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Characteristics and outcomes of patients who did not respond to a national spine surgery registry.

https://doi.org/10.21203/rs.3.rs-2011098/v1

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Journal Publication

published 04 Mar, 2023

Read the published version in BMC Musculoskeletal Disorders →

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Background

Loss to follow-up may bias outcome assessments in medical registries. This cohort study aimed to analyze and compare patients who failed to respond with those that responded to the Norwegian spine surgery register (NORspine).

Methods

We analyzed a cohort of 474 consecutive patients operated for lumbar spinal stenosis at four public hospitals in Norway during a two-year period. These patients reported sociodemographic data, preoperative symptoms, and Oswestry Disability Index (ODI), numerical rating scales (NRS) for back and leg pain to NORspine at baseline and at 3 and 12 months postoperatively. At a median of 50 (36–64) months after surgery, we contacted all patients who did not respond to NORspine after 12 months. Those who responded were termed responsive non-respondents and compared to 12 months respondents.

Results

123 (27%) of patients did not respond to NORspine 12 months after surgery, and 64 (52%) of those responded to our renewed survey at a median of 50 (36–64) months after surgery. At baseline, non-respondents were younger 63 (SD 11.7) vs 68 (SD 9.9) years, mean difference (95% CI) 4.7 years (2.6 to 6.7); p = < 0.001. Non-respondents were more frequently smokers 41 (30%) vs 70 (21%) RR (95%CI) = 1.40 (1.01 to 1.95); p = 0.044. There were no other relevant differences in other sociodemographic variables or preoperative symptoms. We found no differences in effect of surgery on non-respondents vs respondents (ODI (SD) = 28.2 (19.9) vs 25.2 (18.9), MD (95%CI) = 3.0 ( -2.1 to 8.1); p = 0.250.

Conclusion

We found that 27% of patients did not respond to NORspine 12 months after surgery for lumbar spinal stenosis. Non-respondents were somewhat younger and more frequently smokers. However, responsive non-respondents reported similar clinical outcomes compared to those who responded.

Validation

Loss to follow-up

Spine surgery

Attrition bias

Registry

Non-response

Lumbar spinal stenosis

Medical registries provide clinicians with large data sets of high external validity and complement randomized controlled trials that examine more targeted populations and treatments (1, 2). Medical registries can guide decision-making and improve the quality of care by monitoring patient-reported outcome measures (PROMs) stratified by different populations, diagnoses, and treatments (3, 4). Medical registries face higher attrition rates compared to clinical trials - rigorous attempts to attain data are costly and impractical in a registry setting (5). Still, sufficient follow-up rates are crucial for the quality of registries and awareness of follow-up rates is important when interpreting register data.

Non-respondents may systematically differ from respondents and introduce attrition bias that compromises the validity of register data (1, 6–8). However, some studies suggest that non-response occurs at random (2, 4, 5). The last assessment of non-respondents in NORspine was conducted in 2007 and reported a loss to follow-up of 22% at two years postoperatively and did not reveal any differences in outcomes between non-respondents and respondents (5). This study was conducted before NORspine expanded to a national registry and a reassessment is warranted. In this observational, we aimed to compare baseline characteristics and clinical outcomes for patients who did and who did not respond to NORspine.

This cohort study was based on retrospective analyses of prospectively collected NORspine data. We reached out those who did not respond to NORspine at 12 months after surgery, and performed an additional cross-sectional survey at a median of 50 (36–64) months after surgery.

NORspine

All Norwegian hospitals that offer spine surgery are obliged to report to NORspine. Currently, 70% of all degenerative spine surgeries done in Norway are registered in NORspine (9). NORspine is a consent-based register. Patients with primary infections of the spine, fractures of the spine, and patients who are unable to comprehend questionaries in Norwegian, are not invited to participate.

