Introduction

The olfactory nerve (CN I) is a sensory nerve responsible for the sense of smell.

Injury causes anosmia (loss of sense of smell). This is the most common cranial nerve defect.

Anatomy

The olfactory nerve receptors are in the mucosa of the upper nasal cavity. From here, a large number of fine sensory fibres pass through cribriform plate in the ethmoid bone to the olfactory bulb and then on to the amygdala, hippocampus and other brain regions.

Testing

Ask the patient if they have any problems with their sense of smell. Each nostril can be tested formally using clove oil, though this is seldom done.

Investigation

All patients presenting with olfactory dysfunction, particularly following head trauma, should be considered for CT scan to identify possible causes, or associated injuries.

In trauma, scanning is used to identify associated intracranial injury such as extradural haemorrhage or brain contusions from the contrecoup injury.

In atraumatic cases, CT scanning may identify frontal lobe tumours or local nasal pathology.

Trauma

Trauma is the most common cause of olfactory nerve injury. The most common event is an occipital head injury with transient loss of consciousness. In such cases, the contrecoup injury causes shearing of the nerves against the cribriform.

Medical Causes of Damage

Medical causes of damage to the olfactory nerve include:

• Local nasal disease
• External pressure from tumour (frontal lobe)
• Meningitis