Media playback is unsupported on your device Media caption During one episode, Sarah remembers seeing her hands in 2D

For people living with depersonalisation disorder the world appears unreal, as if through a haze or fog - or even in 2D. One in 100 people is thought to have the condition, but experts are warning it is not included in any GP training.

"Relationships you know you value deeply lose their essential quality," Sarah tells the BBC's Victoria Derbyshire programme.

"You know you love your family, but you know it academically - rather than feeling it in the normal way."

Sarah is an actress. She's used to playing roles and projecting emotion. Yet for long stretches of her own adult life, she herself has been emotionally numb - unable to feel.

This is the result of a little-known mental-health condition called depersonalisation disorder.

Sarah has had three chronic episodes of the disorder. The first came while studying for her final exams.

It is characterised by a sense of disconnect from the world around you, and your own body, and is believed to start as a defence mechanism - helping people to cope in periods of acute anxiety or trauma by switching off reality. It can also be triggered by drugs such as cannabis.

Image caption Sarah says, with depersonalisation disorder, familiar places feel like film sets

For those with it, the world can change in an instant.

"It was a sudden flick of a switch. Things felt really alien and threatening," says Sarah.

"Suddenly your flat or somewhere that is very, very familiar to you will suddenly feel like a film set, and your possessions feel like props."

Others have terrifying out-of-body experiences, feel like their own body parts don't belong to them and even see the world in 2D - as if it were flat.

For Sarah, this is what happened during her second episode.

"I was reading, holding a book, and suddenly my hands looked like a picture of a pair of hands.

"I felt this separation between the physical world and my perception of it."

Find out more

Watch Adam Eley's full film on depersonalisation disorder on the Victoria Derbyshire programme's website.

This isn't, however, a rare disorder.

One in 100 people is estimated to have it as a mental health condition, according to three separate studies.

Experts say it is as widespread as conditions such as obsessive compulsive disorder (OCD) and schizophrenia, and it has been a medically recognised disorder for decades.

Some patients, if untreated, can have it for their entire lives.

Yet few medical professionals have heard of it.

One newly qualified doctor - who has the condition - says there was no mention of depersonalisation disorder in either his GP training or at medical school.

He admits to wrongly diagnosing at least two of his own patients in the past, and says he would be "very surprised" if any of his colleagues had heard of the condition.

Sarah says she has seen "possibly up to 20 people over the years who haven't known what I was talking about - community psychiatric nurses, GPs, therapists, counsellors".

Image caption Dr Elaine Hunter runs the only specialist clinic for depersonalisation disorder in the UK

The Royal College of GPs said mental health was a "key part" of its "extensive training curriculum".

Areas of training "including complex mental health problems... are being developed," it added.

The Royal College of Psychiatrists said "we must ensure that knowledge of this condition is improved".

Poor diagnosis rates, however, are only one part of the problem. Another is access to treatment.

There is only one specialist clinic in the UK, and it has limited resources - seeing fewer than 80 patients a year, compared with the 650,000 people estimated to be living with the condition.

Access to the clinic on the NHS requires funding from the local clinical commissioning group, but even with a diagnosis this can take months or even longer.

After a year of waiting, Sarah decided her only choice was to self-fund.

"I was having panic attacks regularly. I was really, really scared.

"I felt like I was in crisis," she says.

Turning children away

The specialist clinic - the Depersonalisation Disorder Service at South London and Maudsley NHS Trust - is also limited to over-18s, having been designated an adult-only service.

This is despite the condition often forming in adolescence.

Dr Elaine Hunter, who runs it, is concerned that she is having to turn children and young people away.

"We sometimes get very distressed and upset patients who have a 15-year-old who is terrified… and there's not very much we can do."

One patient she has recently seen as an adult developed the condition aged 13, and was unable to leave the house for two years - having 10 or more panic attacks a day as a result of the anxiety depersonalisation disorder caused.

At first she was unable to recognise her own parents.

Dr Hunter hopes in time services for under-18s will become available.

She also wants treatment to be made available to patients locally, by training "depersonalisation disorder leads" at each NHS mental health trust, who can then share their knowledge with others.

She has also developed a form of cognitive behavioural therapy (CBT) specifically designed for depersonalisation disorder, which she believes medical professions with a grounding in the talking therapy would be able to learn with training.

Image caption Sarah Ashley "couldn't eat, couldn't sleep" before she began her CBT with Dr Hunter

One of Dr Hunter's patients, Sarah Ashley, a sales manager, says she has noticed a "huge, huge difference" in her mental health, despite being initially sceptical.

"[Before CBT] I'd look down at my hands or parts of my body and feel they're not mine. I'd look in the mirror and it would be like looking at someone else," she says.

"I couldn't eat, couldn't sleep… [weighed] six and a half stone from all the anxiety from it.

"Whereas now, I do get a bit of depersonalisation, but I can deal with it really quickly."

Treatment, then, is available, but currently limited in scope and difficult to access.

Dr Hunter says it should not be the case that people are "finding their symptoms through Google" and having to tell their GP: "This is what I've got", rather than the other way round.