An epidemiological study of acute herniated lumbar intervertebral discs.

Rheumatology and Rehabilitation, 1975, 14,144.

AN EPIDEMIOLOGICAL STUDY OF ACUTE HERNIATED LUMBAR INTERVERTEBRAL DISCS BY JENNIFER L. KELSEY Department of Epidemiology and Public Health, Yale University School of Medicine, New Haven, Connecticut, U.S.A.

A SIGNIFICANT proportion of severe and prolonged low-back pain is attributable to the hemiated lumbar intervertebral disc (Benn and Wood, 1975), yet little is known of the causation of this condition. Therefore, an epidemiological study of acute hemiated lumbar intervertebral discs was undertaken in the New Haven, Connecticut (U.S.A.), area, in order to learn more about the demographic characteristics of persons who develop this condition and about possible aetiologic agents. Some of the findings of this study are described elsewhere, including the demographic characteristics of those affected (Kelsey and Ostfeld, 1975), and the risks from working in different types of occupations (Kelsey, 1975), from driving of motor vehicles (Kelsey and Hardy, 1975), and from pregnancy (Kelsey, Greenberg, Hardy, and Johnson, 1975). This paper summarizes these previously reported results and presents other data not given elsewhere. METHODS The general approach of this study involved obtaining information on symptoms and signs, demographic characteristics, and exposure to possible aetiologic factors in persons with recent hemiated lumbar intervertebral discs and comparing the demographic characteristics and exposure to possible aetiologic factors in these cases to similar information obtained from (a) persons without low-back problems of the same sex and about the same age and (b) persons who had also had low-back X-ray examinations for a recent complaint but whose problem was thought not to be a hemiated lumbar disc. The study design is described in detail elsewhere (Kelsey and Ostfeld, 1975), and will be presented briefly here. CASES

Cases were found from among persons in the age group 20-64 years from the New Haven Standard Metropolitan Statistical Area who had lumbar X-ray examinations in all three of the hospitals in the New Haven area and at the office of two of the private radiologists in New Haven during the period June 1971 to May 1973. Although lumbar 144

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SUMMARY In an epidemiological study of acute hemiated lumbar intervertebral discs in the New Haven, Connecticut (U.S.A.), area, it was found that this condition was most likely to be diagnosed among persons in the age group 30-39 years, and that the most important risk factors among the variables considered in this study were driving of motor vehicles at or away from work, sedentary occupations, suburban residence, and previous full-term pregnancies. Variables for which there was some suggestion of an association but for which the evidence was inconclusive were the male sex, high social class among females, chronic cough and chronic bronchitis, participation in baseball, golf and bowling, the spring and fall seasons, and possibly lack of physical activity other than at work. No increase in risk for this condition was related to race, social class in males, smoking habits, participation in sports other than baseball, golf and bowling, weight or body bulk, recent episodes of emotional stress, pregnancies which were not full-term, and jobs involving lifting, pushing, pulling, or carrying.

KELSEY: AN EPIDEMIOLOGICAL STUDY

145

CONTROLS

In order to find ways in which people with herniated lumbar discs differ from people who remain free of this disease, two control groups of individuals without known * The descriptions 'ruptured', 'free fragments', 'herniated', 'prolapsed', 'bulging', and 'extruded' are included, but not disc degeneration without evidence of nerve root involvement.


radiography alone is of limited value in diagnosing herniated discs, the assumption is made that a wide spectrum of persons with herniated discs can be ascertained from among these patients. Also, the radiologists' reports were important in finding out whether any other condition could be bringing about symptoms and signs similar to those usually experienced by persons with herniated lumbar discs such as tumours and spondylolisthesis. In order to determine which of these people having low-back X-ray examinations were likely to have herniated lumbar discs, they were interviewed within a few weeks of the time the radiographs were taken and their medical records were reviewed. During the interview they were asked about their symptoms and were given a few simple diagnostic tests by the interviewers. Then, the medical records of those whose symptoms and signs were consistent with a herniated disc were abstracted; the radiologists' reports were used to exclude persons with other conditions which could produce the same signs and symptoms; the surgeons' reports were reviewed for patients who had undergone surgery; and information on the straight-leg raising test, reflexes, and weakness of the big-toe extensor was recorded in instances in which the patient was not interviewed until after surgery or could not be interviewed until more than a few weeks had elapsed since he sought medical care. Using the information obtained during the interview and from the medical records, the following diagnostic criteria were applied: Surgical cases were those in which all of the following three criteria were fulfilled: (a) the surgeon stated on the hospital chart that he saw the herniated disc during surgery;* (b) the patient gave evidence in his answers to the questionnaire that his pain was distributed along the sciatic nerve; and (c) the patient had a positive straight leg raising test and/or the symptoms of increased pain in the low back or along the sciatic nerve when stretching or extending his leg from a sitting position and/or the symptoms of increased pain along the sciatic nerve when coughing or sneezing. Probable cases were similar to the surgical except that the herniation need not have been observed at surgery. Included were cases in which the sciatic pain was felt in both the thigh and lower leg and cases in which there was sciatic pain in part of the leg and numbness in another part. Possible cases differed from the probable in that the sciatic pain was only in the thigh or the lower leg but not in both. Also, if the leg was numb so that the distribution of pain was unknown but straight leg raising brought about an increase of pain in the low back, the person was classified as a possible case. Only people who had recently acquired their disease were included in these analyses since the fact that a person has had a herniated disc for some time may have brought about changes in his way of life, and it was desired to study factors leading to the development of the herniated disc rather than changes occurring as a result of it. Therefore, people were excluded from the analysis who had previously had herniated lumbar discs or other serious back problems and who had experienced their symptoms of herniated discs for more than one year prior to the time they were interviewed.

