Abstract
Background
Despite the magnitude of dizziness/vertigo in primary health care, prospective studies are scarce, and few studies have focused on vocational consequences. Using the International Classification of Primary Health Care (ICPC), GPs have two alternative diagnoses, H82 (vertiginous syndrome) and N17 (vertigo/dizziness), when issuing sickness certificates to these patients.
Aim
To assess the incidence of dizziness/vertigo in long-term sickness absence and to identify sociodemographic and diagnostic predictors for transition into disability pension.
Design of study
Register-based prospective study, 5-year follow-up.
Setting
All individuals in Norway eligible for sickness absence in 1997 (registered employed or unemployed).
Method
The risk of disability pension was assessed with Cox proportional hazards analysis, with medical and sociodemographic information as independent variables, stratified for sex.
Results
Six-hundred and ninety-four women and 326 men were included. Dizziness/vertigo made up 0.9% of long-term sickness absence among women and 0.7% among men. Among both women and men, 41% was certified with H82 and 59% with N17: 23% of women and 24% of men obtained a disability pension. Age was the strongest predictor for obtaining a disability pension. Subjects with only basic education had an almost doubled risk of obtaining a disability pension compared to the highest educational group. Women with H82 had significantly higher risk for obtaining a disability pension than those with N17. The difference increased after adjustment for sociodemographic variables. Sex had no effect when all other variables were controlled for.
Conclusion
Dizziness/vertigo is an infrequent cause of certified sickness absence, but long-term sickness absentees with dizziness/vertigo have a considerable risk of obtaining a disability pension in the future.
Keywords: disability insurance, dizziness, health insurance, risk factors, sick leave, vertigo
INTRODUCTION
Dizziness/vertigo challenges clinicians and researchers due to its subjective nature. Traditionally dizziness is subclassified into vertigo, presyncope, disequilibrium, and ‘other types of dizziness’.1 This classification was introduced 35 years ago and is still in use.2,3 Vertigo indicates vestibular disorder: benign paroxysmal positional vertigo (BPPV), vestibular neuritis, or Ménière's disease, and has constituted 30% of cases in previous studies.3–7 It has been claimed that vertigo can easily be distinguished from other causes of dizziness.3,8
Dizziness/vertigo is a common symptom, increasing with age,7 and the frequency is higher in women than men.7,9–15 Lifetime prevalence was estimated to be 30% in a German survey,11 while a study from Scotland reported 21% vertigo, 29% unsteadiness, and 13% other types of dizziness.10 In a sample of general practices in London, 24% reported patients with dizziness in the last month.16 The studies differ, with unequal populations, different case definitions, and different measures of prevalence.
Prospective studies indicate that nearly 30% of the patients have symptoms after 12–18 months;17,18 however, knowledge of the vocational consequences is scarce. In a UK survey, 23% reported symptoms of dizziness, but only 1.5% had taken days off work.7 Sickness absence is an important measure of ill-health and social functioning,19,20 and permanent withdrawal from the labour market is common after longer spells of absence.21,22
How this fits in
Few prospective studies have assessed the long-term prognoses of subjects with dizziness/vertigo. This study adds that dizziness/vertigo is a relatively rare cause of certified sickness absence, but long-term sickness absentees with dizziness/vertigo have a considerable risk of obtaining a disability pension in the future.
Aims of the study
The aims of this study were to assess the incidence of dizziness/vertigo in long-term sickness absence in Norway, and to identify sociodemographic and diagnostic predictors for transition into disability pension.
METHOD
The Norwegian sickness absence benefit scheme
All employed and registered unemployed subjects are covered by the scheme. After 8 weeks, GPs must fill in an ‘8 weeks’ sickness absence certificate' that includes a main diagnosis based on the International Classification of Primary Care (ICPC).23,24
The disability pension scheme
All legal inhabitants aged 16–66 years are covered by the scheme. Disability pension is granted in cases of permanent, at least 50%, incapacity for work, when all relevant treatment and rehabilitation approaches have been applied.
Setting and participants
In 1997, 920 139 women and 1 019 216 men in Norway were eligible for sickness absence benefits. All cases with at least 8 weeks' sickness absence were recorded by The National Insurance Services. Anonymous data were transferred to a research database established by The National Insurance Services and Statistics Norway. Information from patients aged below 63 years with a main diagnosis of dizziness/vertigo were included in the study.
