Loss of Sense of Smell

The loss of smell (anosmia) is not a serious disorder, but it can significantly affect one’s quality of life. Taste and smell are closely linked, so people with anosmia are also unable to taste food properly. Anosmia occurs when the olfactory sensory neurons in the nose cannot transmit odor messages to the brain. Generally, the loss of sense of smell is a relatively unusual condition that is more common in men than in women. The risk of anosmia increases with age, and at least a partial loss is common by age 60 years. However, acute anosmia has also emerged as one of the most common indicators of COVID-19 infection. It does not appear that anosmia caused by COVID-19 results in permanent damage, and data suggest that the loss of taste and smell is temporary.

Ability to Taste and Enjoy Food May Also Be Affected

The sense of smell requires olfactory sensory neurons, sensing cells found in the nasal lining that detect odors and relay this information to the brain. When food is chewed, the chemicals that carry food scents travel up the back of the throat to the nasal passages. The senses of smell and taste combine near the back of the throat, and the receptors coordinate to create the perception of flavor. If there is a blockage or swelling in the nasal passages, odor messages are not transmitted to the brain. Likewise, if the specialized neurons are destroyed, the brain cannot receive these sensory messages.

Common Causes of Anosmia

The three primary causes of anosmia include obstruction (blockage) in the nasal passageways, destruction of the tissue lining the nose caused by inflammation or swelling, and a loss of nerve transmission from sensory neurons in the nose to the brain. With COVID-19 infection, it is thought that damage caused by the virus to the skin cells that line the nasal cavity is to blame for the loss of smell and taste. It does not appear that the SARS-CoV-2 virus causes permanent damage to the nerve cells themselves.

Nasal-passage obstruction caused by benign growths (nasal polyps) or tumors and congestion from a cold, sinus infection, or allergy flare-up are common causes of temporary loss of smell and taste. If chronic inflammation of the nasal lining from infection or allergy destroys the nasal tissue, a permanent inability to sense odors can result. Certain drugs or exposure to toxic chemicals can also cause anosmia for the same reason. Intranasal zinc products, decongestant nose sprays, and certain oral medications, such as nifedipine and phenothiazines, are examples of drugs that may cause permanent loss of smell. Anosmia may also result from diseases of the nerve pathways that transmit smells to the brain. Nerve damage can occur with head trauma, surgery, infection, tumors in the central nervous system, or Alzheimer’s disease.

Diagnosis and Management

The diagnosis of anosmia is made by taking a thorough history of symptoms and considering any past events that could cause anosmia, such as recent head trauma or nasal congestion from an upper respiratory infection. The nasal passageways are examined, and a test kit that includes materials with intense odors may be used to determine whether there is a detectable loss of smell. If the cause of anosmia is not apparent, a computed tomography scan or magnetic resonance imaging of the head may be performed to identify other causes.

Management of anosmia is aimed at eliminating an identifiable and treatable cause. Many causes are not treatable, and the loss of smell is permanent. Even if a cause is successfully eliminated, the loss of smell may persist. If anosmia is permanent, it is vital to be aware of associated dangers. Loss of smell includes the inability to detect harmful odors such as natural gas in cooking and heating systems or smoke from a fire. Homes should be equipped with smoke and natural gas detectors.

The content contained in this article is for informational purposes only. The content is not intended to be a substitute for professional advice. Reliance on any information provided in this article is solely at your own risk.