The External Auditory Canal

The external auditory canal (EAC) consists of a cartilaginous outer one-third and a bony inner two-thirds. Figure 3.2 shows its relationship with deeper structures. The entire length of the canal is a little over 3 cm from the external meatus to the TM. From the outside in, the outer third is directed slightly posteriorly, whereas the inner two-thirds is directed anteroinferiorly and has an anterior bony hump that occludes a view of the very front portion of the TM. Because of this curvature, the canal can best be straightened, as for eardrop insertion, by pulling the auricle backward. The skin of the external canal changes greatly from the outside in. In the outer cartilaginous segment, it is thick and contains hairs and numer-

Santorini Fissure Ear

Fig. 3.2 The external auditory canal and its relationships:

l. Cartilaginous part; 2. Parotid gland; 3. Bony part; 4. Lateral wall of epitympanum (attic); 5. Mastoid antrum; 6. Attic (epitympanum); 7. Temporomandibular joint fossa; 8. Facial, vestibular, and auditory nerves (VII and VIII); 9. Eustachian tube; 10. Bone of tympanic ring; 11. Fissure of Santorini in cartilage of external canal. (Source: Becker W, Naumann HH, Pfaltz CR. Ear, Nose, and Throat Diseases. Stuttgart: Thieme; 1994)

Fig. 3.2 The external auditory canal and its relationships:

l. Cartilaginous part; 2. Parotid gland; 3. Bony part; 4. Lateral wall of epitympanum (attic); 5. Mastoid antrum; 6. Attic (epitympanum); 7. Temporomandibular joint fossa; 8. Facial, vestibular, and auditory nerves (VII and VIII); 9. Eustachian tube; 10. Bone of tympanic ring; 11. Fissure of Santorini in cartilage of external canal. (Source: Becker W, Naumann HH, Pfaltz CR. Ear, Nose, and Throat Diseases. Stuttgart: Thieme; 1994)

The Middle Ear 15

ous glands, both sebaceous and cerumen-producing. These three elements exist together as "apopilosebaceous" units. The skin lining the inner bony two-thirds is very thin, with little hair and no glands, but it is well-innervated and very sensitive to touch, in contrast to the skin of the auricle. The bony portion of the external canal is also known as the tympanic ring.

The "fissures of Santorini" (discontinuities in the cartilage of the outer canal) enable infection or neoplasm to easily spread down and forward into the adjacent parotid gland. Hence, a patient with a severe external otitis can develop cellulitis and parotitis adjacent to the ear. The outer third of the canal can be very narrow in width in some patients, limiting good access to the inner two-thirds and the TM. However, these individuals usually have normal-caliber bony canals further in. In general, the external canal is about 9 mm in height and 7 mm in width (due to the anterior bony hump).

The EAC is innervated by branches from four nerves: cranial nerves V (anteriorly), VII, IX, and X. The last three, innervating most of the canal, send their branches into the brainstem via the nervus intermedius and Arnold's nerve. The relays of Arnold's nerve near the nucleus ambiguus explain why stimulation of the canal during cleaning will often produce a cough reflex.

It is significant that other areas innervated by these four cranial nerves can transmit referred pain to the ear. Examples include post-tonsillectomy otalgia (ear pain) via cranial nerve IX or, more ominously, otalgia from malignancies in the tonsil, hypopharynx, or supraglottic larynx via cranial nerves IX and X. Several years ago I was referred a patient whose only complaint was a left earache, not even during swallowing. On laryngoscopy, he had a massive squamous cell carcinoma of the left hypopharynx! Finally, be advised that otalgia may even be referred from organs in the chest— disease of the heart, lungs, great vessels, and esophagus may all cause ear pain via the vagus nerve (X).

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