Explanatory variables
The main diagnosis for each case based on the ICPC. The ICPC is organised in chapters corresponding to organ systems: H includes otological and N neurological conditions. With respect to dizziness/vertigo, two diagnoses are available: H82 (vertiginous syndrome), and N17 (vertigo/dizziness). Sociodemographic variables were age, sex, income before tax in 1996, and educational level (years).
Follow-up and endpoint
The sample was followed from the start of the sickness absence spell until 31 December 2002, with granting of disability pension as the endpoint.
Analysis
Survival analysis was carried out with Cox proportional hazards analysis for the full sample, and stratified for sex, with disability pension as the dependent variable. Sociodemographic variables and diagnoses were entered as explanatory variables. The hazard ratio (HR) for obtaining disability pension, with 95% confidence intervals (CI), was identified for each variable. The statistical software program SPSS (version 13.0) was used.
RESULTS
Among all spells of long-term sickness absence in 1997, cases with dizziness/vertigo made up 0.9% of women and 0.7% of men. The descriptive statistics of the study sample are shown in Table 1; 694 women and 326 men were included, corresponding to an annual incidence for women of 7.5/10 000 at risk (vocationally active) and for men of 3.2/10 000 at risk (vocationally active). Among both women and men, 41% were certified with H82, and 59% with N17.
Table 1.
Women | Men | |||||||||
---|---|---|---|---|---|---|---|---|---|---|
Variables | n | % | 95% CI | DP, % | 95% CI | n | % | 95% CI | DP, % | 95% CI |
Diagnosis | ||||||||||
H82 | 282 | 41 | 37 to 44 | 29 | 24–34 | 134 | 41 | 36 to 46 | 23 | 16 to 30 |
N17 | 412 | 59 | 56 to 63 | 18 | 15–22 | 192 | 59 | 54 to 64 | 24 | 18 to 30 |
All | 694 | 100 | 23 | 18–27 | 326 | 100 | 24 | 20 to 27 | ||
Age, years | ||||||||||
16–29 | 85 | 12 | 10 to 15 | 2 | 0–6 | 36 | 11 | 8 to 14 | 3 | 0 to 8 |
30–39 | 204 | 29 | 26 to 33 | 9 | 5–13 | 79 | 24 | 20 to 29 | 9 | 3 to 15 |
40–49 | 212 | 31 | 27 to 34 | 22 | 17–28 | 87 | 27 | 22 to 31 | 17 | 9 to 25 |
50–62 | 193 | 28 | 24 to 31 | 47 | 38–52 | 124 | 38 | 33 to 43 | 44 | 33 to 52 |
Education, number of years | ||||||||||
Basic, 7–9 | 82 | 12 | 9 to 14 | 46 | 36–57 | 59 | 18 | 14 to 22 | 44 | 31 to 57 |
Lower middle, 10–12 | 333 | 48 | 44 to 52 | 25 | 21–30 | 131 | 40 | 35 to 46 | 21 | 14 to 28 |
Higher middle, 13–14 | 163 | 23 | 20 to 27 | 12 | 7–17 | 102 | 31 | 26 to 36 | 15 | 8 to 22 |
Academic, ≥15 | 113 | 16 | 14 to 19 | 14 | 8–21 | 32 | 10 | 7 to 13 | 19 | 5 to 32 |
Annual income 1996, NOK | ||||||||||
0–79 999 | 63 | 9 | 7 to 11 | 22 | 19–25 | 18 | 6 | 3 to 10 | 13 | 9 to 16 |
80 000–159 999 | 236 | 34 | 30 to 38 | 25 | 22–29 | 37 | 11 | 8 to 13 | 27 | 23 to 32 |
160 000–239 999 | 311 | 45 | 41 to 49 | 18 | 15–21 | 143 | 44 | 38 to 49 | 25 | 20 to 30 |
≥240 000 | 84 | 12 | 10 to 15 | 26 | 23–30 | 128 | 39 | 34 to 45 | 20 | 15 to 24 |
DP = disability pension. NOK = Norwegian krones (1 GBP = 9.7 NOK).