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RHEUMATOLOGY AND REHABILITATION VOL. XTV NO. 3

INTERVIEWING

All information on symptoms, demographic variables, and exposure to possible risk factors in cases and controls was obtained by a standardized, structured questionnaire administered by carefully trained non-medical interviewers. Usually the interviewing was done in the homes of the subjects, although when feasible they were interviewed in hospital. The response rate was 79 % for persons having low-back radiographs and 77 % for matched controls. Of the 1405 persons with low-back radiographs who were successfully interviewed, almost one-sixth (223) were subsequently classified as having surgical, probable, or possible new herniated lumbar discs. A total of 217 pairs* (89 females, 128 males) was obtained for the comparison of cases and matched controls. Two hundred and twentythree cases (91 females, 132 males) and 494 controls (225 females, 269 males) were available for the comparison of cases and unmatched controls. STATISTICAL PROCEDURES

In comparing characteristics of cases and matched controls, the tests of statistical significance are based on differences between the two members of each pair. In considering a dichotomously distributed variable such as place of residence (which in this study population had to be either urban or suburban), the number of pairs in which the case lived in the suburbs and the control in the city is compared to the number of pairs in * Matched controls could not be obtained for six of the cases seen by private radiologists because of problems related to confidentiality of names of persons seeking medical care from private physicians.


herniated discs were asked the same questions as the cases, so that their demographic characteristics and exposure to possible risk factors could be ascertained and compared to those of people with surgical, probable, and possible herniated lumbar discs. The reason for using two control groups chosen in different ways was that if the same variables are found to be risk factors in the comparisons of cases with both control groups, this provides strong evidence that the associations are real and not merely attributable to particular characteristics of one of the control groups. In the first group, controls were individually matched to cases. For each surgical, probable, and possible case, the next person of the same sex and about the same age who was admitted to the same hospital service or to the same radiologist's office for a condition not related to the spine was chosen as a control. Cases and controls seen at emergency rooms were matched within two years of age in two hospitals and within three years of age in the other hospital; the ages of all others were matched within ten years. People who would have been classified as surgical, probable or possible cases or who had previously had a herniated disc or chronic low-back pain could not serve as controls. Also excluded as controls were people who reported that they had symptoms of the condition for which they sought medical care for more than one year, so that, like the cases, they had recently developed their disease. Each person who failed to qualify as a control was replaced by another person of about the same age and same sex admitted to the appropriate hospital service The second control group was composed of people who had low-back radiographs taken during the period June 1971 to May 1973 and were thus interviewed in the course of finding cases, but who were not classified as surgical, probable, or possible cases and who had not had symptoms for more than one year.


147

SUMMARY OF RESULTS REPORTED ELSEWHERE DEMOGRAPHIC VARIABLES

Table I shows the age and sex distribution of the cases. When the surgical, probable, and possible cases are considered together, there are more males than females in the ratio of 1.45 to 1; however, it may be seen that this is almost entirely attributable to the excess of males among surgical cases, since the ratio of males to females among probable and possible cases is only 1.13 to 1. The median age for all the males, 38.0 years, is very close to the median age for the females, 40.0 years. As would be expected, the probable and possible cases tended to be younger than the surgical cases. Tables II and III summarize information obtained from comparisons of cases with matched controls. (Results of comparisons between cases and unmatched controls are not presented in these summary tables, but will be described in the text where appropriate.) In respect to demographic variables, Table II shows that suburban residence was associated with an increased risk for herniated lumbar discs compared to urban residence, a trend seen in both sexes and also in the comparison between cases and unmatched controls. In females, but not males, high social class was related to an increased risk. There was no significantly increased or decreased risk in whites compared to blacks or in


