The home secretary's decision to accept doctors' advice that General Augusto Pinochet is too ill to stand trial brings to mind some recent cases in British law.

Three years ago murder charges against the alleged Nazi war criminal, 85-year-old Syzmon Serafinowicz, were dropped after a jury declared he was unfit to stand trial.

The collapse of the £7m case provoked an outcry, with some MPs condemning it as a waste of time and public money.

The case highlighted the fact that when it comes to the elderly and infirm, legal proceedings can be overridden by medical concerns.

In the Pinochet case, medical reports on the former Chilean dictator appear to have won him a reprieve under exemptions set out in the Extradition Act 1989.

But when is poor health enough to change the course of the law?

In the Serafinowicz case, the Old Bailey jury sided with the defendant's lawyers, who claimed their client was suffering from Alzheimer's disease. The defendant died later that year.

Reflecting on the case, Mr Serafinowicz's solicitor, Nicholas Bowers, says a defendant must have the mental faculties to follow the proceedings in court and advise his lawyers.

"If for medical reasons the defendant cannot do that, and there's medical evidence to say so, then the trial cannot go ahead," says Mr Bowers.

During an eight-day hearing, the court heard from doctors who agreed that Serafinowicz was suffering from dementia, probably the progressive and incurable Alzheimer's disease, to such an extent that he was unfit to be tried.

The defendant's GP, a consultant age psychologist and a general psychologist were all called by Mr Serafinowicz's defence. The Crown insisted he was fit for trial.

Most cases of ill health are resolved between the two parties before ever reaching the courtroom.

"Where you have some issue or debate is where the prosecution feel they ought to go to the medical experts as well," says lawyer Robert Roscoe.

If the prosecution feel their report puts a convincing case to the contrary, then it is up to a jury to decide whether a trial can proceed.

The Pinochet issue is one of extradition rather than trial in the UK, and would therefore better be likened to the case of Roisin McAliskey, says Mr Roscoe.

In 1998, Home Secretary Jack Straw controversially turned down a request from Germany to extradite Ms McAliskey, on health grounds, under section 12 of the Extradition Act.

Lawyers acting for Ms McAliskey, who was accused of taking part in a mortar attack on a British Army barracks in Osnabruck, successfully showed that sending her to Germany would be "unjust and oppressive" under the Act.

During her detention she was said to have suffered severe psychological damage, resulting in a diagnosis of post-traumatic stress disorder.