What is post-traumatic olfactory loss and what causes it?
As the names suggests, post-traumatic olfactory loss describes anosmia or hyposmia which results from a head injury. The extent of loss is determined not only the severity of the injury but also the part of the head damaged; smell loss is more likely to occur from injuries to the back of the head.
Why is this? Well, look at the diagram on the right. This shows an impact to the front of the skull rather than the rear, but the principle remains the same. The brain essentially floats in a body of fluid inside the skull, and a sudden blow to the head can cause it to collide with the inside front of the skull, which has a rough surface. The olfactory system is situated at the lower-front of the brain and is therefore very susceptible to damage in this way.
People who have suffered a loss of smell in this way may have been told that the olfactory loss has been caused by the olfactory nerves being severed. They travel from the olfactory receptor cells through a layer of bone in the skull called the cribriform plate which is covered in tiny holes, almost like a sieve. As the brain bounces back and forth as described above the nerves fibres can be pulled and snag on the edges of the holes in the bone and break.
This isn’t necessarily the full story, however. To start with, the olfactory nerve fibres are too tiny to appear on any scans, so whilst it’s quite likely that some damage to the nerves has occured, this may not be the only reason for the loss of smell.
Bruising and subsequent scarring of the frontal lobes of the brain is also common in a head injury of this kind. The olfactory bulb, the structure situated at the front of the brain that processes the signals it receives from the receptor cells in the nose, can suffer damage in this way.
Smell loss can result from damage to the front of the head or face too. For example, direct injury to the nose can prevent odours travelling to the olfactory cleft, the space at the top of the nose where the receptor cells are located.
How can it be treated and what are the challenges?
There are no standard treatments for directly repairing the damage caused by post-traumatic olfactory loss, for example to the olfactory nerve or bulb. We know that patients are commonly told by doctors that their sense of smell isn’t going to come back and there is nothing that can be done to treat the problem. Whilst the prognosis for post-traumatic patients is admittedly poor, some people may experience some improvement.
Some degree of recovery may take place over time, but the extent of this, if indeed any recovery takes place at all, will largely be determined by the severity of the damage to the olfactory system. The olfactory neurons regenerate and can grow new nerve fibres, or axons, which can attach themselves to the olfactory bulb, thus restoring the connection between the receptor cells and the bulb. However, damage sustained in the injury can lead to the development of scar tissue which can prevent the axons reaching the bulb. In addition the axons may be unable to find their way back through the holes in the cribriform plate.
Any recovery can take quite some time. We’ve heard of patients being told by their doctors that if no recovery occurs within 6-12 months then it’s unlikely that anything will change. This isn’t entirely true; we’re aware of people who have regained some ability to detect odours several years after the initial loss. The problem is that at this time there is no way of predicting if any recovery will occur or how long after injury this may take.
Head trauma is responsible for 8% of cases of olfactory loss
What can I do?
Smell training can help you to firstly establish whether you are able to detect any odours and then maximise any olfactory ability that does remain. It is certainly worth trying and we also feel there is an element of self-empowerment in doing this, particualrly if you’ve been told that nothing can be done by your doctor.
Some informal studies have been undertaken using alpha-lipoic acid, which is used for nerve-related symptoms of diabetes. There is evidence that it improves the function and conduction of neurons in patients with diabetes, and it is thought that it may be helpful to people who have suffered post-traumatic or post-viral olfactory loss by helping in the regeneration of olfactory neurons. Some clinicians do suggest it to patients and it is readily available from to buy from health food stores or other retailers that sell vitamins and other supplements.
Regeneration and Rewiring the Olfactory Bulb: http://chemse.oxfordjournals.org/content/30/suppl_1/i133.full
Alpha-lipoic acid in the treatment of smell dysfunction following viral infection of the upper respiratory tract: http://www.ncbi.nlm.nih.gov/pubmed/12439184
Further information and resources
Season to Taste: How I Lost My Sense of Smell and Found my Way
Molly Birnbaum, Granta Books 2011 ISBN: 1846273900, 9781846273902
After losing her sense of smell as the result of a head injury Molly set off on a journey to try to regain it and learn more about her condition and the the sense of smell itself. Highly recommended reading for anyone affected by post-traumatic olfactory loss.