Descriptive study of sensitization to methylchloroisothiazolinone and methylisothiazolinone in a skin allergy unit

Methylchloroisothiazolinone (MCI) and methylisothiazolinone (MI) are heterocyclic compounds used as preservatives in cosmetic and industrial products. They continue to be common allergens, causing positive patch test reactions in 2% to 4% of patients tested.

We searched the database at our skin allergy unit for all cases of sensitization to MCI/MI and MI diagnosed between January 1980 and March 2013.

Patch tests were performed with MCI/MI in 8705 patients and with MI in 404 patients. In total, 222 patients (2.55%) were sensitized to MCI/MI and 21 (5.19%) were sensitized to MI. The incidence of MCI/MI cases peaked between 1998 and 2005 and again between 2009 and 2013. Of the 222 patients with MCI/MI sensitization, 142 were women (64%) and 49 were men (36%); the mean age was 43 years. The most frequently affected areas were the hands (54% of cases), the arms (36%), and the face (35%); 75.67% of cases were due to cosmetics and 2.25% were due to paint. Of the 21 patients with MI sensitization (mean age, 50 years), 12 were women (57%) and 9 were men (43%). The most common site of involvement in this group was the face (71% of cases), followed by the arms (38%) and the hands (29%). All the cases were due to cosmetics.

Our data show that sensitization to the combination of MCI and MI and MI alone has increased in recent years. It would appear to be necessary to add MI to the baseline patch test series, although the test concentration has yet to be determined.