It's well known that grief is not linear - but for a small percentage of people, it's also never ending.
It's estimated that 10 per cent of people who have lost a loved one experience prolonged grief disorder (PGD), which is defined as having very difficult feelings such as sadness or guilt which last more than six months.
PGD is a psychiatric disorder, and was formally classified by the World Health Organisation in 2018. It is characterised by elevated levels of bereavement-related distress.
According to the NHS, people with PGD spend a lot of time thinking about the person who has died, have difficulty accepting the death, have suicidal thoughts and struggle to return to everyday activities.
They may feel that life has lost its meaning, or they have lost part of their identity, or simply cannot accept the death, even though they know it has occurred.
People who have lost a loved one to a traumatic or sudden death are more likely experience PGD, but it doesn't affect everyone who has experienced such a loss - leaving scientists puzzled.
In a new review, published today in medical journal Trends in Neurosciences, researchers examined what is known about the neurobiology of PGD and why it persists in some individuals.
The team, based at the University of New South Wales in Australia, argue that PGD shares some of the brain patterns seen in patients with depression and anxiety, and there's also activity in the systems involved in reward and attachment.
This could mean that people are 'craving' the presence of their lost loved on on a deep psychological level, making it harder for them to effectively move on.
Lead author Richard Bryant said: 'Prolonged grief disorder is the new kid on the block in terms of psychiatric diagnoses.
'It's not that it's a different type of grief. It's just more that the person is stuck in the grief.
'It sort of gelled with this notion that grief is characterised by a craving or a longing for the deceased.'
Bryant's team looked at the findings from previous studies into PGD to make their argument.
As PGD is a relatively new psychiatric diagnosis, there is a limited amount of data available and it mostly takes the form of functional MRI (fMRI) brain scans which measure changes in blood flow in the brain as people look at photos of the deceased or think about their loss, and grief provocation tasks.
Eureka reports that 'across these studies, PGD has been repeatedly linked to changes in reward-related brain circuits. These regions include the nucleus accumbens and orbitofrontal cortex, which are involved in desire and motivation, as well as the amygdala and insula, which play roles in emotion processing.'
Some of the observed brain patterns are also seen in cases of depression and post-traumatic stress disorder, which share the same traits (such as rumination and emotional distress) as PGD.
'It would be very strange if we didn't get that overlap,' said Bryant, who hopes to expand his research in the future by working with larger groups of bereaved people to monitor how their brain activity changes as they move through - or get stuck in - grief.
He said: 'I hope to raise awareness. 'To actually deal with prolonged grief, we need to recognise it as a disorder.
'We do have treatments that can address it, but we can't do that if we can't identify these people.'