Anatomy of the Ear PDF

# Anatomy The ear consists of 3 parts: - External ear - Middle ear - Inner ear ## External Ear The external ear is formed of 3 parts: - Auricle: Formed of cartilage covered by skin. - Helix: The outer rolled edge. - Antihelix: Y-shaped ridge anterior to the helix. - Concha: Depression...

# Anatomy The ear consists of 3 parts: - External ear - Middle ear - Inner ear ## External Ear The external ear is formed of 3 parts: - Auricle: Formed of cartilage covered by skin. - Helix: The outer rolled edge. - Antihelix: Y-shaped ridge anterior to the helix. - Concha: Depression anterior and below to antihelix. - Meatus: The opening of the external auditory canal. - Tragus: The ridge covering the meatus. - Lobule: Fornmed of fat covered by skin. - External Auditory Canal (EAC): - Its adult length is about 24 mm (1 inch). - It is formed of outer cartilaginous 1/3 and medial bony 2/3. - The outer cartilaginous 1/3 is continuous with the cartilage of auricle; lined by thick skin containing hair follicles, sebaceous and ceruminous glands, its direction is upwards, backwards and medially. - The medial bony 2/3 is a part of temporal bone, lined by thin skin devoid of hair follicles and glands, its direction is downwards, forwards and medially. - It has 2 constrictions: at the bony-cartilaginous junction and 5 mm from the drum (isthmus). - The medial end of the bony part has a narrow groove called the tympanic sulcus in which the tympanic annulus of the drum is lodged. - Tympanic Membrane (TM, Drum): - It is oval in shape, 9 x 8 mm, pearly white in colour, concave, surrounded by fibrous ring called tympanic annulus (deficient above) which is lodged in the tympanic sulcus, oblique (55° with the floor). - It is formed of 2 parts: - Pars Flaccida: It is the upper flaccid part, which has no fibrous layer and is not surrounded by fibrous annulus. - Pars Tensa: It is the lower tense part, which has a fibrous layer and is surrounded by fibrous annulus. - Layers of the Drum: - Outer epithelial layer (skin): continuous with that of external auditory canal. - Middle fibrous layer: Fornmed of circular and radial layers, not present in the pars flaccida. The handle of malleus embedded in this layer. - Inner mucosal layer: continuous with that of the middle ear. - The drum is 0.1 mm in thickness in adults but it is thicker in children. - The cone of light appears during examination in the antero-inferior part due to the oblique position of the drum (55° with the floor). So, this part reflects the light of the otoscope as a cone. ## Middle Ear Cleft It is formed of 3 parts: - Middle ear cavity (ME) -- Also known as the tympanum. - Eustachian tube (ET) - Mastoid air cells ### Middle Ear Cavity (ME) It has 6 walls and is formed of 3 parts: - Attic (epitympanum): It is the upper part, present above the level of the drum, continuous posteriorly with the mastoid antrum through the aditus ad antrum. - Mesotympanum: It is the middle part, present at the level of the drum. - Hypotympanum: It is the lower part, present below the level of the drum **Walls of ME Cavity:** - Roof: Fornmed of thin plate of bone called tegmen tympani separating the cavity from temporal lobe of the brain. - Floor: Formed of thin plate of bone separating the cavity from the jugular bulb (it is sometimes dehiscent i.e. deficient, leads to high jugular bulb). - Anterior wall: Shows 3 canals from above downwards: - Canal for tensor tympani muscle. - Eustachian tube. - Carotid canal: Contains the internal carotid artery (ICA). - Posterior wall: Shows 3 features fronm above downwards: - Aditus Ad Antrum (communicating the attic to the antrum). - The pyramid: Contains the stapedius muscle. - Vertical part of facial nerve in its bony canal. - Lateral wall: Fornmed of the drum, and bone above and below it. - Medial wall: Shows 6 features: - Promontory: First (basal) turn of the cochlea. - Oval window: Above and behind the promontory, closed by the footplate of stapes. - Round window: Below and behind the promontory, closed by the secondary tympanic membrane - Sinus tympani: Depression between oval and round windows behind the promontory. - Processes cochleariformis: Pulley-like projection above and anterior to the promontory (the tensor tympani tendon turns 90° around it). - Transverse part of facial nerve in its bony canal above the promontory and oval window. **Contents of the Middle Ear Cavity:** - 3 bones (ossicles): Malleus, Incus and Stapes. - 2 muscles: - Tensor tympani: Supplied by trigeminal nerve. - Stapedius: Supplied by facial nerve. - 2 nerves: - Chorda tympani (of 7th nerve) - Tympanic plexus (of 9th nerve) - Air: Coming from Eustachian tube. ## Eustachian Tube (ET) - It is 36 mm in length. - It connects the middle ear cavity to the nasopharynx. - It is directed downwards, forwards and medially. - It is formed of upper bony 1/3 and lower cartilaginous 2/3. - The lower end opening present 1 cm behind the posterior end of inferior turbinate (of nose). - It is normally closed, open during swallowing and yawning by the action of levator and tensor palati muscles of the soft palate. - ET is shorter, wider and more horizontal