Anatomy of the olfactory nerve

1. Big picture

The olfactory nerve, cranial nerve I (CN I), is the nerve of smell. In the neurology exam, the examiner mainly wants you to know:

1. The anatomical pathway from nasal mucosa to brain.

2. Why olfactory lesions cause anosmia/hyposmia.

3. Why temporal lobe disease can cause olfactory hallucinations.

4. How to examine CN I correctly.

5. The important trap: do not test smell with ammonia, because ammonia stimulates the trigeminal nerve, not pure olfaction.

CN I is clinically important because smell disturbance can be caused by simple nasal disease, but also by head trauma, frontal basal tumors, temporal lobe epilepsy, neurodegenerative disease, and raised intracranial pressure syndromes.