which the control came from the suburbs and the case from the city. For a continuously distributed variable (such as weight), tests of significance are based on whether the mean difference (in weight) between the cases and matched controls is different from zere. Details of the procedures may be found in Snedecor and Cochran (1967). Somewhat different procedures are used to test the statistical significance of differences between cases and unmatched controls. More detail may be found elsewhere (Kelsey and Ostfeld, 1975; Kelsey and Hardy, 1975), but briefly, because different types of people tended to use the various hospital services, comparisons of these cases and unmatched controls were made in seven separate groups for males and six groups for females according to the hospital service. (The age distributions of the cases and unmatched controls within each group were similar, so they did not have to be divided into separate age-groups.) To test the significance of over-all differences between cases and unmatched controls, a weighted average of the differences between cases and controls in the individual groups is computed. Methods of calculating the specific weights vary slightly depending on whether the variable is dichotomously or continuously distributed, but in both instances, the smaller the variability (and therefore the greater the reliability) in a group, the larger the weight applied to that group. Most of the variables in this study are dichotomously distributed; the chief summary statistic used in the tables involving dichotomously distributed variables is the relative risk, which indicates the magnitude of the increase in risk in a person exposed to a given factor compared to the risk in a person not exposed. A relative risk of 2, for instance, would indicate that a person exposed to the risk factor is twice as likely to develop this disease as a person not exposed. Details of procedures for estimating relative risk from comparisons of cases with matched controls and of cases with unmatched controls are given in such books as MacMahon and Pugh (1970) and Fleiss (1973). For continuously distributed variables, the chief summary statistic used is the difference between the mean for cases and the mean for controls. Except where noted, the trends were similar in the surgical, probable, and possible cases, so they have been combined in most of tliese summary tables. Only in considering the age and sex distributions, have ths surgical, probable, and possible cases been separated.

148

RHEUMATOLOGY AND REHABILITATION VOL. XIV NO. 3 TABLE I DISTRIBUTION OF CASES OF ACUTE HERNIATED LUMBAR DISCS BY SEX, AGE AND TYPE OF CASE

Females

Males

Age Group (in years) Number 8 23 14 15 2 62

20-29 30-39 40-49 50-59 60-64 Total

28 14 16 10 2 70

20-29 30-39 40-49 50-59 60-64 Total

36 37 30 25 4 132

Surgical Cases 12.9 37.1 22.6 24.2 3.2

100.0

Number

Percentage

4 8 9 6 2 29

13.8 27.6 31.0 20.7 6.9

100.0

Probable and Possible Cases

40.0 20.0 22.9 14.3 2.9

100.1

12 20 23 7 0 62

19.3 32.3 37.1 11.3 0

100.0

All Cases

27.3 28.0 22.7 18.9 3.0

99.9 Median iige=38.O years

16 28 32 13 2 91

17.6 30.8 35.2 14.3 2.2

100.1 Median age =40.0 years

persons who had ever been married compared to persons who had never been married in the comparison of cases and matched controls shown in Table II or in the comparison of cases and unmatched controls. It might be mentioned, however, that among females there were somewhat fewer cases than controls who had never been married, but with the small numbers of never married females this did not reach statistical significance. OCCUPATIONAL VARIABLES

Cases and controls were asked details about all occupations which they had had for at least a year since they had left school. The next section of Table II indicates that people who said that they sat half the time or more on their present job had about a 50-60 % increased risk of developing a herniated lumbar disc compared to people who sat less than half the time on their jobs. From the comparison of cases and unmatched controls, the relative risk was estimated to be 1.70. On subdividing by age, this trend is seen for those of age 35 years and older but not for persons younger than age 35. On analysing the data according to length of time in sedentary occupations (not shown here), it was found that at leastfiveyears in a sedentary occupation were necessary before an increased risk was seen. Table II further shows that the relative risk was particularly high among those males who sat driving motor vehicles on their jobs. (Females could not be considered here since


20-29 30-39 40-49 50-59 60-64 Total

Percentage


149

TABLE II ESTIMATES OF RELATIVE RISK FOR SELECTED VARIABLES

Relative Risk Variables Estimate

Probability

1.54

0.05

0.79 1.50 1.27 1.32

>0.10 >0.10 >0.10 >0.10

Occupational Sedentary jobs, all ages Case < 35 years Case > 35 years Jobs requiring driving (males only) Truck driving (males only) Jobs involving any lifting* Jobs involving any pushing* Jobs involving any pulling* Jobs involving any carrying*

1.58 0.81 2.40 2.75 4.67 1.25 1.12 1.16 1.13

0.06 >0.10 0.005 0.02 0.02 >0.10 >0.10 >0.10 >0.10

Other Driving other than on job (drivers/non-drivers)

2.16

0.007

* Includes pairs in which both members worked at least 20 hours per week. TABLE III MEAN NUMBER OF CHILDREN, PREGNANCIES RESULTINGIN LIVE-BIRTHS, AND PREGNANCIES NOT RESULTING IN LIVE-BIRTHS, CASES AND MATCHED CONTROLS

Variable Number of children Females Males Pregnancies resulting in live-births Pregnancies not resulting in live-births

Number of pairs

Mean for Mean for cases matched controls

Difference in means

Probability P

88* 125

2.8 1.9

2.1 1.9

0.7 0

0.10

88

2.8

2.2

0.6

0.10

* Answers were not recorded for one female. very few of them had jobs involving driving motor vehicles.) This table indicates that truck drivers, who constituted the largest group of men driving on their jobs, were over four times as likely to develop a herniated lumbar disc as males who were not truck drivers. There was no increased or decreased risk for herniated discs found among people (working at least 20 hours per week) whose jobs involved lifting, carrying, pulling, or pushing. There is a slight suggestion of a positive association between lifting and herniated discs in the comparison of cases and matched controls shown in Table II, but in the comparison of cases and unmatched controls, the relative risk was estimated to be


