While recent investigations have revealed viral, inflammatory and vascular factors involved in the pulmonary pathogenesis of SARS-CoV-2, the pathophysiology of neurological disorders in COVID-19 remains poorly understood. However, olfactory and gustatory dysfunction are quite common in COVID-19, especially in paucisymptomatic patients, which is the most frequent clinical manifestation of the infection. An elevated temperature or fever and a continuous cough are some of the first signs of COVID-19 infection; since May 2020, loss of the sense of smell has also been recognised as a common symptom. According to the British Medical Journal, half of COVID-19 patients appear to lose their sense of smell, while 16% of them experience this ‘anosmia’ as an isolated symptom.
Further evidence has since emerged, with a study on the evolution of COVID-19 symptoms in the JAMA Network concluding that loss of smell or taste was among the most ‘common and persistent’ symptoms, with more than one in ten patients reporting signs of altered senses after all other symptoms had disappeared.
Reports and anecdotal evidence suggest that an increasing number of patients also experience parosmia – the medical term for distortions of smell. Patients are still able to perceive smells, but the smells may not be what they usually experience, for example, the normally pleasant smell of coffee may instead smell extremely unpleasant.
Post-viral anosmia is common, and not only with COVID-19 but also with other viruses. Scientists investigating why COVID-19 triggers anosmia have suggested several theories, including preliminary research on the observation that support cells, which provide support for the olfactory system in the nasal cavities, are affected by the virus, triggering an immune response to fight the infection. This causes the nerves to stop functioning properly.
Post-viral anosmia is one of the main causes of loss of sense of smell in adults. “The viruses that give rise to the common cold are well known to cause post-infectious loss, and over 200 different viruses are known to cause upper respiratory tract infections. It is therefore perhaps no surprise that the new COVID-19 virus can also cause anosmia in infected patients.” Anosmia can also be triggered by neurological conditions, head trauma, nasal polyps, sinusitis or allergies, so these factors should be ruled out before declaring them a symptom of COVID-19.
The loss of smell may seem like a minor symptom, but patients with a loss of smell may experience significant damage to their quality of life, with increased feelings of frustration with their altered senses and a sense of longing for the return of their sense of smell. A recent study also found that depression and anxiety are positively associated with COVID-19 patients presenting with a decreased sense of smell and taste.
When COVID-19-related olfactory disturbances improve spontaneously, specific treatment may not be necessary. However, when the impairment persists beyond 2 weeks, it may be reasonable to consider treatment. The efficacy of available treatments for patients with COVID-19-related olfactory disorders is unknown, although treatments targeting post-infectious olfactory disorders may be potentially useful for COVID-19.
Olfactory training involves repeated and deliberate sniffing of a set of odours (corresponding to the primary odours) for 20 seconds each at least twice a day for at least 3 months (or longer if possible). Studies have shown an improvement in the sense of smell in patients with post-infectious olfactory disorders after olfactory training. Olfactory training may be considered for patients with persistent olfactory disorders related to COVID-19 infection because this therapy has a low cost and negligible adverse effects.
Oral and intranasal corticosteroids have been used to rule out an inflammatory component in patients with postinfectious olfactory disorders. However, corticosteroids are not currently recommended for individuals with postinfectious olfactory disorders because evidence of benefit is lacking and there is a potential risk of harm. Because of safety concerns, the administration of systemic corticosteroids for the routine management of acute COVID-19 is not recommended. In the absence of demonstrable inflammatory disease observed by endoscopy or imaging, initiation of corticosteroid treatment is unlikely to benefit post-COVID-19 olfactory complaints, as it does for other causes of post-infectious olfactory complaints. However, for patients who were using intranasal steroids before developing COVID-19 (e.g., for allergic rhinitis), such medication should be continued.
Other drugs that have shown promise in postinfectious olfactory disorders include intranasal sodium citrate, which is thought to modulate olfactory receptor transduction cascades; intranasal vitamin A, which may act to promote olfactory neurogenesis; and systemic omega-3s, which may act through neuroregenerative or anti-inflammatory means. The latter 2 drugs may serve as adjuvant therapies in olfactory training. However, to date, there is no evidence that these therapies are effective in patients with COVID-19-related olfactory disorders.
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