Questions Over Redback Spider Antivenom

Although not life-threatening, most redback spider bites cause severe and persistent pain, more severe than previously thought by many health care workers.

Redback spider antivenom is usually given intramuscularly, but this method is ineffective and should be reviewed, according to research published in the current issue of the Medical Journal of Australia.

Co-author of the research, Newcastle University toxicologist Dr Geoffrey Isbister, said almost all other antivenoms, including snake antivenom, are given intravenously with a high degree of success.

"The intramuscular administration of redback antivenom was introduced without controlled studies to demonstrate efficacy or safety, and the use of intravenous antivenom may be more effective," Dr Isbister said.

During the study, 68 people from around Australia with redback spider bites were followed up from poisons information centres and hospital emergency departments. Pain occurred after all bites and was severe in over 60 per cent of cases. The severity of the pain prevented the patient from sleeping in a third of cases.

"The research shows treatment with intramuscular antivenom makes little difference to the post bite pain experienced by patients," Dr Isbister said.

"Only 17 per cent of patients treated with intramuscular antivenom were pain free after 24 hours, compared with 12 per cent of untreated patients An acceptable treatment outcome would see 80 to 90 per cent of patients pain free after 24 hours. This did not happen.

"This is a poor outcome for any antivenom as they usually have dramatic effects.

"Many doctors in Australia believe most redback spider bites cause minor symptoms and although a safe, intramuscular antivenom exists, many are reluctant to use it unless there is severe envenomation because they believe it is unnecessary and there is a perceived risk of adverse reactions.

"Most patients should be given antivenom to ease the symptoms, which can be severe. A randomised, controlled trial of intravenous versus intramuscular antivenom is needed," Dr Isbister said.

The Medical Journal of Australia is a publication of the Australian Medical Association.

CONTACT:
Dr Geoffrey Isbister, 0438 466 471
Judith Tokley, AMA, 0408 824 306