Transition to disability pension
Disability pension was obtained by 23% of the women and 24% of the men. Among women with an H82 diagnosis, 29% (95% CI = 24 to 34) obtained disability pension versus 18% (95% CI = 15 to 22) for those with N17. There was no difference in the male groups. Disability pension rates increased linearly with age. Among those with only basic education, 46% of women and 44% of men obtained a disability pension.
Multivariate analysis
Cox proportional hazards analysis (Table 2) showed age to be the strongest predictor of obtaining a disability pension. Subjects with an H82 or N17 diagnosis aged above 49 years had a HR of 6.6 compared to those aged less than 40 years. The elevated risk related to H82 diagnosis increased after adjusting for sociodemographic variables. Subjects with only basic education had an almost doubled risk compared to those in the highest educational group, controlled for age and income. After adjusting for diagnosis, age, and education, there was no income effect. There was no sex difference when all other variables were controlled for. The increased risk of disability pension among women with an H82 diagnosis remained in the stratified regressions.
Table 2.
All | Men | Women | |||||||
---|---|---|---|---|---|---|---|---|---|
Variables | HR | 95% CI | P-value | HR | 95% CI | P-value | HR | 95% CI | P-value |
Diagnosis | |||||||||
N17 | 1.0 | 1.0 | 1.0 | ||||||
H82 | 1.5 | 1.1 to 1.9 | 0.003 | 1.0 | 0.6 to 1.6 | 0.992 | 1.7 | 1.3 to 2.4 | <0.001 |
Age, years | |||||||||
16–39 | 1.0 | 1.0 | 1.0 | ||||||
40–49 | 2.7 | 1.7 to 4.2 | 2.4 | 1.0 to 5.7 | 2.7 | 1.6 to 4.7 | |||
50–62 | 6.6 | 4.3 to 10.1 | <0.001 | 6.2 | 2.8 to 13.5 | <0.001 | 6.6 | 3.9 to 10.9 | <0.001 |
Education, number of years | |||||||||
Academic, ≥15 | 1.0 | 1.0 | 1.0 | ||||||
Higher middle, 12–14 | 1.0 | 0.6 to 1.6 | 0.9 | 0.4 to 2.1 | 0.9 | 0.5 to 1.7 | |||
Lower middle, 10–11 | 1.6 | 1.1 to 2.5 | 1.3 | 0.5 to 3.1 | 1.7 | 1.0 to 2.8 | |||
Basic, 7–9 | 2.0 | 1.2 to 3.2 | 0.001 | 1.8 | 0.7 to 4.4 | 0.024 | 1.9 | 1.1 to 3.4 | 0.004 |
Annual income 1996, NOK | |||||||||
≥240 000 | 1.0 | 1.0 | 1,0 | ||||||
160 000–239 999 | 1.0 | 0.7 to 1.5 | 1.2 | 0.7 to 2.0 | 1.0 | 0.6 to 1.7 | |||
80 000–159 999 | 1.2 | 0.8 to 1.8 | 0.8 | 0.3 to 2.1 | 1.3 | 0.7 to 2.3 | |||
0–79 999 | 1.2 | 0.6 to 2.2 | 0.120 | 0.4 | 0.0 to 2.8 | 0.490 | 1.4 | 0.7 to 2.8 | 0.170 |
Sex | |||||||||
Men | 1.0 | ||||||||
Women | 1.1 | 0.8 to 1.4 | 0.670 |
HR = hazard ratio. DP = disability pension. NOK = Norwegian Krones (1 GBP = 9.7 NOK).
DISCUSSION
Summary of main findings
Dizziness/vertigo is an infrequent cause of long-term sickness absence; the annual incidence for women of 7.5/10 000 at risk (vocationally active) and for men of 3.2/10 000 at risk (vocationally active). Among both sexes, 41% were diagnosed with vertigo. One-quarter of women and men obtained a disability pension. Low education was a strong predictor of obtaining a disability pension, while an H82 diagnosis was a risk factor for women only. The strongest predictor was age, probably caused by increasing comorbidity, a difficult labour market, and more lenient treatment of applications with increasing age.25,26
Strength and limitations of the study
This is the first population-based study on occupational consequences of dizziness. Sick-leave data are regarded as reliable and objective. The ICPC is a well-known classification instrument,23,24 and confidentiality reduced the risk of ‘false’ diagnoses.27 Weaknesses of register-based studies are lack of clinical data except for diagnoses, lack of information related to work conditions, and the fact that the diagnoses cannot be validated. Since cases were identified through the ICPC codes, patients whose dizziness was linked to mental health or cardiovascular problems were not included. The baseline data were recorded in 1997. The process of application, approval and delivery from Statistics Norway meant that the data were received in late 2005. There has been no secular change since baseline. No major alteration in the rules for the sickness absence benefit scheme or disability pension has been introduced, and the ICPC is still used on the sickness certificates.
