Anosmia (loss of smell) and ageusia (loss of taste) became widely discussed symptoms during the COVID-19 pandemic. While the virus brought attention to these sensory dysfunctions, they are not exclusive to a single pathogen. Flavor perception heavily depends on the sense of smell; the tongue only registers basic tastes like sweet, sour, bitter, salty, and umami. The appearance of anosmia and ageusia signals that the intricate sensory pathways in the nasal cavity or brain have been disrupted by various underlying causes.
The Specific Role of COVID-19
The SARS-CoV-2 virus induces sensory loss through a mechanism distinct from the simple congestion of a common cold. The virus does not primarily infect the olfactory sensory neurons. Instead, it targets and damages the non-neuronal supporting cells, known as sustentacular cells, which maintain the environment around the sensory neurons. Viral entry is facilitated by the ACE2 receptor and the TMPRSS2 enzyme on the surface of these cells.
Damage to these supporting structures leads to inflammation and disruption of the olfactory epithelium, indirectly impairing the function of adjacent sensory neurons. This mechanism explains why COVID-19-related anosmia often occurs suddenly and completely, frequently without the nasal blockage typical of other respiratory infections. Newer variants have shown a lower incidence of this symptom, though it remains a recognized manifestation of the disease.
Non-Viral Causes of Sensory Loss
Loss of smell and taste is associated with many conditions besides COVID-19. Many infectious causes other than SARS-CoV-2 can temporarily induce anosmia by causing inflammation and swelling in the nasal passages. Common upper respiratory infections, such as the flu or a cold, cause mucus buildup that physically blocks odor molecules from reaching the olfactory receptors. This is referred to as a conductive loss, and the sense of smell generally returns once the congestion clears, typically within one to two weeks.
Structural issues can also lead to a persistent conductive loss of smell. Nasal polyps, which are non-cancerous growths, can physically obstruct airflow. Similarly, a severely deviated septum can block the pathway necessary for smell perception. Chronic rhinosinusitis, a long-term inflammation of the sinuses, often involves swelling and mucus buildup that restricts access to the olfactory nerve endings.
A different category of causes involves direct damage to the neurological components of the olfactory system. Head trauma, such as a concussion, can physically sever the delicate nerve fibers that pass into the brain, resulting in a sudden and potentially permanent loss of smell. Certain systemic conditions and chronic exposures can also impair the olfactory nerves over time. Long-term heavy smoking exposes the nasal lining to toxic chemicals that can injure sensory cells and taste buds.
The natural process of aging also leads to a gradual decline in the ability to smell, called presbyosmia, which becomes noticeable after age 60 as the number of olfactory nerve fibers decreases. Furthermore, a variety of medications, including some antibiotics and blood pressure drugs, have been linked to changes in smell and taste as a side effect. In rare cases, a sudden, unexplained loss of smell can be an early indicator of neurodegenerative diseases, such as Parkinson’s or Alzheimer’s disease.
Recovery Outlook and When to See a Doctor
The outlook for recovering the sense of smell depends on the underlying cause. When anosmia is due to temporary factors like a cold or sinus infection, recovery is usually rapid and complete within a couple of weeks. Recovery from COVID-19-related loss is often longer; most individuals see significant improvement within six months, though a small percentage may experience persistent issues for a year or more.
As the olfactory system recovers, some people experience distorted sensations, indicating that the nerves are attempting to reconnect. Parosmia is a condition where familiar odors are perceived as unpleasant, often described as rancid or chemical. Phantosmia, or an olfactory hallucination, is the perception of a smell when no odor source is present, such as smelling smoke or a metallic scent.
It is prudent to seek medical advice if the loss of smell or taste is sudden and cannot be attributed to an obvious cold or flu, or if it results from a head injury. If the loss persists for more than four weeks following a viral illness, a specialist evaluation is warranted to rule out structural problems like polyps or chronic sinusitis. Loss of smell accompanied by other neurological symptoms, such as changes in memory, movement, or vision, requires immediate medical attention.