Echographic evaluation with image analysis of irritant reactions induced by nonanoic acid and hydrochloric acid

Skin reactions to 40% nonanoic acid in propanol (NON) and to 4% hydrochloric acid were evaluated by image analysis of 20 MHz B scan recordings (Dermascan C, Cortex Technology). 18 women, aged 18 to 45, were patch tested with 24-h application time, and clinical and instrumental evaluations were performed at the beginning of the experiment, and 24 and 48 h after patch testing. To check possible regional variations in reactivity to NON and HCl, both substances were applied 2 x on the same forearm. As control tests, saline solution, propanol and sodium lauryl sulfate (SLS) 3% were applied. Echographic images were processed by a program enabling numerical representation of picture data, based on attribution of fictional values to the echoes' amplitudes, selection of amplitude bands of interest, binary transformation of the image, and calculation of the extension of areas reflecting within the same amplitude range. Sonographic recordings were evaluated by an amplitude interval marking hyporeflecting parts of the dermis (corresponding to edema and inflammatory infiltration), and by a band highlighting the entrance echo (epidermis). Our evaluations showed that extension of the hypoechogenic area of the dermis increases according to intensity of inflammatory reaction for all irritant substances. A clear decrease in reflectivity of the epidermis echo at 24 h was visible at SLS patch test test sites, whereas at patch test sites performed with NON and HCl, there was a trend towards an increase in values of hyperreflecting pixels. No significant variations between data recorded at proximal compared to distal skin sites were observed for NON and HCl. In conclusion, the echographic technique provides a method for quantification of the dermal inflammatory reaction, and also for description of skin reactions induced by irritant substances, according to their specific epidermal effects.