Ghostly figures that drift in and out of the shadows. Disembodied voices and the doorbell constantly ringing when no one's there. An alarming smell of smoke or the feeling of insects crawling all over the skin.
All of these are the disturbing and often terrifying hallucinations commonly described by Parkinson's disease sufferers.
The main symptoms of the incurable brain disorder that affects 145,000 Britons are tremors and problems with movement, but up to three-quarters of patients also experience visions and delusions.
According to the charity Parkinson's UK, one in five sufferers are so frightened by them they're left housebound.
A ground-breaking NHS trial will start next month, offering 200 volunteers the chance to try a drug called ondansetron which has been shown in studies to help halt hallucinations. Pictured: Stock image
But there may now be some relief, thanks to a 65p-a-day drug originally designed to treat cancer-related sickness.
A ground-breaking NHS trial will start next month, offering 200 volunteers the chance to try a drug called ondansetron which has been shown in studies to help halt hallucinations.
First developed 30 years ago, ondansetron works by blocking the effects of the brain chemical serotonin, which can cause nausea and vomiting but is also thought to play a part in triggering psychotic episodes.
Scientists discovered the drug was effective in treating the symptoms of acute psychiatric problems, including hallucinations in people with schizophrenia, and dampen down symptoms in those with obsessive compulsive disorder.
Ondansetron is already known to be safe, and small trials have shown it can completely stop hallucinations in the majority of Parkinson's patients who suffer them.
Parkinson's disease is caused by a loss of nerve cells in the central part of the brain called the substantia nigra. It is not clear exactly what triggers the cell death, but it is thought there may be some genetic element.
As the cells die, there is a decline in levels of a brain chemical called dopamine, vital for regulating the movement of the body.
Without adequate dopamine, controlling limbs becomes erratic and leads to telltale Parkinson's tremors as well as freezing of muscles and balance problems.
As the condition progresses, it can also result in psychiatric problems, including hallucinations, depression and anxiety.
Parkinson's disease is caused by a loss of nerve cells in the central part of the brain called the substantia nigra. It is not clear exactly what triggers the cell death, but it is thought there may be some genetic element. Pictured: Stock image
'There's a delicate balance between different chemicals in the brain that help it to make sense of all the visual information it is receiving,' says Professor Suzanne Reeves, an expert in old age psychiatry at University College London.
'Parkinson's disease disrupts that delicate balance, so the brain reaches the wrong conclusion about the visual information it is getting – hence the hallucinations.'
High doses of medications given to increase dopamine, to treat other symptoms of the condition, can also upset the delicate balance of chemicals, worsening hallucinations.
Currently, potent anti-psychotic drugs are used to reduce hallucinations. But these not only make Parkinson's symptoms worse, they can also lead to a four-fold increase in the chances of having a stroke.
Parkinson's UK say the problem worsened during the Covid-19 lockdown, with calls to its helpline surging, and one patient in ten reporting more disturbing visions.
The £1 million trial, which will run at more than 20 NHS Parkinson's clinics across the UK, is being funded by the charity's drug development arm – Parkinson's Virtual Biotech.
One patient set to benefit is 54-year-old Michelle Ellis, who was diagnosed with Parkinson's in 2012. She recalls her first frightening 'vision' while driving home from a family lunch in 2016.
She says: 'As the car pulled away from the petrol station at the motorway services, I glanced towards the back seat to check Amy, my granddaughter, who was asleep, and let out a scream – there was a stranger sitting right next to her.'
The former car industry worker from Leicestershire screamed at her husband, Peter, who was driving, to stop the car. 'I yelled at him, "There's somebody in the back with Amy."
But he was adamant nobody was there, and when I glanced around again it was like they had just vanished into thin air. I was absolutely terrified.'
After the incident, Michelle initially blamed tiredness – something she had struggled with since her diagnosis – but months later, more phantom figures emerged.
'I thought I could see people walking round the house when I knew nobody else was home,' says Michelle, who is a mother-of-four with ten grandchildren.
'And then I thought I could see spiders everywhere – crawling on the floors and walls – out of the corner of my eye.'
Doctors managed to reduce Michelle's hallucinations slightly by lowering the dosage of one of her medications, Madopar, given to control tremors, but they returned during lockdown.
She says: 'I was shielding, so wasn't able to get out. The hallucinations started to get worse and more frequent. But I'm learning to live with the visions as just another part of the disease.
'I just hope this drug will be the answer we are all looking for.'