A man accused of murdering his girlfriend has admitted manslaughter with his lawyer claiming that pre-existing medical conditions caused her death.

Ruth Binns' body was discovered by her sister the morning after she had a violent argument with Arnold Shawn Astwood at her Southampton home. Astwood's lawyer claims the row sparked a goitre – a swelling in the neck from enlargement of the thyroid gland – causing her fatal breathing difficulties.

No details of the circumstances of the killing were given at Supreme Court when Astwood, 36, of Green Acres Road, Devonshire, entered his plea to the lesser charge on the morning his trial for murder was due to begin. It was accepted by prosecutors who then dropped the murder charge.

Speaking afterwards, his lawyer Charles Richardson said Ms Binns' death was the unintentional result of a "regular domestic dispute" and not murder.

"From the outset, forensically speaking, this is one of the most unusual cases that either myself, the Director of Public Prosecutions, the Police and the forensic experts have ever seen," he said.

"What we have is a very unfortunate case of a young woman who suffered from a goitre condition. She had a very large thyroid gland. When she became excited in any way it would swell and cause difficulty with her breathing. Forensics also discovered that she had a heart condition too.

"I think it was accepted by all that, had it not been for these two medical conditions, the altercation with Mr. Astwood would not have caused her death. As I understand it, the goitre swelled up and blocked off her breathing. That, coupled with the heart condition, the two together coupled with the stress of the altercation she had with Mr. Astwood, caused her death."

The victim's devastated sister Alea Dill said after the hearing: "I don't believe whatever happened that night, whatever (he did) causing my sister to die, I don't think he had plans to kill her, but he killed her."

Mr. Richardson said of the domestic dispute: "Yes, it is being said that there was some violence, but it was the garden variety sort of fight, not extreme violence."

He stressed that while people were aware of Ms Binns' medical condition, "nobody ever guessed" it could kill her.

He said of Astwood: "He's very contrite. He's been contrite since day one. He's extremely sorry this happened to her. He never ever intended for her to be seriously hurt."

There were emotional scenes in the courtroom after Astwood's plea to manslaughter was accepted. One friend of Ms Binns' who was present in the public gallery attempted to get into the dock where Astwood was seated. She was restrained by prison officers and taken out of the courtroom.

Brought before the judge later, the woman said she hated Astwood for what he did. She was banned from coming within 100 yards of his future court appearances.

Mr. Richardson said: "I can understand emotions running high but I have to reiterate that I've never come across anyone who's as genuinely remorseful."

Astwood was remanded into custody, and is due back before the Supreme Court on February 2, when a date for his sentencing will be set. Mr. Richardson has asked for background reports to be prepared in the meantime, and said it was too early yesterday to discuss what sentence he'll seek.

No comment was available from the Department of Public Prosecutions.