Epidemiology of Multiple Sclerosis in Glasgow

Stan Murray, Consultant in Public Health Medicine, Greater Glasgow NHS Board

Khalid Bashir, Principal Information Officer, Greater Glasgow NHS Board

Gillian Penrice, Specialist Registrar in Public Health Medicine, Greater Glasgow NHS Board

John Womersley, Consultant in Public Health Medicine, Greater Glasgow NHS Board

Correspondence to: stan.murray@gghb.scot.nhs.uk

SMJ 2004 49(3): 100-104

Abstract 

Study objective:  The objectives of the study were to establish the prevalence and incidence of multiple sclerosis in Glasgow and to assess the epidemiological importance of deprivation and ethnicity.

 Design:  The study was a descriptive cross-sectional study.  Multiple sources of ascertainment were used to identify cases.

 Setting and patients:  The study was carried out in the population of three Local Health Care Cooperatives (LHCCs) in the area of Greater Glasgow NHS Board.  The total population was approximately 169,000.

 Main results:  In total, 245 patients with multiple sclerosis were identified.  The mean age of patients was 49.8 years, the female to male ratio was 3.2:1, and the mean duration of disease was 16 years. The overall prevalence was 14.5 per 10,000, and the overall incidence 5.7 per 100,000 per year.  Both the prevalence and incidence of multiple sclerosis were higher in the more affluent population. The crude prevalence in the Asian population was 6.3 per 10,000.  

Conclusions:  The epidemiology of multiple sclerosis in Glasgow was similar, in most respects, to the epidemiology described in other parts of the U.K.  There was evidence for the importance of deprivation as a determinant of both incidence and prevalence of disease.  The reasons for the higher incidence and prevalence of multiple sclerosis in less deprived populations are not clear. 

Introduction

Multiple sclerosis, the most common disabling neurological disease of young adults, is an important problem in public health.¹  The needs of patients with multiple sclerosis are wide-ranging and depend on both the type of clinical presentation and the stage of disease.  In the last few years, the task of needs assessment has been complicated by the need to consider provision of new and expensive treatments.²

The assessment of need of patients with multiple sclerosis in Glasgow has been hampered by the absence of local epidemiological information, and for this reason, an assessment of need was carried out at the beginning of 2000.  In this paper, the principal epidemiological findings of this study will be described.

Objectives

The main objectives of the study were to estimate the prevalence and incidence of multiple sclerosis in Glasgow and to investigate the epidemiological importance of deprivation and ethnicity. 

Methods

Design of study

A descriptive cross-sectional study was carried out in the catchment populations of three of the fifteen Local Health Care Cooperatives (LHCCs), a total population of approximately 169,000.  LHCCs are local groups of general practices which have been established in Scotland for the purpose of implementing clinical governance in primary care.  The population of each LHCC is about 55,000.  The selection of the three LHCCs from the fifteen in the Board's area was not random.  At the time of the study, the creation of LHCCs was relatively recent, and the willingness of doctors in different LHCCs to participate in epidemiological projects was in doubt.  For this reason, an approach was made to the lead general practitioners of three LHCCs who were known to be interested in epidemiology. These LHCCs covered the Maryhill & Woodside, Cambuslang & Rutherglen and Strathkelvin areas of the city.   

All of the general practitioners in the three LHCCs agreed to allow access to their records.  Originally, it was intended that the Poser diagnostic criteria would be used in the definition of cases.³  In the course of the pilot study, it became clear that insufficient clinical information was available in the case records in primary care for Poser diagnostic categories to be allocated.  For this reason, a more pragmatic definition was used.  Patients were considered to have multiple sclerosis if the case-records contained documented evidence of assessment and diagnosis by a neurologist.

Population

The only available source of information about population, by practice or by LHCC, is the Board's Community Health Index (CHI), a register of all patients registered locally with a general practitioner.  The most important shortcoming of the CHI is its inflation, usually considered to reflect delay in removal from practice lists of patients who have died or moved elsewhere.  Comparison of the CHI with the Registrar General's mid-year estimate of population for Glasgow in 1998 showed that the level of inflation was approximately 5%.  Correction factors specific for age-group and sex were calculated by dividing the number of people in each category derived from the Registrar General's mid-year population estimate by the corresponding number derived from the CHI.

Ascertainment

Multiple methods of ascertainment of cases were used, similar to those which have been used in other epidemiological studies.^{4 5 6 7}  Five sources of data were used to identify cases.  These were primary care data, routinely-collected hospital discharge data, neurologists and two Community Physical Disability Teams (CPDT) in the north and south of Glasgow.  Cases were also identified from records of the Multiple Sclerosis Therapy Centre (MSTC), a voluntary organisation which receives financial support from health boards in the West of Scotland.  Data from neurologists, CPDT and the MSTC were only available by post-code sector of residence, rather than by practice.  For this reason, data from these sources were selected by the post-codes of residence which residents of each LHCC were most likely to have.  A request for data was also made to the Multiple Sclerosis Society.  Unfortunately, the Society was unable to release named information to us for the purposes of the survey.