Demographic Residence (urban/suburban) Social class (high school graduates/non-graduates) Males Females Race (whites/non-whites) Marital Status (ever married/never married)

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RHEUMATOLOGY AND REHABILITATION VOL. XIV NO. 3

only 0 • 94. In addition, no association was found between the amount usually lifted or the frequency of lifting and herniated lumbar discs (not shown here). DRIVING OF MOTOR VEHICLES

VARIABLES RELATED TO PREGNANCY

Table III contains information on variables related to pregnancy. Since these are continuously distributed variables, differences between means are shown rather than relative risks. There were- no differences in the number of children of male cases and controls, but female cases had more children than females in either control group. On further examination of the pregnancy histories of the females, it appeared that there was no tendency for cases to have more miscarriages than the controls, but that cases had had more live-births than controls in either group. It was also found (but not shown here) that among surgical cases the association between herniated lumbar discs and number of live-births is almost entirely attributable to those with L5 herniations, and that there was little difference between cases with herniations at the L4 level and their controls. There was no evidence from this study that women with herniated lumbar disc were more likely to have experienced low-back and sciatic pain during previous pregnancies than were their controls. DISTRIBUTION OF SYMPTOMS

Other data reported previously but not shown in tabular form here indicated that among the 91 surgical cases, the L5 and L4 levels were most frequently involved; no difference was found in the age distribution of those affected at the L4 and L5 levels, but the four cases with herniations at higher levels were all over age 50. Ninety-one % of the males and 93 % of the females had pain in their low back as well as their legs, the remainder having pain (or numbness) in their legs only. Males were almost twice as likely to have pain or numbness along the sciatic nerve of their left leg as their right leg, whereas the left and right legs were involved with equal frequency among the females. RESULTS NOT PREVIOUSLY REPORTED PRECIPITATING EVENTS

Cases who reported pain in their low back were asked what they were doing when their back started to hurt and all cases were asked what they were doing when their leg(s) started to hurt (since to qualify as a case a person had to have pain or numbness in his legs, but not necessarily in his back). Table IV shows that the low-back pain generally appeared to be initiated by some physically traumatic event, such as lifting, falling, bending, or being in a motor vehicle accident, whereas the leg involvement tended to come on without any specific traumatic event. When the responses were analysed according to the symptom that came first (regardless of whether it involved the low back or leg), lifting was the most frequent answer when the sexes are combined, accounting for 13.1 % of answers; falling was second, making up 12.7 % of responses, and automobile accidents were third, constituting 9.0% of answers.


Not only did people who drove motor vehicles on their jobs appear to have a substantially increased risk for developing a herniated lumbar disc, but it appeared that driving other than professionally was also associated with an increased risk (see Table II). It was estimated that persons who answered "yes" to the question, "Do you drive a car?" were about twice as likely to develop a herniated lumbar disc as persons who said "no" to this question. This difference could not be attributed to other characteristics of persons who drive, such as age, social class, race, or place of residence.


151

TABLE IV EVENTS MOST FREQUENTLY REPORTED TO PRECIPITATE SYMPTOMS OF HERNIATED LUMBAR DISCS, BY SEX

Answers to question, "What were you doing when your back started to hurt?" Males (n = 119*) Females (n=85) Both sexes (n=204)

Answer

17.6 15.1 8.4 7.6

Falling Bending Auto accident Resting

Percentage of all answers Answer 12.9 10.6 10.2 9.4

Falling Lifting Bending Auto accident

Percentage of all answers 14.2 13.7 9.3 8.8

Answers to question, "What were you doing when your leg started to hurt?" Males (n=130*) Females (n =91) Both sexes (n=221)

Answer Gradually Can't remember Resting Falling


Standing Can't remember Falling Gradually


Answers considering whichever symptom appeared first Males (n=130*) Females (n=91)

Answer

Percentage of all answers Answer

Can't remember Gradually Falling Resting Lifting Walking

Percentage of all answers 10.0 9.5 9.0 7.7 7.7 7.7

Both sexes (n =221)


Percentage of all answers

Falling 11.9 Lifting 13.1 Lifting 16.1 Falling 13.1 Standing 11.0 Falling 12.7 7.7 9.0 Auto accident Auto accident 11.0 Auto accident 6.2 8.8 6.2 Bending Lifting Standing 6.2 Can't remember * Answers were not given for onset of low-back pain by one male, for leg pain by two males, and for the pain that started first in two males. Although the data are not presented here, it was of interest to note that when cases for which the treatment was financed by workmen's compensation were considered separately, lifting, falling, bending, carrying, pulling, and pushing made up 70.9% of the responses for males and 71.4% for females to the question, "What were you doing when your back started to hurt?" In contrast, when private insurance companies were paying for the treatment, these answers accounted for only 35.3% of responses in males and 23.6 in females. MONTH OF ONSET OF SYMPTOMS

