External and Middle Ear Anatomy PDF

ENT LEC. (1) Dr. Baseem N. Abdulhadi The External Ear: Learning Objectives: By the end of this lecture, you should be able to: 1) Describe the applied anatomy of the external ear. 2) List functions of the e...

ENT LEC. (1) Dr. Baseem N. Abdulhadi The External Ear: Learning Objectives: By the end of this lecture, you should be able to: 1) Describe the applied anatomy of the external ear. 2) List functions of the external ear. 3) Describe the applied anatomy of the middle ear and its adjacent structures. 4) List the differences of the eustachian tube between adults and children and their impact. 5) Recognize the eustachian tube functions. 6) Demonstrate how the middle ear works. Anatomy of the external ear: Auricle: is also known as the pinna. A major portion of it is made up of a single plate of elastic cartilage presenting elevations and depressions. It is covered with skin on the outer and inner surfaces. The lowest part of the auricle is soft to the touch. It consists of connecting tissue covered with skin and is known as the lobule of the ear. The auricle presents an outer rim known as the helix. Inside the helix lies another curved ridge, starting from the antitragus. It is known as antihelix. It curves upwards and divides into two limbs to enclose the depression known as the triangular fossa. Under the helix lies the scaphoid fossa. Postero-superior margin of the helix presents the tubercle known as Darwin’s tubercle. In front of the antitragus, there is an intertragic notch followed by an elevation known as the tragus. The area bounded by the antitragus is divided into two: 1. Cymba conchae above and 2. The concha below. Blood supply of the Auricle: Anteriorly it is supplied by the superficial temporal artery and posteriorly by the posterior auricular. + Branch from the occipital artery (artery) Lymphatic Drainage: Lymphatics of the auricle drain into the pre-auricular, post- auricular, and superficial cervical lymph nodes. + Retroauricular (Mastoid) Nodes Nerve Supply: 1) The upper 2/3rd of the lateral surface of the auricle is supplied by the auriculotemporal nerve, while the lower 1/3rd by the great auricular nerve. 2) The medial surface of its 2/3rd surface is supplied by the lesser occipital nerve, and the lower 1/3rd by the great auricular nerve. 3) The root of the auricle is supplied by the auricular branch of the vagus (Nerve of Arnold). 1 External Auditory Canal: (EAC) (The external auditory canal (EAC) or external auditory meatus (EAM) External auditory canal runs from the concha to the tympanic membrane. Its direction is complicated; however, it is mainly directed medially with tendency to go forwards, upwards and downwards. The “S” shaped course of the canal prevents the examiner from having a look at the tympanic membrane. The curve can be obliterated by pulling the auricle laterally upwards and backwards. The length of the canal is 24-25 mm. Its medial 2/3rd is bony and the lateral 1/3rd is cartilaginous. Bony canal measures 16 mm while the cartilaginous canal measures only 8 mm. The cartilaginous part is wider than the bony part. The tympanic membrane is set obliquely at an angle of 55 degrees to the floor of the canal. Skin of the meatus is thin in the bony part and is firmly attached to the periosteum. The cartilaginous incomplete ring of the tube is formed by the “C” shaped cartilaginous plate, while the deficient part of the ring is completed by the fibrous tissue. Skin of the cartilaginous part is attached to the perichondrium and is thicker than the skin of the bony part. It contains sebaceous and ceruminous glands (modified sweat glands) and hair. Maceration is defined as the softening and breaking down of skin resulting from prolonged exposure to moisture. Ceruminous glands produce wax like material which prevents maceration of the tissue in the presence of water. Due to the firm fixity of the skin to the underlying cartilage inflammations of the cartilaginous portion are very painful. It is a known site for the furuncle (Furuncle is the staphylococcal infection of the hair follicles). The bony meatus presents the narrow part known as isthmus which is situated 6 mm lateral to the tympanic membrane. Foreign bodies get lodged medial to the isthmus and get impacted which makes it difficult to remove. There is a small recess situated medial to the isthmus known as the anterior recess. It acts as a cesspool for the discharge of debris in cases of infections of the ear. The anteroinferior part of the bony meatus has the deficiency known as the foramena Huschke. It remains patent up to the age of 4 years, and is closed in adults. It acts as the gate for the infections of the parotid to enter the canal. The cartilage of the meatus is continuous with the cartilage of the external ear. It has two fissures known as the fissures of Santorini. They act as the gate of entry for infections from the parotid and the superficial mastoid regions. (Pinna ?) Physiology of the external ear: 1. It gathers sound and directs it into the canal. (‫)ﯾﻛﺑر ﯾﺿﺧم‬ 2. With the external canal, it acts as a resonator to slightly amplify lower frequency sounds. 