Comparison with existing literature
Vertigo and dizziness reflect a distinction between vestibular and non-vestibular symptoms.1,3 The possibility of differentiating between types of dizziness is disputed,3,11 although a high degree of sensitivity has been reported in identifying vertigo by history alone.28 The proportion of subjects with vertigo (41%) was slightly higher than in previous studies.3–7 In the present study, as well as in previous studies, dizziness/vertigo was more common for women than for men in all age groups.7,9–14,29 However, women also have higher rates of sickness absence in general,19,30,31 often explained by occupational differences.19,32,33 Less control of working conditions increases the burden on employees, especially on women in low-paid jobs.19 The present study confirmed findings of no sex difference in relation to the risk of transition from sickness absence to disability pension,22,30,34 implying that the threshold for taking sick leave is not lower among women than men.
Disability pension is granted when ‘all relevant treatment and rehabilitation approaches have been applied’. Treatment in primary care typically consists of reassurance and medication to relieve symptoms,3,5,6,35 even though medication is not recommended beyond the acute stage.3,5,36 Referral to vestibular rehabilitation37–40 is uncommon in primary care.17,36,41 Lack of guidelines may lead to persistent complaints with disability pension as the endpoint.25 Vestibular rehabilitation could play an important role in preventing disability if applied at an early stage.42
Implications for future research
Further studies should focus on GPs' role in the management of patients with dizziness/vertigo. Attention should particularly be directed towards female patients.
Acknowledgments
The data used in this study were provided by Statistics Norway, with permission from the Data Inspectorate. Statistics Norway is not responsible for the authors' analyses.
Funding body
The Research Council of Norway (Health Economy Bergen). Norwegian Fund for Postgraduate Training in Physiotherapy
Ethical approval
Not applicable
Competing interests
The authors have stated that there are none
Discuss this article
Contribute and read comments about this article on the Discussion Forum: http://www.rcgp.org.uk/bjgp-discuss
REFERENCES
- 1.Drachman D, Hart C. An approach to the dizzy patient. Neurology. 1972;22:323–334. doi: 10.1212/wnl.22.4.323. [DOI] [PubMed] [Google Scholar]
- 2.Sloane P, Coeytaux R, Beck R, Dallera J. Dizziness: state of the science. Ann Intern Med. 2001;134(9 pt 2):823–831. doi: 10.7326/0003-4819-134-9_part_2-200105011-00005. [DOI] [PubMed] [Google Scholar]
- 3.Hanley K, O'Dowd T, Considine N. A systematic review of vertigo in primary care. Br J Gen Pract. 2001;51(469):666–671. [PMC free article] [PubMed] [Google Scholar]
- 4.Sixt E, Landahl S. Postural disturbances in a 75-year-old population: I. Prevalence and functional consequences. Age Ageing. 1987;16(6):393–398. doi: 10.1093/ageing/16.6.393. [DOI] [PubMed] [Google Scholar]
- 5.Colledge N, Barr-Hamilton R, Lewis S, et al. Evaluations of investigations to diagnose the cause of dizziness in elderly people: a community-based controlled study. BMJ. 1996;313(7060):788–792. doi: 10.1136/bmj.313.7060.788. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Bird J, Benyon G, Prevost A, Baguley D. An analysis of referral patterns in the primary care setting. Br J Gen Pract. 1998;48(437):1828–1832. [PMC free article] [PubMed] [Google Scholar]
- 7.Yardley L, Owen N, Nazareth I, Luxon L. Prevalence and presentation of dizziness in a general practice community sample of working age people. Br J Gen Pract. 1998;48(429):1131–1135. [PMC free article] [PubMed] [Google Scholar]
