Safe use of radiographic contrast media

Editor, – I would like to commend Kenneth Thomson and Dinesh Varma for their succinct discussion of the safety profile of iodinated radiographic contrast media (Aust Prescr 2010;33:19-22).

However a noticeable absence in the article is the discussion of oral contrast – particularly the increasing use of injectable iodinated radiographic contrast media as oral contrast (after dilution) for abdominal CT.

One of the issues related to iodinated media like iohexol or diatrizoate sodium is the alleged cumulative nephrotoxicity of these media when given orally in addition to the intravenous dose. This perception appears to be in error. From what I can tell, iohexol is poorly absorbed in the intact gastrointestinal tract and about 1% of the dose is excreted by the kidney. There is however a theoretical potential to cause renal dysfunction in a dehydrated patient as the hypertonic oral iodinated media can cause excessive excretion of water into the gastrointestinal lumen, precipitating a body fluid loss into the third space.

I would appreciate it if the authors can comment on this as the use of oral iodinated media is becoming more common in Australia, replacing the cheaper but less palatable barium meal.

Shyan Goh
Locum orthopaedic registrar
Sydney, NSW

Dr Dinesh Varma, author of the article, comments:

We did not include oral contrast media mainly because the adverse effects and complications are extremely rare, as are the contraindications.

The most commonly used oral contrast media are barium sulfate-based agents or water soluble iodinated contrast agents. The use of injectable iodinated contrast media as oral contrast agent is extremely rare and if alternative contrast media are required for CT examinations, water is more commonly used as negative oral contrast media. Some centres have replaced positive oral contrast media with water in their CT abdomen protocols.

Some of the recognised adverse effects of iodine-based oral contrast agents are a mild laxative effect attributable to high osmolarity of diatrizoate meglumine and diatrizoate sodium. This can also result in dehydration with shift of fluid in the third space as you have mentioned. We also agree with your comments that renal impairment is usually a secondary effect of this phenomenon as these agents are sparingly absorbed from an intact gastrointestinal tract.

Other rare complications include aspiration, which may result in serious pulmonary complications. Anaphylactic reactions have also been reported.