Consideration was given to the month of onset of the first reported symptoms of the herniated lumbar disc for cases from the two hospitals in which ascertainment was


Lifting Falling Bending Auto accident


152


conducted over the entire two-year period of the study. Numbers were too small to enable any conclusions to be drawn, but there was some tendency for the onset of the first symptoms to occur more frequently in spring and fall than in summer or winter. When the months of onset of symptoms in the low back and legs were examined separately (not shown here), it was found that both follow the same over-all pattern, although the distribution of month of onset of symptoms in the leg showed the spring and fall excess to a slightly greater extent than the distribution of month of onset of symptoms in the low back.

MEAN HEIGHTS, IN INCHES, OF CASES AND UNMATCHED CONTROLS, BY SEX AND HOSPITAL SERVICE

Males Hospital Service

Females

Cases No. Mean

Controls No. Mean

Cases No. Mean

Controls No. Mean

I.

Yale-New Haven Hospital Orthopaedic and Neurosurgicalinpatients

54 69.09

25 68.86

24 64.35

10 63.75

II.

Hospital of St. Raphael Orthopaedic and Neurosurgical inpatients

22 70.00

21 68.71

19 65.03

13 63.35

III. Yale-New Haven Hospital Emergency Room patients

17 68.74

79 68.97

17 64.03

71 63.44

IV. Hospital of St. Raphael Emergency Roompatients

14 70.29

74 68.58

13 63.19

75 63.84

Other Yale-New Haven and St. Raphael patients

6 70.25

13 69.69

11 64.50

40 64.04

Private Radiologists' patients

4 68.25

12 68.25

4 65.50

11 64.50

13 68.92

37 68.14

V. VI

VII. V.A. Hospital patients

—

—

Weighted mean difference between cases and controls=0.488, Z = l .91, P=0.06. * The body bulk indices used here are weight/height2 for males and weight/height for females, as recommended by Florey (1970), Goldbourt and Medalie (1974) and Keys et al. (1972). These give an indication of a person's weight, controlling for his height.


BODY BUILD

There were no differences between the weights or body bulk indices* of cases and people in either control group, and the data are not shown here. There was some tendency for female cases to be taller than their matched controls and a slight tendency for male cases to be shorter than their matched controls. When cases are compared to the unmatched controls (Table V), both male and female cases tended to be taller than their controls. Cases and controls were also asked whether they had gained or lost any weight during the year before their onset of symptoms, and if so, how much they had gained or lost. Differences between cases and controls were negligible, and the data are not shown here. TABLE V


153

RESPIRATORY SYMPTOMS AND SMOKING

TABLE VI DISTRIBUTION OF CASE-MATCHED CONTROL PAIRS ACCORDING TO WHETHER THEY REPORTED SYMPTOMS OF CHRONIC COUGH, CHRONIC PHLEGM, AND CHRONIC BRONCHITIS, BY SEX

Number of pairs*

Symptom

Case with symptom, control with symptom

Case with symptom, control without symptom

Males Chronic cough Chronic phlegm Chronic bronchitis

3 5 1

22 23 14

1 2 0

12 17 11

Females

Case without symptom, control with symptom

Case without symptom, control without symptom

Estimated relative Probability risk

20 18 14

78 70 84

1.10 1.28 1.00

>0.10 >0.10 >0.10

13 6 3 In some instances answers were not recorded.

59 58 67

0.92 2.83 3.67

>0.10 0.04 0.06

Chronic cough Chronic phlegm Chronic bronchitis

PHYSICAL ACTIVITY AND SPORTS

The question of whether physical activity and exercise play a role in the aetiology of herniated lumbar disc was approached in several ways. As was mentioned above, there was no tendency for persons doing heavy manual labour to have either a greater or lesser risk for herniated lumbar discs than individuals not doing heavy manual labour. However, it was also desired to see if physical activity away from work was involved, or if any specific sports either increased or decreased the risk for herniated lumbar discs. Table VII, which includes only pairs in which both members work full-time, shows that there was a tendency for cases to do less housework and gardening than their matched controls. The trends were similar whether only those working full-time or all people are included. When the data were grouped according to the age of the cases (not shown here), it was found that this trend was present only in those of age 35 and older. When cases were compared to unmatched controls, similar trends were seen in the males, but there was no difference between the female cases and controls. Another approach to physical activity of the cases and controls involved asking t Questions on the respiratory symptoms were adapted from the British Medical Research Council's (1960) standardized questionnaire on respiratory symptoms, which has been used extensively in epidemiological studies of chronic bronchitis.