A NORspine dataset consists of both patient- and surgeon-reported variables. Patients complete a standardized questionnaire preoperatively on sociodemographic data and preoperative symptoms, as assessed by validated PROMs: Oswestry Disability Index (ODI) ranging from 0 (minimal disability) to 100 (bedbound), Numeric Rating Scales (NRS) ranging from 0 (no pain) to 10 (worst imaginable pain) for back and leg pain, and quality of life as assessed by EuroQol 5 Dimension 3 level − 0.59 (worse than dead) to 1,0 (perfect health)(10–13).

Surgeons report directly after the surgery on diagnoses, relevant comorbidities, and perioperative details such as type of surgery. The NORspine sends follow-up questionnaires to patients at 3 and 12 months after surgery by regular mail, including one reminder if the patient does not reply. Patients report directly to NORspine at follow-ups using PROMs (ODI, NRS back and leg pain, and EQ5D, and Global Perceived Effect (GPE) - a seven-point Likert scale (1 = completely recovered, 2 = much improved, 3 = slightly improved, 4 = unchanged, 5 = slightly worse, 6 = much worse, 7 = worse than ever)(14).

Data collection

We analyzed prospectively collected NORspine data on patients operated for lumbar spinal stenosis (LSS) at four hospitals between January 1st, 2015 and December 31st, 2016. Patients that had returned NORspine questionnaire at 12-months postoperatively are termed as respondents, while those who did not return the questionnaire are termed as non-respondents. At a median of 50 (36–64) months after surgery, we contacted the non-respondents treated at our hospitals and sent standard postoperative questionnaires by mail. We sent one reminder by mail, and one by SMS, to those who still did not respond. Those who responded at a median of 50 (36–64) months after surgery are termed as “responsive non-respondents” and those who did not respond to any contact are termed as “resistant non-respondents”.

Baseline and outcome measures

We compared the baseline characteristics of the non-respondents and respondents and clinical outcomes reported by PROMs between responsive non-respondents (median 50 months after surgery) and respondents (12-months after surgery). We also dichotomized clinical outcome using the GPE scale, defining success as “completely recovered” and “much improved”, and compared the proportions of successfully treated non-respondents versus respondents. Finally, we compared the baseline characteristics of the responsive non-respondents and the resistant non-respondents.

Statistics

We used descriptive statistics presented by means (SD) for continuous variables and numbers (percentages) for categorical variables. We analyzed between-group differences by mean difference (95%CI) and Students T test for continuous variables, or relative risk (95%CI) and z-statistics for categorical variables. Statistical analyses were performed using SPSS, version 26 (IBM Corp., Armonk, N.Y. USA) and MedCalc Software Ltd. Relative risk calculator. https://www.medcalc.org/calc/relative_risk.php (Version 20.027; accessed March 14, 2022).

Ethical considerations

All patients provided an informed consent when entering the registry. The Norwegian national ethical board (Regional committees for medical and health research ethics, reference number 2017/2157) approved this study, as did the data protection officers at the four participating hospitals. This study was conducted in accordance with the Helsinki declaration and we have reported the results according to the STROBE recommendations (15, 16).

Of 474 eligible patients, 17 were deceased at the time we performed the cross-sectional data collection, leaving 457 patients for main analyses. At 12 months after surgery, 123 (27%) were non-respondents while 334 (73%) were respondents (Fig. 1). Of the 123 non-respondents, 64 (52%) patients returned questionnaires at a median of 50 (36–64) months after surgery (“responsive non-respondents”) while 59 (48%) were “resistant non-respondents” (Fig. 1).

Baseline characteristics

The non-respondents were younger than the respondents 63 (SD 11.7) vs 68 (SD 9.9) years, mean difference (95% CI) 4.7 years (2.59 to 6.74); p = < 0.001. Non-respondents were more frequently smokers compared to respondents: 41 (30%) vs 70 (21%), RR (95%CI) 1.40 (1.01 to 1.95); p = 0.044. Furthermore, non-respondents had a lower proportion of surgeon-reported relevant comorbidities compared to respondents 93 (69%) vs 243 (78%), RR (95%CI) 0.89 (0.77 to 1.00); p = 0.047. However, we found no difference in ASA-classification between non-respondents and respondents: the number (%) of ASA grade 1 and 2 was 111 (79%) vs 242 (72%) RR (95% CI) 1.09 (0.98 to 1.22); p = 0.100. As shown in Table 1, there were no other differences between the non-respondents and respondents at baseline. Also, we found no differences in type of surgery (decompression only vs decompression and additional fusion) among the non-respondents and respondents.

