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I. Anatomy of Ear, Physiology of Hearing, Assessement of Hearing, Hearing Loss Vestibular Function Tests

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I. Anatomy of Ear, Physiology of Hearing, Assessement of Hearing, Hearing Loss Vestibular Function Tests I. ANATOMY OF EAR, PHYSIOLOGY OF HEARING, ASSESSEMENT OF HEARING, HEARING LOSS VESTIBULAR FUNCTION TESTS ANATOMY OF EAR AURICLE OR PINNA • The entire pinna (except its lobule and outer part of external acoustic canal) is made up of a framework of a single piece of yellow elastic cartilage • There is no cartilage between the tragus and crus of the helix – incisura terminalis • An incision made in this area will not cut through the cartilage – used for endaural approach in surgery of external auditory canal and mastoid EXTERNAL ACOUSTIC CANAL • 24 mm long • Outer part – directed upwards, backwards and medially • Inner part – directed downwards, forwards and medially • To see the tympanic membrane – pinna has to be pulled – upwards, backwards and laterally • Cartilaginous part Outer one-third ( 8mm ) Two deficiencies – fissures of Santorini – through them the parotid and superficial mastoid infections can appear in the canal or vice versa Skin is thick and contains ceruminous and pilosebaceous glands which secrete wax Hair is only confined to the outer canal and therefore furuncles are seen only in the outer third of the canal • Bony part Inner two-thirds ( 16mm ) Devoid of hair and ceruminous glands Isthmus – bony projection lateral to TM – foreign bodies lodged medial to isthmus are difficult to remove Antero-inferior part of bony canal may present a deficiency – foramen of Hushke - in children up to 4 years – permitting infections to and from the parotid TYMPANIC MEMBRANE • Obliquely set so that postero-superior part is more lateral than antero-inferior part • 9-10mm tall; 8-9mm wide; 0.1mm thick Pars tensa • Forms most part of tympanic membrane • Periphery thickened to form annulus tympanicus • Central part is tented inwards at the level of tip of malleus and is called the umbo • Umbo is the most reliable landmark in otoscopy • A bright cone of light seen radiating from the tip of malleus to the periphery in the anteroinferior quadrant Pars flaccida (Shrapnel's membrane) • Situated above the lateral process of malleus between the notch of Rivinus and the anterior and posterior malleal (earlier called malleolar) folds NERVE SUPPLY OF EXTERNAL EAR Pinna • Greater auricular nerve(C2C3) • Lesser occipital nerve(C2) • Auriculotemporal nerve (mandibular branch of 5th nerve) • Auricular branch of Vagus (Arnold's N) • Facial nerve External auditory canal • Anterior and superior walls – auriculotemporal branch of mandibular nerve • Posterior and inferior walls – auricular branch of vagus – Arnold's nerve • Part of posterior wall – sensory branch of facial nerve – Nerve of Wrisberg Tympanic membrane • Anterior half of lateral surface- auriculotemporal • Posterior half of lateral surface- Arnolds nerve • Medial surface - tympanic branch of 9th nerve ( Jacobson's N ) MIDDLE EAR • Mesotympanum – part of middle ear lying opposite to pars tensa (narrowest part) • Epitympanum or attic – lying above pars tensa but medial to Shrapnell's membrane • Hypotympanum – lying below the level of pars tensa • Protympanum – around the tympanic orifice of Eustachian tube Boundaries • Roof (tegmental wall): tegmen tympani ; separates middle ear from middle cranial fossa • Floor (jugular wall): bone which separates middle ear from jugular bulb • Lateral wall : formed largely by TM ; To a lesser extent by bony outer attic wall ( scutum ) • Anterior wall: Bone which separates middle ear from internal carotid artery. It has 2 openings upper- canal for tensor tympani lower- ET • Posterior wall Pyramid – bony projection where tendon of stapedius arises Facial nerve runs in the posterior wall just behind the pyramid Two recesses – Facial recess (suprapyramidal) and Sinus tympani (infrapyramidal) These recesses are the most common location of cholesteatoma persistence after ear surgery Sinus tympani is very difficult to access surgically Facial recess is lateral to facial nerve and its relations are o Medial – pyramid o Lateral – posterosuperior meatal wall o Below - chorda tympani o Above - fossa incudis • Medial wall: formed by labyrinth Promontory - formed by basal turn of cochlea Oval window - foot plate of stapes fixed Round window or fenestra cochlea- covered by secondary TM Canal for facial nerve – above the oval window Processus cochleariformis - anterior to oval window, marks the genu of facial nerve ( landmark of surgery ) • Prussak's space medially neck of malleus laterally pars flaccida above lateral process of malleus anteriorly, posteriorly and superiorly by lateral malleal ligament Contents of middle ear • Ear ossicles – Malleus, Incus, Stapes (the smallest bone) • Ligaments of ear