- Email: [email protected]
Oral contraception and other factors in relation to back disorders in women: findings in a large cohort study
ORIGINAL RESEARCH ARTICLE
Oral Contraception and Other Factors in Relation to Back Disorders in Women: Findings in a Large Cohort Study Martin Vessey,* Rosemary Painter,* and Jonathan Mant† The Oxford-Family Planning Association contraceptive study includes 17,032 women, initially aged 25–39 years, recruited at 17 British family planning centers during the interval 1968 –1974 and subsequently followed-up for periods up to 26 years. This article examines the pattern of referral to hospital for back disorders among these women. Certain back disorders have been reported to occur more frequently in oral contraceptive users than in other women, and back pain has also been reported in some women consequent to using an intrauterine device. The disorders considered were spinal osteoarthritis, displaced cervical disc, displaced lumbar disc, other and unspecified displaced disc, cervicalgia, unspecified back pain, and sprains and strains of the back. Spinal osteoarthritis and unspecified backache were the only two conditions significantly related (both positively) to age. Displaced lumbar disc and other and unspecified displaced disc were strongly positively related to height and weight. Unspecified backache showed similar, but less striking (in terms of the magnitude of the relative risks), associations with height and weight. Little evidence was found of any association between oral contraceptive use and any of the back disorders, and the same was true for intrauterine device use. CONTRACEPTION 2000;60:331–335 © 2000 Elsevier Science Inc. All rights reserved.
(CSAG).1 It was found that approximately10% of the adult British population responding to a questionnaire reports some restriction of activities as a consequence of back pain in the preceding month. Clinically relevant disability shows a 1-year prevalence of 3%– 6%. The CSAG also reported on risk factors for back pain.1 Increasing age up to the 40s or 50s, heavy physical occupations, and cigarette smoking were all positively associated with back pain, whereas sex and social class (in women) seemed to have little effect. Surprisingly, the CSAG report did not refer to the importance of body size (weight, height, body mass index) in the etiology of back pain, but a number of other authors have done so.2–5 In addition, there has been some interest in the possibility that oral contraceptive use might increase the risk of back pain,6,7 but this relationship remains uncertain.8,9 The Oxford-Family Planning Association (OxfordFPA) contraceptive study collected information about referral to hospital for disease and injury in a population of 17,032 women during the interval 1968 –1994. We considered that the data on back pain might be of interest, and report here our findings for certain diagnostic groupings.
KEY WORDS:
Materials and Methods
back pain, cohort study, oral contraceptives, intrauterine device, displaced intervertebral disc
Introduction Back pain is an important cause of discomfort and disability in the adult population. The epidemiology of the disorder has been reviewed in depth recently by the British Clinical Standards Advisory Group *University Department of Public Health, Institute of Health Sciences, Headington, Oxford, England; and †University Department of General Practice, The Medical School, Edgbaston, Birmingham, England Name and address for correspondence: Professor Martin Vessey, University Department of Public Health, Institute of Health Sciences, Old Road, Headington, Oxford, OX3 7LF, England; Tel.: ⫹44 (0)1865-227030; Fax: ⫹44 (0)1865226655; e-mail: [email protected] Submitted for publication September 23, 1999 Revised November 15, 1999 Accepted for publication November 16. 1999
© 2000 Elsevier Science Inc. All rights reserved. 655 Avenue of the Americas, New York, NY 10010
A detailed description of the methods used in the Oxford-FPA study has been given elsewhere.10 In brief, 17,032 women were recruited at 17 large family planning clinics in England and Scotland between 1968 and 1974. At the time of recruitment, each woman was required to be: 1) aged 25–39 years, 2) married, 3) white and British, 4) willing to cooperate, and 5) either a current user of oral contraceptives of ⱖ5 months standing or a current user of a diaphragm or an intrauterine device of ⱖ5 months standing without previous exposure to the oral contraceptive pill. Among other items, each woman was asked questions at entry to the study about her age, childbearing history, contraceptive history, height and weight, social class, smoking behavior, and past medical history. ISSN 0010-7824/00/$20.00 PII S0010-7824(99)00103-1
332
Vessey et al.
Contraception 1999;60:331–335
Table 1. Summary of associations between first hospital referral rates for back disorders under consideration (ICD 8 codes in parentheses) and age, parity, social class, smoking, height, weight, and body mass index Disorder Spinal osteoarthritis (713.1) Displaced cervical disc (725.0) Displaced lumbar disc (725.1) Other and unspecified displaced disc (725.8, 725.9) Cervicalgia (728.0) Backache not otherwise specified (728.9) Sprains and strains of back (846, 847)
No. of women
Age (y)
Parity
Social class
Smoking
Height
Weight
Body mass index
207 80 316
111 ⫺ ⫺
⫺ ⫺ ⫺
2 ⫺ ⫺
1 ⫺ ⫺
⫺ ⫺ 111
⫺ ⫺ 111
⫺ ⫺ ⫺
311 243 966 292
⫺ ⫺ 11 ⫺
⫺ ⫺ ⫺ ⫺
⫺ ⫺ ⫺ ⫺
1 ⫺ ⫺ ⫺
111 ⫺ 111 ⫺
111 ⫺ 111 ⫺
111 ⫺ 1 ⫺
The rates on which the table is based were standardized for age. 1positive association; 2negative association; ⫺no association; one arrow p ⱕ0.05; two arrows p ⱕ0.01; three arrows p ⱕ0.001 (based on 2 trend) Groupings used in trend analysis: Age (y) 25–34, 35–39, 40 – 44, 45– 49, 50 –54, 55–59, 60⫹. Parity (term births) 0, 1, 2, 3, 4⫹. Social class (Registrar General’s Classification) higher (I–II), middle (III), lower (IV–V plus armed forces, unemployed). Smoking (cigarettes/day) never, ex-smoker, 1–14, 15⫹. Height (cm) ⬍155, 155–159, 160 –164, 165–169, ⬎170. Weight (kg) ⬍51, 51–57, 58 – 63, 64 –70, ⬎70. Body mass index (kg/m2) ⬍20, 20 –21.9, 22–23.9, 24 –25.9, 26 –27.9, ⬎28.