Table 1

Baseline characteristics and perioperative data of 474 patients with lumbar stenosis who reported to NORspine.
	N Missing	Non-respondents (SD, %)	N Missing	Respondents (SD, %)	Mean difference (95% CI) or relative risk (95% CI)	P value
Age (years)	N = 140 Missing = 0	63.0 (11.7)	N = 334 Missing = 0	67.7 (9.9)	-4.7 (-6.74 to -2.59)	< 0.001
Female	N = 140 Missing = 0	71 (51%)	N = 334 Missing = 0	183 (55%)	0.93 (0.77 to 1.12)	0.426
BMI	N = 136 Missing = 4	28.6 (4.5)	N = 329 Missing = 5	28.2 (4.5)	0.4 (-0.48 to 1.33)	0.362
Comorbidities*	N = 135 Missing = 5	93 (69%)	N = 310 Missing = 24	243 (78%)	0.89 (0.77 to 1.00)	0.047
ASA grade I and II	N = 140 Missing = 0	111 (79%)	N = 334 Missing = 0	242 (72%)	1.09 (0.98 to 1.22)	0.100
Smokers	N = 138 Missing = 2	41 (30%)	N = 331 Missing = 3	70 (21%)	1.40 (1.01 to 1.95)	0.044
Norwegian as first language	N = 138 Missing = 2	130 (94%)	N = 331 Missing = 3	324 (98%)	0.96 (0.92 to 1.01)	0.090
University or college education > 4 years	N = 136 Missing = 4	30 (22%)	N = 328 Missing = 6	83 (25%)	0.87 (0.60 to 1.26)	0.463
Single civil status	N = 139 Missing = 1	30 (22%)	N = 332 Missing = 2	85 (26%)	0.84 (0.58 to 1.22)	0.361
Preoperative ODI	N = 136 Missing = 4	42.3 (16.1)	N = 329 Missing = 5	40.4 (15.8)	1.87 (-1.31 to 5.06)	0.248
Preoperative NRS back pain	N = 127 Missing = 13	6.9 (2.0)	N = 312 Missing = 22	6.8 (2.1)	0.17 (-0.26 to 0.61)	0.430
Preoperative NRS leg pain	N = 123 Missing = 17	6.9 (2.2)	N = 311 Missing = 23	7.0 (2.1)	-0.04 (-0.49 to 0.40)	0.844
Decompression only type surgery	N = 140 Missing = 0	122 (87%)	N = 334 Missing = 0	301 (90%)	0.98 (0.84 to 1.15)	0.820
Fusion type surgery	N = 140 Missing = 0	18 (13%)	N = 334 Missing = 0	33 (10%)	1.27 (0.74 to 2.18)	0.393
* Comorbidities that were assessed as relevant by the reporting surgeon.

Clinical outcomes

The median follow-up time for responsive non-respondents was 50 (36–64) months, whereas respondents replied at 12 months after surgery. As presented in Table 2, we did not find any differences in mean (SD) ODI scores between the responsive non-respondents and respondents postoperatively 28.2 (19.9) vs 25.2 (18.9), mean difference (95% CI) = 3.0 (-2.1 to 8.1); p = 0.250. Nor did we find any differences between responsive non-respondents versus respondents for NRS back pain, 4.6 (3.0) vs 4.1 (2.9), mean difference (95% CI) 0.43 (-0.3 to 1.2); p = 0.271 or NRS leg pain score 4.0 (3.2) vs 3.9 (3.1) mean difference (95% CI) 0.15 (-0.7 to 1.0); p = 0.719. Finally, we found similar proportions of successively treated patients among non-respondents and respondents, as assessed by GPE (63 (70%) vs 330 (79%), RR (95%CI) 0.89 (0.75 to 1.06); p = 0.183).

