Helen knew something was wrong as soon as she woke up. The world was suddenly mono instead of stereo: she couldn't hear out of her right ear.

She hadn't had a cold or been unwell, so at first she brushed it off as nothing. But overnight, ordinary events — conversations with friends, the open plan office at work — became distressing.

"It was frightening. I went out with my husband … we went out for dinner. It was just dreadful. The sound was bouncing off the walls, making me feel ill, making me dizzy," she said.

And when it hadn't gone away by the next morning, Helen's concerns grew.

"I thought I'd just pop along to the GP and see if they could say what it is or tell me I've got nothing to worry about," she said.

Unfortunately, she did have something to worry about.

"She [the GP] had a look of horror on her face when I said this had suddenly come on. And she said 'this is really serious'."

The GP urged Helen to get to an emergency room as soon as possible. Though Helen didn't know it, she was at risk of permanently losing hearing in that ear unless doctors acted fast.

Sudden sensorineural hearing loss

"Sudden sensorineural hearing loss" (SSNHL), otherwise known as sudden deafness, is a loss of hearing over hours or days with no obvious cause.

Helen is a typical case — an otherwise healthy person who goes to sleep and wakes up without hearing in one (or rarely, both) ears.

Estimates of how many people are affected by SSNHL vary. The medical literature suggests it's anywhere from five to 160 people per 100,000 each year — uncommon, but not unheard of.

And it's often missed by doctors, according to ear, nose and throat surgeon Peter Friedland.

"Unfortunately, it's often brushed off and the patient isn't taken seriously or they're dismissed," Professor Friedland said.

"It can be related to other causes that are more common. Hence it's not investigated as a possibility of causing this sudden loss of hearing, which can be permanent."

Those other causes of hearing loss can include infection and wax build-up. But when they're ruled out, and when the person doesn't have another illness, SSNHL is a distinct possibility.

And while it usually only affects one ear, Professor Friedland said it can still have a huge impact.

"The loss of hearing is devastating because we need both ears to hear with. For direction, binaural hearing, safety crossing a road, conversations with background noise," he said.

What causes the loss of hearing?

In the majority of cases we don't know what causes the loss of hearing.

"We think it may be viral … and sometimes if a patient's had a herpes sore we can think it may be related to herpes simplex, the respiratory virus, not the genital virus," Professor Friedland said.

"Other causes we think about are vascular, this could be a bleed in the inner ear, in the labyrinth of the inner ear. Or it could be decreased blood supply to the inner ear. We know the blood supply to the inner ear is very tenuous, there's one little artery."

But though these conditions appear somehow linked to SSNHL, the mechanism that actually causes the loss of hearing remains unclear.

Experts speculate the virus or inflammation may be somehow disrupting the workings of the cochlea, a spiral-shaped cavity in the inner ear which helps convert vibrations into electrical signals for the brain.

"We only find a cause in one in ten [cases]," Professor Friedland said.

Treatment on a timer

After heading to emergency, the registrar at the hospital couldn't find anything wrong with Helen's hearing, she says.

"He did some basic tests with me … and said, 'Nah, I can't see there's a problem'," Helen said.

"He vaguely mentioned steroids, and to go and see an audiologist at some stage. And that was it."

But the clock was ticking for Helen. The main treatment for SSNHL is steroids, and for the best chance of recovery they have to be given in the first 72 hours of hearing loss. While some people will spontaneously get better without treatment, many don't.

"They can [in the short term] cause mood disorders, a bit of anxiety, some dizziness, inflammation of the stomach, insomnia," Professor Friedland said.

"But if you weigh this up against the effects, the life-long effects of hearing loss … I think it has to be offered to patients."

After a visit to an audiologist who confirmed her hearing loss, Helen went back to see another GP. It was now three days since she'd first noticed the change.

"He said, 'you should be on steroids'. So I started them within an hour," she said.

While the precise mechanism by which steroids treat hearing loss is unknown, and evidence for their benefit is mixed, it's thought they may reduce inflammation and swelling in the hearing organs.

'Speed is of the essence'

A few days later, Helen got in to see Professor Friedland.

"He said [the GP] could have saved my hearing — the fact he moved so quickly, and got me on to the steroids so quickly," Helen said.

Within days of going on steroids, Helen's hearing had dramatically improved. She's since made a full recovery and says she's become an advocate for letting others know about the condition.

"Anyone who cares to listen, I will tell them, if your hearing suddenly goes, don't just dismiss it," she said.

"It's a question of moving quickly and pushing to make sure you get the treatment you need early. Speed is of the essence."

Hyperbaric oxygen therapy?

For those who don't respond to the steroids, hyperbaric oxygen therapy is another option.

It involves being in a special chamber where you breathe in 100 per cent oxygen. By contrast, the air we breathe in every day is about 20 per cent oxygen.

The rationale for this treatment is that injured tissue may need more oxygen than usual to promote healing.

Hyperbaric oxygen therapy increases the amount of oxygen your blood can carry, which may encourage healing and the fighting of infection.

"If we believe there's been decreased blood supply … to the cochlea, then by giving hyperbaric oxygen we're increasing oxygen perfusion to the cochlea and improving the healing and repair of the inner hair cells and the membranes within the cochlea," Professor Friedland said.

But hyperbaric oxygen therapy is expensive — around $500 an hour — with up to 20 hours of therapy recommended for the treatment of SSNHL.

And the evidence of its effectiveness in treating the condition is mixed.

A recent article in JAMA Otolaryngology found there did seem to be some general benefit to hyperbaric oxygen for SSNHL, but that the inconsistent nature of the research made it difficult to draw strong conclusions about how effective it is, or how it should be used in conjunction with other treatments.