Olfactory Nerve; Nose, Olfaction
The nose is the first part of the airway, and in it inspired air is humidified warmed and smelt. It comprises two arched, narrow cavities situated above the palate and below the anterior and middle cranial fossae, and extending from the anterior nares to the nasopharynx. The cavities are separated by the midline septum and they receive the openings of the paranasal air sinuses on each side.
The olfactory epithelium is situated at the upper part of each cavity, where 15–20 olfactory nerves, surrounded by individual dural sheaths, pass through the cribriform plate of the ethmoid bone to the olfactory bulb.
Defective fusion of the frontonasal and maxillary processes may produce nasal deformity, and medial and lateral dermoid cysts over the bridge of the nose and lateral to the orbit. A number of uncommon inflammatory conditions may produce nasal deformity. These include Wegener’s granulomatosis, and infection with syphilis (congenital and acquired), tuberculosis and nocardia. The nose is commonly damaged by direct trauma, and may be enlarged in acromegaly and myxedema, or become reddened or enlarged in alcoholism. The commonest cause of nasal obstruction and discharge is infection, such as a cold, but foreign bodies, polyps and septal deviation are frequently encountered, and occasionally neoplasms. Epistaxis (bleeding nose) may follow minor trauma; the bleeding point is usually anterior, particularly over the nasal septum (Little’s area). High facial fractures may extend into the anterior cranial fossa. Leaking CSF may be distinguished from other clear nasal drips by its sugar content, and when mixed with blood, by a separate diffusion ring on filter paper.
The anterior nares are examined with a nasal speculum (figure 3). This is introduced anteroposteriorly to avoid stimulating the sensitive nasal septum. With the use of a head mirror, light may be directed into the anterior nares to observe the inferior concha, the anterior nasal cavity and the nasal septum. Bleeding points may be detected and cauterized.
Nasal endoscopes are used to examine the interior of the nose and the openings of the nasal sinuses.
The posterior nares and adenoids may be observed, using a nasal endoscope or head mirror and a small, long stemmed angled mirror placed behind the palate.
Apply pressure (gently at first) over the frontal (medial eyebrow) and maxillary (cheek) air sinuses, to look for tenderness due to underlying inflammation.
The olfactory nerves of each side are tested in turn by compressing the contralateral nostril and applying preparations such as cloves, peppermint and a fetid odor (figure 4a,b). Pungent preparations, such as ammonia, should be avoided, as these also stimulate the fifth nerve and the signs may be misinterpreted. More sophisticated tests require dilution of the odorant to determine threshold levels. Loss of smell (anosmia) markedly effects the appreciation of food.