During follow-up, each woman was questioned by a doctor or a nurse at return visits to the clinic, and certain items of information were noted on a special form. These included details of pregnancies and their outcome, changes in contraceptive practices and reasons for the changes, and particulars of any referrals to hospital as either an outpatient or an inpatient. Diagnoses on discharge from hospital were confirmed by obtaining copies of discharge letters, summaries, and pathology reports. Women who stopped attending the clinic were sent a postal version of the follow-up form and, if this was not returned, were interviewed by telephone or at a home visit. The work was coordinated by a part-time research assistant in each clinic, and yearly follow-up was maintained until age 45 years, with an annual loss of contact of only about 0.4%. On reaching the age of 45 years, each woman was allocated to one of three groups: 1) oral contraceptives never used, 2) oral contraceptives used for a total of ⱖ8 years, and 3) other durations of oral contraceptive use. Only the women in the first two groups were followed-up annually in the detailed way already described until the end of the study in mid-1994. Accordingly, women in the third group have been omitted from the present analysis from age 45 years onwards. This analysis is based on the computation of woman-years of observation terminated by the occurrence of the first referral to hospital during follow-up for the back condition under consideration, by release from follow-up (emigration, death, short- term pill users reaching age 45 years), by loss to follow-up, by withdrawal of cooperation, or by the end of the study (July 1994). Both inpatient and outpatient referrals to hospital were included in the analysis. The diagnostic groups considered in the analysis were: pondylitis osteoarthritica, hereafter referred to as spinal osteo-
arthritis (International Classification of Diseases (ICD), Eighth Revision, code 713.1), displacement of cervical intervertebral disc (ICD code 725.0), displacement of lumbar (or lumbosacral) intervertebral disc (ICD code 725.1), displacement of other or unspecified intervertebral disc (ICD codes 725.8 and 725.9), cervicalgia (ICD code 728.0), backache not otherwise specified (ICD code 728.9), and sprains and strains of back (ICD codes 846 and 847). All diagnostic coding throughout the study was done by one of us (M.P.V.) to avoid interobserver variation. Discharge summary information was generally available for women admitted to hospital, but the coding of outpatient diagnoses mostly depended on information provided by the women themselves.
Results Table 1 shows the conditions under consideration and the numbers of women with hospital referral for the different conditions. Table 1 also shows, in summary form, which relationships between the various first hospital referral rates and a range of other factors of potential interest reached statistical significance. The direction of the significant relationships is indicated as well. A highly significant (p ⬍0.001) positive association with age was found for spinal osteoarthritis (ICD code 713.1) and a similar but less highly significant (p ⬍0.01) association was found for unspecified backache (ICD code 728.9). Highly significant positive associations were also found between displaced lumbar disc (ICD code 725.1), other and unspecified displaced disc (ICD codes 725.8 and 725.9), and unspecified backache (ICD code 728.9) and both height and weight. The relationship between these three conditions and body mass index was less consistent, failing to reach significance for displaced lumbar disc
Contraception 1999;60:331–335
Oral Contraception and Back Disorders
333
Table 2. Association between first referral to hospital for certain back conditions and measurements of body size Displaced lumbar disc (725.1)
Other and unspecified displaced disc (725.8, 725.9)
Unspecified backache (728.9)
Height (cm) ⬍155 155–159 160–164 165–169 ⬎170 2 (1) trend
1.0 1.2 (0.7–2.4) 2.0 (1.2–3.8) 1.9 (1.1–3.7) 2.3 (1.3–4.4) 14.3, p ⬍0.001
1.0 1.6 (0.9–2.9) 1.4 (0.8–2.6) 2.0 (1.1–3.7) 2.1 (1.2–4.0) 10.6, p ⫽ 0.001
1.0 1.2 (0.9–1.6) 1.4 (1.0–1.8) 1.4 (1.0–1.9) 1.6 (1.2–2.2) 14.1, p ⬍0.001
Weight (kg) ⬍51 51–57 58–63 64–70 ⬎70 2 (1) trend
1.0 1.4 (0.9–2.3) 2.0 (1.3–3.2) 1.7 (1.0–2.8) 2.2 (1.3–3.7) 10.3, p ⫽ 0.001
1.0 1.4 (0.9–2.4) 2.0 (1.3–3.3) 1.8 (1.1–3.0) 2.7 (1.6–4.6) 17.8, p ⬍0.001
1.0 1.1 (0.9–1.5) 1.4 (1.1–1.7) 1.7 (1.3–2.2) 1.5 (1.1–2.0) 22.4, p ⬍0.001
1.0 1.2 (0.8–1.8) 1.3 (0.9–1.9) 1.2 (0.8–1.9) 1.4 (0.8–2.4) 1.3 (0.7–2.3) 1.0, p ⫽ 0.310
1.0 1.5 (1.0–2.4) 1.9 (1.3–3.0) 1.8 (1.1–3.0) 2.2 (1.2–3.9) 2.3 (1.2–4.1) 11.6, p ⬍0.001
1.0 1.2 (1.0–1.5) 1.4 (1.1–1.7) 1.1 (0.9–1.5) 1.6 (1.2–2.2) 1.4 (1.0–1.9) 5.7, p ⫽ 0.017
Body mass index (kg/m2) ⬍20 20–21.9 22–23.9 24–25.9 26–27.9 ⬎28 2 (1) trend
Data are relative risks with 95% confidence intervals (in parentheses) adjusted for age.
