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ANATOMY OF THE MIDDLE EAR

BY DR: MAAMON AMEEN

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Consist of

• Auditory tube • Tympanic Cavity• Aditus• Mastoid Air Cells

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Embryology

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The eustachian tube ,tympanic cavity ,attic,mastoid air cells develop from the endoderm of tubotympanic recess which arises from the first pharyngeal pouches.

Embryology

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Malleus and incus are derived from mesoderm of the first arch while the stapes develop from the second arch except its footplate and annular ligament which are derived from the ottic capsule.

Embryology

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Eustachian Tube

• 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

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

Eustachian Tube

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

Eustachian Tube

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CHARACTERISTIC ADULT INFANTLength 36 mm 18mm

Angle with horizontal 45° 10°

Lumen Narrow Wide

Angulation at Isthmus Present Absent

Cartilage Rigid Flaccid

Elastic recoil Effective Ineffective

Ostmann’s fat More Less

Difference in adult & infant<7years ET

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Blood supply of ET:• Ascending pharyngeal artery• Middle meningeal artery• Artery of pterygoid canal

• Veins drain into pterygoid venous plexus

Eustachian Tube

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TYMPANIC MEMBRANE• Pearly white semi-

translucent membrane• Obliquely placed faces

laterally,inferiorly ,anteriorly

• 2 parts : pars tensa & pars flaccida

• 3 layers of pars tensa : outer epithelial, middle fibrous & inner mucosal

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• Fibrous layer is disorganized in pars flaccida (Sharpnell’s membrane)

• Pars tensa thickened peripherally into fibrocartilagenous annulus called annulus tympanicus which fits into tympanic sulcus of temporal bone.

• Annulus deficient superiorly in pars flaccida and is called notch of Rivinus

TYMPANIC MEMBRANE

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Nerve supply of TM:• Anterior half of lateral surface :

Auriculo temporal nerve

• Posterior half of lateral surface : Auricular branch of vagus

• Medial surface : Tympanic branch of glossopharyngeal nerve(jacobson nerve)

TYMPANIC MEMBRANE

V

X

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TYMPANIC CAVITY

• Epitympanum

• Mesotympanum

• Hypotympanum

Parts of tympanic cavity:

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Walls of the tympanic cavity:TYMPANIC CAVITY

Lateral Medial

Posterior Ant

erio

r

RoofFloor

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ROOF • Thin plate of bone called TEGMEN

TYMPANI .• It also extends posteriorly to form the roof

of the aditus and antrum.• It seperates tympanic cavity from the

middle cranial fossa.

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The floor

• Thin plate of bone which seperates tympanic cavity from the jugular bulb.

• 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

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The anterior wallLower part• Thin plate of bone which separates the

cavity from the internal carotid artery.

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The anterior wallUpper part• Canal of tensor tympani muscle.• Eustachian tube opening

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Lateral wallBony :• Lateral attic wall above pars flaccida• lateral wall of hypotympanum

Membranous :• Tympanic membrane

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Medial wall• Promontory• Oval window• Round window• Tympanic part of bony facial nerve canal• Lateral semicircular canal• Processus cochlearformis

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The promontory is a rounded elevation occupying much of the central portion of the medial wall.

formed by basal turn of cochlea

usually has small grooves on its surface containing the nerves which form the tympanic plexus

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Oval window

lies Behind and above the promontory A kidney-shaped opening that connects

the tympanic cavity with the vestibule. Close by a footplate of the stapes. Its size average 3.25 mm long and 1.75

mm wide.

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Round window

• Lies below and behind the oval window

• Separated by subiculum(post extension of promontory)

• Ponticulus.another ridge above subiculum and runs to pyramid on the posterior wall

• Sinus tympani is where ponticulus and subiculum meet

• RWM is 2.3×1.9.• It is placed at right angles to

plane of stapes foot plate

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Facial nerve canal• The facial nerve canal (or Fallopian canal) runs above

the promontory and oval window in an anteroposterior direction.

• The facial nerve canal is marked anteriorly by the processus cochleariformis, a curved projection of bone, concave anteriorly, which houses the tendon of the tensor tympani muscle as it turns laterally to the handle of the malleus.

• The region above the level of the facial nerve canal forms the medial wall of the epitympanum.

• Behind the oval window, the facial canal starts to turn inferiorly as it begins its descent in the posterior wall of the tympanic cavity.

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• The dome of the lateral semicircular canal is the major feature of the posterior portion of the epitympanum, lying posterior and extending a little lateral to the facial canal

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Posterior wall

• Aditus ad antrum• Fossa incudis for short process of incus• Bulge produced by lateral semicircular canal• Pyramidal eminence for stapedius tendon• Buldge produced by vertical part of facial

nerve• Sinus tympani• Facial recess

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The posterior wall is wider above than below. Upper part a large irregular opening - the aditus ad antrum, that

leads back from the posterior epitympanum into the mastoid antrum.

Below the aditus is a small depression, the fossa incudis, which houses the short process of the incus and its suspensory ligament.

