The Epidemiology of Myasthenia Gravis

Lawrence H. Phillips II, M.D.

Disclosures

Semin Neurol. 2004;24(1) 

In This Article

Abstract and Introduction

Abstract

Epidemiology is the study of a disease in a population. The epidemiology of myasthenia gravis (MG) has been studied for over 50 years, and over 50 studies have been published in one form or another. The published prevalence has steadily increased, and one can estimate that there are ~60,000 patients with MG in the United States. The current trends indicate that, as the population ages, an increasing number of patients with MG can be expected, but the clinical patterns of the disease may change.

Introduction

Epidemiology is the study of a disease within a population. This is distinct from the study of a disease in an individual patient or a group of patients. Each physician encounter with a patient generates information in the form of history, physical examination findings, and, often, laboratory data. As no one patient is likely to have all of the manifestations of a particular disease, a physician-patient encounter is inherently biased in favor of the observations made during that encounter. Any aspects of the disease not present in the individual patient will be neglected, and the information generated from the encounter should properly be considered to be an anecdote. An old academic trick is to describe the first example of an unusual disease as "the only time I have seen such a case." The second time the disease is encountered, one can say, "I have seen this before." When the third case is encountered, some would consider it reasonable to say, "I have seen case after case after case of this disease."

Let us consider for the moment the meaning of the word "anecdote." In everyday usage, it is a noun that usually indicates a "short, often amusing story about an event."[1] In the context of analysis of medical information, however, we think of it as an adjective (i.e., "anecdotal information"). In this context, the meaning is "based on reports or things someone saw rather than on proven facts."[1] Another, more pithy way of stating this is that data are not the plural of anecdote. This is not to deny the value of clinical anecdotes, but we should keep in mind that there are many reasons why they may not be representative of a disease.

The only way to be absolutely certain that one has all possible information about a disease is to examine every patient with it. This is, of course, impossible and absurd to even contemplate. It is possible to come close to that standard by collecting information about the disease in a defined population. The role of the epidemiologist is thus to identify characteristics of a disease that may not be apparent in individual patients but are clearly present in a population of affected patients. The larger the population, the more comprehensive the insights one can obtain.

Classical epidemiology has concentrated primarily on infectious diseases, hence the derivation of the term from "epidemic." The epidemiologic approach has been used to identify patterns that help to identify the source of epidemics, and important clues regarding etiology have been found. This is the basis for public health investigations into outbreaks of such infectious diseases as meningococcal meningitis, poliomyelitis, and West Nile virus encephalitis. More recently, investigators have realized that the same techniques can be applied to the study of chronic illness.

Most epidemiologic investigations begin with enumeration of the number of patients affected in a population. The numbers are expressed as a "rate," or fraction of the population. The most commonly defined rates are incidence, prevalence, and mortality. Incidence is the number of new cases (incident cases) that occur in a population in a defined period of time. Most commonly it is expressed as the average annual incidence. Prevalence (or point prevalence) is the number of cases of the diseases present in a population at a specified time point, known as the prevalence day. Mortality is a rate similar to incidence except that it defines the number of deaths of affected individuals in a specified period of time. It can be expressed as cause-specific (i.e., the number of individuals who die from the effects of the disease under study) but it is most commonly expressed as the overall number of deaths from all causes.

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