The porphyrias 

The porphyrias are metabolic diseases resulting from deficiency, or in one disease, an increase in the activity, of specific enzymes in the haem biosynthetic pathway (1, 2). Each of the eight main types of porphyria is defined by the association of characteristic clinical features with a specific pattern of excess production of pathway intermediates. Each pattern indicates the site of the underlying enzymatic abnormality (Fig. 12.3.3.1).

The porphyrias can therefore be defined clinically as either an acute porphyria, characterized by acute neurovisceral attacks that are associated with the overproduction of the porphyrin precursor, 5-aminolaevulinic acid (ALA, OMIM 125270), usually accompanied by porphobilinogen, or a nonacute porphyria in which these attacks are absent and photocutaneous symptoms due to excess formation of porphyrins are the main clinical manifestation. Other classifications include division into erythropoietic or hepatic, depending on the principal site of expression of the specific enzymatic defect.