By: Varsha Kumari
Basic anatomy:
The olfactory nerve is the first cranial nerve that originates from the cell bodies of bipolar olfactory neurons in the olfactory epithelium, a specialized tissue that is present in the posterosuperior region of each nasal cavity. The apical portion of these cells has dendrites that project into the epithelial surface, where they interact with exogenous odoriferous particles via G protein-coupled receptors (Helwany & Bordoni, 2023). The axons of these bipolar neurons continue along the lamina propria, passing through the cribriform plate of the ethmoid bone and the subarachnoid space containing cerebrospinal fluid (CSF) and terminate on mitral cells in the olfactory bulbs (Masurkar & Chen, 2009). From the olfactory bulbs, some olfactory projections move medially to the septal area and the contralateral bulb via the anterior commissure, whereas other fibers travel laterally to the amygdala and piriform cortex, often referred to as the primary olfactory cortex, a sensory area where conscious odorant sense is interpreted (Sonne & Lopez-Ojeda, 2022).
Functions of the olfactory nerve:
The olfactory nerve has solely afferent sensory nerve fibers and, like other cranial nerves, is paired (Helwany & Bordoni, 2023). Its function is to mediate the perception of smell by transmitting sensory information from the nasal cavity to the central nervous system (Masurkar & Chen, 2009). Olfaction is the chemical sensation that arises from gaseous odorants, also referred to as the ability to smell. The olfactory nerve (CN1) coordinates with other neuroanatomical components in the cerebral cortex, nasal passages, and neurotransmitters to carry out this complex chemosensory function. Olfaction in humans has a profound connection to other intricate activities such as gustation (taste) and involuntary memory formation (Branigan & Tadi, 2023).
It also has a significant impact on animal behavior and communication. Various species rely on olfaction to identify resources for nutrition, mates, poisons, predators, and potential threats (Branigan & Tadi, 2023).
Complications that can result due to damage:
Olfactory disorders can be classified as congenital or acquired. However, the prior condition is rare.(Thomas et al., 2020) Damage to the CN1 results from local upper respiratory tract infection or brain lesions resulting from head trauma and neurodegenerative diseases (Boesveldt et al., 2017; Thomas et al., 2020).
They can be categorized quantitatively into three groups: hyperosmia, hyposmia, and anosmia (Thomas et al., 2020).
Anosmia: the inability to perceive the sense of smell, which roughly affects 3–20% of the population (Boesveldt et al., 2017), possibly due to mechanical obstruction that results from mucus plugging and nasal polyps (Li & Lui, 2023).
Hyposmia: reduced ability to smell.
Hyperosmia: enhanced ability to smell, a relatively rare condition.
Qualitatively, they are categorized as phantosmia and parosmia
- Parosmia: an altered perception of an odor after stimulus presentation
- Phantosmia: perception of odors without a stimulus, a form of olfactory hallucination (Thomas et al., 2020).
Complications are closely related to olfaction’s three key functions: (1) ingestion, (2) hazard avoidance, and (3) social communication. Odors act as alerts, such as when sensing natural gas or hygiene-related domains, and dysfunction could lead to safety problems (Thomas et al., 2020).
Anosmia by itself is unlikely to cause any complications. However, the inability to identify harmful smells can be life-threatening (Li & Lui, 2023).
Moreover, it can be an early indication for developing neurological illnesses such as Parkinson’s disease or Alzheimer’s disease, requiring prompt medical evaluation. It raises the risk of malnutrition and has been linked to higher mortality rates among older adults (Boesveldt et al., 2017).
