The data presented here suggest an increased risk for MS among female nurse anaesthetists compared with other female nurses and teachers. The risk deduced from our data concern exposure prior to 1985; the exposure to volatile anaesthetic agents may thus reflect an earlier situation with less stringent restrictions which should be kept in mind when discussing safety in anaesthetic departments. From our clinical experience we have the impression that the exposure levels may have been high. The nurse anaesthetists that we encountered as MS patients have to a large extent had potentially very high exposure to ether when working with paediatric anaesthesia, often with the child on their lap. Several of them describe toxic symptoms afterwards, such as headache, nausea, and fatigue.
The specificity based on our number of cases may be considered adequate, based on existing register information and therefore of totally objective character. The sensitivity, however, is low as several cases have not been admitted to hospital or do not have a disability pension due to the disease. There is hardly any reason, however, why MS nurse anaesthetists should have been registered in any other way than the other nurses or teachers, which could have systematically biased the results obtained. With regard to the number of exposed subjects, it is noteworthy that we, in a separate analysis without age restrictions, only found 997 anaesthetist nurses, whereas there were about 2000 in 1985 according to information from statistics in the National Board for Health and Welfare database. This difference in number can partly be explained by the exclusion of males, but there are other reasons. The questionnaire used in the census does not require information regarding specificity of working tasks. Furthermore, the information asked for in the census, where only work in one specified week was asked for, resulted in a certain loss because of vacation, and maternity and sick leave for some individuals. Thus all specialties are under‐reported, which in our case leads to the nurse anaesthetists being too few, both in nominator and denominator. The number of other female nurses can be assumed too high, both for cases and healthy subjects (that is, in nominator and denominator), since an under‐reporting of anaesthetic nurses was made in the census of 1985, before the onset of MS. Hence, no distortion of the CIR is to be anticipated.
- Nurse anaesthetists can be exposed to volatile anaesthetic agents which, due to their toxicological effects, can be classified as organic solvents.
- Exposure to organic solvents has been associated with an increased risk for MS. An increased incidence was also found in this study of nurse anaesthetists.
However, there are distinct problems in making a good age standardisation when cases are so few. Even single cases will change the prevalence in the actual stratum because the total number of cases is small.
The MS incidence densities of the reference groups (17/100
000 person‐years for female teachers and 19/100
000 person‐years for other female nurses) are not in complete agreement with existing MS incidence densities of two groups of women from Denmark and Gothenburg, respectively, being approximately 10–12/100
000 person‐years in this age group.22,27
According to the Danish researchers, the MS incidence among Danish women in a cohort of nurses28
000 person‐years in the age range 30–34 years (personal communication, Koch‐Henriksen).
The higher incidence in our reference groups compared with the Danish and Gothenburg incidence data might be due to selection bias, consisting of a higher incidence of MS in middle and upper socioeconomic classes compared with the average population.30,31,32
Exposure to microbial agents in public places, especially viruses, is a potential risk in all the groups investigated, nurses as well as teachers. By using teachers and other nurses as reference groups, the possible effect of a potential infectious agent was taken care of in the analysis of the effect of anaesthetic agents.
After the publication of our first paper on the risk for MS in nurse anaesthetists,18
another paper on that topic has been published.29
In that Danish study, no excess risk for MS was observed among Danish nurse anaesthetists. However, the material was rather small, and a power analysis reveals that the possibility of finding a tentative risk from anaesthetic agents is limited (about 30% chance).
In conclusion, despite weaknesses in our data extraction procedure, this somewhat crude study has the benefit of utilising only objectively registered data, and seems to further support previous results suggesting an increased risk for MS in solvent exposed populations, including nurse anaesthetists.
- Exposure to organic solvents/volatile anaesthetic agents should be minimised globally.