Because coughing increases the pressure on the discs, it was thought possible that chronic coughing or chronic bronchitis might be associated with an increased risk for herniated lumbar discs, and that because of the strong association between smoking and coughing, cases might have tended to smoke more than controls. Table VI shows that among females there were more cases than matched controls who reported chronic production of phlegm and symptoms of chronic bronchitis, the latter being defined as chronic cough and chronic phlegm.t Although not shown here, there were also more cases than unmatched controls who reported chronic cough, chronic phlegm, and chronic bronchitis, but for none of these symptoms was statistical significance quite reached. There were no differences at all in smoking habits of cases and controls in either group.

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RHEUMATOLOGY AND REHABILITATION VOL. XIV NO. 3 TABLE VII

DISTRIBUTION OF CASE-MATCHED CONTROL PAIRS, ACCORDING TO WHETHER THEY REPORTED SPENDING 20 HOURS OR MORE PER WEEK DOING HOUSEWORK AND GARDENING (>20 HOURS) OR LESS THAN 20 HOURS PER WEEK DOING HOUSEWORK AND GARDENING (0.10 >0.10 0.08

them to rank three types of household and leisure time activities according to the amount of time they spent in each. The activities were (a) household chores and working in the garden; (b) social activities, such as visiting and entertaining; and (c) reading, watching television, or listening to the radio. There were no differences between cases and controls in the answers to this question. In an additional attempt to assess physical activity, cases and controls were shown a card with a list of sports; they were asked to name the ones that they had played during the past two years, and were questioned about the frequency with which they played them while ,'n season, ranging from five or more hours per week to rarely. Cases and controls in general played about the same number of sports, but there was some tendency for the cases to play them rarely and the controls to play them more frequently. (Data are not shown here.) In respect to specific sports, bowling, golf and baseball were played more frequently by cases than their matched controls, but none of these differences between cases and controls was statistically significant. In the comparisons of cases with unmatched controls, there were again no significant differences, although golf and baseball were played rrore frequently among the cases, and bowling somewhat more often. STRESSFUL EVENTS

Cases and controls were shown a list of stressful life events, and were asked if any had happened to them in the year before their onset of symptoms. There was no tendency for cases to have been exposed to a greater number of stressful events than persons in either control group. DISCUSSION This epidemiological study of acute herniated lumbar intervertebral discs has certain limitations, some of which have been discussed elsewhere (Kelsey and Ostfeld, 1975; Kelsey and Hardy, 1975). Among these are the disproportionate numbers of cases in this study population seen at hospital, the somewhat disappointing response rate (about 78%), the lack of a control group from the general population, the relatively small sample size, and the difficulty of diagnosing herniated discs. However, with so little known about the epidemiology of this condition, it seemed that a case-control study based on persons seeking medical care was the approach of choice at this time and that any leads arising from this study could be examined more thoroughly in a detailed study based on a more general population. The consistency of the trends among the surgical, probable, and possible cases, and the ability of many of the same risk factors to differentiate between cases and other persons having low-back radiography as well as between cases and matched controls who had no apparent spinal problems, gives one some confidence that a large proportion of persons who probably had herniated discs were in


Males Females Both sexes

Case 5= 20 h Case > 20 h Case < 20 h Case < 2 0 h Control < 2 0 h Control > 2 0 h Control < 2 0 h Controls 20 h


155


fact picked out of the population having low-back X-ray examination. Also, it would be hard to see why differences between these cases and controls would not be indicative of differences between persons with and without herniated lumbar disc in a more general population. The risk factors which appeared to be most strongly associated with herniated lumbar disc in this study were being in the age group 30-39 years, having a sedentary occupation, driving a motor vehicle (either at work or away from work), living in the suburbs, and having full-term pregnancies. The age distribution of the surgical cases is consistent with other series (Hudgins, 1970; Hirsch and Nachemson, 1963; Aitken, 1952; Eckert and Decker, 1947; O'Connell, 1960; Waris, 1948; Yaskin, 1944), most of which show mean ages of around 40 for cases undergoing surgery, report similar age distributions for male and female cases, and indicate that the age group at greatest risk is the 30-39 year group. In a series composed of a greater proportion of non-surgical cases, the age distribution would be expected to be younger. The precise reason for the distribution is not known; however, as disc degeneration progresses, large portions of the disc are transformed into fibrous tissue that has lost much of its turgor and elasticity, and this would be expected to decrease the chances of a disc herniating (Hult, 1954&). The association with sedentary occupation is not surprising, since it is known that sitting puts more pressure on the discs than either standing or lying down (Keegan, 1953; Caillet, 1968; Nachemson, 1965), and it would thus seem reasonable that stress from several years in a sedentary occupation would bring about an increased risk for a herniated lumbar disc. The still greater risk from sitting while driving a motor vehicle had not been hypothesized when the study began, and there has apparently been no experimental work on the effects of sitting while driving a motor vehicle on the intervertebral discs. However, the magnitude of the relative risk and our inability to attribute it to any other variables associated with driving (Kelsey and Hardy, 1975) lends support to the belief that this relationship is a real one. Furthermore, it is not difficult to think of reasons for such an association. Keegan (1953) has called attention to the insufficient support for the low back provided by most motor vehicle seats. Also, people while driving generally sit with their legs extended to the floor pedals rather than with their feet flat on the floor and are subject to continual vibration and mechanical stress from starting and stopping. The increased risk for herniated lumbar disc among suburban residents compared to city dwellers was seen in both sexes and in comparisons with both control groups. However, when examined in more detail (Kelsey and Hardy, 1975), it was found to be attributable at least in part to the greater tendency of persons who live in the suburbs to drive, and was not a statistically significant variable when driving status was held constant. The observation that female but not male cases had more children than controls suggests that pregnancy itself may be involved in the aetiology of herniated lumbar discs rather than caring for children. (The latter possibility of course cannot be excluded, since there are also differences between the sexes in the amount and type of care given to children.) The association of herniated lumbar discs with number of live-births but not with miscarriages suggests that something towards the end of pregnancy is responsible for this effect. Two such factors could be the mechanical stress from carrying and delivering the baby, and the ligamentous laxity brought about towards the end of pregnancy by relaxin from the corpus luteum. The fact that the association of herniated lumbar discs with live-births was seen only in women with herniations at the L5 level and not at