3. It also helps to localize sounds, especially in conjunction with the other ear. 4. Ear wax (cerumen) production, which has a protective role in the EAC. 2 The Middle Ear: The middle ear cleft: The Eustachian tube and antero-inferior part of the tympanic cavity are lined with respiratory epithelium (columnar ciliated). Elsewhere the epithelium is cuboidal. The cleft consists of: 1) Eustachian (pharyngotympanic) tube: It is named after Bartolomeo Eustachio (died 1574), the Italian anatomist who identified and described it. The Eustachian tube connects the tympanic cavity with the nasopharynx. The lower orifice lies in the lateral wall of the nasopharynx on a level with the posterior end of the inferior turbinate. In the adult this 3.7 cm tube runs laterally and posterosuperiorly, to open in the anterior wall of the tympanic cavity. The lateral third is bony, while the lower and medial parts are cartilaginous. The tube is closed at rest, but is opened on yawning or swallowing by the combined actions of the sphincter of the nasopharyngeal isthmus and the tensor palatini muscle (and to lesser extent by the action of levator palatini and salpingopharyngeus), which is attached to the cartilaginous medial wall of the tube. The tube is more horizontal and relatively wider and shorter in the infant than in the adult and these differences might explain the higher incidence of middle ear infections in pediatric age group. The functions of the Eustachian tube are: 1) Pressure equalization and ventilation of the middle ear. 2) Mucociliary clearance of secretions from the middle ear. 3) Protection of the middle ear from sounds, and from pathogens and secretions from the nasopharynx. Its main function is to equalize pressure in the middle ear with the atmospheric pressure, thereby allowing free movement of the tympanic membrane. By allowing air to enter and leave the tympanic cavity, this tube balances the pressure on both sides of the membrane. 2) Tympanic cavity: It is an air-filled, mucous membrane-lined cavity in the temporal bone between the tympanic membrane laterally and the lateral wall of the inner ear medially. 3 It consists of two parts: 1. The tympanic cavity proper: immediately adjacent to the tympanic membrane. 2. The epitympanic recess: superiorly. The cavity is also artificially divided into three parts: a. Mesotympanum: lying medial to the membrane. b. Epitympanum (attic): lying medial to the bone of the horizontal part of the squama (outer attic wall), above the membrane. c. Hypotympanum: below the drumhead, medial to the tympanic plate. Boundaries (Walls) of the middle ear: The middle ear has 6 walls: 1) Roof (tegmental wall): formed by tegmen tympani which is a thin plate of bone, separates the tympanic cavity from the middle cranial fossa. 2) Floor (jugular wall): is a thin plate of bone separating the cavity from the jugular bulb. A dehiscence may be present. Near the medial border of the floor is a small aperture through which the tympanic branch from the glossopharyngeal nerve (IX) enters the middle ear. 3) Lateral wall (membranous wall): the tympanic membrane forms the central portion of the lateral wall; above and below the tympanic membrane, the lateral wall is formed by bone. Tympanic membrane is pearly grey in color, oval in shape and lies obliquely. It is convex towards the tympanic cavity; and It consists of three layers: An outer epithelial layer: continuous with the epithelium of the external auditory canal. A middle fibrous layer: containing (inner) circular and (outer) radial fibers, and the handle of malleus. An inner mucosal layer. Tympanic membrane is divided by the anterior and posterior malleal folds into part below the folds called the pars tensa which is thickened peripherally into a fibrocartilaginous annulus which fits into the grooved tympanic sulcus of the temporal 4 bone, and part above the folds called pars flaccida which it lacks the middle fibrous layer (present only in pars tensa). The handle of malleus descends downwards and posteriorly between the inner mucosal and middle fibrous layers of tympanic membrane. The lowest point of the handle of malleus is called Umbo, from the umbo a cone of light extends downwards and forwards when the tympanic membrane is seen by otoscopy. Nerve supply: The outer surface by: a. The auriculotemporal n. b. The auricular branch of the vagus n. c. A branch from the facial n. The inner surface by: The glossopharyngeal nerve. 4) Medial wall (labyrinthine wall): has several obvious features which are: a) The promontory: the bony projection covering the basal turn of the cochlea. b) The foramen ovale (oval window): occupied by the footplate of the stapes and the annular ligament, closes the middle ear from the scala vestibuli. It lies postero-superior to the promontory. c) The facial nerve: the horizontal portion of the nerve in its bony canal lies just above the oval window. d) The ampullary end of the horizontal semicircular canal, which lies just above the second genu of the facial nerve. e) The foramen rotunda (round window): closed by the round window membrane separates the middle ear from the scala tympani. It lies postero-inferior to the promontory. 5) Posterior wall (mastoid wall): separates the tympanic cavity from the mastoid air cells; it features an opening in its superior part, (the aditus ad antrum) to the mastoid antrum, connecting the tympanic cavity to the mastoid cells; the canal for the facial nerve descends between the posterior wall and the antrum, medial to the aditus. Associated with the mastoid wall are: 5  a. The pyramidal eminence, a small elevation through which the tendon of the stapedius muscle enters the middle ear, which is inserted into the neck of the stapes. b. The opening through which the chorda tympani nerve, a branch of the facial nerve enters the middle ear. 6) Anterior wall (carotid wall): separates the tympanic cavity from the carotid canal; superiorly, it has the opening of the pharyngotympanic tube, the canal for the tensor tympani, the canal through which the chorda tympani nerve escapes from the middle ear. The contents of the middle ear are the: 1) Auditory ossicles (malleus, incus, and stapes): 2) Stapedius and tensor tympani muscles. 