in children (so acute otitis media is more common in children). - Children with cleft palate usually have secretory otitis media (deficient palatal muscles). ## Mastoid Air Cells - They are many cavities lined by mucous membrane, continuous together and with ME through aditus ad antrum. - They are formed of: - Antrum: The largest air cell, communicating to attic through aditus, presenting 2 features in its medial wall, the lateral semicircular canal and 2nd genu of facial nerve. - Tip cells: At the tip of mastoid. - Peri-sinus cells: Around sigmoid sinus. - Sinu-dural cells: Between dura and sigmoid sinus. - Retro-facial cells: Behind te vertical part of facial nerve. - Zygomatic cells: In zygomatic process. - Peri-antral cells: Around the antrum. **Types of Mastoid:** - Cellular (pneumatic): 85% - Acellular (sclerotic): 15% - Diploic (contain bone marrow): In children. # Inner Ear It is formed of bony labyrinth that contains inside it the membranous labyrinth. ## Bony Labyrinth It consists of cochlea, vestibule and 3 semicircular canals (SCC), filled with perilymph and enclosing the membranous labyrinth. ### The Vestibule It is the central part of the bony labyrinth, present posterior to the cochlea and anterior to SCC. Its lateral wall is directed to the middle ear and it shows the oval and round windows. It houses the utricle and saccule of membranous labyrinth. ### The Semicircular Canals (SCC) The superior, posterior and lateral canals open into the posterior part of the vestibule by 5 openings. Each canal has a swelling at one end called ampulla. They house the semicircular ducts of membranous labyrinth and positioned at right angle to each other. ## The Cochlea It resembles a snail cell and it opens in the anterior part of the vestibule. It makes 2 1/2 turns around the modiolus, its narrow apex is directed antrolaterally and its wide base is directed posteromedially. Its first turn (base) makes the promontory in the medial wall of the middle ear. It contains 3 compartments; scala vestibuli (it contains perilymph and closed by the footplate of stapes), scala tympani (it contains perilymph and closed by the round window), and scala media (which is the membranous cochlea or cochlear duct that contains endolymph). The scala vestibuli and scala tympani are communicated together at the apex of the cochlea (helicotrema). ## Membranous Labyrinth It is situated in the bony labyrinth, and it contains endolymph and surrounded with perilymph. It consists of cochlear duct inside the cochlea, utricle and saccule inside the vestibule, 3 semicircular ducts inside the SCC, and endolymphatic duct and sac (the duct inside the vestibular aqueduct and the sac lies beneath the dura). **N.B.:** - The cochlear duct contains the sensory neuroepithelium of hearing which is called Organ of Corti. - The sensory organ of utricle and saccule is called macula, while of SCC is called crista ampullaris (which are concerned with equilibrium). ## Organ of Corti - The cochlear duct (scala media) is triangular in cross section and it has 3 walls; the basilar membrane that supports the organ of Corti, the Reissner's membrane which separates it from the scala vestibuli, and stria vascularis which secrets endolymph. - Organ of Corti is formed of inner and outer hair cells, supporting cells and tectorial membrane. The bottoms of hair cells are connected to the spiral ganglion in the modiolus that is connected to the cochlear nerve. The tops of hair cells contain stereocilia that touching the tectorial membrane. ## Equilibrium Sensory Organs - **Macula** - **Crista ampullaris** # Physiology **1- Auricle:** Its function is collection and localization of sound. **2- EAC:** Its function is conduction of sound. **3- Middle Ear:** Its function is - Conduction of sound from tympanic membrane to oval window through the ossicles. - Amplification of sound by about 20 times through two factors: - Areal ratio: The difference between surface area of the drum and that of oval window is about 17:1. - Lever action of ossicular chain. **N.B.:** This amplification compensates the decrease in sound when the sound transmitted to the fluid of inner ear (impedance matching mechanism). **N.B.:** When the oval window becomes in, the round window will be out, this mechanism is called phase difference preventing increase of inner ear pressure. **4- Eustachian Tube:** Its function is - Equalization of pressure in both sides of the drum (Ventilation) - Drainage of middle ear secretion (ME is lined by mucous membrane). **5- Mastoid Air Cells:** It may act as an air reservoir for ME cavity and may insulate the labyrinth protecting it from temperature variations. **6- Inner Ear:** - **Cochlear part:** Concerned with perception of sound (hearing) by organ of Corti. - **Vestibular part:** Concerned with equilibrium, utricle and saccule are responsible for equilibrium in linear acceleration (by macula) while SCCs are responsible for equilibrium in angular acceleration (by crista ampullaris). ## Mechanism of Hearing The sound waves are collected by the auricle and pass through the EAC. Vibrations of the tympanic membrane are transmitted to the stapes