[Histopathology of exogenous and idiopathic lucites and provocation phototests]

Provocative phototests are used regularly to explore photobiology in patients with photodermatoses. Our objective was to determine the reliability of the phototest in the diagnosis of idiopathic and exogenous light eruption in order to identify the ideal date for the histological examination by correlation between the histological aspects of light- triggered and spontaneous lesions.

Among 66 patients, there were 48 with polymorphous light eruptions, 13 with exogenous eruptions and 5 with chronic actinodermatitis. The histological findings of the provocative phototests were correlated with the clinical aspect, the date of biopsy and the histological aspect of the spontaneous lesions.

In polymorphous light eruptions (48 cases) this test triggered a photoallergic reaction in 48% of the patients. There was a good correlation between the histological findings and clinical features in 96% of the cases. The histological image of the light-triggered lesion on day 6 was the closest to that of the spontaneous lesion. In exogenous eruptions and chronic actinodermatitis (18 cases), only 33% of the clinically positive phototests were also positive at histological examination. The histological image was the same for the provocative tests whether they were clinically positive or negative, with signs of epidermal phototoxicity, regeneration acanthosis and moderate perivascular lymphocyte infiltration of the superficial dermal layer.

Our findings confirmed that the phototest provides a good correlation between histological findings and clinical presentation in idiopathic polymorphous light eruptions, justifying simple clinical interpretation, but did not demonstrate a rate of positive results as high as reported in the literature. Day 6 is the ideal date for a phototest biopsy. In exogenous light eruptions, the phototest is less reliable since the correlation between histology and clinical presentation is weaker. It is difficult to reproduce spontaneous lesions with the phototest, but if used, the phototest biopsy should be done on day 18.