The olfactory sense is yet another means by which diagnosticians can find clues about ailments or uncover little fibs embarrassed patients might be telling. Dr Bridget Osborne warns us not to turn up our noses at using smells in diagnosis.
Introduction
I n the twenty-first century we are subjected to relentless olfactory stimulation. Simply walking down the High Street, we are enveloped by smells, which we barely register. We pass fragrant coffee bars, delicious-smelling restaurants, bakeries with the wonderful smell of freshly baked bread, and our fellow shoppers, each with their individual scent of aftershave or perfume.
These ‘loud’ smells, many of them artificial, can desensitise us to the underlying odour of human kind, including the smell of earwax, sweaty armpits, feet and menstrual blood. Compared to other animals, such as dogs, our sense of smell is almost vestigial. However, although we have only six million olfactory receptors, compared to 300 million in dogs,1 we are still able to detect substances in dilutions of less than one part in several billion parts of air.
We do not have the vocabulary to describe smell in the same way that we are able to describe colour – for example we all understand that blue can be anything from navy through petrol to the palest duck shell blue. However, to the majority of people, vinegar smells of vinegar, although cooks and connoisseurs are able to differentiate balsamic from malt. Much like a wine taster educates his or her palate to fine wine, distinguishing all the different notes, all clinicians can educate themselves to detect smells in a clinically useful way too.
The primary olfactory cortex is linked to the amygdala and hippocampus, which are involved with emotional and long-term memory, with the result that certain smells can provoke vivid recall of people and events. Olfactory memory may well be the trigger that makes experienced clinicians worry about a patient, even when they cannot clearly identify their concerns. What they are doing is subconsciously remembering a different patient from the past, who had a ‘bad smell’.
A nose for pheromones
In addition to smelling with their noses, animals as diverse as cattle and snakes have an accessory olfactory system, known as the vomeronasal organ, located in the roof of the mouth. This is primarily used to detect pheromones, rather than the inhaled chemical molecules detected by the nose. The presence of a similar structure in humans was doubted for many years, but its existence has recently been histologically proven in about 60 percent of people, using cadaveric dissection.2 Animals using their vomeronasal organ show a ‘Flehman response’, demonstrated, for example, when a ram following a receptive ewe can be seen to lift his head and curl back his top lip.
Pheromones, also known as ecto-hormones, are substances used for olfactory signalling throughout the insect and animal world. The first pheromone to be isolated was bombykol, a chemical produced by the female silkworm moth (Bombyx mori) to attract a mate. There has been much controversy about human ability to either produce or detect pheromones, but research has increasingly shown that not only do humans produce these substances, but that they also influence human behaviour, including sexual and social relationships.
This research has been exploited commercially and a brief browse of the internet will show numerous artificially produced substances available. One example of an online advert for a product we’ll call Product X, reads: “Product X is intended to attract women by helping you radiate pure alpha dominance. It helps create gut level attraction, by emitting the pheromone signature of an aggressive, sexually active young man who gets what he wants. It makes women feel butterflies in their stomach any time they talk to you. Product X gets its best results with women who are naturally attracted to aggressive ‘bad boy’ types, and looking for fun.”
Substances which may act as pheromones are produced by apocrine glands, which are found principally in the axillae, areola and genitalia in humans. Almost 400 different volatile compounds have been identified in axillary sweat using gas chromatograph-mass spectrometry, which have individually distinct and reproducible spectrometric fingerprints with reproducible differences between the sexes.3
Short chain fatty acids, known as ‘copulins’, whose composition varies throughout the menstrual cycle, occur in vaginal secretions. It has been suggested that there are two types of pheromone: ‘signal pheromones’, which produce short term behavioural change, and ‘primer pheromones’, which activate the hypothalamic pituitary axis and trigger release of GnRH, inducing longer-term changes through subsequent release of gonadotrophins.4
Although humans are largely unaware of being able to detect pheromones, we undoubtedly are able to do so, as shown by the synchronisation of menstrual cycles in groups of women living or working in close proximity.5 Work in mice has replicated this effect, but also shown other interesting effects, such as earlier maturation of females in the presence of an unrelated male, known as the ‘Vandenbergh effect’, as well as the rather more sinister ‘Bruce effect’, in which pregnant mice housed with a male that was not their original mate had significantly more miscarriages before mating with the new male.4
There are also a number of studies looking at the effect of androstenol on male sexual attractiveness to females, but the effects seem to depend upon the phase of the menstrual cycle and to be blunted by oral contraception.4
Another interesting effect of hormonal