The Anaesthesia Viva

The clinical effects of porphyria are due to overproduction of haem precursors, which are highly toxic, due to the overactivity of a small, readily induced enzyme. The enzyme is d-aminolaevulinic acid (ALA) synthetase, and the disease is due to a relative deficiency of another enzyme later on in the synthetic process, thus allowing accumulation of intermediate metabolites. The position of the deficient enzyme predicts the type of precursor to accumulate and the pattern of the disease. Smaller intermediates cross the blood-brain barrier and cause neuropsychiatric disturbance, while larger ones cause cutaneous manifestations.

The two important conditions are Acute Intermittent Porphyria (AIP) and Variegate Porphyria (VP). Both are inherited in an autosomal dominant form. Both are precipitated by induction of ALA synthetase by pregnancy, dieting, and drugs of importance such as barbiturates, steroids, sulphonamides and griseofulvin. Management of an attack involves analgesia, carbohydrate loading, beta-blockade, fluids and haematin solutions to suppress ALA synthetase activity.