Olfactory nerve (CN I)
Function and clinical significance
The olfactory nerve mediates the sense of smell. Loss or reduction of smell (anosmia or hyposmia) is the commonest cranial nerve deficit encountered in practice. In the ED it matters for three reasons:
- It may be the only sign of skull‑base or intracranial injury after head trauma.
- It carries immediate safety risks (inability to detect smoke, gas, spoiled food).
- Permanent anosmia has medico‑legal and compensation implications.
Ask about smell as part of a focused cranial nerve examination (RCEM curriculum expectations) and document findings clearly (GOV.UK).
Anatomy and mechanisms of injury
Olfactory receptor neurons are located in the mucosa of the superior nasal cavity. Their unmyelinated axons traverse the cribriform plate of the ethmoid bone to synapse in the olfactory bulb and project to limbic structures (amygdala, hippocampus). The nerve is vulnerable to:
- Shearing of fibres at the cribriform plate after blunt force and rapid deceleration (classically after occipital impact/contrecoup).
- Direct anterior skull‑base fractures involving the cribriform plate.
- Local conductive obstruction from polyps, rhinosinusitis, or other mucosal disease.
- Intracranial compression (frontal lobe tumours), meningitis, or iatrogenic/toxic causes.