Table 2

Postoperative clinical outcomes for a selection of NORspine patients operated for lumbar spinal stenosis.
	N Missing	Responsive non-respondents Mean (SD) / n (%)	N Missing	Respondents Mean (SD)/ n (%)	Mean diff (95% CI) or Relative risk (95% CI)	P-value
ODI	N = 64 Missing = 0	28.2 (19.9)	N = 333 Missing = 1	25.2 (18.9)	2.99 (-2.1 to 8.1)	0.250
NRS back pain	N = 64 Missing = 0	4.6 (3.0)	N = 328 Missing = 5	4.1 (2.9)	0.43 (-0.3 to 1.2)	0.271
NRS leg pain	N = 63 Missing 1	4.0 (3.2)	N = 321 Missing = 12	3.9 (3.1)	0.15 (-0.7 to 1.0)	0.719
Success by GPE*	N = 63 Missing 1	63 (70%)	N = 330 Missing = 3	330 (79%)	0.89 (0.8 to 1.1)	0.183
*Success defined as “completely recovered” or “much recovered” on GPE scale.

Resistant non-respondents

Appendix Table 1 compares the resistant non-respondents (59 (48%)) to responsive non-respondents (64 (52%)) at a median of 50 (36–64) months post surgery.

The main findings from this Norwegian register-based cohort study of patients who had spinal surgery due to lumbar spinal stenosis, were that responsive non-respondents were somewhat younger and tended to smoke more often than those who responded. Moreover, we found no differences in clinical outcomes or proportions of successfully treated responsive non-respondents compared to respondents.

Several studies have demonstrated that non-respondents are younger than respondents (2, 4, 17–19). Completing and posting questionnaires consumes time, and younger patients may be more busy due to work and family obligations. Our finding that non-respondents were more frequently smokers has also been supported by others (2, 4, 6, 18, 19). Also, we found that surgeons reported fewer relevant comorbidities for non-respondents than for respondents. However, the variable “relevant comorbidity” is subject to interpretation by the treating surgeon. The registration of relevant comorbidities by the treating surgeon may be questioned. A validation study of NORspine data found that surgeons tended to underreport relevant comorbidities and that ASA-grading done by the anesthetist could be more reliable in assessment of comorbidity (20). In our study, there was no difference in proportions of ASA 1 and 2 grade patients among non-respondents compared to respondents.

In addition to young age and smoking, previous studies of non-respondents also reported a predominance of the male gender, living alone, and higher preoperative disability at baseline (2, 4–6, 17–19). However, these findings were not reproduced in our study. At baseline, we found no differences in pre-operative ODI between the non-respondents and respondents (Table 1). However, a Swedish spine register study found that non-respondents had higher ODI scores, lower quality of life (EuroQol 5D), and lower function (Short form health survey - SF-36) preoperatively. The aforementioned study implies that non-respondents had a worse starting point and were not quite representative of the entire register population (2).

Another Swedish spine register study reported that non-respondents had inferior clinical outcomes (6), while other studies support our findings of similar postoperative outcomes for non-respondents versus respondents (2, 4, 5, 17–19). Small differences in PROMs have been reported between non-respondents and respondents, but the magnitudes of these differences were assessed as clinically irrelevant (21).

