Excessive hoarding may be a distinct type of obsessive-compulsive disorder not treatable in the standard way, research suggests.
Many people with OCD wash their hands compulsively
Hoarding and saving has long been thought to be a classic symptom of OCD.
But an American Journal of Psychiatry study found hoarders show different brain activity patterns.
University of California Los Angeles researchers say their work suggests some hoarders may have been receiving inappropriate treatment.
OCD is an anxiety disorder in which sufferers are compelled by irrational fears and thoughts to repeat seemingly needless actions over and over again.
It is often associated with excessive hand washing, cleaning or repeated checking.
The cause of OCD is not fully understood, but it has been linked to an imbalance of the brain signalling chemical serotonin and is often treated with drugs that alter the way the brain processes this chemical.
Many experts believe excessive hoarding to be another manifestation of the condition.
However, researcher Dr Sanjaya Saxena believes that his work challenges this view.
He said: "Our work shows that hoarding and saving compulsions long associated with OCD may spring from unique, previously unrecognized neurobiological malfunctions that standard treatments do not necessarily address.
"In addition, the results emphasize the need to rethink how we categorize psychiatric disorders.
"Diagnosis and treatment should be driven by biology rather than symptoms."
The UCLA team carried out sophisticated PET scans to measure brain activity in 45 adults with OCD, of which 12 were hoarders, and 17 people without mental health problems.
The hoarders showed a unique pattern of activity, including less activity in brain regions known as the posterior cingulate gyrus and cuneus.
It is estimated that hoarding and saving symptoms are found in up to 30% of patients currently recognised as having OCD. These people are often also indecisive and perfectionists.
Standard therapies for OCD often seem to have little effect at reducing these particular symptoms.
Failure to come forward
Dr Naomi Fineberg, an expert in OCD at Queen Elizabeth Hospital, Welwyn Garden City, told BBC News Online that the finding was "no surprise".
"When you are looking at obsessional patients, hoarders seem to stand apart, and they don't respond well - if at all - to standard anti-obsessional treatments, which makes you think they might be a bit different.
"OCD is probably a mix of different disorders. In fact there is probably a fairly broad spectrum of obsessional disorders."
Dr Fineberg said many hoarders did not come forward for treatment.
Unlike other forms of OCD in which patients often recognise their behaviour as irrational and senseless, hoarders tended to believe they were acting rationally, and did not need help.