First Cranial Nerve Lesions

Anatomy

Course of the Olfactory Nerve

The olfactory nerves are paired structures.

Olfactory nerve fibres pass from the mucous membranes of the nasal cavity upward through the cribriform plate of the ethmoid bone, to join the olfactory bulb within the anterior cranial fossa.

The olfactory bulb, lying on the cribriform plate, passes backwards to form the olfactory tract, which runs beneath the frontal lobes to enter the cerebrum via medial and lateral olfactory stria.

Olfactory Nerve Innervations

The olfactory nerves carry sensory fibres to the brain, which convey the sense of smell.

Pathology

Cause	Comments
Trauma	Most common cause. Direct injury to olfactory nerves, often from trauma to the anterior cranial fossa.
Space-occupying lesions	Tumours, rarely aneurysms or abscesses affecting the nasal cavity or anterior cranial fossa.
Mononeuritis	Various causes, e.g. diabetes, toxins, microvascular disease, paraneoplastic disease, connective tissue disease, infections (HIV, Lyme disease, syphilis).
Idiopathic	In some cases, no clear cause is found.

Differential Diagnoses of Loss of Sense of Smell

Condition	Comments
Seizure activity	May result in altered perception of smell.
Upper respiratory tract infections	Common cause of transient anosmia due to inflammation and excess mucus affecting olfactory function.

Factors That Reduce Sensitivity of Smell

Factor	Comments
Relationship of smell to taste	Flavour perception is largely olfactory; this explains why food tastes bland during a cold.
Adaptation	Prolonged exposure to strong odours can reduce perception.
Age	Sensitivity declines with age.
Smoking	Reduces olfactory function.
Some drugs	Certain medications can reduce smell sensitivity in some individuals.

Clinical Assessment

• Anosmia = complete loss of smell.
• Hyposmia = reduced sensitivity to smell.

Most cases of anosmia are bilateral.

• The First Cranial Nerve is not routinely tested when examining cranial nerves.
• If anosmia is reported, a neurologist may perform formal testing with familiar smells (coffee, vanilla, peppermint).
• Pungent odours (e.g. ammonia) are avoided as they stimulate Trigeminal nerve (CN V) fibres.

If anosmia is suspected, clinicians should look for:

• Signs of intracranial mass lesions.
• Signs of frontal lobe dysfunction.

Investigations

Blood Tests

According to clinical suspicion, consider:

Test	Purpose
FBC	General screen.
U&Es / Glucose	Assess metabolic causes.
CRP	Inflammation.
ESR	Inflammation.
Other tests as indicated	e.g. blood lead levels.

Imaging

Modality	Purpose
CT / CT angiogram	Good initial screening for mass lesions; CT angiogram for suspected aneurysms.
MRI	Most sensitive for detecting lesions affecting the olfactory nerve and for visualising the nerve itself.

Management

Management is directed at the underlying cause, where identified.

Olfactory Neuron Regeneration

The olfactory neurons are unique in that they can regenerate after injury.

• This provides potential for temporary rather than permanent anosmia in some cases.

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References

Publications

• Brazis PW, Masdeu JC, Biller J. Localization in Clinical Neurology. 8e 2021
• Fuller G. Neurological Examination Made Easy. 6e 2019
• O’Brien M. Aids to the Examination of the Peripheral Nervous System. 6e 2023

FOAMed

• Coni R. Neuro 101: Cranial Nerves. LITFL
• Nickson C. Cranial nerve lesions DDx. LITFL