By Jane Elliott
Health reporter, BBC News
When Helen Swain started to suffer blackouts at the age of seven her doctors were mystified.
Helen's blackouts left doctors mystified
Food allergies and epilepsy were suspected, but tests proved inconclusive.
Her episodes would start with a feeling "something was not quite right", and within a few hours she would have an attack.
The trigger always seemed to be exhaustion, stress or major life changes, and the episodes usually occurred at night.
Helen's legs would start to feel restless and uncomfortable, she would start to feel very sick and start gulping air.
She would get hot and cold flushes and a headache and need to lie down in bed.
After two or three hours she would get an overwhelming urge to be sick, but would then black out.
On coming round, she would feel extremely clammy and sweaty before she actually was sick.
Helen would then be on a cycle of blacking out, coming round and being sick every few minutes, with episodes lasting up to 15 hours.
Helen would then be physically and mentally exhausted and would need to be in bed for one or two days.
Fear of an attack left Helen, now 32, from Braintree, Essex, too worried to venture far from home.
She even gave up her university place and plans to be a teacher, and took a job at her father's law firm to stay near home.
She was terrified throughout both her pregnancies that her condition could end up harming her children, but was becoming resigned to the fact that doctors seemed unable to find the cause.
But a blackout a year ago led to an accurate diagnosis.
She says: "I was going to London with my husband to celebrate his birthday. I started to panic on the way home on the train and had a turn.
"I banged my head badly as I blacked out in the ambulance a heart monitor was fitted to my finger. Suddenly my heart stopped and the paramedics had to start resuscitation."
Helen worried through both pregnancies
Doctors realised there was a heart rhythm irregularity - a heart block - and Helen was fitted with a pacemaker.
Her blackouts have now stopped completely, but Helen still gets a few attacks each year with other symptoms, though they are nothing like as severe.
"My life has changed dramatically and I feel fine. I no longer have to worry about banging my head."
But Stars, a charity for people who have black outs, warns that many people - like Helen - have either a wrong diagnosis or no diagnosis and that this is putting lives at risk.
Stars say many blackouts are actually caused by heart rhythm irregularities, which happen when the blood supply to the brain is interrupted - known as syncope.
Because the symptoms are similar to epilepsy this is often wrongly given as the diagnosis and it is estimated that up to a third of those diagnosed with epilepsy may have been misdiagnosed.
Statistics show that irregular heart rhythms cause more than 100,000 deaths a year.
So now a checklist, available from the charity, alerts sufferers to the danger signs.
Trudie Lobban, who founded Stars after a three-year search to get a diagnosis for her daughter Francesca, said: "When patients are empowered with important information they can help their doctor better understand the symptoms and nature of their blackouts.
"This can help avoid a long list of referrals, misdiagnosis and inappropriate treatment before an accurate diagnosis is secured."
Dr Adam Fitzpatrick, heart rhythm specialist at Manchester Royal Infirmary, agreed: "Heart checks are certainly not done enough. When the doctor suspects epilepsy he should still do the ECG [an electrocardiogram which; measures heart activity] as it could show up any heart problems, which could be life threatening.
"I know of several cases where people have not had an ECG which could have picked up their condition and they have died."
He said this was despite the fact NICE guidance advises an ECG when diagnosing epilepsy.
He said that in his area the specialists worked closely together to try to avoid diagnostic confusion, adding that his hospital has a new rapid access clinic opening soon to combine neurology, cardiology and falls.