and reaching only the 5% level of significance for unspecified backache. The relationships between the three conditions and measurements of body size are shown in more detail in Table 2. It is apparent that the disc lesions are more strongly associated with height and weight (as is indicated by the higher values of the relative risks) than is unspecified backache. As well as the age and body size associations shown in Table 1, it can also be seen that cigarette smoking was positively associated (at the 5% level of significance) with spinal osteoarthritis and other and unspecified displaced disc, whereas the former condition occurred less commonly in women from the higher
social classes than in women from the lower social classes (p ⬍0.05). The relationships between the various back disorders and some simple measures of oral contraceptive use (adjusted for age) are summarized in Table 3. The only condition showing any sizeable association with oral contraceptive use is displaced cervical disc, but none of the elevated relative risks for this disorder reaches statistical significance. Indeed, only one relative risk in Table 3 reaches the 5% level of statistical significance (past use of oral contraceptives in women with unspecified backache), and it has a value of only 1.2.
Table 3. Relative risks and 95% confidence intervals (in parentheses) for first referral to hospital for various back disorders in relation to oral contraceptive use Oral contraceptives Disorder Spinal osteoarthritis Displaced cervical disc Displaced lumbar disc Other displaced disc Cervicalgia Backache not otherwise specified Sprains/strains of back
No. of women
Ever used
207 80 316 311 243 966 292
1.3 (0.9–1.7) 1.5 (0.9–2.5) 1.1 (0.9–1.4) 1.0 (0.8–1.3) 0.9 (0.7–1.1) 1.1 (0.9–1.2) 1.0 (0.8–1.2)
Recently used†
Used in past
1.0 (0.6–1.6) 1.3 (0.7–2.6) 1.1 (0.8–1.5) 1.0 (0.8–1.4) 1.0 (0.7–1.4) 0.9 (0.7–1.1) 1.0 (0.8–1.4)
1.3 (1.0–1.8) 1.6 (0.9–2.8) 1.1 (0.8–1.4) 1.0 (0.8–1.3) 0.8 (0.6–1.0) 1.2 (1.0–1.3)* 0.9 (0.7–1.2)
Never users of oral contraceptives have been taken as the reference category; relative risks are adjusted for age. † Current use or use within the 12 preceding months; *p ⱕ0.05.
334
Vessey et al.
We also examined the relationships between the different back disorders and total duration of oral contraceptive use (nonuser, ⱕ12 months, 13–24 months, 25– 48 months, 49 –72 months, 73–96 months, ⱖ97 months) and interval since oral contraceptives last used (never used, used currently or within 12 preceding months, used 13–24 months ago, 25– 48 months ago, 49 –72 months ago, 73–96 months ago, 97–120 months ago, and ⱖ121 months ago). There were no statistically significant associations between any of the back disorders and total duration of oral contraceptive use. With regard to interval since last use, the data were significantly heterogeneous (but did not follow any pattern) with regard to unspecified backache, while the relative risks were significantly elevated in the two most extreme groups with regard to spinal osteoarthritis (last used 97–120 months ago, relative risk 2.0 [CI 1.2–3.3], last used ⱖ121 months ago, relative risk 1.8 [CI 1.2–2.6]). There were no significant associations between interval since last oral contraceptive use and any of the other back disorders. Finally, because intrauterine device users sometimes complain of back pain, which they attribute to the device, we recomputed the data given in Table 3, substituting intrauterine device use for oral contraceptive use. Only one relative risk reached statistical significance; this concerned the relationship between unspecified backache and past intrauterine device use (relative risk 1.3 [CI 1.1–1.5]).
