Melasma

Melasma is a chronic disorder of pigmentation, not a simple stain. Brown to grey-brown patches appear symmetrically in centrofacial, malar or mandibular distributions, most often in women and in Fitzpatrick skin types three to five. Histology shows hyperactive rather than more numerous melanocytes, increased epidermal melanin, and in many lesions dermal melanophages with increased vascularity and solar elastosis, which is why the older epidermal-versus-dermal split by Wood lamp has fallen out of favour.

Trials quantify it with the Melasma Area and Severity Index, which weights four facial regions by area, darkness and homogeneity for a score from zero to forty-eight; the modified version drops homogeneity and runs to twenty-four. Treatment evidence is dominated by the fixed triple combination of hydroquinone, tretinoin and a topical corticosteroid, with oral tranexamic acid the best-studied systemic option. Relapse after stopping is the rule.

The practical consequence is that melasma is a photoprotection problem before it is an active-ingredient problem, and visible light provokes it as well as ultraviolet, so tinted iron-oxide sunscreens outperform transparent ones. A product tested on top of a rigorous sun-protection regimen is being credited with work that regimen is doing.

The specific misreading is seasonal. Melasma darkens through summer and fades through winter, so an uncontrolled study recruiting in August and reporting in January shows improvement in almost any arm, and regression to the mean adds to it because subjects enrol when their pigmentation is worst. A vehicle-controlled, blinded design with standardised photography is the minimum for a claim, and split-face designs suit the symmetry.