CN I is unique among cranial nerves — it is a direct extension of the brain (the olfactory bulb) rather than a peripheral nerve. The receptor cells pass directly through the cribriform plate foramina, making them vulnerable to cribriform plate fractures producing traumatic anosmia (the most common permanent neurological deficit after head injury). Olfactory neuroblastoma (esthesioneuroblastoma) arises from these receptor cells.
| Origin | Olfactory receptor neurons in the nasal olfactory epithelium (roof of nasal cavity) |
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Post-traumatic anosmia from cribriform plate fracture is permanent in 30% of cases. MRI shows olfactory bulb atrophy in chronic anosmia. Olfactory testing (University of Pennsylvania Smell Identification Test — UPSIT) objectively quantifies smell loss. COVID-19 anosmia from direct viral damage to olfactory epithelium is the most common recent cause of acquired anosmia.
Cribriform plate fracture shearing the olfactory nerve filaments producing permanent smell loss managed supportively with safety counselling regarding gas and smoke detection.
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