title: Objective assessment of the minimal irritant reaction induced by a 70% solution of glycolic acid.
abstract: OBJECTIVE
  Glycolic acid has become a very popular cutaneous improving agent, but, despite its increasing employment, its biophysiological effects on the skin have not been adequately examined. Recently, we described subclinical skin damage induced by a 70% solution of glycolic acid applied on forearm skin by means of non-invasive measuring techniques. In the present study we instrumentally assessed the cutaneous effects induced by a glycolic acid minimal irritant dose on the volar aspect of the right forearm.
  
  
  METHODS
  Twenty subjects, seventeen women and three men, aged 16 to 56, were tested with 40 nl of a 70% solution of glycolic acid applied on one volar forearm area until a clincally evident cutaneous reaction occurred. Visual scoring, TEWL measurement, skin colour, capacitance and echographic recordings were performed before the beginning of the experiment and 30 min and 3, 24, 48 and 72 h after glycolic acid removal. The echo-graphic recordings were carried out using a 20 MHz B scanner (Dermascan C), and then were processed by dedicated software based on segmentation procedures. Two amplitude intervals were selected for the elaboration of the images, the first marking the hyporeflecting areas of the dermis, the second highlighting both the hyperreflecting parts of the epidermis and the lower dermis, separately.
  
  
  RESULTS
  Mean application-time before the appearance of minimal inflammatory skin reactions was 25±7 min. Clinical evaluation was confirmed by the analysis of the colorimetric parameter a*. TEWL and capacitance values showed a marked increase, reaching their maximum at 30 min after solution removal, followed by a progressive decrease. Values of both parameters were statistically significant for up to 48 h. The elaboration of echographic images by a low amplitude interval showed dermal edema more pronounced at 30 min and 24 h, while by highlighting hyperreflecting areas an evident reduction of the entry echo for up to 3 h and a prolonged decrease of the reflectivity of the lower dermis were observable.
  
  
  CONCLUSIONS
  Our data demonstrate that despite the persistence of higher TEWL values, glycolic acid induced irritation causes a prolonged increase in capacitance that corresponds to modifications in epidermal reflectivity. Epidermal alterations are associated with dermal edema, as evaluated by ultrasound.
authors.affiliation: Department of Dermatology, University of Modena, Italy.
authors.forename: L
authors.initials: L
authors.lastname: Mantovani
authors.affiliation: Department of Dermatology, University of Modena, Italy.
authors.forename: B
authors.initials: B
authors.lastname: Belletti
authors.affiliation: Department of Dermatology, University of Modena, Italy.
authors.forename: S
authors.initials: S
authors.lastname: Seidenari
keywords: capacitance; colorimetry; echography; glycolic acid; transepidermal water loss