Sunday, 19 September 2010

MRCP revision battle 15.4: Porphyrias

Porphyrias are rare genetic disorders causing abnormalities of enzymes responsible for haem synthesis, resulting in accumulation of intermittent compounds (porphobilinogen, aminolaevulinic acid) and porphyrins.

This battle will focus on 2 acute porphyrias (acute intermittent and variegate) and 1 chronic (porphyria cutanea tarda).

1. Acute intermittent porphyria

Acute intermittent porphyria is an autosomal dominant condition in which there is a defect in porphobilinogen deaminase

Females aged 20-40 are most affected.

There is always raised urinary porphobilinogen and aminolaevulinic acid, but the rise is greater during acute attacks.  The importance of these substances is that the urine goes deep red on being left.

Signs and symptoms include:
  • abdominal pain
  • hypertension
  • low sodium
  • low potassium
  • hypotonia
  • psychosis
  • paralysis
  • seizures
Note there are NO skin symptoms.

A way to remember this list is to think of a patient saying to you "oh doctor, my abdo pain is giving me high blood pressure and stopping me being able to stand (paralysis)"... to which you may think "ah yes, you are a teaspoon (tsp = low tone, sodium, potassium) of crazy (psychosis) aren't you?"

Drugs that can precipitate acute porphyria include:
  • alcohol
  • benzos
  • rifampicin
  • tetracyclines
  • phenytoin
  • OCP
  • halothane
  • sulphonamides

Treatment is:
  • remove precipitating factores
  • IV fluid to correct low sodium/potassium
  • high carb diet
  • IV haematin
  • prochlorperazine for nausea
  • other symptomatic treatment

Although a rare diagnosis, porphyria should always be a differential in someone presenting with abdominal pain.

2. Variegate porphyria

This is an autosomal dominant condition characterised by a deficit in protoporphyrin oxidase.

It can cause abdominal pain and neuro symptoms, but in contrast to acute intermittent porphyria it also has a photosensitive blistering rash.

It is commoner in South Africans (afrikaans)

3. Porphyria Cutanea Tarda

This is due to uroporphyrinogen decarboxylase deficiency.

It results in a photosensitive rash with bulla.
It is also associated with hypertrichosis.
(these two factors may be the basis of werewolf legends...)

Urine has raised uroporphyrinogen which glows pink under a Woods light

Attacks are precipitated by alcohol, iron and oestrogen.

Treatment is with chloroquine.

Wow, that was quite long.  The next battle is far shorter, and is on the second condition I'd never heard of before today...