Background: Multiple chemical sensitivity (MCS) is characterized by chemically induced symptoms from multiple organs. These symptoms occur in response to demonstrable exposure to chemically unrelated compounds at doses far below those known to cause harmful effects in the general population. Although the mechanism of this action remains unclear and no acceptable and well-documented treatment for MCS has yet been established, regarding neurogenic inflammation, it has been hypothesized that an increased density of C-fiber neurons is found in symptomatic tissues.
Methods: Using capsaicin, we examined the sensitivity of the cough reflex in patients with MCS and chronic cough (CC) and compared the findings with those in control subjects. Fifteen patients (four males, 11 females; mean (± SD) age 38.3 ± 16.3 years) suffering from MCS and 29 patients (10 males, 19 females; mean age 46.4 ± 15.9 years) who had cough symptoms lasting 4 weeks or longer and normal chest radiograph findings (CC) were enrolled in the present study. Twenty-nine healthy subjects (14 males, 15 females; mean age 37.9 ± 9.5 years) who had no history of coughing during the previous 6 months and no chronic respiratory diseases were enrolled as controls. Subjects inhaled stepwise incremental concentrations of capsaicin (0.122−62.5 µmol/L) for 15 s. Inhalation was performed at 45 s intervals and the number of coughs per minute was counted. The provocation was terminated when the subject coughed five or more times. Ventilatory functions (forced vital capacity (FVC), forced expiratory volume in 1 s and the expiratory flow rate at 50 and 75% FVC (V50 and V25, respectively)) were also measured.
Results: No significant differences were observed in ventilatory function test findings between the three groups. The log concentration of capsaicin causing five or more coughs (C5) was 0.150 ± 0.630, 0.611 ± 0.691 and 1.120 ± 0.612 µmol/L in MCS, CC and control subjects, respectively. The log C5 in MCS subjects was significantly lower than that in CC and control subjects.
Conclusions: Capsaicin is a cough-inducing agent in humans that possibly acts on non-myelinated C-fiber endings. The findings of the present study indicate that the mechanisms underlying MCS may originate in the sensory nervous system.