From the Guidelines
The olfactory pathway is a complex sensory system that enables us to perceive and process smells, starting from the binding of odor molecules to olfactory receptors in the nasal cavity and culminating in the processing of information in various brain regions, including the piriform cortex, amygdala, and entorhinal cortex, as described in the 2022 update of the ACR Appropriateness Criteria for cranial neuropathy 1. The olfactory pathway begins when odor molecules enter the nasal cavity and dissolve in the mucus covering the olfactory epithelium, where they bind to olfactory receptors on specialized neurons called olfactory receptor neurons.
- These molecules trigger a response that generates action potentials in the olfactory receptor neurons, which travel along their axons to form the olfactory nerve (cranial nerve I) 1.
- The olfactory nerve fibers then pass through small openings in the cribriform plate of the ethmoid bone to enter the cranial cavity, where they synapse in the olfactory bulb.
- In the olfactory bulb, the information is initially processed in structures called glomeruli, and from there, secondary neurons project via the olfactory tract to several brain regions, including the piriform cortex (primary olfactory cortex), amygdala, and entorhinal cortex, as noted in the study published in the Journal of the American College of Radiology 1. The unique aspect of the olfactory pathway is that it bypasses the thalamus before reaching the cortex, making it a direct sensory pathway that can trigger immediate emotional responses and vivid memories.
- This direct route is thought to be responsible for the powerful emotional and memory-related effects of smells, and is supported by the findings of the 2022 study on cranial neuropathy 1.
- The olfactory pathway is also closely linked to other sensory systems, including the sense of taste, and can be affected by a range of factors, including trauma, aging, and neurodegenerative diseases, as discussed in the study published in 2022 1.
From the Research
Olfactory Pathway Overview
- The olfactory pathway begins with the binding of airborne odour molecules to olfactory receptors at the top of the nasal cavity, giving rise to our rich olfactory experience 2
- Nasal airflow plays a role in human olfactory perception beyond the transportation of odorants, with the perceived intensity of a unilaterally presented odour decreasing systematically with the amount of contralateral nasal airflow 2
Central Olfactory Processing
- Nasal flow spontaneously engages central olfactory processing and serves as an integral part of the olfactory percept in humans 2
- The effect of nasal flow on olfactory perception is due to the sensed rather than the factual amount of nasal flow, as applying a local anaesthetic to the contralateral nostril produces the same effect as physically blocking it 2
Olfactory Dysfunction and Treatment
- Postviral olfactory dysfunction (PVOD) is a clinical challenge due to limited therapeutic options and poor prognosis, but treatments including topical steroids, systemic steroids, classical olfactory training (COT), and modified olfactory training (MOT) have been shown to be effective 3
- Direct injection of steroid or nasal steroid spray into the olfactory cleft significantly improved the olfactory function in patients with PVOD, and olfactory improvement is greater than that of the natural course of the disease with short-term COT 3
- Classical olfactory training can be a promising intervention for the recovery of olfactory function in individuals with olfactory dysfunction, with improvements in at least 20%-30% of patients affected by different etiologies of smell loss 4
- Corticosteroid therapy has been shown to be effective in the treatment of long-lasting olfactory disorders in COVID-19 patients, with significant improvements in olfactory function compared to untreated controls 5
- Treatment with corticosteroids and olfactory training has been shown to be promising in persistent posttraumatic olfactory dysfunction, with significant improvements in threshold, discrimination, and identification (TDI) scores 6