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the L4 level gives support to the mechanical stress hypothesis, since ligamentous laxity would be expected to affect discs at all levels to about the same extent, whereas mechanical stress would presumably have the greatest effect at the L5 level. Numbers of women with herniations seen at surgery were small, however, and it would be helpful to have this observation confirmed in another study. Other variables for which there was some suggestion of a relationship with herniated lumbar discs were being of the male sex, being of high social class for females, being tall, having a chronic cough or chronic bronchitis, getting insufficient exercise away from work, participating in baseball, golf, or bowling, and being exposed to the spring and autumn seasons. It has generally been assumed on the basis of cases seen at surgery that herniated lumbar discs occur more frequently among males than females, and indeed, in the present study, the ratio of males to females among surgical cases of 2.1 to 1 was almost the same as the 2.0 to 1 ratio reported as the average for the 52 published case series reviewed by Spangfort (1972). However, there is apparently no evidence of a male excess among cases not coming for surgery; in fact, there are two reports (Hudgins, 1970; Hanraets, 1959) in addition to the present one suggesting that there is no male excess among the non-surgical cases. The actual sex ratio, which is most likely less than 2 : 1 , could only be determined in a study of a general population which included surgical and non-surgical cases in their proper proportions. The distortion of the sex ratio among persons coming for surgery is probably attributable to the tendency of females to be more willing to wait for improvement from conservative therapy while males are more anxious to return to work quickly. The possible association between high social class and herniated lumbar discs in females is difficult to evaluate. The indicators of social class used in this study are of necessity only rough approximations to a person's actual social standing. Furthermore, the difference in the social class distribution of cases and matched controls was not strong, and the difference in the social class distribution of cases and unmatched controls was mostly attributable to the low social class of female controls seen at the two emergency rooms. These considerations, together with the knowledge that no association was found for males, leads one to the conclusion that this result should be confirmed in other studies before it is accepted as a real trend. Cases of both sexes were on the average taller than their unmatched controls, and the female cases tended to be taller than their matched controls. These results by themselves thus might not warrant much attention, but Lawrence (1955) observed that taller miners complained more frequently of "back-hip-sciatic pain" than other miners, thus providing some additional support for an association with tallness. On the other hand, Hirsch et al. (1969) did not find that persons with sciatica were any different in height from persons without this symptom, so that height is another variable that needs to be examined further in other studies. The association of chronic coughing and chronic bronchitis with herniated lumbar discs was again not entirely consistent and was not statistically significant. A possible aetiological role for chronic coughing has not been considered in any other investigation, although it does seem reasonable that chronic coughing would in fact put more pressure on the discs. Since cigarette smoking is by far the most important factor in the aetiology of chronic cough, it is difficult to see why there was some association of coughing with herniated lumbar discs but no relationship at all with smoking. The possibility that lack of exercise might play a role in the aetiology of herniated lumbar discs is again very difficult to evaluate. The evidence in this study for such an


