Just before 2020 began, a novel coronavirus, severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), brought for humans a potentially fatal disease known as coronavirus disease 2019 (COVID-19). The world has thoroughly been affected by COVID-19, while there has been little progress towards understanding the pathogenesis of COVID-19. Patients with a severe phenotype of disease and those who died from the disease have shown hyperinflammation and were more likely to develop neurological manifestations, linking the clinical disease with neuroimmunological features. Anosmia frequently occurs early in the course of COVID-19. The prevalence of anosmia would be influenced by self-diagnosis as well as self-misdiagnosis in patients with COVID-19. Despite this, the association between anosmia and COVID-19 has been a hope for research, aiming to understand the pathogenesis of COVID-19. Studies have suggested differently probable mechanisms for the development of anosmia in COVID-19, including olfactory cleft syndrome, postviral anosmia syndrome, cytokine storm, direct damage of olfactory sensory neurons, and impairment of the olfactory perception center in the brain. Thus, the observation of anosmia would direct us to find the pathogenesis of COVID-19 in the central nervous system, and this is consistent with numerous neurological manifestations related to COVID-19. Like other neurotropic viruses, SARS-CoV-2 might be able to enter the central nervous system via the olfactory epithelium and induce innate immune responses at the site of entry. Viral replication in the nonneural olfactory cells indirectly causes damage to the olfactory receptor nerves, and as a consequence, anosmia occurs. Further studies are required to investigate the neuroimmunology of COVID-19 in relation to anosmia.

中文翻译：

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厌食症：2019年冠状病毒疾病（COVID-19）的神经免疫学中缺少一个环节。

就在2020年之前，一种新型冠状病毒即严重急性呼吸系统综合症冠状病毒2（SARS-CoV-2）为人类带来了一种潜在的致命疾病，称为冠状病毒病2019（COVID-19）。世界已完全受到COVID-19的影响，而在了解COVID-19的发病机理方面进展甚微。患有严重疾病表型的患者和死于该疾病的患者已表现出过度炎症，并且更有可能出现神经学表现，将临床疾病与神经免疫学特征联系起来。厌食症经常发生在COVID-19的早期。患有COVID-19的患者的自我诊断以及自我误诊会影响到贫血症的患病率。尽管如此，失眠症与COVID-19之间的关联仍是研究的希望，旨在了解COVID-19的发病机理。研究表明，在COVID-19中发生嗅觉异常的机制可能不同，包括嗅觉裂综合征，病毒后嗅觉异常综合征，细胞因子风暴，嗅觉感觉神经元的直接损伤以及大脑嗅觉感知中心的损伤。因此，对失眠症的观察将指导我们寻找中枢神经系统中COVID-19的发病机制，这与与COVID-19相关的众多神经学表现是一致的。像其他神经病毒一样，SARS-CoV-2可能能够通过嗅觉上皮进入中枢神经系统，并在进入位点诱导先天免疫应答。病毒在非神经嗅觉细胞中的复制会间接导致嗅觉受体神经受损，因此，发生失眠。需要进行进一步的研究以调查COVID-19与失眠相关的神经免疫学。