Discussion The data reported here have some important limitations and require cautious interpretation. First, only those episodes of disease requiring referral to hospital were recorded in the Oxford-FPA study. Second, the diagnoses for episodes managed entirely on an outpatient basis were, in general, self-reported by the women concerned. Third, at best, the diagnoses given reflect routine clinical practice and have not been made according to strict research criteria. Finally, approximately 75% of the exposure to the pill in the Oxford-FPA study concerns preparations containing ⱖ50 g estrogen. Pills containing such a dose of estrogen are now rarely used. Spinal osteoarthritis showed the expected strong positive relationship with age. Unspecified backache showed a similar but weaker association, but none of the other conditions was significantly related to age. Hospital referral for a displaced cervical disc was unrelated to measures of body size and, indeed, was unrelated to any of the other factors considered in Table 1. The same was true for the rather nonspecific diagnosis of cervicalgia (or neck pain), suggesting,
Contraception 1999;60:331–335
perhaps, a considerable overlap in the use of these two diagnostic labels. Hospital referral for a displaced lumbar disc, on the other hand, was strongly positively related both to height and to weight, and the same was true of other and unspecified disc displacements (again suggesting diagnostic overlap). Surprisingly, although hospital referral for the latter diagnostic group was also strongly positively related to body mass index, the same was not true for the former group. The other condition showing a positive association with height and weight (and marginally with body mass index) was unspecified backache, but no such relationship was found for spinal osteoarthritis or for sprains and strains of the back. An association between back pain of various types and height and weight has been reported by other authors,2–5 but the availability of information for seven different diagnostic groups within the same study is of particular interest. In accordance with the findings of the CSAG,1 social class was largely unrelated to hospital referral for the back disorders considered in our analysis. The only statistically significant finding was a lower risk of referral for spinal osteoarthritis in women of higher social class. The CSAG1 also reported a positive association between cigarette smoking and back pain; a statistically significant association of this type was present for two of our diagnostic categories, spinal osteoarthritis, and other, and unspecified displaced disc. Our analyses of the possible relationship between oral contraceptive use and back disorders have essentially produced negative results. Although one or two statistically significant differences were found, they were not part of a coherent pattern and may well reflect the play of chance, especially bearing in mind the large number of comparisons made in the analyses. Concern about the possible relationship between oral contraceptive use and back disorders appears mostly to emanate from the Scandinavian countries. Wreje et al.7 conducted a register based study in Tierp, Sweden, a municipality of about 20,000 inhabitants. They investigated the number of primary health care visits for low back pain among women aged 14 – 44 years, comparing oral contraceptive users with age-matched nonusers. The percentage of women seeking primary health care for low back pain was higher among oral contraceptive users than nonusers in each of the years 1980 –1985, and the difference was statistically significant in 1981 and in 1984. When pregnant women were excluded, the results were essentially unchanged. Brynhildsen et al., deeming the evidence linking back pain and oral contraceptives to be slender, sent a questionnaire to 225 physicians, physiotherapists, and midwives to en-
Contraception 1999;60:331–335
quire about their knowledge of and attitudes to this issue. Of the responders, 16% of responders thought that oral contraceptives and low back pain were related, 30% said they had seen patients with low back pain that was interpreted as being influenced by use of oral contraceptives, and 25% had recommended at least some patients with low back pain to change their contraceptive method. Brynhildsen et al.6 considered that more work needed to be done on the possible relationship between oral contraceptives and back pain to determine whether the action of the health professionals surveyed was justifiable. Two further publications have been reported from this group. One concerned a prospective study of 50 female soccer players.9 In an analysis of 296 menstrual cycles, no influence of the phase of the cycle or of the use of oral contraceptives on back pain could be detected. In another study,8 female volleyball players (205) and basketball players (150), as well as soccer players (361) were investigated. An age-matched control group (113) was also included. The prevalence of low back pain in the athletes averaged 30%, whereas, among the controls, the prevalence was 18%. Among both the athletes and the controls, the prevalence of low back pain was similar in oral contraceptive users and nonusers. Clearly, the studies reported in the literature and the study we report here are very heterogeneous in terms of the characteristics of the subjects, the ways in which data were collected, and the types of disorder included. Uncertainty must remain about the possible back pain/oral contraceptive association, but our results strengthen the view that there is no important relationship. This view is strengthened further by the fact that no significant relationships between back disorders and oral contraceptives were found in the Royal College of General Practitioners oral contraception study.11 Although back pain is recognized as one of the unwanted effects of intrauterine device use, especially the older types of devices such as the Lippes Loop and SaF-T-Coil used in the Oxford-FPA study, the symptom usually disappears with continued use. Our analysis of this possible association produced essentially negative results, although rates of referral for unspecified backache were significantly higher in past intrauterine device users than in recent users or nonusers. In conclusion, the analysis presented here has shown up some interesting relationships between certain back disorders and a number of variables.
Oral Contraception and Back Disorders
335
Most important are the strong associations between disc displacement (other than cervical disc displacement) and height and weight, and the similar but somewhat less strong association (as indicated by the size of the relative risks) between unspecified backache and the same measures of body size. With regard to the oral contraceptive associations studied, the results are essentially negative, and the same is true for the intrauterine device.
Acknowledgments We thank Mrs. D. Collinge, Mrs. J. Winfield, and the research assistants, doctors, and nurses who worked in the participating clinics for their important contribution. We also thank the Medical Research Council and the James Knott Family Trust for financial support.
References 1. Clinical Standards Advisory Group. Epidemiology Review: The Epidemiology and Cost of Back Pain. London: HMSO, 1994. 2. Kuh DJ, Coggan D, Mann S, et al. Height, occupation and back pain in a national prospective study. Br J Rheumatol 1993;32:911– 6. 3. Heliovaara M, Makela M, Knekt P, et al. Determinants of sciatica and low-back pain. Spine 1991;16:608 –14. 4. Hurwitz EL, Morgenstern H. Correlates of back problems and back-related disability in the United States. J Clin Epidemiol 1997;50:669 – 81. 5. Breen TW, Ransil BJ, Groves PA, Oriol NE. Factors associated with back pain after childbirth. Anesthesiology 1994;81:29 –34. 6. Bryndhildsen J, Ekblad S, Hammar M. Oral contraceptives and low back pain. Attitudes among physicians, midwives and physiotherapists. Acta Obstet Gynecol Scand 1995;74:714 –7. 7. Wreje U, Isacsson D, Aberg H. Oral contraceptives and back pain in women in a Swedish community. Int J Epidemiol 1997;26:71– 4. 8. Brynhildsen J, Lennartsson H, Klemetz M, et al. Oral contraceptive use among female elite athletes and agematched controls and its relation to low back pain. Acta Obstet Gynecol Scand 1997;76:873– 8. 9. Brynhildsen JO, Hammar J, Hammar ML. Does the menstrual cycle and use of oral contraceptives influence the risk of low back pain? A prospective study among female soccer players. Scand J Med Sci Sports 1997;7:348 –53. 10. Vessey M, Doll R, Peto R, et al. A long-term follow-up study of women using different methods of contraception–an interim report. J Biosoc Sci 1976;8:373– 427. 11. Royal College of General Practitioners. Oral Contraceptives and Health. London: Pitman Medical, 1974.
