Arachnoiditis is a little-understood condition in which the middle membrane protecting the spinal cord becomes scarred. Nerves atrophy and become enmeshed in dense scar tissue, which presses constantly on the spine. Minneapolis' Dr Burton, one of the few medics to make a study of lumbar sacral adhesive arachnoiditis (LSAA), estimates that it accounts for 11 per cent of patients with 'failed back surgery syndrome'.

Although LSAA results from a number of different causes, in Dr Burton's view, it essentially reflects the introduction of foreign substances into the human subarachnoid space. Dr Burton says the foreign body most often identified in victims is iophendylate (known as Pantopaque in the US, Myodil in the UK), the oil-based dye used for myelograms. In LSAA, he says, iophendylate is often found in a cyst within the scar tissue mass. In his view, a million people worldwide suffer from arachnoiditis caused by this dye, and this view could be conservative. Until the 1980s, nearly half a million myelograms were being performed in the US every year.

Pantopaque was introduced in the US in 1944 after one study convinced the medical profession that it was safe. This was despite animal studies showing that Pantopaque caused arachnoiditis, says Burton (the Swedes banned the product from use in humans in 1948). Even though the product is no longer manufactured by Glaxo, since the onset of water-based dyes and imaging techniques, iophendylate continues to be used around the world, says Burton, and many back specialists continue to maintain that iophendylate is safe.

The US Food and Drug Administration and the British government have made no moves to ban oil-based myelograms. 'Despite the fact that iophendylate was identified as being causally related to the production of arachnoiditis from the time of its introduction, its use in the US has never been restricted by industry, government or the medical profession,' says Burton. It has needed patients with myelogram-induced LSAA to bring legal suits against the manufacturers before anyone else took notice. In the UK the Arachnoiditis Society now has some 1000 members and a class-action suit is underway against Glaxo.

The water-based dyes now being used instead are not without risk. One woman being investigated for sciatic (back-caused leg) pain with iopamidol (Niopam 200), a water-soluble contrast medium, was immediately rendered paraplegic (The Lancet 27 July 1991), as was another middle-aged woman given a myelogram with iohexol (Omnipaque), another water-soluble dye (The Lancet 16 March 1991). Burton says that some new mediums have caused such pain that they had to be performed under general anaesthesia.

He concludes: 'The medical profession has not yet succeeded in finding a benign, effective myelographic medium.'

Lynne McTaggart