- 8.Labuguen RH. Initial evaluation of vertigo. Am Fam Physician. 2006;73(2):244–251. [PubMed] [Google Scholar]
- 9.Sloane P. Dizziness in primary care. Results from the National Ambulatory Medical Survey. J Fam Pract. 1989;29(1):33–38. [PubMed] [Google Scholar]
- 10.Hannaford P, Simpson J, Bisset A, et al. The prevalence of ear, nose and throat problems in the community: results from a national cross-sectional postal survey in Scotland. Fam Pract. 2005;22(3):227–233. doi: 10.1093/fampra/cmi004. [DOI] [PubMed] [Google Scholar]
- 11.Neuhauser H, Von Brevern M, Radke A, et al. Epidemiology of vestibular vertigo: a neurotologic survey of the general population. Neurology. 2005;65(6):898–904. doi: 10.1212/01.wnl.0000175987.59991.3d. [DOI] [PubMed] [Google Scholar]
- 12.Ihlebæk C, Eriksen H, Ursin H. Prevalence of subjective health complaints (SHC) in Norway. Scand J Public Health. 2002;30(1):20–29. [PubMed] [Google Scholar]
- 13.Tibblin G, Bengtsson C, Furunes B, Lapidus L. Symptoms by age and sex. Scand J Prim Health Care. 1990;8(1):9–17. doi: 10.3109/02813439008994923. [DOI] [PubMed] [Google Scholar]
- 14.Nazareth I, Yardley L. A clinical approach to dizziness. J Com Med. 2002;9(3):159–167. [Google Scholar]
- 15.Guilemany JM, Martinez P, Prades E, et al. Clinical and epidemiological study of vertigo at an outpatient clinic. Acta Otolaryngol. 2004;124(1):49–52. doi: 10.1080/00016480310002122. [DOI] [PubMed] [Google Scholar]
- 16.Nazareth I, Landau S, Yardley L, Luxon L. Patterns of presentations of dizziness in primary care — a cross-sectional cluster analysis study. J Psychosom Res. 2006;60(4):395–401. doi: 10.1016/j.jpsychores.2005.07.011. [DOI] [PubMed] [Google Scholar]
- 17.Nazareth I, Yardley L, Owen N, Luxon L. Outcome of symptoms of dizziness in a general practice community sample. Fam Pract. 1999;16(6):616–618. doi: 10.1093/fampra/16.6.616. [DOI] [PubMed] [Google Scholar]
- 18.Kroenke K, Lucas C, Rosenberg ML, et al. One-year outcome for patients with a chief complaint of dizziness. J Gen Intern Med. 1994;9(12):684–689. doi: 10.1007/BF02599010. [DOI] [PubMed] [Google Scholar]
- 19.Marmot M, Feeney A, Shipley M, et al. Sickness absence as a measure of health status and functioning: from the UK Whitehall II study. J Epidemiol Community Health. 1995;49(2):124–130. doi: 10.1136/jech.49.2.124. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 20.Kivimaki M, Head J, Ferrie JE, et al. Sickness absence as a global measure of health: evidence from mortality in the Whitehall II prospective cohort study. BMJ. 2003;327(7411):364. doi: 10.1136/bmj.327.7411.364. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 21.Shiels C, Gabbay M, Ford F. Patient factors associated with duration of certified sickness absence and transition to long-term incapacity. Br J Gen Pract. 2004;54(499):86–91. [PMC free article] [PubMed] [Google Scholar]
- 22.Gjesdal S, Ringdal PR, Haug K, Maeland JG. Predictors of disability pension in long-term sickness absence: results from a population-based and prospective study in Norway 1994–1999. Eur J Public Health. 2004;14(4):398–405. doi: 10.1093/eurpub/14.4.398. [DOI] [PubMed] [Google Scholar]
- 23.Brage S, Bentsen BG, Bjerkedal T, et al. ICPC as a standard classification in Norway. Fam Pract. 1996;13(4):391–396. doi: 10.1093/fampra/13.4.391. [DOI] [PubMed] [Google Scholar]
- 24.Hofmans-Okkes IM, Lamberts H. The International Classification of Primary Care (ICPC): new applications in research and computer-based patient records in family practice. Fam Pract. 1996;13(3):294–302. doi: 10.1093/fampra/13.3.294. [DOI] [PubMed] [Google Scholar]
