The epidemiology of motor neuron diseases: a review of recent studies

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Abstract

As the incidence and mortality rates of motor neuron diseases (MNDs) have been reported to increase steadily over the 1950s–1980s decades, we compared the results of the most recent studies (1990s decade) with the ones reported for those earlier periods.

The relevant literature was retrieved on a keyword basis from online medical and official death statistics databases. Fifteen European and North American studies were analyzed, for comparison with the results reported in review papers.

The 1990s incidence and mortality rates of MND average at 1.89 per 100,000/year and 1.91 per 100,000/year, respectively, thus yielding increases of 46% and 57% over the 1960s–1970s decades, respectively. This increase appears mainly due to Southern Europe countries, to female gender and to patients aged 75 years and over.

Thus, the results of this analysis (i) confirm that the incidence of, and mortality from, MNDs continued to increase during the 1990s and, (ii) suggest that this increase could be partly due to increased life expectancy. Other factors might also contribute, such as better diagnosis since El Escorial criteria, and better accuracy of death certificate collection.

However, a real increase in the incidence of MNDs, possibly related to environmental factors, cannot be excluded.

Introduction

Most of the epidemiological parameters (i.e. incidence and mortality rates) which characterize motor neuron disease (MNDs), including amyotrophic lateral sclerosis (ALS), have been reported to have increased steadily between the 1950s and the 1980s decades [1], [2], [3].

Thus, on average, estimated increases of 70% in incidence and, of 100% in mortality (calculated from [1], [2], [3]) have been reported in Europe and in North America, globally between the 1960s and the 1980s.

Various hypotheses, including environmental factors, better health coverage and ageing of the population, have been raised to explain this increase.

In addition, geographical variations have been evidenced, including a south to north increasing gradient in Europe [1], [2].

In 1991, at the El Escorial meeting, the World Federation of Neurology (WFN) Research Group on Neuromuscular Diseases has proposed consensual criteria for the diagnosis of ALS, leading to a reference publication in 1994 [4].

As, since that date, a number of epidemiological studies have been performed in order to provide new accurate data as to the frequency of ALS in Europe and in North America, it appeared interesting to analyze the results yielded by those studies and to compare them to the ante-El Escorial ones, in term of secular trends, in the various regions concerned.

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Material and methods

A keyword-based literature search was performed in September 2000 on the following databases: Medline, Biosis, Embase and Pascal. In addition, supplement issues of medical journals dedicated to recent congress and meetings (1999–2000) were screened.

Finally, official French (INSERM) and USA (NCHS) mortality and vital statistics were analyzed.

For the most recent data, covering the 1990s decade, only those studies conducted during this period or initiated earlier but completed recently, were taken

Incidence

As shown in Table 1, the incidence rates of ALS in the 1990s decade range from 1.47 to 2.70 per 100,000/year (average: 1.89 per 100,000/year). No difference between the various regions is apparent. The male/female sex ratio is close to 1.3. Only two studies, conducted in Italy, addressed the issue of changes in incidence over time [5], [6]. They show a regular increase of incidence rates since the 1960s (incidence rates per 100,000/year. 1960s: 0.80; 1970s: 1.05; 1980s: 1.11; 1990s: 1.83).

From

Discussion

These results show that the incidence and the mortality rates reported for MND/ALS during the 1990s decade are, on average, higher than those reported for earlier periods.

Thus, on average, the 1990s incidence of MND/ALS is close to 1.9 per 100,000 persons-year, when that reported for the 1960s–1970s averages at 1.3 per 100,000 persons-year (all regions), thus leading to an increase of 46%. However, this global figure uncovers broad regional differences, with increases of 162% and 17% in

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