Middle ear by arifa

MIDDLE EAR

BY ARIFA HUSSAIN

MIDDLE EAR

Middle ear together with the eustachian tube ,aditus ,antrum and mastoid air cells is called as MIDDLE EAR CLEFT.

The middle ear extends beyond the limits of tympanic membrane which forms its lateral boundary and sometimes divided into

Epitympanum(above pars tensa) mesotympanum (opp:pars tensa) hypotympanum(below pars tensa) Middle ear can be linked to a six sided box with a

roof ,a floor ,medial ,lateral ,anterior and posterior walls.

ANATOMY OF MIDDLE EAR

Divisions of middle ear

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.

The roof

Thin plate of bone which seperates tympanic cavity from the jugular bulb.sometimes,it is congenitally deficient and the jugular bulb may then project into middle ear ;seperated from the cavity only by the mucosa.

THE FLOOR

The walls:1.Canal for tensor tympani2.Eustachian tube opening3.Oval window4.Round window5.Processus cochleariformis6.Horizontal canal7.Facial nerve8.Pyramid9.Aditus10.Chorda tympani11.Carotid artery12.Jugular bulb

Thin plate of bone which seperates the cavity from the internal carotid artery.it also has two openings ; the lower one for the eustachian tube and the upper one for the canal of tensor tympani muscle.

The anterior wall

It lies close to the mastoid aircells.It presents a bony projection called the pyramid through the summit of which appears the tendon of the stapedius muscle to get attachment to the neck of stapes .

Aditus, an opening through which attic communicates with the antrum.

Facial nerve runs in the posterior wall just behind the pyramid.

Facial recess is a depression in the posterior wall lateral to the pyramid.

It is bounded medially by the vertical part of7th nerve laterally by the chorda tympani and above by the fossa incudis.

Posterior wall

Facial recess

It is formed by labyrinth. It has 1)a bulge called promontory 2)oval window into which foot plate of stapes is

fixed. 3)round window which is covered by secondary

tympanic membrane. 4)just anterior to the oval window, the medial

wall presents a hook like projection called the processus cochleari formis, which marks the level of the genu of facial nerve.

Medial wall

Medial wall

1.Promontory2.Processus cochleariformis3.CN 7th

4.Oval window5.Horizontal canal6.Pyramid7.Ponticulus8.Sinus tympani9.Subiculum10.Round window11.Tympanic plexus

It is formed largely by the tympanic membrane .

The tympanic membrane is semi transparent and forms a window into the middle ear.

Lateral wall

ADITUS: is an opening through which the attic communicates with the antrum.

Facial nerve courses just below the aditus. MASTOID ANTRUM: is a large air containing space in

the upper part of mastoid and communicates with the attic through the aditus.

The mastoid consists of bone cortex with a “honeycomb” of aircells underneath.

Its roof is formed by the tegmen antri which is a continuation of the tegmen tympani and seperates it from the middle cranial fossa.

The lateral wall of the antrum is formed by a plate of bone.

Its is marked externally on the surface of mastoid by supra meatal(MACEWEN’S ) triangle.

Aditus and mastoid antrum

Macewen’s triangle

Mastoid air cells

Mastoid air cells

There are three ossicles in the middle ear malleus,incus and stapes.

ossicles of middle ear

EAR OSSICLES

MALLEUS: has head ,neck ,handle (manubrium), lateral and anterior process.

INCUS: has body , short process , long process and a lenticular process .

STAPES: has a head,neck, anterior and posterior crura and a foot plate.

The ossicles conduct sound energy from the tympanic membrane to the oval window and then to the inner ear fluid.

The malleus,incus, stapes

Middle ear is supplied by the following1)Two main arteriesa)Anterior tympanic branch of maxillary arteryb)Stylomastoid branch of posterior auricular

artery2)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

Blood supply of middle ear

TYMPANIC PLEXUS: 1)tympanic branch of glossopharyngeal 2)sympathetic fibres from plexus round the

ICA.CHORDA TYMPANI NERVE: branch of facial

nerve, passes through the middle ear.

Nerve supply

MIDDLE EAR: Retropharyngeal and parotid nodes.

EUSTACHIAN TUBES :retropharyngeal group.

Lymphatic drainage

Endoscopic view of ear showing important structres

o Pathology can be well estimated on otoscopic examination,further questions like extent of disease,exact location,structures involved and possible bone erosions ,will be addressed in detail only with CT&/MRI.

o The majority of everyday practise cases will consists of inflammatory disease ,either chronic/acute&sometime tumours.

o On CT examination,these entities will demonstrate a MASS /DIFFUSE OPACIFICATION OF MIDDLE EAR CAVITY.

RADIOLOGICAL INVESTIGATIONS

Lateral view of skull showing normal mastoid.

Ct scan showing normal middle ear

Images should be viewed in correct bone window setting –large window width(4000HU)&low window level(0-200HU)

In order to evaluate possible ossicular erosion, axial CT images should be searched for ‘3’ distinctive signs:

1.Icecream cone . 2.Two parallel lines. 3.Two dots.

FOR BEST DIAGNOSTIC ACCURACY

Axial CT bone window:long arrow –malleus head and short arrow- incus body.(1.ICE CREAM CONE APPEARANCE)

Normal anatomy of middle ear

2.TWO PARALLEL LINES:Long arrow-Malleus handle and short arrow- incus long process

3.TWO DOTS:Malleus neck,incus lenticular process and stapes head(Incudostapedial joint)

Thank you-