Diagnosing a changing sense of smell

Can changes in our sense of smell say anything about our health? Trainee doctor Isabelle Cochrane investigates...

Isabelle - From sniffing out food, to telling fresh from rotten, to catching a whiff of a sneaky predator hiding behind a bush, smelling is key to survival. In many animals, the olfactory system - which processes smell - is intimately connected with some of the most primitive areas of the brain. These areas are known as the limbic system, and are responsible for driving behaviours that we would often consider to be instinctive, such as feeding and mating. One component of this system is the hippocampus, which deals with associative memory: forging links between disparate pieces of information such as a specific location and a particularly tasty crop of berries. Another important part of the limbic system is the amygdala, which has a key role in the regulation of pleasure and emotion - another powerful motivator for behaviours of evolutionary importance. In humans, these links between olfaction and the limbic system are no longer as significant, as much of our behaviour is driven through conscious decision-making rather than instinct. But we all know that smell can be strongly evocative, with a particular scent bringing back a flood of memories, often with a strong emotional component.

For something of such importance in our evolutionary past, our sense of smell is perhaps a little overlooked in modern clinical practice. We don’t know much about what can cause a disorder of the sense of smell, and should this occur, we aren’t quite clear on how best to treat it. Nevertheless, anosmia (the inability to smell) and hyposmia (a weak sense of smell) are relatively common, with studies putting the prevalence in the general population at somewhere between 3 and 20 percent.

Usually people are born with a normal sense of smell and problems occur later in life, with the incidence of olfactory problems increasing with age. Most commonly people lose their sense of smell after an infection of the nose or sinuses, or due to allergic inflammation of the lining of the nose as might occur in hay fever. This loss is normally temporary, and resolves without intervention. However, in a small proportion of people the loss of smell can persist, particularly following a viral infection of the nose. Another cause of smell loss is head trauma. The olfactory nerve, which transmits signals from the cells lining the nose to the brain, passes through narrow gaps in the base of the skull, and is therefore very vulnerable to injury if the head is violently shaken or there is a fracture to the skull. Very, very occasionally, a loss of smell can indicate the impending onset of a degenerative neurological disease. Both Parkinson’s disease and Alzheimer’s disease are known to be preceded by a loss of sense of smell in some patients, sometimes by as much as five years - meaning that this can be the very first indication that something is amiss. Much more commonly, in up to a quarter of people who lose their sense of smell, no cause is ever found. This is known as idiopathic anosmia.

In some rare cases, some people are born with no sense of smell at all. This can be an isolated problem, or may represent part of a broader issue. Some people have the unusual experience of smelling much too much: in phantosmia, patients experience ‘phantom smells’, also known as olfactory hallucinations. This can occur in ‘organic’ brain disease such as epilepsy, and is thought to be due to abnormal nerve cell activity in an area of the brain called the temporal lobe. This phenomenon can also occur in the context of psychiatric disease such as schizophrenia, which is characterised by hallucinations - although these are much more commonly either auditory or visual.

Though it may not seem obvious, losing one’s sense of smell can have a real impact. Smell is almost as important as taste in stimulating appetite and enjoyment of food. People with a disordered sense of smell often struggle with eating too little, or indeed too much to compensate for the lack of enjoyment. Personal hygiene becomes a concern due to the inability to detect unpleasant smells, and this can affect personal relationships and confidence. Perhaps it’s no surprise that people with problems in their sense of smell are more prone to depression. Additionally, an inability to smell food that has gone bad, or to detect dangerous substances such as natural gas, leads to sufferers of anosmia to report greater levels of anxiety and feeling less safe than those whose sense of smell is normal.

So how do we treat someone who has a problem with their sense of smell? In inflammatory conditions such as allergies and sinusitis, or following infections, nasal steroid drops are often given - with some success. Surgery to the nose and sinuses is also commonly attempted, but the results of this approach are not always good. Patients who have lost their sense of smell following infection or trauma in particular can undertake a course of ‘smell training’, in which they are instructed to smell a number of different substances at regular intervals in an attempt to ‘re-sensitise’ their nose. There is some evidence that this can be effective if used regularly over a long period of time, although it is unclear if it works as well in other patient groups. For the unfortunate significant minority with idiopathic anosmia, as well as for sufferers of phantosmia, treatment relies much more on guesswork than solid scientific evidence, and is relatively ineffective.

However, olfaction is a field that is attracting more and more medical research, and with rapid advances in areas including gene therapy and olfactory stem cells, it may not be long before we sniff out a cure.