3) Chorda tympani nerve, a branch of the facial nerve (CN VII): it enters the middle ear cavity through the posterior wall, lateral to the pyramid then it passes forward, lateral to the incus and medial to the malleus, to escape from the cavity through its anterior wall. 4) Tympanic plexus of nerves. The mastoid antrum: antrum=cavity It is a cavity in the mastoid process of the temporal bone. It is, like the tympanic cavity, separated from the middle cranial fossa by a thin plate of bone, called the tegmen tympani. The antrum and mastoid air cells are lined by a mucous membrane that is continuous with the lining of the middle ear. Auditory ossicles: The bones of the middle ear consist of the malleus, incus, and stapes. They form an osseous chain across the middle ear from the tympanic membrane to the oval window of the internal ear. These ossicles are the first bones to be fully ossified during development and are essentially mature at birth. Malleus: is the largest ossicle and is attached to the tympanic membrane. Identifiable parts include the head of the malleus, neck, anterior and lateral processes, and handle of the malleus. The head lies in the epitympanic recess and its posterior surface articulates with the incus. The lateral process is attached to the anterior and posterior malleolar folds of the tympanic membrane. The downward extension of the malleus is the handle, which is attached to the tympanic membrane. 6 Incus: the second bone in the series of auditory ossicles. It consists of the body of the incus and long and short limbs. The enlarged body of the incus articulates with the head of the malleus and is in the epitympanic recess. Stapes: the smallest and most medial bone in the osseous chain is attached to the oval window. It consists of the head of the stapes, anterior and posterior limbs and the base (footplate) of the stapes. The head is directed laterally and articulates with the long process of the incus. The neck is narrow and receives the insertion of the stapedius muscle. The footplate is attached to the oval window with a loose annular ligament. Muscles associated with the ossicles: Tensor tympani: It lies in a bony canal above the pharyngotympanic tube. It originates from the cartilaginous part of the Eustachian tube, the greater wing of the sphenoid, and its own bony canal, ending in a rounded tendon that inserts into the upper part of the handle of the malleus. Nerve supply: a branch from the mandibular nerve [V3]. Contraction of this muscle will pull the handle of the malleus medially; this tenses the tympanic membrane reducing the force of vibrations in response to loud noises. The importance of the tensor tympani muscle in middle-ear physiology remains unclear. It has been related to the attenuation of sounds produced during the mastication process. the reduction of the force, effect, or value of something. Stapedius: This is a very small muscle that originates from inside the pyramidal eminence, which is a small projection on the mastoid wall of the middle ear. Its tendon emerges from the apex of the pyramidal eminence and passes forward to attach to the posterior surface of the neck of the stapes. Nerve supply: a branch from the facial nerve [VII]. Contraction of the stapedius muscle, usually in response to loud noises, pulls the stapes posteriorly and prevents excessive oscillation, so it plays a role in the protection of the inner ear against loud noises. 7 How the middle ear works: Sound energy from the air is conducted to the inner ear via the tympanic membrane and the ossicles which provide hydraulic (area) advantage and a lever effect. The middle ear transforms acoustic energy from the medium of air to the medium of liquid. It is an impedance-matching system that ensures energy is not lost. (In dry air at 20 °C, the speed of sound is 343.2 meters per second. It travels about 4.3 times as fast in water (1,484 m/s). This impedance matching is accomplished by the following factors: A. The area effect of the tympanic membrane: the ratio of the vibrating portion of the tympanic membrane to that of the stapes footplate results in a 14:1 increase in sound energy by concentrating it into a smaller area. B. Lever action of the ossicular chain: As the eardrum vibrates, the ossicular chain is set into motion about an axis of rotation from the anterior process of the malleus through the short process of the incus. Because the handle of the malleus is approximately 1.3 times longer than the incus long process, the force (pressure) received at the stapes footplate, through the use of leverage, is greater than that at the malleus by about 1.3:1. Thus, the transformer ratio of the middle ear is approximately 18:1 (the combination of the area effect of the tympanic membrane and the lever action of the ossicles: 14 × 1.3 = 18) which translates to approximately 25 dB. A lever amplifies an input force to provide a greater output force 8