Oral Contraception and Other Factors in Relation to Back Disorders in Women: Findings in a Large Cohort Study Martin Vessey,* Rosemary Painter,* and Jonathan Mant† The Oxford-Family Planning Association contraceptive study includes 17,032 women, initially aged 25–39 years, recruited at 17 British family planning centers during the interval 1968 –1974 and subsequently followed-up for periods up to 26 years. This article examines the pattern of referral to hospital for back disorders among these women. Certain back disorders have been reported to occur more frequently in oral contraceptive users than in other women, and back pain has also been reported in some women consequent to using an intrauterine device. The disorders considered were spinal osteoarthritis, displaced cervical disc, displaced lumbar disc, other and unspecified displaced disc, cervicalgia, unspecified back pain, and sprains and strains of the back. Spinal osteoarthritis and unspecified backache were the only two conditions significantly related (both positively) to age. Displaced lumbar disc and other and unspecified displaced disc were strongly positively related to height and weight. Unspecified backache showed similar, but less striking (in terms of the magnitude of the relative risks), associations with height and weight. Little evidence was found of any association between oral contraceptive use and any of the back disorders, and the same was true for intrauterine device use. CONTRACEPTION 2000;60:331–335 © 2000 Elsevier Science Inc. All rights reserved.
(CSAG).1 It was found that approximately10% of the adult British population responding to a questionnaire reports some restriction of activities as a consequence of back pain in the preceding month. Clinically relevant disability shows a 1-year prevalence of 3%– 6%. The CSAG also reported on risk factors for back pain.1 Increasing age up to the 40s or 50s, heavy physical occupations, and cigarette smoking were all positively associated with back pain, whereas sex and social class (in women) seemed to have little effect. Surprisingly, the CSAG report did not refer to the importance of body size (weight, height, body mass index) in the etiology of back pain, but a number of other authors have done so.2–5 In addition, there has been some interest in the possibility that oral contraceptive use might increase the risk of back pain,6,7 but this relationship remains uncertain.8,9 The Oxford-Family Planning Association (OxfordFPA) contraceptive study collected information about referral to hospital for disease and injury in a population of 17,032 women during the interval 1968 –1994. We considered that the data on back pain might be of interest, and report here our findings for certain diagnostic groupings.
KEY WORDS:
Materials and Methods
back pain, cohort study, oral contraceptives, intrauterine device, displaced intervertebral disc
Introduction Back pain is an important cause of discomfort and disability in the adult population. The epidemiology of the disorder has been reviewed in depth recently by the British Clinical Standards Advisory Group *University Department of Public Health, Institute of Health Sciences, Headington, Oxford, England; and †University Department of General Practice, The Medical School, Edgbaston, Birmingham, England Name and address for correspondence: Professor Martin Vessey, University Department of Public Health, Institute of Health Sciences, Old Road, Headington, Oxford, OX3 7LF, England; Tel.: ⫹44 (0)1865-227030; Fax: ⫹44 (0)1865226655; e-mail: [email protected] Submitted for publication September 23, 1999 Revised November 15, 1999 Accepted for publication November 16. 1999
© 2000 Elsevier Science Inc. All rights reserved. 655 Avenue of the Americas, New York, NY 10010
A detailed description of the methods used in the Oxford-FPA study has been given elsewhere.10 In brief, 17,032 women were recruited at 17 large family planning clinics in England and Scotland between 1968 and 1974. At the time of recruitment, each woman was required to be: 1) aged 25–39 years, 2) married, 3) white and British, 4) willing to cooperate, and 5) either a current user of oral contraceptives of ⱖ5 months standing or a current user of a diaphragm or an intrauterine device of ⱖ5 months standing without previous exposure to the oral contraceptive pill. Among other items, each woman was asked questions at entry to the study about her age, childbearing history, contraceptive history, height and weight, social class, smoking behavior, and past medical history. ISSN 0010-7824/00/$20.00 PII S0010-7824(99)00103-1
332
Vessey et al.
Contraception 1999;60:331–335
Table 1. Summary of associations between first hospital referral rates for back disorders under consideration (ICD 8 codes in parentheses) and age, parity, social class, smoking, height, weight, and body mass index Disorder Spinal osteoarthritis (713.1) Displaced cervical disc (725.0) Displaced lumbar disc (725.1) Other and unspecified displaced disc (725.8, 725.9) Cervicalgia (728.0) Backache not otherwise specified (728.9) Sprains and strains of back (846, 847)
No. of women
Age (y)
Parity
Social class
Smoking
Height
Weight
Body mass index
207 80 316
111 ⫺ ⫺
⫺ ⫺ ⫺
2 ⫺ ⫺
1 ⫺ ⫺
⫺ ⫺ 111
⫺ ⫺ 111
⫺ ⫺ ⫺
311 243 966 292
⫺ ⫺ 11 ⫺
⫺ ⫺ ⫺ ⫺
⫺ ⫺ ⫺ ⫺
1 ⫺ ⫺ ⫺
111 ⫺ 111 ⫺
111 ⫺ 111 ⫺
111 ⫺ 1 ⫺
The rates on which the table is based were standardized for age. 1positive association; 2negative association; ⫺no association; one arrow p ⱕ0.05; two arrows p ⱕ0.01; three arrows p ⱕ0.001 (based on 2 trend) Groupings used in trend analysis: Age (y) 25–34, 35–39, 40 – 44, 45– 49, 50 –54, 55–59, 60⫹. Parity (term births) 0, 1, 2, 3, 4⫹. Social class (Registrar General’s Classification) higher (I–II), middle (III), lower (IV–V plus armed forces, unemployed). Smoking (cigarettes/day) never, ex-smoker, 1–14, 15⫹. Height (cm) ⬍155, 155–159, 160 –164, 165–169, ⬎170. Weight (kg) ⬍51, 51–57, 58 – 63, 64 –70, ⬎70. Body mass index (kg/m2) ⬍20, 20 –21.9, 22–23.9, 24 –25.9, 26 –27.9, ⬎28.
