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Table of Contents
LETTER TO EDITOR
Year : 2021  |  Volume : 70  |  Issue : 4  |  Page : 262-263

An anatomical model for SARS-CoV-2 entry into mastoid and middle ear in COVID-19 patients


1 Department of Anatomy, Postgraduate Institute of Medical Education and Research, Chandigarh, India
2 Department of Virology, Postgraduate Institute of Medical Education and Research, Chandigarh, India
3 Department of Anatomy, All India Institute of Medical Sciences, Patna, Bihar, India

Date of Submission05-Mar-2021
Date of Acceptance31-Oct-2021
Date of Web Publication21-Dec-2021

Correspondence Address:
Dr. Ashutosh Kumar
Department of Anatomy, All India Institute of Medical Sciences, Patna, Bihar
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jasi.jasi_45_21

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How to cite this article:
Kumari C, Ishani B, Ravi K N, Kumar A. An anatomical model for SARS-CoV-2 entry into mastoid and middle ear in COVID-19 patients. J Anat Soc India 2021;70:262-3

How to cite this URL:
Kumari C, Ishani B, Ravi K N, Kumar A. An anatomical model for SARS-CoV-2 entry into mastoid and middle ear in COVID-19 patients. J Anat Soc India [serial online] 2021 [cited 2022 Jan 21];70:262-3. Available from: https://www.jasi.org.in/text.asp?2021/70/4/262/333194



Dear Editor,

Recently, Frazier et al. detected SARS-CoV-2 RNA in the PCR-based testing of postmortem biopsy specimens from the middle ear and mastoid cavity of two out of three COVID-19 patients.[1] Detection of SARS-CoV-2 in the deeper parts of the ear is a finding of utmost clinical significance and deserves extensive deliberation. However, how did SARS-CoV-2 reach these parts of the ear, which are not directly exposed to the external environment, is intriguing and deserves an anatomical explanation. In this letter, we suggest an anatomical model which can effectively explain the route of the viral entry from the nose to the middle ear and to the mastoid.

To invade and replicate into a human tissue, SARS-CoV-2 binds to a human cell surface receptor called angiotensin-converting enzyme 2 (ACE-2) through the receptor-binding domain (RBD) present at its spike (S) protein.[2] For the successful host cell membrane fusion and infectivity, following ACE-2 binding, cleavage of the viral spike protein (S) by the proteases like transmembrane serine protease-2 (TMPRSS-2) is essential.[2] Recent studies showed enriched expression of ACE-2 (and also TMPRSS-2) in the epithelial lining of the upper respiratory tract including the nose in humans.[3] Similar evidence has been found for the epithelial lining of the middle ear and mastoid cavities in a recent mouse study.[4]

The nasopharynx, from where swab is collected for viral testing in COVID-19, is anatomically unique in the sense that it presents a common meeting place for the ear, nose, and mouth cavities.[5] The middle ear opens into the lateral wall of the nasopharynx through the “Eustachian tube (ET)” – an osteocartilaginous canal which is about 36 mm in length[5] [Figure 1]. ET has important functions of draining the mucosal secretions from the middle ear cavity to the throat and maintaining the air pressure in the middle ear cavity allowing the controlled passage of the breathing air through the slit-shaped nasopharyngeal opening of the tube.[5] The middle ear is further connected to the mastoid cavity through a very short passage called aditus[5] [Figure 1]. ET has a lining of the respiratory epithelium, and the middle ear and mastoid cavities have simple squamous secretory epithelium, which may allow them to harbor SARS-CoV-2, owing to abundant expression of the viral cell entry factors.[3],[4] The nasopharynx has a mucosal continuity with the middle ear and mastoid cavity through the ET that presents a convenient route for the spread of the virus to the deep auricular parts [Figure 1].
Figure 1: A model depicting anatomical route for transmucosal spread of SARS-CoV-2 from the nasopharynx to the middle ear and mastoid cavities. [There is an anatomical continuity from the nasopharynx to the middle ear and then to the mastoid through the Eustachian tube. This route is initially lined by the mucosal respiratory (nasopharynx to the Eustachian tube) epithelium and then secretory simple squamous (middle ear and mastoid) epithelium which express angiotensin-converting enzyme 2 receptors that can bind SARS-CoV-2, hence presents a transmucosal route of spread for entry of the virus while inhaling air through the nose].

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  References Top

1.
Frazier KM, Hooper JE, Mostafa HH, Stewart CM. SARS-CoV-2 virus isolated from the mastoid and middle ear: Implications for COVID-19 precautions during ear surgery. JAMA Otolaryngol Head Neck Surg 2020;146:964-6.  Back to cited text no. 1
    
2.
Hoffmann M, Kleine-Weber H, Schroeder S, Krüger N, Herrler T, Erichsen S, et al. SARS-CoV-2 cell entry depends on ACE2 and TMPRSS2 and is blocked by a clinically proven protease inhibitor. Cell 2020;181:271-80.e8.  Back to cited text no. 2
    
3.
Sungnak W, Huang N, Bécavin C, Berg M, Queen R, Litvinukova M, et al. SARS-CoV-2 entry factors are highly expressed in nasal epithelial cells together with innate immune genes. Nat Med 2020;26:681-7.  Back to cited text no. 3
    
4.
Uranaka T, Kashio A, Ueha R, et al. Expression of ACE2, TMPRSS2, and Furin in Mouse Ear Tissue, and the Implications for SARS-CoV-2 Infection. Laryngoscope 2021;131: E2013-E2017. doi:10.1002/lary.29324.  Back to cited text no. 4
    
5.
Bluestone CD, Doyle WJ. Anatomy and physiology of eustachian tube and middle ear related to otitis media. J Allergy Clin Immunol 1988;81:997-1003.  Back to cited text no. 5
    


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