ossicles • Muscles – Tensor tympani and Stapedius • Vessels of Middle ear • Nerves – Chorda tympani and tympanic plexus Arterial supply • Anterior tympanic branch – from maxillary artery • Posterior tympanic branch – from posterior auricular artery MASTOID ANTRUM • Large air containing space in the upper part of mastoid • Roof formed by tegmen antri (continuation of tegmen tympani) • Marked externally on the surface of mastoid by suprameatal or MacEwen's triangle • Boundaries of suprameatal triangle Supramastoid crest or temporal line – above (base) Posterosuperior margin of bony external auditory canal – anteroinferiorly Tangential line joining supramastoid crest to spine of Henle - posteriorly • The petrosquamosal suture may persist as a bony plate – Korner's septum • Korner's septum – surgically important – mastoid antrum cannot be reached unless it is removed • Middle ear cleft: Middle ear together with Eustachian tube, aditus, antrum and mastoid air cells. INNER EAR or LABYRINGH • Bony labyrinth • Membranous labyrinth –filled with endolymph • Space between membranous and bony labyrinth filled with – perilymph BONY LABYRINTH • Vestibule In its lateral wall lies the oval window D Medial wall has two recesses o Spherical recess - lodges saccule o Elliptical recess – lodges utricle Posterosuperior part receives five openings of semicircular canals • Semicircular canals Lateral, posterior and superior semicircular canals Lie in planes at right angles to one another Responds to angular acceleration and deceleration • Cochlea Coiled tube making 2.5 to 2.75 turns round a central pyramid of bone called modiolus Three compartments – Scala vestibule, Scala tympani and Scala media MEMBRANOUS LABYRINTH Cochlear ducts Also called the membranous cochlea or scala media Triangular in cross section and the three walls are formed by o Basilar membrane o Reissner's membrane o Stria vascularis • Utricule and Saccule: Utricle responds to linear acceleration and deceleration or gravitational pull • Semicircular ducts • Endolymphatic duct and sac Composition of inner ear fluids Endolymph Perilymph CSF Sodium (mEq/L) 5 140 152 Potassium (mEq/L) 144 10 4 Protein (mg/dL) 126 200 -400 20 -50 Glucose (mg/dL) 10 -40 85 70 EUSTACHIAN TUBE/AUDITORY TUBE/PHARYNGOTYMPANIC TUBE • 36mm long in adults • Runs downwards, forwards and medially from tympanic end • Two parts – Postero lateral bony part (12mm) • Antero medial fibro cartilaginous part (24mm) • Two parts meet at the isthmus – narrowest part of ET • Tympanic end (bony) - anterior wall of middle ear • Pharyngeal end - lateral wall of nasopharynx , it raises an elevation called torus tubarius 1 cm behind the posterior end of inferior turbinate • 3 muscles - 1. tensor veli palatini(dilator tubae) opens tubal lumen 2. levator veli palatini 3. salpingopharyngeus • Ostmann's pad of fat- helps the tube closed at pharyngeal end • Lining- pseudo stratified ciliated columnar , cilia beat in the direction of nasopharynx • Nerve supply : tympanic branch of 9 th cranial nerve supplies sensory as well as secretomotor fibres • Tensor veli palatine- mandibular branch of 5 th nerve • Levator veli palatine and salpingopharyngeus – pharyngeal plexus • Blood supply Ascending pharyngeal artery Middle meningeal artery Artery of pterygoid canal Infant and adult Eustachian tube differences Infant Adult 13 – 18 mm at birth 36mm At birth - 10° with horizontal 45° with horizontal At 7 years - 45° with horizontal No angulation at isthmus Angulation present at isthmus Bony part slightly longer than 1/3 rd of Bony part 1/3 rd ; the total length Cartilaginous part 2/3 rd Tubal cartilage – flaccid; reflux of Tubal cartilage - nasopharyngeal secretions can occur comparatively rigid Ostmann's pad of fat – less in volume More in volume and helps to keep the tube closed • Functions Ventilation & regulation of middle ear pressure Protection against nasopharyngeal sound pressure and reflux of nasopharyngeal secretions Clearance of middle ear secretions • ET function tests: Valsalva test, Politzer test, Toynbee's test, Tympanometry, Sonotubometry Lymphatic drainage of ear Area Nodes Concha, Tragus, Fossa triangularis, Pre auricular and parotid nodes External cartilaginous canal Lobule and antitragus Infra -auricular nodes Helix and anti -helix Post auricular nodes, deep jugular and spinal accessory nodes Middle ear and Eustachian tube Retropharyngeal nodes – upper jugular chain Inner ear No lymphatics KEY POINTS: • Bill's bar – in the internal acoustic meatus separates facial nerve from superior vestibular nerve • Glasserial fissure – in the middle ear transmits anterior tympanic branch of maxillary artery, anterior ligament of malleus and Chorda tympani nerve through canal of Huguier • Tympanic membrane develops from all the three germinal layers • Embryology of inner ear I arch – malleus and incus II arch – stapes except its foot
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