Introduction

The ultimate aim of epidemiologic research is to find means to prevent disease onset (primary prevention) and to restore health once a disease has been developed (secondary prevention). Additional purposes are to evaluate and optimize healthcare. It is not surprising that the main clinical areas covered by epidemiologic research are those associated with a high mortality rate in the population as a whole, such as tumors, and those involving a major disability problem, for example chronic inflammatory diseases. I believe that hidradenitis suppurativa (HS) belongs to this second group. However, the disease has been the subject of limited epidemiologic interest.

8.2 Descriptive Epidemiology

Contents

HS appears to be one of the most disabling dermatological diseases, with about half of patients reporting it as a relevant problem and causing severe distress The development of effective preventive mea sures starts by assessing the distribution of a disease in a given population and, by comparing measures between countries, making correlations with other factors, i.e., ecological correlations. The usual measures of disease distribution are incidence and prevalence. Incidence

Introduction 58 r refers to those cases newly developed in a popu-

Descriptive Epidemiology 58 lation over a given period of time. More specifi-

Analytic Epidemiology 60 cally, incidence density represents the number

Clinical Epidemiology: of new cases per person-time and is applicable

Natural History and Prognosis 62 to a dynamic population reflecting the speed of

Perspectives 63 dissemination of the disease in the population, while cumulative incidence is the proportion of

cases that develop in a fixed group over a given period of time. Such a measure reflects the probability of a member of the group developing the disease over the time considered. It should be

Table 8.1. Prevalence studies of hidradenitis suppurativa (HS)

Reference

Country

Sample

Assessment

Measure

Estimate

Fitzsimmons, 1984 [1]

UK

Unclear

Unclear

Unclear

1:3000

Fitzsimmons, 1985 [2]

UK

Unclear

Unclear

Unclear

1:300

Lookingbill, 1988 [3]

USA

1106 new dermatology patients

Clinical examination for hidden or unrecognized conditions

Point prevalence

1:1000

Jemec, 1988 [4]

Denmark

70 female hospital employees

Interview

Lifetime prevalence

4:100

Harrison, 1991 [5]

UK (South Wales)

Unclear

Clinical examination

Point prevalence

1:600

Jemec, 1996 [6]

Denmark

599 adults

Interview

1-year prevalence

1:100

Jemec, 1996 [6]

Denmark

507 patients at an STD clinic

Clinical examination

Point prevalence

4:100

Mahe, 1998 [7]

Mali

10,575

Clinical examination

Proportion of patientsa

1:3000

TNS Sofres, 2005 [8]

France

6887

Interview

1-year prevalence

1:100

a Not a true prevalence estimate.

a Not a true prevalence estimate.

noted that incidence estimates require an onset for the disease to be precisely defined. For many chronic disorders characterized by prodromal signs and symptoms, such as HS, such an onset may be difficult to establish. Prevalence refers to those cases that are present in a given population, irrespective of their onset, at a point in time (point prevalence) or at any time point over a longer interval (period and lifetime prevalence). Prevalence depends on incidence and on the average duration of the disease. If a disease persists without a cure for a long time, it may give rise to significant prevalence rates even if the incidence is low. Prevalence reflects the burden of the disease in the population and it is a useful measure for planning health services.

Data concerning the descriptive epidemiology of HS are sparse and are derived from studies that adopt different methods, making comparisons difficult (Table 8.1) [1-8]. No incidence data are available, while a number of prevalence studies have been conducted. Critical issues to consider when comparing figures are: representativeness of the examined sample, the timeframe of the study, diagnostic criteria, and methods to assess cases (interview versus clini cal examination). The last one is a particularly crucial issue. No validated criteria for HS are available in the published literature. The best instrument was developed in the framework of a genetic study in the UK [9]. A consensus approach, combining information from the published literature and the views of leading experts in the field of HS research, was adopted. By consensus, three key elements were required to make a diagnosis of HS: (1) typical lesions; (2) a characteristic distribution; and (3) the recurring nature over time. Based on these premise a set of typical lesions was compiled and termed "primary lesions". These were: (1) painful and/ or tender erythematous papules (<1 cm in diameter); (2) painful and/or tender erythematous nodules (>1 cm in diameter); (3) painful and/or tender abscesses (i.e., inflamed, discharging papule or nodule); (4) dermal contractures (i.e., a rope-like elevation of skin); (5) double-ended comedones. The characteristic sites were chosen in accordance with the two areas most frequently affected by HS, namely axillae and groin. These areas were defined by anatomical borders and termed designated sites. To be classified as a case of HS a person must have either: (1) active

disease: one or more primary lesion(s) in a designated site plus a history of three or more discharging or painful lumps (unspecified) in designated sites since the age of 10, or (2) inactive disease: a history of five or more discharging or painful lumps (unspecified) in designated sites since the age of 10, in the absence of current primary lesions. Such a disease definition was able to detect most cases that were judged clinically to represent HS in a previous study. A possible lack of sensitivity of the definition was acknowledged. Formal validation would be necessary before its use in epidemiologic or other studies could be recommended.

The best way to assess disease frequency would be by taking a randomized representative sample of the general population. Actually, most of the available studies rely on convenient population samples based on selection procedures other than randomization. Prevalence estimates range from 3:10,000 to 4:100. These variations may be largely explained by different selection procedures and variations in the sex and age distribution of the examined samples. Interestingly, studies based on population samples are rather consistent in suggesting a 1-year prevalence of about 1%. HS is more common in females, with reported female:male ratios ranging from 2:1 to 5:1. Moreover, the age of onset may vary from childhood to middle age, with a reported average of 21-23 years [6, 9]. Recently, a French survey (TNS Sofres population survey) has provided a weighted estimate of HS 1-year prevalence based on a large representative sample of the general population [8]. The survey showed a convincingly higher prevalence of HS in females as compared to males (1.4% versus 0.6%) and a decreasing prevalence with age from 1.5% in the age group <25 years to 0.5% in the age group >55 years. The distribution of lesions may vary according to gender. In a Danish study, active genitofemoral lesions occurred significantly more often in female patients, while no sex difference was seen in the rarer axillary lesions. Non-inflamed quiescent nodules were also more prevalent in women and in genito-femoral lesions [6].

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