Phantosmia (smelling odours that aren't there)

It is also known as an "olfactory hallucination".

The smell is unique to the person and is usually unpleasant, spoiling the taste of any food or drink consumed. It can be in one or both nostrils.

Most phantom smells go away in time and are not caused by anything serious. But if the problem persists and you're worried, see your doctor.

How does your sense of smell work?

When something gives off a smell, it means tiny molecules have evaporated from its surface and reached your nose.

High up inside your nose is tissue containing specialised nerve cells, called olfactory sensory neurons, which connect directly to your brain.

These nerve cells have receptors that detect the microscopic odour molecules and send electrical signals to the brain. The brain receives these signals and identifies the smell.

There are two ways smells can reach the olfactory sensory neurons:

• By the 'front door' through the nostrils.
• By the 'back door' through the throat to the back of the nasal cavity. Chewing food releases odour molecules and when you swallow, the tongue pushes this air into the back of the nasal cavity (this is known as "retro-nasal olfaction").

What could cause phantom smells?

There are a wide range of possible causes of phantosmia, which include:

• nasal infection – some people start to perceive a smell after they develop an infection that affects their nose or sinuses (read about
  sinusitis
  ). The smell may become noticeable after a sudden change in airflow through their nose – for example, just after they've sneezed. It usually goes away when the infection clears
• nasal polyps, which are abnormal tissue growths that form inside the nasal passages and sinuses (read more about
  nasal polyps
  )
• migraine with aura – some people smell phantom odours just before or during a migraine (read more about
  migraines
  )
• dental problems
• smoking
• exposure to certain chemicals such as insecticides or solvents
• radiation for treatment of head and neck cancer

Neurological (nervous system) conditions

Less commonly, the cause of phantosmia is either nerve cells sending abnormal smell signals to the brain, or a problem with the brain itself.

This may be the result of:

• a disease affecting the nervous system, such as epilepsy,
  Parkinson’s disease
  or
  Alzheimer’s disease
• a
  stroke
• a
  head injury
• cancer – usually a
  brain tumour
  or
  neuroblastoma
  (a rare cancer that may start in the olfactory nerve)

How is phantosmia diagnosed?

Your doctor will want to know if the problem is definitely with your sense of smell, and not with your sense of taste (it's easy to confuse these).

They will also want to determine whether the smell is perceived, as in phantosmia, or actually real – you might be giving off and detecting a body odour, for example.

You'll have a head and neck examination, to see if there is any obvious problem such as something in your nasal passages. The doctor will want to know if the smell transmits through one nostril or both.

You may be referred to an ear, nose and throat specialist for further tests, which may include:

• a nasal endoscopy, where a fine, flexible tube with a tiny camera at the end is inserted up through your nose and images are viewed on a television screen (read more about an endoscopy)
• an MRI scan or
  CT scan
  of your brain and nasal cavities, to rule out tumours, infections or obstructions

How is phantosmia treated?

Some people with phantosmia will find that the smell gradually fades over a few months, and no treatment is needed.

If it is caused by an illness such as sinusitis, it should go when you recover from the illness.

Otherwise, if it persists, the following treatments may be tried:

• Rinsing out the nasal passages with saline solution, which may provide temporary relief for short periods of time. This may temporarily block the smell signals going to your brain, but you'll need to keep doing it.
• Nasal drops or spray – you may be prescribed oxymetazoline, which narrows blood vessels in the nose to help reduce nasal congestion, a spray containing an anaesthetic, which numbs the nerve cells, or a nasal steroid spray.
• Sedatives, antidepressants or anti-epileptic drugs, which may work for some people.

You must always weigh up the benefits of these treatments with the possible side effects – speak to your doctor about this.