Observations of phototest and heat test responses in patients with visible light-induced solar urticaria revealed a type of urticaria that was either a new type or a variant of previously described type 3. In this variant, urticarial responses in the patient were easily elicited by full-spectrum visible light exposure, but not as expected by monochromatic light exposures within the same wavelength range. Surprisingly, urticaria was readily elicited in the patient with the variant type by exposure to radiant heat.
Exacerbations in both the variant and the usual type 3 case were easily controlled by intermittent low-dose systemic chloroquine therapy.
Willis I, Epstein JH. Solar- vs Heat-Induced Urticaria. Arch Dermatol. 1974;110(3):389-392. doi:10.1001/archderm.1974.01630090027006