What is the difference between porphyria cutanea tarda and pseudoporphyria?

Porphyria cutaneous tarda is clinically characterized by skin fragility, tense blisters, scarring, and milia in sun-exposed areas, particularly the dorsal hands (Fig. 10-3). Patients have decreased levels of uroporphyrinogen decarboxylase, sometimes due to alcoholic liver disease or to drugs such as estrogen and iron. Hypertrichosis may develop on the face. The diagnosis is established by skin biopsy for routine histology, as well as porphyrin studies, including a 24-hour urine collection for uroporphyrins. Other porphyrias, including variegate porphyria and hereditary coproporphyria, may present with identical cutaneous findings, and should be separated from porphyria cutanea tarda on the basis of associated clinical findings and complete porphyrin studies. Direct immunofluorescence of the skin may be a helpful test for porphyria but does not distinguish one type from another. Pseudoporphyria demonstrates similar cutaneous findings but porphyrin studies are normal. It is associated with uremia, hemodialysis, and some drugs, especially, nonsterioidal antiinflammatory drugs (NSAID)s.

LaDuca JR, Bouman PH, Gaspari AA: Nonsteroidal anti-inflammatory drug-induced pseudoporphyria: a case series J Cutan Med Surg 6:320–326, 2002, Epub.