Wednesday February 16 2011
It is unclear why hearing loss was linked to dementia
“Hearing loss could be an 'early warning' for dementia,” reported The Daily Telegraph. It said that this finding from a new study “could lead to early interventions against Alzheimer's disease”.
This news report is of a study that followed 639 adults, aged 39-90 years old, for an average of 12 years to see if those with hearing loss were more likely to develop dementia. About 9% of the participants developed dementia during this time, and those with hearing loss at the start of the study were at greater risk of developing the condition.
The study has strengths in that it tested hearing and ruled out dementia for some participants at its start. It also has some limitations, however, including its relatively small size, and larger studies are needed.
The study has shown an association between hearing loss in older adults and later dementia. However, it is not possible to say, based on this study alone, why an association might exist. It is unclear whether hearing loss contributes to the risk of dementia, is a sign of early dementia, or whether dementia and age-related hearing loss involve similar processes. If the last two scenarios are correct, interventions to improve hearing are unlikely to reduce the risk of dementia.
Where did the story come from?
The study was carried out by researchers from the Johns Hopkins School of Medicine and other research institutions in the US. Funding was provided by the National Institute on Aging and the National Institute on Deafness and Other Communication Disorders. The study was published in the peer-reviewed medical journal Archives of Neurology.
The Daily Telegraph gave a balanced coverage of this study.
What kind of research was this?
This prospective cohort study investigated whether there is a relationship between hearing loss and the risk of developing dementia.
This type of study is ideal for investigating whether an exposure might cause an outcome, or whether a particular phenomenon (in this case, hearing loss) might be an early predictor of increased risk of an outcome (in this case, dementia). One of the study’s strengths is that the participants were given hearing tests and assessments to rule out dementia at the start of the study. This means we can be reasonably sure that the measurements of hearing are accurate and that any hearing loss preceded the onset of detectable dementia.
What did the research involve?
The participants in this study were taking part in an ongoing study called the Baltimore Longitudinal Study of Aging. For the current study, the researchers analysed people who did not have dementia, and measured their hearing ability at the start of the study. They followed these people over time to identify anyone who developed dementia. They then compared the risk of developing dementia in people with and without hearing loss at the beginning of the study to see if there were any differences.
The study included 639 adults, aged 36-90 years old (average about 64 years), who were given a thorough assessment and were found to be free of dementia between 1990 and 1994. A standard hearing test identified those with normal hearing (<25 decibels [dB], 455 people), mild hearing loss (25-40 dB, 125 people), moderate hearing loss (41-70 dB, 53 people) or severe hearing loss (>70 dB, 6 people).
The participants also provided information about their lifestyles and were tested for diabetes and high blood pressure. They were followed up until 2008, an average (median) of about 11.9 years. Depending on their age, participants were given thorough cognitive assessments at intervals from every year to every four years, and standard criteria were used to diagnose dementia.
In their analyses, the researchers took into account factors that could influence the results, such as age, gender, race, education, smoking and having diabetes or high blood pressure.
What were the basic results?
At the start of the study, participants with greater hearing loss were more likely to be older, male and have high blood pressure. During follow-up, 58 people (9.1%) developed dementia of any type. Of these, 37 cases were Alzheimer’s disease.
The greater a person’s hearing loss at the beginning of the study, the more likely they were to develop dementia during follow-up:
- In the normal hearing group, 20 out of 455 people developed dementia (4.4%).
- In the mild hearing loss group, 21 out of 125 people developed dementia (16.8%).
- In the moderate hearing loss group, 15 out of 53 people developed dementia (28.3%).
- In the severe hearing loss group, 2 out of 6 people developed dementia (33.3%).
After differences between the groups, such as age, were taken into account, for every 10 decibels of hearing loss, there was a 27% increase in the risk of developing dementia over the follow-up period (hazard ratio 1.27, 95% confidence interval 1.06 to 1.50).
How did the researchers interpret the results?
The researchers concluded that hearing loss is independently associated with dementia. They say that further study is needed to determine whether hearing loss is a marker for early-stage dementia, or whether hearing loss directly affects the risk of dementia.
This study suggests that there is a link between hearing loss and the risk of developing dementia. Strengths of this study include its prospective assessment of hearing, inclusion of people without evidence of dementia at the start of the study, and regular, thorough assessment of cognitive function. There are some points to note:
- The study was relatively small, and the numbers of people in some subgroups, such as those with severe hearing loss (six people), were very small. Therefore, the results for these subgroups may not be representative of all people with this level of hearing loss and may not be very reliable.
- The development of dementia is a slow process, and people in the very early stages of the disease may not show any detectable signs. Therefore, some people included in the study may have already been in the very early stages of the disease. The authors tried to test whether this was the case by carrying out analyses that excluded people who developed detectable dementia shortly after the start of the study (up to six years). These analyses still showed the link between hearing loss and dementia.
- The study took into account some factors that could affect the risk of dementia, such as age, education, smoking and certain medical conditions. However, there may be other factors, such as genetic factors, that affect the risk of dementia but which were not taken into account. These could have affected the results.
- The authors note that the participants had all volunteered to take part and were generally from high socioeconomic backgrounds. Therefore, they were not representative of the community as a whole.
- Though the cause of hearing loss among the participants was not specified, it seems likely that many cases would be due to the common condition of age-related hearing loss (presbycusis). This occurs when the hair cells in the ear gradually deteriorate, and is more common with increasing age. It is not possible to say from this study whether hearing loss might directly contribute to the risk of dementia, or whether it indicates increased risk of the disease. As both presbycusis and dementia are related to increasing age, it is possible that similar physiological processes of cellular ageing are common to both conditions.
Further investigation of this association is needed. However, if hearing loss is only a marker for dementia, or if common disease-related processes underlie both conditions, interventions to improve hearing are unlikely to reduce the risk of dementia. Ideally, these findings need confirmation in larger studies in more representative groups in the community, as the authors themselves acknowledge.