Diagnosis:
To diagnose anosmia, a complete history and physical examination, followed by oronasal smell identification tests such as SS or UPSIT, should be utilized as the first-line method for diagnosing anosmia (Saltagi et al., 2021). General examination of the external nose is required for evidence of trauma and visible nasal deformity, as well as an inspection of the entrance of the nasal cavity, which may reveal nasal discharge, a deviated nasal septum, large nasal polyps or masses, and foreign bodies. Moreover, a neurological examination should be conducted according to the patient’s history. In general, a cranial nerve evaluation that focuses on the optic, trigeminal, and face nerves is helpful. Imaging will often be considered in secondary care and determined by the suspected etiology. For example, conductive causes like nasal polyps may necessitate a CT scan. In situations where no reason can be detected or a sensorineural cause is suspected, magnetic resonance imaging (MRI) may be used to rule out intracranial disorders such as anterior cranial fossa tumors or demyelinating illnesses (Deutsch et al., 2021).
Management:
As most cases are related to primary nasal pathologies such as chronic rhinosinusitis, allergic rhinitis, or acute sinusitis; the mainstay of treatment for such patients is intra-nasal corticosteroids (Deutsch et al., 2021).
For post viral anosmia, intranasal and oral corticosteroids, as well as other remedies, are preferred (Rashid et al., 2021).
In cases of idiopathic causes, several different dietary supplements such as zinc, alpha lipoic acid, vitamin A, and omega 3 are recommended, However, poor efficacy is reported (Deutsch et al., 2021).
Conclusion: The olfactory nerve is the first paired cranial nerve, which performs the function of transmitting olfactory information to perceive conscious sensation of the sense of smell. Various olfactory disorders can occur due to damage arising from local infection to complex neurogenerative diseases that can be diagnosed through imaging modalities along with history and clinical examination. Intra-nasal/oral corticosteroids serve as the mainstay of treatment.
References:
Boesveldt, S., Postma, E. M., Boak, D., Welge-Luessen, A., Schöpf, V., Mainland, J. D., Martens, J., Ngai, J., & Duffy, V. B. (2017). Anosmia—A Clinical Review. Chemical Senses, 42(7), 513. https://doi.org/10.1093/CHEMSE/BJX025
Branigan, B., & Tadi, P. (2023). Physiology, Olfactory. StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK542239/
Deutsch, P. G., Evans, C., Wahid, N. W., Amlani, A. D., & Khanna, A. (2021). Anosmia: an evidence-based approach to diagnosis and management in primary care. The British Journal of General Practice, 71(704), 135. https://doi.org/10.3399/BJGP21X715181
Helwany, M., & Bordoni, B. (2023). Neuroanatomy, Cranial Nerve 1 (Olfactory). StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK556051/
Li, X., & Lui, F. (2023). Anosmia. The Journal of Laryngology, Rhinology, and Otology, 15(11), 579–581. https://doi.org/10.1017/S1755146300168532
Masurkar, A. V., & Chen, W. R. (2009). Olfactory Bulb Physiology. Encyclopedia of Neuroscience, 77–86. https://doi.org/10.1016/B978-008045046-9.01687-9
Rashid, R. A., Zgair, A., & Al-Ani, R. M. (2021). Effect of nasal corticosteroid in the treatment of anosmia due to COVID-19: A randomized double-blind placebo-controlled study. American Journal of Otolaryngology, 42(5), 103033. https://doi.org/10.1016/J.AMJOTO.2021.103033
Saltagi, A. K., Saltagi, M. Z., Nag, A. K., Wu, A. W., Higgins, T. S., Knisely, A., Ting, J. Y., & Illing, E. A. (2021). Diagnosis of Anosmia and Hyposmia: A Systematic Review. Allergy & Rhinology, 12. https://doi.org/10.1177/21526567211026568
Sonne, J., & Lopez-Ojeda, W. (2022). Neuroanatomy, Cranial Nerve. StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK470353/
Thomas, D. C., Baddireddy, S. M., & Kohli, D. (2020). Anosmia: A review in the context of coronavirus disease 2019 and orofacial pain. Journal of the American Dental Association (1939), 151(9), 696. https://doi.org/10.1016/J.ADAJ.2020.06.039
