The olfactory nerve subserves the sense of smell. Smell
receptors are situated high in the nasal cavity, and these bipolar olfactory cells project axons through the cribriform plate of the ethmoid bone to the olfactory bulb. Second-order neurones (mitral cells) then project centrally together with the pyramidal cells of the olfactory nucleus down the olfactory tracts and stria to the primary olfactory cortex (medial temporal lobe) and the ipsilateral amygdaloid body, (lie anterior perforated substance and the septal area. The primary olfactory cortex has numerous connections with the hypothalamus, reticular system and the limbic lobe.
Commonly used test substances include vials of peppermint, vanilla, coffee, almond oil or even stronger odours such as whisky or isopropyl alcohol used for skin cleaning.
□ Check that the nasal passages are clear.
□ Test the .sense of smell for each nostril separately, n Occlude one nostril by digital pressure-
□ Ask the patient, with eyes closed, to sniff and identify in turn the test substances.
The commonest cause of anosmia is obstruction of the nasal passages. The commonest neurological cause of anosmia is a previous head injury causing shearing damage to the olfactory filaments as they pass through the cribriform plate to the olfactory bulbs. Nasofrontal tumours such as olfactory groove meningiomas and carcinomas arising from the paranasal air sinus are less common causes.
Perversion of smell (parosmia) is sometimes of psychological origin but may occur following partial recovery of the olfactory nerves after trauma. Certain drugs and sinus infection also cause this phenomenon. Olfactory hallucinations, usually of an unpleasant nature, are a characteristic of seizures arising from the temporal lobe. These olfactory hallucinations are often associated with smacking movements of the lips and unusual feelings in the epigastrium.
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