ANATOMY OF EXTERNAL AUDITORY CANAL

ANATOMY OF EXTERNAL AUDITORY CANAL

1. Anatomy of External Auditory Canal Dr Kanu Lal Saha Associate Professor Otology Division Department of Otolaryngology - Head and Neck Surgery Bangabandhu Sheikh Mujib Medical University, Dhaka

2. The external ear The external ear comprises Auricle ( pinna ) External auditory canal ( meatus )

3. Auricle (Pinna) Attaches to the side of the head at an angle of about 30 degrees. Its growth parallels overall body growth until approximately 9 years of age. Has a single plate of elastic cartilage measuring between 0.5 and 2.0 mm in thickness. Is continuous with the cartilage of the external meatus. Skin is thin and firmly attached to the cartilage, particularly on its lateral aspect, which contains little subcutaneous fat. Contains many hair-containing sebaceous glands, which are particularly abundant in the concha and the scaphoid fossa. Fissures of Santorini, which are not constant in number, extent, or course, are located in the tragal cartilage and filled by fibrous tissue.

4. Structure of Auricle(Pinna)

5. Lateral Surface of the Auricle Helix (or snail) - Is the prominent rim of the auricle. -A small tubercle where the helix turns inferiorly is called Darwin's tubercle. - Antihelix is the curved prominence parallel and anterior to the helix. Triangular fossa lies between the two crura of the antihelix. Scaphoid fossa (scoop or shovel) is a narrow, curved depression between the helix and the antihelix. Concha (shell) is partially divided by the crus of the helix into two portions: Upper ( cymba conchae) Lower (cavum conchae) Lies at an angle of 90 degrees (± 15 degrees) to the cortex of the mastoid bone.

6. Lateral Surface of the Auricle Tragus: Is the eminence projecting posteriorly over the external auditory canal. From the Greek word meaning goat, the tragus is so called because hair growth on its medial surface is said to resemble a goat's beard. Antitragus is located posteriorly, opposite the tragus, and is separated from it by the intertragic notch. Lobule Consists of fibrous and fatty tissues and is devoid of cartilage. A strong fibrous band connects the tragus to the beginning of the helix,completing the meatus anteriorly. The endaural Lempert incision passes through this fibrous band.

7. Muscles of the auricle

8. Blood supply of the Auricle Posterior auricular artery, a branch of the external carotid artery. Anterior auricular artery, a branch of the superficial temporal artery. A branch from the occipital artery. Venous drainage accompanying arteries

9. I nnervation of the ear 1.Auricular branch of CN IX 2. Auricular branch of CN X 3. G reater auricular nerve 4. L esser occipital nerve 5. Auriculotemporal branch of CN V 6. Inferior alveolar nerve 7. Lingual n. and chorda tympani n 8 . CN VII (facial nerve)

10. Nerve Supply of Auricle A temporal branch of CN VII supplies the anterior auricular muscle and the superior auricular muscle. The posterior auricular branch of CN VII supplies the posterior auricular muscle and the intrinsic muscles of the cranial aspect of the pinna. Sensory nerves of the auricle include

11. Lymphatic drainage of Auricle

12. THE EXTERNAL ACOUSTIC MEATUS (AUDITORY CANAL) The external acoustic meatus is a bony-cartilaginous canal extending from the concha of the auricle to the tympanic membrane Owing to the obliquity of the tympanic membrane, the posterosuperior wall of the meatus is slightly shorter than the anteroinferior wall (approximately 25 mm and 31 mm respectively). Its internal diameter is 7 mm by 9 mm, and it is elliptical and not circular. The cartilaginous portion is slightly concave towards anterior and the osseous portion towards posterior. Therefore, the canal has an italic “S” shape. Deep to the isthmus, the anteroinferior portion of the canal dips forwards forming a wedge-shaped anterior recess between the tympanic membrane and the canal. The EAC length is approx. 2.4 cm and volume is 15.4 cm3 in the adult. Two constrictions: one at the junction of the cartilaginous and bony portions and the other, the isthmus (narrowest), 5mm from the tympanic membrane where a prominence of the anterior canal wall reduces the diameter. The resonance of the external auditory canal in the adult is approximately 2.5–3 kilohertz (kHz). The EAC is covered by skin which is continuous with that of the auricle and also forms the external layer of the tympanic membrane.

13. THE EXTERNAL ACOUSTIC MEATUS (AUDITORY CANAL) EAC-Cartilaginous segment Measures 8 mm long and forms the lateral one-third of the canal. It Is continuous with the cartilage of the auricle and strongly attached to the bony canal. The medial border of the meatal cartilage is attached to the rim of the bony canal by fibrous bands. There are two horizontal fissures (of Santorini) that lie anteroinferiorly in the cartilaginous portion that possibly increase flexibility but allow passage of infection or tumour into the parotid gland. EAC-Bony Segment Measures 16mm long and forms the medial two-third of the canal. The anterior wall, floor and lower posterior wall formed by the tympanic portion of the temporal bone. Its roof and the upper part of the posterior wall are formed by the squamous portion. In infants, the bony canal is very short, and the tympanic membrane is more horizon- tally situated. Pneumatization of the bone is rare. Foramen of Huschke of the External Auditory Canal Is a dehiscence in the anteroinferior wall of the bony canal up-to 4 years of age . Infections may pass through it between the parotid gland and the meatus.

14. Skin of the Cartilaginous and Bony canal Skin of the Cartilaginous Canal: The skin is tightly bound to the perichondrium Thickness : 0.5–1 mm The skin has sebaceous and ceruminous glands as well as hair follicles in the superior and posterior portion Skin of the Osseous Canal: The skin is not firmly bound to the dermis Continuous with the external (lateral) aspect of the tympanic membrane It is thin (0.2 mm) No hair follicles or sebaceous glands

15. Relationships of the External Auditory Canal

16. Blood supply of EAC The arterial supply of the external meatus is derived from branches of the external carotid. The auricular branches of the superficial temporal artery supply the roof and anterior portion of the canal. The deep auricular branch of the first part of the maxillary artery arises in the parotid gland behind the temporomandibular joint pierces the cartilage or bone of the external meatus and supplies the anterior meatal wall skin and the epithelium of the outer surface of the tympanic membrane. Finally, auricular branches of the posterior auricular artery pierce the cartilage of the auricle and supply the posterior portions of the canal. The veins drain into the external jugular vein, the maxillary veins and the pterygoid plexus. The lymphatic drainage follows that of the auricle.

17. Nerve Supply of the External Auditory Canal The external auditory canal receives its sensory innervation from the Trigeminal-V(auriculotemporal nerve) Facial-VII, Vagus -X(Arnold’s Nerve) nerve and Greater auricular nerve-C3 Arnold’s nerve: Originates in the superior ganglion (jugular) of CN X. Passes through the mastoid canaliculus in the lateral wall of the jugular fossa and exits the skull via the tympanomastoid fissure. Receives a branch from CN VII within the temporal bone. Receives a branch from the posterior auricular branch of CN VII after it exits from the skull. This branch contains most of the sensory fibers of CN VII. Anatomically, it is not possible to identify the individual distribution of CN VII, IX, and X. However, the distribution of vesicles in herpes zoster suggests that all three nerves contribute to this cutaneous innervation.

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