Below the fossa incudis and medial to the opening of the chorda tympani nerve is the pyramid, a small hollow conical projection with its apex pointing anteriorly.

This houses the stapedius muscle and tendon, which inserts into the posterior aspect of the head of stapes.

The canal within the pyramid curves downwards and backwards to join the descending portion of the facial nerve canal.

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Facial Recess

The facial recess is bounded:

– Medially by the facial nerve and

– Laterally by the tympanic annulus,

– with the chorda tympani nerve running obliquely through the wall between the two.

The chorda always runs medial to the tympanic membrane.

The facial recess is a groove which lies between the pyramid with facial nerve, and the annulus of the tympanic membrane .

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Facial Recess The angle between the facial nerve and the chorda allows a posterior

tympanotomy, allowing access to the middle ear from the mastoid without disruption the tympanic membrane.

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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

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Contents of middle ear cavity• Air• 3 ossicles : malleus, incus and stapes• 2 muscles : tensor tympani & stapedius• 2 nerves : chorda tympani & tympanic

plexus• Mucosal folds & ligaments• Blood vessels

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•

ossicles

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MUSCLE ORIGIN INSERTION NERVE SUPPLY

ACTION

Tensor typmani

Cartilaginous part of ET, its own bony canal

Upper part of handle of malleus

Branch from mandibular nerve [V3]

tensing tympanic membrane to reduce the force of vibrations in response to loud noises

Muscles of the middle ear

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Stapedius pyramidal eminence

Neck of stapes

Branch of facial nerve [VII]

pulls the stapes posteriorly and prevents excessive oscillation in loud noises

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THE CHORDA TYMPANI NERVE It enters the tympanic cavity from

the posterior canaliculus at the junction of the lateral and posterior walls.

It runs across the medial surface of the tympanic membrane between the mucosal and fibrous layers.

Then passes medial to the upper portion of the handle of the malleus above the tendon of tensor tympani.

leaves thorough petrotympanic fissure.

carries taste sensation from the anterior two-thirds of the same side of the tongue and secretomotor fibres to the submandibular gland.

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THE TYMPANIC PLEXUS It is formed by the

– tympanic branch of the glossopharyngeal nerve (Jacobson's nerve) and

– caroticotympanic nerves, which arise from the sympathetic plexus around the internal carotid artery.

The nerves form a plexus on the promontory and provide the branches to the mucous membrane lining the tympanic cavity, Eustachian tube and mastoid antrum and air cells.

The plexus also provides branches to join the greater superficial petrosal nerve and the lesser superficial petrosal nerve that contains all the parasympathetic fibres of the glossopharyngeal nerve.

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Mastoid air cells

• Mastoid processes can be pneumatic, sclerosed or mixed

• Mastoid process develops by the age of 2 yrs

• Antrum is well developed at birth

• Aditus ad antrum is the opening in the posterior wall of middle ear and leads posteriorly to antrum

• Vary considerably in number, form & size• Interconnected & lined by squamous non-ciliated

epithelium

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Mastoid antrum

• The lateral: formed by squamous

temporal bone• Medial wall: related with the posterior and horizontal semicircular canal• Posteriorly :

communicate by several openings with mastoid air cells.Important surgical marks to mastoid antrum is the MacEwen’s Triangle

• The roof of mastoid antrum (tegmin antri) separate it from middle cranial fossa.

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MacEwen’s Triangle• Superior: temporal line

• 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

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Mucosa Of Middle Ear Cleft• Mucus membrane of the nasopharynx is

continuous with that of middle ear ,aditus and antrum.

• Mucus secreting • Respiratory type• Cilia bearing• Lines the bony wall of the tympanic cavity and

wraps the middle ear structure_ossicles ,muscles ligaments and nerves like peritoneum wraps viscera of the abdomen.

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Blood supply• Arteries : Middle ear is supplied by the following1)Two main arteries

a)Anterior tympanic branch of maxillary arteryb)Stylomastoid branch of posterior auricular artery

2)Four minor arteries a)Petrosal branch of middle meningeal arteryb)Superior tympanic branch of middle meningeal arteryc)Branch of artery of pterygoid canald)Tympanic branch of internal carotid

• Veins : Pterygoid venous plexus Superior petrosal sinus

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Lymphatic drainage

• Middle ear : Retropharyngeal & parotid nodes

• ET : Retropharyngeal group of nodes

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Facial Nerve

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• Meatal segment : 8-10 mm

• Labyrinthine segment : 4 mm

• Tympanic segment : 11 mm

• Mastoid segment : 13 mm

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• Gives 3 branches in intratemporal part.

1. Greater petrosal nerve

2. Nerve to the stapedius

3. Chorda tympani

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Surgical landmarks of facial nerve

• Processus cochleariformis• Oval window and horizontal canal• Short process of the incus• Pyramid • Digastric ridge: just deep to or medial to

the mastoid tip anterior to that is the tip of the facial nerve

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• Cartilaginous pointer: sharp traingular piece of cartillage of pinna and points to the nerve which lies 1 cm deep .

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