Some of the nearly 900,000 New Zealanders who suffer hearing loss could be at extra risk of developing dementia.
Professor Peter Thorne, a leading New Zealand academic heading up a project to research hearing loss, says the condition has big consequences for brain health and is "increasingly being shown to be an additional risk factor for developing dementia".
"There is a relationship between hearing loss and good brain health," he says. "The problem increases with age so we are working on ways to identify it earlier, to develop measures to prevent and treat the condition, and to help adults age well."
Thorne, who is based at the University of Auckland's Faculty of Medical and Health Sciences, says there are many unanswered questions around the possible links between the illnesses.
He says one theory suggests the brain, if the hearing loss is untreated, can become overwhelmed by years of straining to hear, making those with hearing loss more vulnerable to dementia; while others include the possibility hearing loss could lead to what is a known factor in dementia - social isolation.
Thorne is leading a team of New Zealand scientists and clinicians working on ways to understand hearing loss and another debilitating condition - balance disorder - at the newly established Eisdell Moore Centre at the university.
A virtual centre, it is named after the late Sir Patrick Eisdell Moore who was a prominent Auckland ear, nose and throat surgeon and brings together teams from Auckland, Canterbury and Otago universities under Thorne's directorship.
"New Zealand has internationally recognised scientists researching these fields and we hope the centre will be able to harness this talent to help people," says Thorne. "We also want to involve people from the community like hearing and balance disorder support groups.
"The impact of hearing loss can be severe and lead to poor communication and isolation from friends and society," says Thorne. "Hearing plays a vital role in maintaining brain health and there is a need to understand more about it; to develop better ways to diagnose, treat or prevent the condition because the number affected is expected to increase substantially as our population ages.
"The question is how to manage the problem, how to get people using hearing aids or doing something about it early enough."
National Foundation for the Deaf (NFD) figures show hearing loss affects almost 19 per cent of the population (880,350 people) and costs the economy over $950m each year, mostly through lost productivity.
Thorne's work is building on international research. A 2011 study into the links between hearing loss and dementia at the John Hopkins School of Medicine in the United States found elderly people with hearing loss are significantly more likely to develop other cognitive problems and dementia than those who retain their hearing.
The study leader Dr Frank Lin, said hearing loss is a slow and insidious process associated with ageing, and because of this a lot of people ignore it, feeling they are not affected.
Thorne says a major cause of hearing loss, apart from ageing, is exposure to loud noise (a World Health Organisation report estimates more than a billion people worldwide aged between 12 and 35 are at risk from high noise levels in recreational settings).
"This can occur at work but it is also a big issue during leisure times when people may be listening to loud music or socialising in noisy bars," he says. "The ears cannot stand too much loud noise, but the good news is it is preventable and we should be looking to change people's habits through public education and programmes."
Listen Hear New Zealand, an NFD report released in February, identified noise as the cause of up to 17 per cent of hearing loss (compared to 37 per cent globally) and said it was a significant reason adults develop the condition later on in life.
Thorne says balance disorders - including dizziness, spinning sensations, fatigue and ringing in the ears - is estimated to affect four out of ten people: "It contributes to falls in older people and, like hearing loss, good balance function plays a vital role in maintaining brain health," he says. "It too is considered a risk to the development of dementia."
Because the condition is connected to the inner ear it can be difficult to diagnose and as a result often goes unreported, Thorne says.
"This makes research more challenging but we are keen to learn more about the disorder," he says. "We are starting to realise it is more common than first thought and needs a lot of research into its prevalence and causes."
Balance disorders have also been the subject of international research. A study by Imperial College London found many patients are incorrectly diagnosed with mental health issues and are stigmatised when seeking treatment.
Dr Barry Seemungal of the college's Faculty of Medicine says sufferers often stop going out because of the overwhelming sensory problems they experience in the outside world; "many are diagnosed as agoraphobic (a fear of open or public spaces) when in fact they have a balance problem."
• Alzheimers New Zealand data shows more than 62,000 Kiwis had dementia in 2016 (up from 48,000 in 2011), a number expected to rise to over 170,000 by 2050. Although not a specific disease, dementia is a term for a range of symptoms associated with a decline in mental ability severe enough to interfere with daily life. Alzheimer's disease accounts for up to 80 per cent of all cases of dementia.