SMOKING MAY BE CONSIDERED AN ESTABLISHED RISK FACTOR FOR SPORADIC ALS

To the Editor:

Armon's¹ review article on smoking and the risk of amyotrophic lateral sclerosis (ALS) referenced our cohort study based on the Swedish Construction Workers Cohort.² The authors classified our study as Class IV evidence for a conclusion regarding the association between smoking and ALS. We do not agree with several of their criticisms.

First, the mean age of the Construction Workers Cohort was 35.5 years at the beginning of follow-up and 55.5 years at the end. Armon contends that this violated the validity of the study results. We do not believe that the population characteristics could influence the validity of a study when only internal comparisons were made.

Considering the generalizability of the results, we were not trying to assume that these results apply to other general populations given the specific features of construction workers. Although mainly young men, this cohort had a wide age range (i.e., age at entry to the cohort was 15–81 years). Age at enrollment is not relevant. However, age at ALS diagnosis could be relevant if smoking had differing effects on risk of ALS dependent on age. Regardless, the majority of our ALS cases were older. The mean age at ALS diagnosis was 64.4 years (range 31–85 years), with 15.6% at age <55 years, 15.6% 55–59 years, 18.1% 60–64 years, 18.8% 65–69 years, 14.4% 70–74 years, and 17.5% ≥75 years.

Secondly, the smoking information in our baseline questionnaire was obtained via the construction workers' personal interviews with a trained nurse. Presumably, this method is more reliable than self-report. Although the smoking status is not validated, as Armon mentions, we believe it is common in other large-scale follow-up studies.^3,4 Furthermore, this was the method employed by another study cited by Armon, which they classified as Class I evidence.³

Disclosure: The authors report no …