Some studies suggest that loss to follow-up of as little as 5% (22, 23) may cause bias, while rates above 20% (24) could potentially lead to serious bias. There is a variation in loss to follow-up rates in spine register studies ranging from 12% (4) to 42% (2). The loss to follow-up at 12 months post surgery in our study was 27%, hence in the middle of numbers reported by other spine registries. Also, previous studies have implied that it is not the extent of loss to follow-up but the type of attrition that is relevant for assessment of bias (1, 7, 25). Classification of missing data based on Rubin´s and Little´s work differentiates between data missing at random (MAR), missing completely at random (MCAR), and missing at non-random (MNAR) (26). In cases of MAR, the non-respondents and respondents differ at baseline but report similar clinical outcomes after treatment; in cases of MCAR, the groups are similar at baseline and report similar outcomes; in cases of MNAR, the two groups compared report different outcomes. The largest risk of bias in a registry setting arises in cases of MNAR - the results are based on respondents only (1). The use of multiple imputations and mixed linear models are used to manage MNAR (25). Parai et al. found loss to follow-up in the Swedish spine registry to be of the MNAR type (6), while Solberg et al. and Højmark et al. found MAR as the mechanism of loss to follow-up in the Norwegian and Danish spine registries (4, 5). In our study data seem to be missing at random, since baseline characteristics differ somewhat between non-respondents and respondents, but the two groups report similar outcomes.

The methods used by registries to collect data may influence patient response. Reasons for patients not responding can be related to forgetfulness, lack of interest, questionnaires being too time-demanding. Clinical visits and telephone interviews have been shown to increase response rates (5), but are time-consuming, costly, and not practical in a register setting. A web-based registry has shown high loss to follow-up (59%) (18). A combination of postal and web-based methods could complement each other and increase response rates. NORspine is planning to implement a combination of methods to increase the follow-up rate.

Strengths and limitations:

Main weaknesses of our study are that we reached out to a sample of all potential register patients and that responsive non-respondents were compared to respondents at different time points, i.e., 12 months vs 50 (36–64) months post surgery. However, previously published data have shown that patient who are followed longer than one year after spinal surgery keep reporting stable symptoms (27).

In this observational study based on data from the national spine registers NORspine, we found a 27% loss to follow-up at 12 months after surgery for lumbar spinal stenosis. We reached out to non-respondents at a median of 50 (36–64) after surgery and found that non-respondents were somewhat younger and more frequently smokers. However, responsive non-respondents reported similar clinical outcomes compared to those who responded. According to previous works, our findings mean that loss to follow-up poses a small risk of bias in NORspine.

NORspine Norwegian registry for spine surgery

ODI Oswestry Disability Index

NRS Numerical rating scale

PROM Patient-reported outcome measure

GPE Global Perceived Effect

EQ-5D European quality of life 5-dimension questionnaire

SF-36 Short form- 36 health survey

MAR Missing at random

MNAR Missing not at random

MCAR Missing completely at random

Ethics approval and consent to participate: The Norwegian national ethical board (Regional committees for medical and health research ethics, reference number 2017/2157) approved this study, as did the data protection officers at the four participating hospitals. All methods were carried out in accordance with relevant guidelines and regulations. We confirm that informed consent was obtained from all subjects and/or their legal guardian(s).
Consent for publication: Not applicable
Availability of data and materials: The datasets generated and analysed during the current study are not publicly available due to the Norwegian data protection law but are available from the corresponding author on reasonable request.
Competing interests: The authors declare that they have no competing interest
Fundings: None
Authors' contributions: All authors read and approved the manuscript. GL is the guarantor and gave the original concept of the study. GL and OKA were involved in the study design. OKA supervised the study, was involved in statistics, and took part in writing of the manuscript. SK was involved in the collection of the data, statistics and writing of the manuscript. FD and TS contributed to the study design and writing of the manuscript.
Acknowledgements: The authors would like to thank the Norwegian Registry for Spine Surgery (NORspine).

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No competing interests reported.

AppendixTable1Baselinecharacteristicsofresponsivenonrespondentsn64andresistantnonrespondentsn59..docx

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You are reading this latest preprint version

Characteristics and outcomes of patients who did not respond to a national spine surgery registry.

Status:

Journal Publication

Version 1

Abstract

Background

Methods

Results

Conclusion

Figures

Background

Methods

NORspine

Data collection

Baseline and outcome measures

Statistics

Ethical considerations

Results

Baseline characteristics

Clinical outcomes

Resistant non-respondents

Discussion

Strengths and limitations:

Conclusion

List Of Abbreviations

Declarations

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1