Smell Disorders​

The sense of smell, part of the chemosensory system, is a complex process. Odor molecules released by substances stimulate the olfactory (smell-related) nerves contained within the nose. These neurons then pass their signals directly to the olfactory areas of the brain for interpretation. The ability to smell is important, not just for enjoying the little pleasures of life such as the fragrance of a flower, but also for your safety. For example, a discerning sense of smell can provide an early alert of leaking gas or smoke from a fire, or a warning that food has spoiled.

Smell disorders affect around 1-2% of people in the US. Such conditions are more frequently found in older adults, and men tend to be more often affected than women. There are a range of conditions coming under the general term of smell disorders. These include:

• Hyposmia: a decreased ability to detect smells
• Anosmia: a complete inability to detect smells
• Parosmia: an abnormal, altered perception of smell
• Phantosmia: the sensation of a smell that isn’t present in reality

What Causes Smell Disorders?

In most cases, a smell disorder is a result of an injury or illness. However, it is possible for a smell disorder to be present at birth, known as a congenital condition, or to have no known cause, which doctors call idiopathic.

A few of the more common causes of smell disorders include hay fever (allergic rhinitis) and other upper respiratory tract infections that may cause a congested sinus, as well as nasal polyps. Head trauma, hormonal imbalances, smoking, exposure to chemicals or certain medications, can also underlie a smell disorder, as can neurological diseases. More recently, infection with COVID-19 has been associated with a loss of smell as a symptom but this is not the only virus known to cause anosmia. Hepatitis and the flu can also induce a loss of smell.

Hyposmia, a reduced ability to smell, has been linked with smoking, colds and flus, and even mental health disorders, such as schizophrenia, depression, and seasonal affective disorder. COVID-19 and older age have also been associated with hyposmia.

Phantosmia, also known as olfactory hallucinations, can be a very individual experience. The odors may be constantly there or may come and go at different times; they may be pleasant or disagreeable; in one nostril or both. Similar to hyposmia and anosmia, olfactory hallucinations can be caused by head trauma, upper respiratory infections, age, certain medications, and neurological conditions, such as Parkinson’s disease. In addition to these, brain tumors and seizures involving the temporal lobe of the brain have also been implicated in phantosmia.

Parosmia typically occurs due to damage to the olfactory neurons in the nerve, distorting their ability to accurately identify odors. This results in perceiving pleasant smells as malodorous or vice versa, or having difficulty in recognizing certain scents. In some cases, smells that once used to be fragrant may be so overwhelming and unpleasant that they can cause you to feel nauseated. The causes of parosmia are typically the same as those for other smell disorders, including brain injury and viral infections.

Diagnosis and Treatment

Diagnosing and managing smell disorders comes under the expertise of an otolaryngologist, also known as an ENT (ear, nose, throat) specialist.

When assessing an abnormal sense of smell, the doctor will conduct a physical examination of the nose and throat, as well as take a thorough medical history. History-taking covers details such as any potential exposure to toxic chemicals, past head or brain injuries, recent viral illnesses, current medications, and any other known medical conditions. You may be recommended to undergo MRI or CT scanning, or take a smell test. Tests assessing your sense of smell measure your ability to detect faint odors or your ability to identify them.

If the smell disorder is due to a viral illness or trauma, your doctor may simply recommend allowing time to see if it will resolve on its own, as often happens, particularly in post-viral smell disorders. Young patients, females, and non-smokers have the best chance of recovery without needing intervention. The treatment of other situations of smell disorders depends on the underlying cause. If an obstruction, such as nasal polyps, is identified as the problem, your doctor will manage this accordingly, whether through medications or a procedure.

A dysfunctional sense of smell can often be an early sign of Alzheimer’s or Parkinson’s diseases. If your doctor suspects this may be the case, you may be referred to another specialist for further investigation into these common neurodegenerative conditions.

Some people may be offered olfactory training, which involves being given exposure to a set of common smells everyday over the course of several months. These scents tend to include rose, eucalyptus, lemon, and clove.

At the moment, there is also research investigating using medications to encourage the regeneration of olfactory nerves. Scientists are also looking into how aging affects the senses of smell and taste, and how they can be better preserved into older age.

Though some people go on with life quite contentedly with a dysfunctional sense of smell, it can have implications for your safety, and even your eating habits. If you realize that your sense of smell is not quite right, see your family physician or ENT specialist.