An initial list of patients with multiple sclerosis was compiled from computerised records in each practice.  Visits were made to each practice, and the diagnosis verified in each case by inspection of the case records.  The list of verified cases was compared with lists derived from other sources.  Data were abstracted directly from case records to a specially designed proforma.  The data were coded and entered into a database in Microsoft Access.

Deprivation and ethnicity

The index developed by Carstairs and Morris was used to measure levels of material deprivation.⁸  Several variables collected in the decennial census are considered to reflect levels of material affluence.  The Carstairs score is an index of deprivation calculated from four of these variables and allocated at the level of post-code sector.  Information about the ethnicity of patients was collected from case-records, using surname.  Information about populations in different ethnic groups was derived from the 1991 Census.

Statistical analysis

Incidence was estimated by making use of data about patients whose date of presentation fell in the last ten years, from 1989 to 1998.  Confidence intervals were calculated for values of incidence and prevalence by making use of the Poisson distribution for analysis of rates.⁹  Techniques of both direct and indirect standardisation were used.  Direct standardisation of prevalence was carried out using the population of Northern Ireland in 1961 as the standard population.¹⁰  Indirect standardisation for age and sex was used to calculate incidence and prevalence rates in different deprivation categories.  In this analysis, age and sex specific rates in the population of GGNHSB were used as the standard rates.⁹  Patterns of incidence and prevalence by deprivation category were tested by linear regression.⁹

Results

Ascertainment

In total, 245 patients with multiple sclerosis were identified.  The two most important sources of information were records in primary care and routinely-collected discharge data (Table 1).  All of the 245 cases were identified from these two sources.  In total, 219 cases were identified from records in primary care, and 106 from routinely collected discharge data.  One hundred and thirty-nine cases were identified from records in primary care alone, 26 cases were identified from routinely collected data alone and 80 cases were identified from both sources.  Smaller numbers of cases were identified from the other sources.  Sixty-one, eleven and eight cases were identified from records held by MSTC, CPDT and neurologists respectively.  No cases were identified from these sources which were not also identified from the two primary sources of information.

Prevalence

Of the 245 cases, 58 were male and 187 female (Table 2).  The mean age of patients identified was 49.8 years, and the female to male ratio was 3.2:1.  In this study, no cases were detected in patients less than 21 years old or more than 75 years old.  The overall prevalence of multiple sclerosis was 14.5 per 10,000 (Table 3).  The prevalence was 7.2 per 10,000 in males and 21.3 per 10,000 in females.  In both sexes, the prevalence was highest in the population aged 45-64 years.  There was evidence of a considerable degree of variation in prevalence by practice.  In two practices, no cases were identified.  In the other practices, the prevalence ranged from 27 to 5.1 per 10,000. 

The distribution of duration of disease is shown in Table 4.  The mean duration of disease was 16 years.   Eight patients had had the disease for less than a year, 47 patients had had the disease for five years or less and 86 patients had had the disease for 10 years or less.  Thirty-six patients had had the disease for more than 30 years.

Incidence

In the study population, there had been 96 new cases of multiple sclerosis in 10 years, or 9.6 each year (Table 5).  The overall incidence was 5.7 per 100,000 per year (Table 6).  The incidence in females, 7.7 per 100,000, was more than twice the incidence in males, 3.5 per 100,000.  The overall incidence was highest in people aged 15-44 years, 9.6 per 100,000.  The commonest clinical features of patients who presented with multiple sclerosis in the last 10 years were paraesthesia of the lower or upper limbs, weakness of the lower limbs, visual symptoms, for example visual loss, diplopia, and vertigo (Table 7). 

Assuming the prevalence found in this study, a total of 1,322 cases would be expected in GGNHB, and the overall prevalence would be 14.5 per 10,000.  The expected number of new cases in Glasgow in each year would be about 52.

Deprivation and ethnicity

Both the prevalence and incidence of multiple sclerosis were related to the level of material deprivation in the population (Figures 2 and 3). The standardised prevalence ratio ranged from 150% in the most affluent population to 57% in the most deprived (p<0.001).  The standardised incidence ratio was 170% in the most affluent population and 56% in the most deprived (p<0.001).  In total, two of the patients with multiple sclerosis were of Asian extraction.  There were no patients belonging to either the Chinese or Afro-Caribbean ethnic minority.  The crude prevalence in the Asian population was 6.3 per 10,000 (Confidence Interval 15.0, -2.4).