During follow-up, each woman was questioned by a doctor or a nurse at return visits to the clinic, and certain items of information were noted on a special form. These included details of pregnancies and their outcome, changes in contraceptive practices and reasons for the changes, and particulars of any referrals to hospital as either an outpatient or an inpatient. Diagnoses on discharge from hospital were confirmed by obtaining copies of discharge letters, summaries, and pathology reports. Women who stopped attending the clinic were sent a postal version of the follow-up form and, if this was not returned, were interviewed by telephone or at a home visit. The work was coordinated by a part-time research assistant in each clinic, and yearly follow-up was maintained until age 45 years, with an annual loss of contact of only about 0.4%. On reaching the age of 45 years, each woman was allocated to one of three groups: 1) oral contraceptives never used, 2) oral contraceptives used for a total of ⱖ8 years, and 3) other durations of oral contraceptive use. Only the women in the first two groups were followed-up annually in the detailed way already described until the end of the study in mid-1994. Accordingly, women in the third group have been omitted from the present analysis from age 45 years onwards. This analysis is based on the computation of woman-years of observation terminated by the occurrence of the first referral to hospital during follow-up for the back condition under consideration, by release from follow-up (emigration, death, short- term pill users reaching age 45 years), by loss to follow-up, by withdrawal of cooperation, or by the end of the study (July 1994). Both inpatient and outpatient referrals to hospital were included in the analysis. The diagnostic groups considered in the analysis were: pondylitis osteoarthritica, hereafter referred to as spinal osteo-
arthritis (International Classification of Diseases (ICD), Eighth Revision, code 713.1), displacement of cervical intervertebral disc (ICD code 725.0), displacement of lumbar (or lumbosacral) intervertebral disc (ICD code 725.1), displacement of other or unspecified intervertebral disc (ICD codes 725.8 and 725.9), cervicalgia (ICD code 728.0), backache not otherwise specified (ICD code 728.9), and sprains and strains of back (ICD codes 846 and 847). All diagnostic coding throughout the study was done by one of us (M.P.V.) to avoid interobserver variation. Discharge summary information was generally available for women admitted to hospital, but the coding of outpatient diagnoses mostly depended on information provided by the women themselves.
Results Table 1 shows the conditions under consideration and the numbers of women with hospital referral for the different conditions. Table 1 also shows, in summary form, which relationships between the various first hospital referral rates and a range of other factors of potential interest reached statistical significance. The direction of the significant relationships is indicated as well. A highly significant (p ⬍0.001) positive association with age was found for spinal osteoarthritis (ICD code 713.1) and a similar but less highly significant (p ⬍0.01) association was found for unspecified backache (ICD code 728.9). Highly significant positive associations were also found between displaced lumbar disc (ICD code 725.1), other and unspecified displaced disc (ICD codes 725.8 and 725.9), and unspecified backache (ICD code 728.9) and both height and weight. The relationship between these three conditions and body mass index was less consistent, failing to reach significance for displaced lumbar disc
Contraception 1999;60:331–335
Oral Contraception and Back Disorders
333
Table 2. Association between first referral to hospital for certain back conditions and measurements of body size Displaced lumbar disc (725.1)
Other and unspecified displaced disc (725.8, 725.9)
Unspecified backache (728.9)
Height (cm) ⬍155 155–159 160–164 165–169 ⬎170 2 (1) trend
1.0 1.2 (0.7–2.4) 2.0 (1.2–3.8) 1.9 (1.1–3.7) 2.3 (1.3–4.4) 14.3, p ⬍0.001
1.0 1.6 (0.9–2.9) 1.4 (0.8–2.6) 2.0 (1.1–3.7) 2.1 (1.2–4.0) 10.6, p ⫽ 0.001
1.0 1.2 (0.9–1.6) 1.4 (1.0–1.8) 1.4 (1.0–1.9) 1.6 (1.2–2.2) 14.1, p ⬍0.001
Weight (kg) ⬍51 51–57 58–63 64–70 ⬎70 2 (1) trend
1.0 1.4 (0.9–2.3) 2.0 (1.3–3.2) 1.7 (1.0–2.8) 2.2 (1.3–3.7) 10.3, p ⫽ 0.001
1.0 1.4 (0.9–2.4) 2.0 (1.3–3.3) 1.8 (1.1–3.0) 2.7 (1.6–4.6) 17.8, p ⬍0.001
1.0 1.1 (0.9–1.5) 1.4 (1.1–1.7) 1.7 (1.3–2.2) 1.5 (1.1–2.0) 22.4, p ⬍0.001
1.0 1.2 (0.8–1.8) 1.3 (0.9–1.9) 1.2 (0.8–1.9) 1.4 (0.8–2.4) 1.3 (0.7–2.3) 1.0, p ⫽ 0.310
1.0 1.5 (1.0–2.4) 1.9 (1.3–3.0) 1.8 (1.1–3.0) 2.2 (1.2–3.9) 2.3 (1.2–4.1) 11.6, p ⬍0.001
1.0 1.2 (1.0–1.5) 1.4 (1.1–1.7) 1.1 (0.9–1.5) 1.6 (1.2–2.2) 1.4 (1.0–1.9) 5.7, p ⫽ 0.017
Body mass index (kg/m2) ⬍20 20–21.9 22–23.9 24–25.9 26–27.9 ⬎28 2 (1) trend
Data are relative risks with 95% confidence intervals (in parentheses) adjusted for age.
