This hereditary metabolic disorder of porphyrin metabolism is unique among the porphyrias in that porphyrins or porphyrin precursors are not excreted in the urine. Also, erythropoietic protoporphyria (EPP) is characterized by an acute sunburn-like photosensitivity, in contrast to the other common porphyrias (porphyria cutanea tarda or variegate porphyria), in which obvious acute photosensitivity is not a presenting complaint.
Causes of Erythropoietic Protoporphyria
The specific enzyme defect occurs at the step in porphyrin metabolism in which protoporphyrin is converted to heme by the enzyme ferrochelatase. This leads to an accumulation of protoporphyrin that is highly photosensitizing.
Symptoms of Erythropoietic Protoporphyria
First symptoms usually appear in infancy or early childhood and present as an uncomfortable or painful burning sensation of the skin after sun exposure. It occurs most often on the tops of the hands and feet, face and ears. In most cases visible changes to the skin are mild. The affected skin may become red and swollen and blistered. Later there are pitted scars and sometimes crusty thickened skin, particularly over the cheeks, nose and knuckles of the hands.
Diagnosis
In EPP there is photosensitivity but no “rash” only an exaggerated sunburn response that appears much earlier than ordinary sunburn erythema. Also, the skin changes occur behind window glass. Finally, there are virtually no photosensitivity disorders in which the symptoms appear so rapidly (minutes after exposure to sunlight). Porphyrin examination establishes the diagnosis with elevated free protoporphyrin levels in the RBCs and in the stool. The fecal protoporphyrin is most consistently elevated. The urinary porphyrins are not elevated. In chronic cases, the waxy thickening and wrinkling of facial skin is diagnostic.
Treatment
There is no treatment for the basic metabolic abnormality, but symptomatic relief of the photosensitivity can be achieved in many patients with oral β-carotene in divided doses of 180 mg/d. Therapeutic levels of carotenoids are achieved in 1 to 2 months. This treatment brings about an amelioration of the photosensitivity but does not completely eliminate the problem of photosensitivity. Patients on β-carotene can remain outdoors longer by a factor of 8 to 10 but still burn if exposures are too long. Nevertheless, many patients can participate in outdoor sports for the first time. There is no toxicity with prolonged treatment with β-carotene. Protection by β-carotene can be considerably enhanced by PUVA-induced tanning.
References
- https://rarediseases.org/rare-diseases/erythropoietic-protoporphyria/
- https://www.dermnetnz.org/topics/erythropoietic-protoporphyria/