- 25.Gjesdal S. From long-term sickness absence to disability pension. Bergen: University of Bergen; 2003. [Google Scholar]
- 26.Aarts L, De Jong P. Economic aspects of disability behaviour. Amsterdam The Netherlands: Nord-Holland; 1992. [Google Scholar]
- 27.Hussey S, Hoddinott P, Wilson P, et al. Sickness certification system in the United Kingdom: qualitative study of views of general practitioners in Scotland. BMJ. 2004;328(7431):88. doi: 10.1136/bmj.37949.656389.EE. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 28.Kroenke K, Lucas CA, Rosenberg ML, et al. Causes of persistent dizziness. A prospective study of 100 patients in ambulatory care. Ann Intern Med. 1992;117(11):898–904. doi: 10.7326/0003-4819-117-11-898. [DOI] [PubMed] [Google Scholar]
- 29.Guilemany J-M, Martínez P, Prades E, et al. Clinical and epidemiological study of vertigo at an outpatient clinic. Acta Otolaryngol. 2004;124(1):49–52. doi: 10.1080/00016480310002122. [DOI] [PubMed] [Google Scholar]
- 30.Gjesdal S, Bratberg E. The role of gender in long-term sickness absence and transition to permanent disability benefits. Results from a multiregister based, prospective study in Norway 1990–1995. Eur J Public Health. 2002;12(3):180–186. doi: 10.1093/eurpub/12.3.180. [DOI] [PubMed] [Google Scholar]
- 31.Alexanderson K, Leijon M, Åkerlind I, et al. Epidemiology of sickness absence in a Swedish county in 1985, 1986 and 1987. Scand J Soc Med. 1994;22(1):27–34. doi: 10.1177/140349489402200105. [DOI] [PubMed] [Google Scholar]
- 32.Blank N, Diederichsen F. Short-term and long-term sick-leave in Sweden: relationship with social circumstances, working conditions and gender. Scand J Soc Med. 1995;23(4):265–272. doi: 10.1177/140349489502300408. [DOI] [PubMed] [Google Scholar]
- 33.Karasek N, Theorell T. Healthy work: stress, productivity, and the reconstruction of working life. New York: Basic Books; 1990. [Google Scholar]
- 34.Brage S, Nygård J, Tellnes G. The gender gap in musculo-sceletal-related long-term sickness absence in Norway. Scand J Soc Med. 1998;26(1):34–43. doi: 10.1177/14034948980260010901. [DOI] [PubMed] [Google Scholar]
- 35.Brandt T. Management of vestibular disorders. J Neurol. 2000;247(7):491–499. doi: 10.1007/s004150070146. [DOI] [PubMed] [Google Scholar]
- 36.Yardley L, Donovan-Hall M, Smith H, et al. Effectiveness of primary care-based rehabilitation for chronic dizziness. Ann Intern Med. 2004;141(8):598–605. doi: 10.7326/0003-4819-141-8-200410190-00007. [DOI] [PubMed] [Google Scholar]
- 37.Yardley L, Luxon L. Treating dizziness with vestibular rehabilitation. BMJ. 1994;308(6939):1252–1253. doi: 10.1136/bmj.308.6939.1252. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 38.Shepard N, Asher A. Treatment of patients with nonvestibular dizziness and disequilibrium. In: Herdman S, editor. Vestibular rehabilitation. Philadelphia: FA Davis; 2000. pp. 534–544. [Google Scholar]
- 39.Luxon L. Theorethical basis of physical exercice regimes and manoeuvres. In: Luxon L, Davies R, editors. Handbook for vestibular rehabilitation. Florence: Singular Publishing Group; 1997. pp. 87–115. [Google Scholar]
- 40.Shepard N, Telian S. Practical management of the balance disorder patient. Florence: Singular Publishing Group; 1996. [Google Scholar]
- 41.Hansson E, Månsson N, Håkansson A. What happens with the dizzy patient in primary health care? Does education influence treatment? Adv Phys. 2004;6:93–96. [Google Scholar]
- 42.Yardley L, Beech S, Zander L, et al. A randomized controlled trial of exercise therapy for dizziness and vertigo in primary care. Br J Gen Pract. 1998;48(429):1136–1140. [PMC free article] [PubMed] [Google Scholar]