Cranial Nerves I – The Olfactory Nerve and Its Disorders

The Olfactory nerves which subserve the sense of smell have their cells of origin in the mucous membrane of the upper and posterior part of the nasal cavity. They are bipolar sensory cells, the distal portions of which consist of ciliated processes which penetrate the mucous membrane in the upper portion of the nasal cavity. The central processes of these nerves, approximately twenty on each side, pass through the cribiform plate of the ethmoid cells, the dendrites of which form brush-like terminals of olfactory glomeruli. The axons of the mitral cells enter the Olfactory groove of the frontal bones to the cerebrum. Posteriorly, the olfactory tract divided into medial and lateral olfactory striae. The medial straie passes to the opposite side through the anterior commisure. Fibers in the lateral striae give off collaterals to the anterior perforated substance and terminate in the medial and cortical nuclei of the amygdaloid complex and prepiriform area. The latter represents the primary olfactory cortex.

Testing: Before evaluating Olfactory sensations, one must ascertain that the nasal passages are not blocked. Local lesions such as allergic rhinitis, plyps and sinusitis which impair olfaction have to be excluded. The test substance should be non-irritant and volatile. Freshly ground coffee power, asafoetida, eucalyptus oil or lemon oul are some of the common test substances used. Substances like chloroform which may stimulate gustatory end-organs or the peripheral endings of the trigeminal nerves in the nasal mucosa instead of stimulating the Olfactory nerves, should be avoided. Each nostril should be tested separately with the other nostril being occluded. The patient should be asked to inhaled and identify the test substance. The perception of odor is more important than identification.

Disorders of Olfaction

1. Quantitative: Loss (anosmia), reduction (hyposmia) or increased acuity (hyperosmia).

2. Qualitative: Distortion of smell (dysosmia or parosmia)

3. Illusions and hallucinations of smell.

Anosmia occurs only if the disease is bilateral. Head injury, Olfactory groove meningioma and aneurysms of anterior cerebral or anterior communicating artery are some of the causes of unilateral anosmia. Hyperosmia is usually a feature of neurotic patients. Dysosmia or parosmia may occur with local abnormalities in the Nose. Olfactory hallucinations and delusions usually suggest psychiatric disorders. However, they may be associated with uncinate seizures where the olfactory experience is brief and accompanied by an alteration of consciousness and other epileptic phenomena.

The Olfactory nerve is one of the 12 Cranial nerves of the human body and it is very important if its function and disorders are known.