and reaching only the 5% level of significance for unspecified backache. The relationships between the three conditions and measurements of body size are shown in more detail in Table 2. It is apparent that the disc lesions are more strongly associated with height and weight (as is indicated by the higher values of the relative risks) than is unspecified backache. As well as the age and body size associations shown in Table 1, it can also be seen that cigarette smoking was positively associated (at the 5% level of significance) with spinal osteoarthritis and other and unspecified displaced disc, whereas the former condition occurred less commonly in women from the higher
social classes than in women from the lower social classes (p ⬍0.05). The relationships between the various back disorders and some simple measures of oral contraceptive use (adjusted for age) are summarized in Table 3. The only condition showing any sizeable association with oral contraceptive use is displaced cervical disc, but none of the elevated relative risks for this disorder reaches statistical significance. Indeed, only one relative risk in Table 3 reaches the 5% level of statistical significance (past use of oral contraceptives in women with unspecified backache), and it has a value of only 1.2.
Table 3. Relative risks and 95% confidence intervals (in parentheses) for first referral to hospital for various back disorders in relation to oral contraceptive use Oral contraceptives Disorder Spinal osteoarthritis Displaced cervical disc Displaced lumbar disc Other displaced disc Cervicalgia Backache not otherwise specified Sprains/strains of back
No. of women
Ever used
207 80 316 311 243 966 292
1.3 (0.9–1.7) 1.5 (0.9–2.5) 1.1 (0.9–1.4) 1.0 (0.8–1.3) 0.9 (0.7–1.1) 1.1 (0.9–1.2) 1.0 (0.8–1.2)
Recently used†
Used in past
1.0 (0.6–1.6) 1.3 (0.7–2.6) 1.1 (0.8–1.5) 1.0 (0.8–1.4) 1.0 (0.7–1.4) 0.9 (0.7–1.1) 1.0 (0.8–1.4)
1.3 (1.0–1.8) 1.6 (0.9–2.8) 1.1 (0.8–1.4) 1.0 (0.8–1.3) 0.8 (0.6–1.0) 1.2 (1.0–1.3)* 0.9 (0.7–1.2)
Never users of oral contraceptives have been taken as the reference category; relative risks are adjusted for age. † Current use or use within the 12 preceding months; *p ⱕ0.05.
334
Vessey et al.
We also examined the relationships between the different back disorders and total duration of oral contraceptive use (nonuser, ⱕ12 months, 13–24 months, 25– 48 months, 49 –72 months, 73–96 months, ⱖ97 months) and interval since oral contraceptives last used (never used, used currently or within 12 preceding months, used 13–24 months ago, 25– 48 months ago, 49 –72 months ago, 73–96 months ago, 97–120 months ago, and ⱖ121 months ago). There were no statistically significant associations between any of the back disorders and total duration of oral contraceptive use. With regard to interval since last use, the data were significantly heterogeneous (but did not follow any pattern) with regard to unspecified backache, while the relative risks were significantly elevated in the two most extreme groups with regard to spinal osteoarthritis (last used 97–120 months ago, relative risk 2.0 [CI 1.2–3.3], last used ⱖ121 months ago, relative risk 1.8 [CI 1.2–2.6]). There were no significant associations between interval since last oral contraceptive use and any of the other back disorders. Finally, because intrauterine device users sometimes complain of back pain, which they attribute to the device, we recomputed the data given in Table 3, substituting intrauterine device use for oral contraceptive use. Only one relative risk reached statistical significance; this concerned the relationship between unspecified backache and past intrauterine device use (relative risk 1.3 [CI 1.1–1.5]).
Discussion The data reported here have some important limitations and require cautious interpretation. First, only those episodes of disease requiring referral to hospital were recorded in the Oxford-FPA study. Second, the diagnoses for episodes managed entirely on an outpatient basis were, in general, self-reported by the women concerned. Third, at best, the diagnoses given reflect routine clinical practice and have not been made according to strict research criteria. Finally, approximately 75% of the exposure to the pill in the Oxford-FPA study concerns preparations containing ⱖ50 g estrogen. Pills containing such a dose of estrogen are now rarely used. Spinal osteoarthritis showed the expected strong positive relationship with age. Unspecified backache showed a similar but weaker association, but none of the other conditions was significantly related to age. Hospital referral for a displaced cervical disc was unrelated to measures of body size and, indeed, was unrelated to any of the other factors considered in Table 1. The same was true for the rather nonspecific diagnosis of cervicalgia (or neck pain), suggesting,
Contraception 1999;60:331–335
perhaps, a considerable overlap in the use of these two diagnostic labels. Hospital referral for a displaced lumbar disc, on the other hand, was strongly positively related both to height and to weight, and the same was true of other and unspecified disc displacements (again suggesting diagnostic overlap). Surprisingly, although hospital referral for the latter diagnostic group was also strongly positively related to body mass index, the same was not true for the former group. The other condition showing a positive association with height and weight (and marginally with body mass index) was unspecified backache, but no such relationship was found for spinal osteoarthritis or for sprains and strains of the back. An association between back pain of various types and height and weight has been reported by other authors,2–5 but the availability of information for seven different diagnostic groups within the same study is of particular interest. In accordance with the findings of the CSAG,1 social class was largely unrelated to hospital referral for the back disorders considered in our analysis. The only statistically significant finding was a lower risk of referral for spinal osteoarthritis in women of higher social class. The CSAG1 also reported a positive association between cigarette smoking and back pain; a statistically significant association of this type was present for two of our diagnostic categories, spinal