Anosmia is the complete loss of the sense of smell, affecting millions globally. This impairment compromises safety, as individuals cannot detect gas leaks, smoke, or spoiled food. Recovery from anosmia is highly variable, depending on the underlying cause and the specific part of the olfactory system affected.
The Different Types of Anosmia
The olfactory system begins in the upper nasal cavity, where specialized olfactory sensory neurons detect odors and send signals to the brain. Anosmia is broadly categorized based on where this pathway is disrupted, leading to either conductive or sensorineural loss.
Conductive anosmia occurs when odor molecules are physically prevented from reaching the sensory neurons in the nasal passages. This blockage is typically caused by issues like severe nasal congestion from a common cold, chronic sinusitis, or nasal polyps. In these cases, the neural machinery for smell remains intact, but the signal cannot reach the receptors.
Sensorineural anosmia involves damage directly to the olfactory sensory neurons or the neural pathways leading to the brain. This type of loss is often caused by a severe viral infection, head trauma, or certain neurodegenerative conditions. The sensory cells or nerves are impaired, meaning the signal cannot be generated or transmitted even if the scent molecule reaches the area.
Expected Recovery Based on Cause
The duration of anosmia is directly linked to the cause, with recovery ranging from days to potentially never, depending on the damage sustained. Anosmia resulting from acute causes, such as the common cold or a temporary sinus infection, is usually short-lived. This conductive loss resolves quickly once the inflammation subsides and airflow to the upper nasal cavity is restored, typically within a few days or weeks.
Post-infectious anosmia, frequently caused by viruses like influenza or SARS-CoV-2, has a more variable and often longer recovery timeline. While a majority of patients experience the return of their sense of smell within two to three weeks, a significant minority—around 10 to 20 percent—suffer from persistent loss. For these individuals, gradual improvement can take many months, with recovery commonly occurring over a 6 to 12-month period as the damaged olfactory neurons regenerate.
Anosmia following head trauma carries a less optimistic prognosis due to the risk of physical nerve damage. The prognosis is particularly guarded if the injury involved fractures to the frontal skull base, indicating a more severe shearing of the olfactory nerves. Spontaneous recovery for post-traumatic anosmia is most likely to occur within the first six to twelve months after the injury.
For those who do recover naturally after trauma, the process can continue for up to two years as the nerves attempt to regenerate and reconnect. Only about 30 percent of patients with post-traumatic anosmia regain their sense of smell, and recovery beyond two years is considered rare. Conversely, age-related smell loss, known as presbyosmia, is a sensorineural condition that tends to be progressive and permanent.
Treatment Options to Regain Smell
Active interventions are available to facilitate the return of the sense of smell. Olfactory training, also called smell training, is the most widely recommended non-pharmacological intervention for persistent anosmia. This technique involves deliberately sniffing a set of four distinct scents—typically rose, lemon, clove, and eucalyptus—twice daily for a duration of at least 16 weeks.
The goal of smell training is to stimulate and reorganize the neural pathways involved in smell perception, essentially retraining the brain. This process can be effective even for those with long-term anosmia, though it may take a year or two of consistent practice to see a full benefit. Intranasal corticosteroids, such as nasal sprays, are often prescribed to reduce inflammation and swelling within the nasal passages, which can improve conductive-type loss.
For anosmia related to chronic rhinosinusitis or nasal polyps, a combination of medical management and sometimes surgery is required. Corticosteroids can be delivered via high-volume saline rinses to directly target inflammation in the olfactory cleft. If a physical obstruction like a large polyp is the cause, surgical removal may be necessary to clear the airway and allow scent molecules to reach the olfactory receptors.