#216 The Smell of Dementia

The Ageing Senses

The quality of your sense of smell is a key indicator of your likelihood of getting dementia.

A five-minute read.

Between 12% and 20% of people suffer from some kind of impairment to their sense of smell. Tests include the ability to recognize common household smells. They also include “discrimination tests”. Three sample smells are presented to an individual. Two are identical, one is different. The job is spotting the matching pair when blindfolded.

Smell loss does increase with age. The sensors within the nose are continuously replaced. With age the process slows. There are other common causes. These include sinonasal disease, viral infections and traumatic injury. Ageing healthily can mean taking one or more medications. For such things as cholesterol, blood pressure or diabetes. It is one of the benefits of modern medicine that such drugs can allow us to live an otherwise normal life. There is a hidden cost. These very same drugs can reduce our sense of smell.

Our nose works by dissolving the scent molecules in the air in the mucus of our nose. The sensors can then identify what we are smelling. Based on last weeks Newsletter #215 we are inhaling pollution. This cannot be good for the smell process! Medicine, disease and pollution all account for some of the loss of smell.

It is strange finding that we are not very sensitive to our "olfactory loss”. Researchers tested women of different ages. All had a clinically proven decline in their sense of smell. In a fifty-five-year-old age group only one third said they had a smelling problem. This dropped to twelve percent for an above eighty age group. The gap gets bigger as we get older. Our mind is "filling in the gaps". However, our "soundscape" is fading. Smell loss can cause depression whether we notice it or not.

Smell, Brain Size and Cognitive Ability.

The Baltimore Longitudinal Study offers a unique combination of measures. Over repeated years, respondents were given a smell test. A large sample also had a brain scan which measured the size of different parts of the brain. Finally, the respondents completed a range of cognitive and memory tests.

Low scores on identifying the odors were a predictor of subsequent greater brain shrinkage. Parts of the brain associated with the emotions, declined in size. A high score was associated with better results on the cognitive and memory tests. A slower subsequent decline in attention, for example.

A similar study in Leipzig showed an even strong association with cognitive abilities. The study had over 7000 participants. There was a strong association between respondents’ cognitive ability and their olfactory ability. A poor score on the smell tests was associated with lower verbal fluency, attention, learning and memory. Researchers are now proposing smell tests as part of the diagnosis of dementia.Even accounting for all the other reasons for smell loss the associations are strong enough.

These are ofcourse associations even though they happen sequentially. Declines in any of the senses are however now thought to be one cause of dementia. Without the stimulation of hearing or sight the brain can atrophy. The strongest predictor of future Alzheimer’s disease is hearing loss. There is evidence that losses are not just the start of the disease. The process can sometimes be reversed. A cataract operation may slow the progression of dementia by restoring sight. Recent research has suggested that smell loss may be as important.

Reviving Your Sense of Smell

There is a growing interest in whether it is possible to “train” our sense of smell. Early indications are good. One study introduced a range of smells to sleeping respondents. Those nighttime scent doses were designed to stimulate the “olfactory bulb”. This processes the signals generated by the sensors in our nose. The results were improved smell test scores. MRI scans also showed changes in the smell pathways in the brain. Other studies have used sniffing of different smells morning and evening. Before you reach for the perfume bottle these are long term effects. The results become visible after 6 months and improve for up to a year.

Returning the “smellscape” to an individual is known to reduce depression. Smell is unique amongst our senses. It has direct access to the emotional parts of the brain. It is able to have a more direct and immediate effect than the other senses. It is the same with unpleasant smells. These are often associated with threats to us. There is almost an instantaneous response triggered inflammation defences.

Perhaps we underestimate our senses of smell at our peril.

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