osteoarthritis, and other, and unspecified displaced disc. Our analyses of the possible relationship between oral contraceptive use and back disorders have essentially produced negative results. Although one or two statistically significant differences were found, they were not part of a coherent pattern and may well reflect the play of chance, especially bearing in mind the large number of comparisons made in the analyses. Concern about the possible relationship between oral contraceptive use and back disorders appears mostly to emanate from the Scandinavian countries. Wreje et al.7 conducted a register based study in Tierp, Sweden, a municipality of about 20,000 inhabitants. They investigated the number of primary health care visits for low back pain among women aged 14 – 44 years, comparing oral contraceptive users with age-matched nonusers. The percentage of women seeking primary health care for low back pain was higher among oral contraceptive users than nonusers in each of the years 1980 –1985, and the difference was statistically significant in 1981 and in 1984. When pregnant women were excluded, the results were essentially unchanged. Brynhildsen et al., deeming the evidence linking back pain and oral contraceptives to be slender, sent a questionnaire to 225 physicians, physiotherapists, and midwives to en-
Contraception 1999;60:331–335
quire about their knowledge of and attitudes to this issue. Of the responders, 16% of responders thought that oral contraceptives and low back pain were related, 30% said they had seen patients with low back pain that was interpreted as being influenced by use of oral contraceptives, and 25% had recommended at least some patients with low back pain to change their contraceptive method. Brynhildsen et al.6 considered that more work needed to be done on the possible relationship between oral contraceptives and back pain to determine whether the action of the health professionals surveyed was justifiable. Two further publications have been reported from this group. One concerned a prospective study of 50 female soccer players.9 In an analysis of 296 menstrual cycles, no influence of the phase of the cycle or of the use of oral contraceptives on back pain could be detected. In another study,8 female volleyball players (205) and basketball players (150), as well as soccer players (361) were investigated. An age-matched control group (113) was also included. The prevalence of low back pain in the athletes averaged 30%, whereas, among the controls, the prevalence was 18%. Among both the athletes and the controls, the prevalence of low back pain was similar in oral contraceptive users and nonusers. Clearly, the studies reported in the literature and the study we report here are very heterogeneous in terms of the characteristics of the subjects, the ways in which data were collected, and the types of disorder included. Uncertainty must remain about the possible back pain/oral contraceptive association, but our results strengthen the view that there is no important relationship. This view is strengthened further by the fact that no significant relationships between back disorders and oral contraceptives were found in the Royal College of General Practitioners oral contraception study.11 Although back pain is recognized as one of the unwanted effects of intrauterine device use, especially the older types of devices such as the Lippes Loop and SaF-T-Coil used in the Oxford-FPA study, the symptom usually disappears with continued use. Our analysis of this possible association produced essentially negative results, although rates of referral for unspecified backache were significantly higher in past intrauterine device users than in recent users or nonusers. In conclusion, the analysis presented here has shown up some interesting relationships between certain back disorders and a number of variables.
Oral Contraception and Back Disorders
335
Most important are the strong associations between disc displacement (other than cervical disc displacement) and height and weight, and the similar but somewhat less strong association (as indicated by the size of the relative risks) between unspecified backache and the same measures of body size. With regard to the oral contraceptive associations studied, the results are essentially negative, and the same is true for the intrauterine device.
Acknowledgments We thank Mrs. D. Collinge, Mrs. J. Winfield, and the research assistants, doctors, and nurses who worked in the participating clinics for their important contribution. We also thank the Medical Research Council and the James Knott Family Trust for financial support.
References 1. Clinical Standards Advisory Group. Epidemiology Review: The Epidemiology and Cost of Back Pain. London: HMSO, 1994. 2. Kuh DJ, Coggan D, Mann S, et al. Height, occupation and back pain in a national prospective study. Br J Rheumatol 1993;32:911– 6. 3. Heliovaara M, Makela M, Knekt P, et al. Determinants of sciatica and low-back pain. Spine 1991;16:608 –14. 4. Hurwitz EL, Morgenstern H. Correlates of back problems and back-related disability in the United States. J Clin Epidemiol 1997;50:669 – 81. 5. Breen TW, Ransil BJ, Groves PA, Oriol NE. Factors associated with back pain after childbirth. Anesthesiology 1994;81:29 –34. 6. Bryndhildsen J, Ekblad S, Hammar M. Oral contraceptives and low back pain. Attitudes among physicians, midwives and physiotherapists. Acta Obstet Gynecol Scand 1995;74:714 –7. 7. Wreje U, Isacsson D, Aberg H. Oral contraceptives and back pain in women in a Swedish community. Int J Epidemiol 1997;26:71– 4. 8. Brynhildsen J, Lennartsson H, Klemetz M, et al. Oral contraceptive use among female elite athletes and agematched controls and its relation to low back pain. Acta Obstet Gynecol Scand 1997;76:873– 8. 9. Brynhildsen JO, Hammar J, Hammar ML. Does the menstrual cycle and use of oral contraceptives influence the risk of low back pain? A prospective study among female soccer players. Scand J Med Sci Sports 1997;7:348 –53. 10. Vessey M, Doll R, Peto R, et al. A long-term follow-up study of women using different methods of contraception–an interim report. J Biosoc Sci 1976;8:373– 427. 11. Royal College of General Practitioners. Oral Contraceptives and Health. London: Pitman Medical, 1974.