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effect is rather scanty, consisting of a tendency for cases of age 35 years and older to report doing less housework and gardening than controls, and for cases, especially females, to play sports more infrequently when they do play them. In addition, the extent of over-all physical activity for many people is largely determined by the nature of their occupation, since many of their potentially active hours are spent at work. The finding in these data that physical activity on the job is not associated either negatively or positively with herniated lumbar disc would thus provide some evidence against an aetiologic role of physical activity itself. However, assessing the amount of exercise that a person takes has repeatedly proved difficult in epidemiological studies, and inaccurate measurement would make it harder to find an association, if it existed. The probable high correlation between prolonged sitting and lack of exercise adds to the difficulty of studying this variable independent of any effect from prolonged sitting. Thus, although it does not appear that lack of physical activity is of much importance, this cannot be stated with a great degree of certainty. A detrimental effect on the discs of playing baseball, golf, or bowling was suggested by the data, but in no instance was statistical significance reached. Since the torsion involved in swinging a bat or club would be expected to increase the pressure on the disc, it is not unreasonable that these activities might have an adverse effect. Likewise, bowling involves holding a heavy object in a position which would undoubtedly be stressful to the spine. However, a larger study population would be needed to determine whether or not these were chance findings or whether participation in these sports really does involve an increased risk for herniated lumbar discs. In these data there was a slight suggestion of a greater than expected number of symptoms beginning in spring and autumn. There seems to be an impression among surgeons and patients that symptoms frequently start in cold and damp times of year, particularly autumn, winter and early spring (Hult, 1954a). Thus, the present study merely adds to this impression, without providing any definitive evidence. Finally, variables which were not found to be associated with herniated lumbar intervertebral discs in this study were race; social class in males; smoking; weight; body bulk; recent stressful events; pregnancies which were not full term; sports other than baseball, golf and bowling; and lifting, pushing, pulling, and carrying on the job. Race, social class, smoking habits, pregnancies which are not full-term, and participation in sports have apparently not been studied by other investigators, and there is no way to confirm or refute the negative findings of the present study. These will not be considered further. Although it would seem that greater body weight would result in increased pressure on the discs and would thus bring about a greater chance of herniation, such an effect was not found in this study. Also, Hirsch et al. (1969) did not find any differences in the average weights of persons with both low-back pain and sciatica compared to healthy controls, and Hult (1954ft) was unable to relate subjective lumbar-spine symptoms to obesity. Thus, it appears that weight is probably not an important risk factor for herniated lumbar disc. The role of lifting and other manual labour in the aetiology of herniated lumbar discs has been the subject of considerable discussion, with some surgeons (Goodsell, 1967; Howorth, 1953; Spurling, 1953) believing that jobs involving heavy physical labour increase the risk for herniated lumbar disc, and others (Semmes, 1964; Coventry, 1968) suggesting that it is sedentary occupations which increase the risk. These data, which obviously support the latter point of view, have some confirmation from data of Hult (1954ft), who also concluded that heavy work is not a fundamental cause of the

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"lumbago-sciatica syndrome". Although lifting was given as the precipitating event more often than any other activity, especially among those receiving workman's compensation, its importance as an underlying cause is doubtful. Nevertheless, it would be desirable for further studies to be carried out in which more detailed and refined methods of measuring amount and type of physical activity at work were used. In fact, in view of the preliminary and exploratory nature of this entire study, it would be important to have all the results presented here re-examined in other investigations.

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ACKNOWLEDGMENTS

This study was supported by USPHS Grant 5-R01-AM-15397 from the National Institute of Arthritis, Metabolism, and Digestive Diseases. The author was supported by Career Development Award 1-K04-NS-70502 from the National Institute of Neurological Diseases and Strokes. The assistance of Adrian M. Ostfeld, M.D., Maryann Bracken, Arlene Finger, Mary Johnson, and Gertrude Laden has been greatly appreciated.


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An epidemiological study of the relationship between occupations and acute herniated lumbar intervertebral discs.

Aquaporin-1 Expression in Herniated Human Lumbar Intervertebral Discs.

Pregnancy and the syndrome of herniated lumbar intervertebral disc; an epidemiological study.

Herniated lumbar intervertebral disk.

Herniated intervertebral discs associated with unstable spinal injuries.

Lumbar epidural injection of steroid in acute prolapsed intervertebral discs. A prospective study.

Driving of motor vehicles as a risk factor for acute herniated lumbar intervertebral disc.

Treatment of Contained Herniated Lumbar Discs With Ozone and Corticosteroid: A Pilot Clinical Study.

Three-dimensional observation of collagen framework of lumbar intervertebral discs.

Quantitative magnetic resonance imaging of the lumbar intervertebral discs.

Intervertebral bone implants following excision of protruded lumbar discs.

Fibroblast Transplantation Results to the Degenerated Rabbit Lumbar Intervertebral Discs.

Variations in size of the bony lumbar canal in patients with prolapse of lumbar intervertebral discs.

A comparison of minimally invasive techniques in percutaneous treatment of lumbar herniated discs. A review.

Mechanotransduction in intervertebral discs.

Double-blind evaluation of chemonucleolysis for herniated lumbar discs. Late results.

SOCS3 signaling pathway in herniated lumbar discs.

Double-blind evaluation of intradiscal chymopapain for herniated lumbar discs. Early results.

Double blind evaluation of extradural methyl prednisolone for herniated lumbar discs.

An unexpectedly progressed lumbar herniated disk.

Thoracic infectious spondylitis after surgical treatments of herniated lumbar intervertebral disc.

Propriospinal Myoclonus Induced by a Herniated Lumbar Intervertebral Disc at a Young Age: A Case Report.

Discoscopic findings of high signal intensity zones on magnetic resonance imaging of lumbar intervertebral discs.

Gene expression profile analysis of human mesenchymal stem cells from herniated and degenerated intervertebral discs reveals different expression of osteopontin.