Discussion

Ascertainment

In this study, a pragmatic definition of multiple sclerosis was used.  A case was defined as one in which the diagnosis had been made after assessment by a neurologist.  The diagnosis of multiple sclerosis had not been questioned or revised in any of the patients identified in the study.  The use of multiple methods of ascertainment is a routine method of maximising the level of ascertainment in studies of prevalence of multiple sclerosis and other types of chronic disease.  In this study, all of the cases were ascertained from two sources, primary care and hospital discharge documents. 

Prevalence

The results of the study were broadly consonant with those of other epidemiological studies carried out in the UK.  The estimate of crude prevalence of multiple sclerosis, 14.5 cases per 10,000, lies in the range of values for prevalence which have been found in other studies.^{11 6} The finding that 76% of the cases were in females was also in accordance with the results of other studies.  In this study, the female to male ratio of cases was 3.2:1, which exceeds the ratio in most other surveys.

Three qualifications need to be taken into account in comparing the results of different studies.  Of these, the need for standardisation may be addressed most simply.¹⁰  The standardised prevalence in this study was 12.9 per 10,000.  Secondly, the estimate of prevalence depends on the completeness of ascertainment.  In view of the fact that estimates of prevalence are known to increase in consecutive surveys of the same area it is important to consider that this is the first study of prevalence in Glasgow. ^{12 13 14}   Although multiple sources of data were used, it is likely that ascertainment may not have been entirely complete, and that the estimate of prevalence would be greater in a subsequent study.  Lastly, the approach to classification may complicate comparison with other studies. In this study, definition of a case was based on a process of neurological assessment and diagnosis.  Although there may have been differences in this process with different consultants, the fact that the diagnosis had not been revised in any of the cases suggests that the diagnosis of multiple sclerosis was unequivocal.

The variation in prevalence by practice has been reported in previous studies. In this study, there were no cases of disease in two practices.  In the others, the prevalence varied more than five-fold, from 5.1 to 27 per 10,000.  In one study in Fife, the prevalence in one practice was 46.4 per 10,000¹ and in another the prevalence was 50 per 10,000.¹⁵  One possible explanation for these variations is that patients with multiple sclerosis may elect to join specific practices. 

Incidence

Two assumptions underpin the method of estimating incidence from the numbers of patients with disease of ten years' duration or less.  The first is that mortality in patients with multiple sclerosis is very low in the first ten years of the disease and the second, that net movement of patients into the area of study is negligible.^{16 17}  The estimate of crude incidence, 5.7 per 100,000 per year is broadly in accordance with estimates made in six other British studies.^{12 4 18 13 19 6} The incidence in females, 7.7 per 100,000 per year, was more than twice the incidence in males, 3.5 per 100,000 per year, a finding that confirms the greatly increased risk of multiple sclerosis in women than in men.  The age and sex specific incidence rates may be used to calculate that the total expected number of new cases in the Board's population each year would be 52.

Ethnicity

Only two subjects belonged to an ethnic minority.  Both patients were of Asian origin, in an estimated Asian population of 3,196 people. The apparent rarity of disease in the population of Asian extraction is a well-recognised feature of multiple sclerosis.²⁰  Two qualifications should be made about the findings in the Asian population.  Firstly, the confidence intervals for prevalence are wide because only two cases were detected.  The fact that the confidence interval for the prevalence rate included zero shows that the finding was not significant.  Secondly, the widely-recognised methodological problems associated with studies of prevalence in the Asian population should be taken into account.²¹  These include difficulty in estimating the catchment population and errors in case-finding.  The latter is, in part, attributable to reluctance on the part of patients and families to accept the diagnosis of multiple sclerosis.

Deprivation

In this study, the evidence for a gradient in both incidence and prevalence by category of deprivation was unexpected.  The prevalence of the disease might have been expected to have been greater in more deprived populations.  This might have reflected declining economic circumstances and possible loss of employment, especially after the onset of disability in chronic disease.

Multiple sclerosis has been reported to be more frequent in populations of higher social class¹⁴ but the importance of deprivation in the epidemiology of multiple sclerosis does not appear to have been addressed in any previous study.  Material deprivation is a well-established determinant of incidence in a wide range of diseases.⁸  In most types of morbidity, the epidemiological importance of deprivation reflects an association of deprivation with a relevant risk factor.  Aetiological uncertainty about multiple sclerosis complicates understanding of the significance of deprivation as a possible risk factor.

The finding that prevalence is greater in more affluent populations, appears to reflect a similar pattern of incidence.  The difficulty in analysing a possible relationship between material deprivation and incidence stems from the dearth of knowledge about risk factors, and the evidence that there may be a critical period in early life for risk of the disease.²²

Acknowledgements:  

The authors wish to acknowledge the support of Drs I A Notman, IMH Gordon and N J Gaw and their colleagues in the Strathkelvin, Cambuslang and Rutherglen and Maryhill and Woodside LHCCs for access to medical records.  The authors are also grateful to Mrs Margaret Bell, who prepared the paper.

Keywords:  Multiple sclerosis, epidemiology, deprivation.

References

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