Anatomy of Ear & Mastoid Dr. Vishal Sharma

Embryology

Parts of ear External ear:  Pinna / auricle  External auditory canal Middle ear cleft:  Eustachian tube  Tympanic cavity (middle ear)  Mastoid air cell system Inner ear (labyrinth):  Cochlea  Vestibule  Semicircular canals

External Ear

External auditory canal 24 mm long; Lateral 1/3 Cartilaginous (8 mm): up + back + medial. Medial 2/3 Bony (16 mm): down + forward + medial. Pinna pulled up + back + lateral to straighten external auditory canal. Only cartilaginous skin has hair follicles: furuncle Cartilaginous fissure of Santorini & bony foramen of Huschke: passage of infection to and from parotid gland

Nerve supply of E.A.C.

Middle ear cleft Consists of: Eustachian tube Tympanic cavity (middle ear) Aditus Mastoid air cell system Develops from tubo-tympanic recess

Middle Ear

Tympanic membrane Pearly white semi-translucent membrane Obliquely placed at 550 to floor 2 parts: pars tensa and pars flaccida 3 layers of pars tensa: outer epithelial, middle fibrous & inner mucosal Fibrous layer disorganized in pars flaccida Annulus deficient superiorly as notch of Rivinus

Otoscopic view

1. Epitympanum: Bounded laterally by scutum + pars flaccida & medially by bone above tympanic part of facial nerve 2. Mesotympanum: Middle ear cavity lying opposite pars tensa & bony tympanic annulus 3. Hypotympanum: Middle ear cavity below level of bony tympanic annulus 4. Posterior tympanum: Middle ear cavity behind bony tympanic annulus 5. Anterior tympanum or Pro-tympanum: Middle ear cavity in front of bony tympanic annulus

Walls of middle ear Roof: Tegmen tympani, middle cranial fossa Floor: Jugular bulb, carotid artery Anterior: Tensor tympani, ET orifice, carotid artery Posterior: Aditus, fossa incudis, pyramidal eminence, facial recess, sinus tympani Lateral: Tympanic membrane, scutum Medial: Promontory, oval window, round window, lateral SCC, facial nerve (tympanic part)

Lateral wall Membranous: a. Tympanic membrane Bony a. Lateral attic wall above pars flaccida (Scutum) b. Lateral wall of hypotympanum

Posterior wall Aditus ad antrum Fossa incudis for short process of incus Bulge produced by lateral semicircular canal Pyramidal eminence for stapedius tendon Bulge produced by vertical part of facial nerve Sinus tympani Facial recess

Sinus Tympani Boundaries: Superior: Ponticulus Inferior: Subiculum Lateral: Mastoid Segment of Facial Nerve Medial: Posterior semicircular canal It evades direct surgical visualization during surgery. Site for cholesteatoma recurrence

Anterior wall Chorda tympani nerve exits via canal of Huguier Canal for tensor tympani on top Eustachian tube opening below Internal carotid artery is related inferiorly

Medial wall Lateral semicircular canal Tympanic part of bony facial nerve canal Processus cochleariformis Promontory: formed by basal turn of cochlea Oval window Round window

Contents of middle ear Air 3 ossicles: malleus, incus and stapes 2 muscles: tensor tympani & stapedius 2 nerves: chorda tympani & tympanic plexus Mucosal folds & ligaments Blood vessels

Eustachian Tube

Anatomy 36 mm long in adults Directed anteriorly, inferiorly & medially from anterior wall of M.E., forming angle of 450 with horizontal & sagittal planes Enters naso-pharynx 1.25 cm behind posterior end of inferior turbinate

Parts Lateral 1/3 is bony Medial 2/3 is fibro-cartilaginous. Junction b/w 2 parts is isthmus, narrowest part of Eustachian Tube.

Anatomy of medial 2/3rd Cartilage plate lies postero-medially & consists of medial + lateral laminae separated by elastin hinge. Fibrous tissue + Ostmann’s fat pad lie antero-laterally.

Adult vs. Child (< 7 yr) INFANT Length 36 mm 18 mm Angle with horizontal 45 0 10 0 Lumen Narrower Wider Angulation at isthmus Present Absent Cartilage Rigid Flaccid Elastic recoil Effective Ineffective Ostmann’s fat More Less

Adult vs. Child (< 7 yr)

Functions 1. Ventilation & maintenance of atmospheric pressure in middle ear for normal hearing 2. Drainage of middle ear secretions into nasopharynx by muco-ciliary clearance, pumping action of Eustachian tube & presence of intra-luminal surface tension

Functions 3. Protection of middle ear from: Ascending nasopharyngeal secretions due to narrow isthmus & angulation between 2 parts of E.T. at isthmus Pressure fluctuations Loud sound coming through pharynx

Inner Ear

Cochlear Hair Cells

Transmit auditory stimuli Modulate inner hair cell Inner Hair Cells Outer Hair Cells 3500 12,000 Single row Three or four rows Flask-shaped Cylindrical Primarily afferent Primarily efferent Transmit auditory stimuli Modulate inner hair cell Resistant to damage Vulnerable to damage

Vestibular Hair Cells

Temporal Bone

MacEwen’s Triangle Superior: supramastoid crest Anterior: postero-superior margin of bony external auditory canal opening Posterior: tangent drawn to mid-point of posterior wall of external auditory canal Contains spine of Henle Mastoid antrum lies 12-15 mm deep to triangle

Trautmann’s triangle Boundaries Superiorly: superior petrosal sinus Posteriorly: sigmoid sinus Anteriorly: solid angle (semi-circular canals) Pathway to posterior cranial fossa from mastoid cavity & vice versa

Mastoid cells Central group Accessory group  Mastoid antrum  Squamous  Peri-antral  Occipital Peripheral group  Styloid  Dural  Peri-tubal  Sino-dural  Zygomatic  Peri-sinus  Petrous apex  Tip  Retro-facial

Facial Nerve

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