footplate through the ossicular chain. Pressure changes in the labyrinthine fluid leads to movement of the basilar membrane (travelling wave theory). This action leads to frictional movements between the stereocilia of the hair cells and the tectorial membrane that result in electric energy and auditory nerve impulses. Each cochlea sends impulses to both sides of the brain. **N.B.:** - **CHL:** (Conductive Hearing Loss): caused by defect in the conductive mechanism, due to lesions in EAC and/or middle ear. - **SNHL** (Sensorineural Hearing Loss): caused by defect in the perceptive and/or neural mechanism, due to lesions in inner ear (cochlear part) or central pathway of hearing. - **MHL:** (Mixed Hearing Loss): CHL+SNHL. - **Vertigo:** Caused by defect in the balance mechanism, due to lesions in the inner ear (vestibular part) or central pathway of equilibrium. # Symptoms and Methods of Ear Examination Before studying ear diseases, you should know what are the symptoms caused by ear diseases, and how to examine the patients (detailed history taking and clinical examination were discussed at the end of this book in clinical ENT) ## Symptoms of Ear Diseases: - Deafness: Diminution of hearing - Tinnitus: Sensation of noise - Discharge: Fluids come out from the ear - Pain (otalgia) and headache - Vertigo: False sensation of rotation - Facial paralysis. - Swelling and deformity. - Eye symptoms. ## Methods of Ear Examination: - External examination: For auricle, preauricular and postauricular (mastoid) areas by Inspection and Palpation. - Otoscopy: By using otoscope to see the EAC, tympanic membrane and ME. - Examination of tympanic membrane mobility (i.e. Eustachian tube is patent or closed) by: - Valsalva's method: Otoscopic examination of drum while the patient is blowing his nose. - Siegalization: By using Siegle's pneumatic otoscope (otoscope connected to a pump). - Tuning fork tests: Rinne and Weber tests to detect the type of hearing loss. - Examination of the ear under the microscope (sometimes needed) to magnify the view. # Clinical Tests of Hearing: Tuning Fork Tests Tuning fork used is 512 Hz (as it is equally heard and felt) ## Rinne's Test It compares air conduction (AC) and bone conduction (BC) in the same ear. - It denotes the type and not the degree of hearing loss. - It gives valuable results when the hearing loss exceeds 25dB. **Results:** - Rinne +ve: Normal hearing (louder near the meatus) i.e. AC better than BC. - Rinne - ve: CHL (louder on mastoid) i.e. BC better than AC. - Reduced Rinne + ve: SNHL i.e. AC is better than BC but both are reduced. - False Rinne - ve: Severe SNHL (unilateral) i.e. the sound heard by the other ear not the examined ear (which has severe SNHL) when the fork is placed on the mastoid. ## Weber's Test - Comparison of hearing by BC between both ears. - With the tuning fork placed on the central incisor, forehead or chin (midline). - Ask the patient if the sound is central of lateralised to the right (Rt) or the left (Lt) ear. **Results:** - Central: Hearing is equal in both ears Normal. - Lateralised to Rt. ear of to Lt. ear: - In CHL: Lateralised to the conductive side (due to the masking effect of noise). - In SNHL: Lateralised to the normal or better hearing ear. ## Schwabach's Test Comparison of bone conduction between the patient and the examiner (regarding that the examiner is normal). **Results:** - Normal: Duration of hearing is equal (over the mastoid of the patient and the examiner). - In CHL: Hearing is prolonged by BC. - In SNHL: Hearing is shortened by BC. # Diseases of the External Ear ## CONGENITAL The auricle develops from 6 mesenchymal proliferations which are derived from the 1st and 2nd branchial arches; they are located around the 1st branchial cleft from which the EAC develops. Congenital anomalies may be unilateral or bilateral. **The anomalies may be:** - Defect in size: - Anotia (no auricle) - Microtia (small auricle) - Macrotia (large auricle) - Treatment: Plastic surgery before school age. - Defect in shape: - Bat ear (protruding or lop ear) - Treatment: Plastic surgery before school age. - Accessory auricle: - Small cartilage anterior to the aurile. - Treatment: surgical removal if it is large. - Pre-auricular cyst and sinus: - It is usually present at the root of the helix, presented with cystic swelling, repeated infection and dicharge. - Treatment: Surgical removal. - Congenital aural (meatal) atresia: - Obstruction of the EAC due to defect in 1st branchial cleft. It is more commonly to be associated with ME hypoplasia or ossicular fixation and it is rarely associated with inner ear anomalies. - Clinical Picture: - Microtia of Anotia of auricle: May be present. - CHL: Rarely mixed hearing loss (if there is inner ear anomaly). - Investigations - CT: To assess the anatomy (Middle ear, Inner ear and length of atresia). - ERA (Evoked Response Audiometry): To assess the hearing (to see the cochlea is functioning or not) (See Audiology) - Treatment: - Hearing aid: In bilateral cases to preserve hearing till the age of surgery. - Meatoplasty: Done before school age. - Plastic surgery to the auricle (if there is microtia).