Introduction

Several outbreaks of Epidemic Conversión Disorder are occurring in different groups of people in the world. Rather than being viewed as a number of people suffering from individual conversion disorder, epidemic hysteria is considered as a social phenomenon involving otherwise healthy people. We received a report letter from Dirección General de Epidemiología, about the existence of a large number of possible food poisoning cases among students, attending morning sessions at a technical high school, located in the downtown area of Mexico City. Twelve students were driven to the Mexican Red Cross Hospital due to fainting. The aims of this study were to determine the cause of such outbreak in a group of adolescents; to get an adequate explanation about the origin of the event; to identify the event dissemination ways and associates risk factors.

Methods Study design:

A matched case-control study was carried out to identify factors associated with the illness. Two control cases were randomly selected from the list of nonill students for each case. Fifty two cases and 104 controls were included.

Hypotheses:

Following the good health status determined by the physician at the hospital, we started the initial interview with the students. We reached the following possible hypotheses regarding the origin of this outbreak: first, the event was due to food poisoning; second, to the inhalation of a toxic gas such as carbon monoxide and thirdly, by exposure to high levels of contaminants. Finnaly, it might be a mass event of conversion disorder.

Variables:

Among the variables included in the study were: sex, age, class group, location of the student at the time of the out-break, and foods eaten during recess and immediately before the outbreak. All the students present at the time of the outbreak were interviewed using a standard questionnaire.

Laboratory

Simultaneously, samples of the food-products sold in and around the school that day were collected for bacteriologic and chemical analyses, the existence of a gas leak, carbon monoxide source, or any other airborne pollutant was investigated by the research team.

Analyses:

The demographic characteristics were analyzed by descriptive statistic; association between risk factors as possible causes of the event was determined by multivariate analysis at 95% confidence interval.

Results:

The outbreak occurred in the building of a downtown public school in Mexico City. The school has three floors, surrounding a central yard. There are 11 classrooms, two laboratories, an art workshop and a school medical clinic. The total duration of outbreak was 15 minutes. There were 455 students enrolled in the morning program, all of them were interviewed. A total of 52 cases was identified, among the 455 students, for an attack rate of 11.4%. There were three groups in which no cases were found. The attack rate in girls was 3.9 times higher than in boys. Sixty five percent of the cases occurred in two of the nine classrooms (1° B and 2° A). All the students of one group had been waiting at the patio for over an hour during an interclass break. Case cero was a girl from this group with a previous history of fainting. The outbreak occurred outside class-room in the central yard. Five female classmates of case cero fainted while they were with her in the yard. Cases then spread rapidly to the first floor with an attack rate of 13.2 percent, the second floor had 7.7 percent, and finally the third floor had 2.1 percent. All cases had fainted as per case definition. Additionally, headache was a prominent symptom occurring in 88 percent, paresthesias in 56 percent, and perceived difficulty in moving arms or legs in 35 percent. Also almost a quarter of the cases complained of irritation of the eyes and nose. Within one hour, all had completely recovered. Five days after the problem, three girls fainted; no outbreak occurred.

Being a girl or belonging to class groups 1°B or 2°A, were the most significant risk factors, with (p 0.001). Also being less than 15 years of age was a significant risk factor for illness. The analysis of food preference data in the cases and controls showed that drinking a fruit beverage "X" was not related to the illness. Foods such as sandwiches, brought from home and cookies, candies and popcorn bought from street venders, had a borderline significant association with the illness. However, the number of cases attributable to these foods was very low. Also, it was difficult to figure out how sandwiches were prepared by mothers of individual students and how this factor could be implicated. No pathogen toxin or toxic chemical were identified in the food samples. Some foods studied in the crude analysis were ruled out in the multivariate analysis. A thorough environmental was negative, there being no evidence of a continuing gas leak or other causes. The pollution levels during that week were reported as being within the normal range, by the Metropolitan Index of Air Quality (IMECA). In order to evaluate psychological factors, individual interviews were carried out. The psychologist found that the cases tended to have one or both parents absent from home due to divorce or death, and their family have been damaged by eco-nomic problems. In addition, psychological testing showed that these cases had higher anxiety levels than controls.

Discussion According to our findings, this outbreak appears as a Epidemic Conversion Disorder. First, no biologic cause was found for the cases. In addition, there was not any evidence to implicate food poisoning, no source of toxic gas could be identified at the school, and the levels of air pollution were not above normal levels. The clinical presentation was not different from the fainting and paresthesia reported in others studies, nor was sex distribution. One possible explanation for the initial case was the time of sun exposure in the schoolyard. Subsequent spread of the outbreak was due to psychological and extra-medical factors, including publicity by the mass media. Interestingly the spread was stopped immediately after closure of the school for one day. All the findings of the psychological reports, applied by another researcher group add further weight to this conclusion.

In support to our results, many studies has been reported in which the clinical manifestations are the same that we found. In these reports, the outbreak occurred frequently among women, teenagers, students of elementary and secondary schools and chorus, in whom no organic etiology or precipitant causes can be identified. Some authors have reported that the phenomena is more evident in groups with hormonal changes, rigid discipline used in music bands, and during periods of exams or situations under stress. Such circumstances are more related to the outbreak. Some studies have demonstrated that dysfunctional families, divorced or dead parents, play a mayor role in comparison with other factors such as socioeconomic level, religion or ethnicity. The mechanisms of these events have not been clearly identified. The typical course of a psychogenic epidemic at a workplace progresses from sudden onset, often with dramatic symptoms, to a rapidly attained peak that draws much publicity and is followed by quick disappearance of the symptoms. Over 90% of the affected people are women, and the signs range from dizziness, vomiting, nausea, and fainting to epileptic type seizures, and hyperventilation. Predisposing factors include boredom, physical stressors, poor labor-management relations, impaired interpersonal communications and lack of social support. The rapid spread in the conversion disorder, is by visual contact; the treatment should be directed towards the underlying stressors but the out-break may be prolonged. In Epidemic Conversion Disorder the abnormality is confined to group interactions. This outbreak shows the importance of psychological support in populations with risk factors of presenting the illness. The social problems among large populations produce an unforgettable painful experience, mainly among teenagers who dealt with the psychological damage with-out any support.

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