In recent weeks, there have been several reports of the loss of smell and taste as possible symptoms of the coronavirus (COVID-19). The evidence is anecdotal at this point, as the virus has not been a concern long enough for formal study (although many are underway, albeit not of this possible characteristic, to the best of my knowledge). However, the consistency with which these symptoms occur indicates formal investigation is necessary.
Sensory Disruptions: A Clue to COVID-19 Infection?
An important clue that the loss of smell (anosmia) and taste (ageusia) are signs of COVID-19 is the report of the losses from multiple countries. Cases in France, Italy, South Korea, Britain, Germany and the U.S. have shown similar patterns. A brief survey in South Korea revealed that 32 percent of 2,000 COVID-19 patients suffered losses of these two senses.1
Victims abruptly lose the ability to smell and taste pungent foods like garlic and onions. Other examples include the inability to detect odors from items such as fuel oil and dirty diapers.1 These occurrences are being identified in multiple age groups.
The losses of smell and taste are preceding other COIVD-19 symptoms by days. Doctors concerned by this pattern are recommending immediate quarantining of patients reporting the losses.
Viruses similar to COVID-19 (SARS-CoV and MERS-CoV) are known to infect the central nervous system. A study in 2008 showed that SARS-CoV entered the brain via the bulb of the first cranial nerve. A theory has been proposed that COVID-19 is following the same path, leading to infection of the medulla area of the brain and respiratory control centers.2
Proof of the relationships here would require studying the brains of COVID-19 fatalities during autopsies. Unfortunately, autopsies have focused primarily on the lungs so far and have been limited due to fear of spreading the virus.2
Until formal investigation leads to more concise information, it is an excellent idea to be mindful of these early symptoms. Practitioners should refamiliarize themselves with the anatomy, function, testing and pathologies related to the senses of smell and taste.
The Role of the Cranial Nerves
The sense of smell is conveyed through the first cranial nerve, the olfactory nerve. The olfactory nerve is a sensory nerve with one function: the sense of smell. The nerve originates from the anterior perforated substance and the uncus of the brain. It terminates above the ethmoid bone, where it sends fibers through the cribriform plate into the nasal cavity.3-4
The sense of taste is conveyed through the seventh cranial nerve, the facial nerve, and the ninth cranial nerve, the glossopharyngeal nerve. The facial nerve originates from the brain stem. It has motor and sensory function. The motor portion terminates primarily in facial muscles, while the sensory portion terminates in the tongue and other visceral structures, such as the lacrimal, submandibular and sublingual glands.3-4
The ninth cranial nerve originates from the medulla. It also has motor and sensory functions. The motor portion terminates in the stylopharyngeus muscle, while the sensory portion terminates in the tongue and other visceral structures, such as the parotid gland, tympanic membrane, carotid sinus and others.3-4
The seventh cranial nerve conducts taste from the anterior two-thirds of the tongue, while the ninth cranial nerve conducts taste from the posterior third of the tongue.3-4
Tests to identify smell and taste dysfunction are seldom performed. There are two reasons for their frequent omission in medicine: 1) Testing is awkward. 2) The loss of smell and taste is prevalent, almost to the point of being normal variants.5
Testing in Chiropractic Practice
The most common reasons for loss of the abilities to smell and taste are advancing age, common colds, the flu and allergies. Because of the frequency of benign loss, Current Procedural Terminology Evaluation and Management guidelines for neurological examination do not include testing of the first cranial nerve.6
The seventh and ninth cranial nerves are tested, but the motor portion of the seventh is the priority, as are the functions of the ninth cranial nerve that work in conjunction with the 10th cranial nerve.
The above reasons are also applicable to chiropractic practice, and a third reason specific to chiropractic is mentionable. Olfactory and glossopharyngeal nerve pathology is generally not related to pathologies amenable to chiropractic care.
However, considering the current public health crisis, chiropractors and practitioners in all fields should be mindful of dysfunctions related to smell and taste.
Testing for smell involves the patient holding one nostril closed with the eyes shut. A substance is then held a few inches from the open nostril. The patient is asked to identify the substance by its odor. The process is repeated on the opposite side. Tobacco, coffee and lemon extract are commonly used substances.7
Testing for taste involves the patient closing the eyes while holding the mouth open. A liquid substance is dripped onto the tongue with an eyedropper. The patient is asked to identify the substance by its taste.7
For practical purposes, assessing smell and taste is easiest to perform through patient history. The examiner can simply ask patients how their sense of smell and taste, are and if the senses have recently changed. Further investigation, as described above, can follow if necessary.
Two rather unexpected dysfunctions are emerging as probable signs of the coronavirus. These usually benign dysfunctions are now of grave concern. The need for practitioners to return to their early training regarding the assessment of smell and taste should be obvious.
References
- Rabin RC. Lost Sense of Smell May Be Peculiar Clue to Coronavirus Infection." The New York Times, March 22, 2020.
- Yeager A. "Lost Smell and Taste Hint COVID-19 Can Target the Nervous System." The Scientist, March 24, 2020.
- Wilson-Pauwels A, Decker S. Cranial Nerves, Anatomy and Clinical Comments. London, 1988.
- Moore K. Clinically Oriented Anatomy, 3rd Edition. Baltimore: Williams and Wilkins, 1992.
- Goldberg S. The Four-Minute Neurological Exam. Miami: Medmaster, 1999.
- CPT 2018 Professional. Chicago: American Medical Association, 2018.
- DeMyer WE. Technique of the Neurologic Examination, 4th Edition. New York: McGraw-Hill, 1994.
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