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Anatomy of the Middle Ear Cleft and Its Surgical Importance

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DEFINITION AND OVERVIEW

1. Tympanic cavity (with its sub-compartments)
2. Eustachian (auditory) tube - the anterior extension
3. Mastoid antrum and mastoid air cell system - the posterior extension

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PART 1: THE TYMPANIC CAVITY

Subdivisions of the Tympanic Cavity

• Retrotympanum - posterior to the promontory (contains sinus tympani, facial recess, oval and round window niches)
• Protympanum - anterior, contiguous with the Eustachian tube opening

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WALLS OF THE TYMPANIC CAVITY (6 Walls)

1. LATERAL WALL (Membranous Wall)

• Formed by the tympanic membrane centrally
• Bony lateral wall of epitympanum (scutum) superiorly
• Bony hypotympanic wall inferiorly

• Oval, ~9-10 mm in vertical and 8-9 mm in horizontal diameter
• Set at ~55° to the floor of the ear canal
• Divided into:
  • Pars tensa - the larger lower portion, with a fibrocartilaginous annulus seated in the tympanic sulcus
  • Pars flaccida (Shrapnell membrane) - small triangular upper portion above the malleolar folds, within the notch of Rivinus; lacks a tympanic annulus; its lamina propria is lax with randomly oriented collagen fibres
• Both portions have 3 layers: outer epidermis (continuous with EAC skin), middle lamina propria (fibrous), inner mucosal layer (continuous with middle ear mucosa)
• The scutum (outer attic wall) is the sharp wedge-shaped inferior portion of the lateral epitympanic wall - it is thin and easily eroded by cholesteatoma, which is the classic CT finding in attic cholesteatoma
• Petrotympanic fissure on the medial surface transmits the anterior malleolar ligament and anterior tympanic branch of the maxillary artery; the chorda tympani exits anteriorly through this fissure

• Pars flaccida is the site where attic/epitympanic cholesteatoma originates due to retraction
• The thin scutum bone is used as a target for drilling in atticotomy
• Petrotympanic fissure transmits chorda tympani - damage during myringoplasty causes taste disturbance

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2. MEDIAL WALL (Labyrinthine Wall)

• Promontory - a rounded eminence formed by the first turn of the cochlea. The tympanic plexus (Jacobson's nerve, branch of CN IX, carrying parasympathetics to parotid) runs over its surface.
• Oval window (Fenestra Vestibuli) - above and behind the promontory; 3.25 mm long, 1.75 mm wide; closed by the stapes footplate and its annular ligament; connects tympanic cavity to the vestibule
• Round window (Fenestra Cochleae) - below and behind the promontory, separated from the oval window by the subiculum (bony ridge); the round window membrane (2.3 × 1.9 mm) faces the scala tympani; partially obscured by overhanging bony niche and mucosal folds
• Facial nerve canal (Fallopian canal) - runs horizontally above the promontory and oval window; may have microdehiscences (in up to 25-55% of cases) making the facial nerve vulnerable during surgery; the two or three straight blood vessels visible along this line are a reliable intraoperative marker
• Processus cochleariformis - a bony shelf at the anterior end of the medial wall where the tensor tympani tendon turns 90° laterally to insert on the malleus handle; used as a key surgical landmark for the facial nerve (geniculate ganglion lies just anterosuperior to it)
• Horizontal (lateral) semicircular canal - lies above the facial nerve; its prominence forms an important intraoperative landmark in mastoid surgery
• Ponticulus - bony ridge from promontory to pyramidal eminence, separating the oval window niche from sinus tympani
• Sinus tympani - a deep recess posterior and medial to the pyramidal eminence; notoriously difficult to visualize microscopically and is a common site for residual cholesteatoma

• The facial nerve dehiscence over the oval window is the major cause of iatrogenic facial palsy in stapedectomy and tympanoplasty
• The cochleariform process is used to locate the geniculate ganglion during middle fossa approaches
• Sinus tympani is the most common site of residual disease after cholesteatoma surgery

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3. POSTERIOR WALL (Mastoid Wall)

• Aditus ad antrum - the opening in the upper part of the posterior wall connecting the epitympanum to the mastoid antrum; this is the pathway through which epitympanic cholesteatoma invades the mastoid
• Pyramidal eminence (Pyramid) - hollow bony projection through which the tendon of stapedius muscle emerges; the facial nerve runs directly behind this structure
• Facial recess (posterior tympanotomy) - a recess bounded medially by the facial nerve, superiorly by the fossa incudis (which holds the short process of the incus), and laterally by the chorda tympani; access to the mesotympanum via this recess is the basis of posterior tympanotomy used in cochlear implant surgery
• Fossa incudis - a small hollow in the posterosuperior part holding the short process of the incus; loss of this bony support contributes to incus dislocation in trauma

• The facial recess is the surgical corridor for cochlear implant electrode insertion without opening the ear canal
• Aditus ad antrum is skeletonized in modified radical mastoidectomy
• The descending facial nerve immediately medial to the pyramid is at risk during posterior canal wall-down mastoidectomy

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4. ANTERIOR WALL (Carotid Wall)

• Contains the opening of the Eustachian tube (lower part) and the canal for tensor tympani muscle (upper part, above the Eustachian tube)
• The internal carotid artery ascends in close relation to the anteromedial wall; the bony partition may be thin or dehiscent, making it vulnerable to injury during anterior tympanotomy or temporal bone surgery
• Petrotympanic fissure transmits chorda tympani anteriorly

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5. ROOF (Tegmental Wall - Tegmen Tympani)

• A thin plate of bone separating the middle ear from the middle cranial fossa (dura mater)
• May contain dehiscences or be eroded by cholesteatoma, allowing intracranial spread
• The arcuate eminence (prominence of the superior semicircular canal) is an adjacent landmark on the middle fossa floor
• The temporal line on the mastoid surface is ~5 mm below the tegmen level

• Tegmen erosion by cholesteatoma permits extradural abscess, meningitis, or temporal lobe abscess
• The tegmen is removed during the middle cranial fossa approach to the internal auditory canal

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6. FLOOR (Jugular Wall)

• Thin plate of bone separating the tympanic cavity from the jugular bulb
• May be absent or dehiscent - a high riding jugular bulb can bulge into the inferior tympanic cavity, mimicking a vascular mass (pulsatile tinnitus)
• Tympanic canaliculus in the floor transmits Jacobson's nerve (tympanic branch of CN IX), which crosses the promontory and contributes to the tympanic plexus

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PART 2: THE AUDITORY OSSICLES

Malleus

• Largest ossicle; consists of head, neck, manubrium (handle), lateral process, anterior process
• The manubrium is embedded in the fibrous layer of the tympanic membrane
• Head lies in the epitympanum; articulates with the incus via the incudomalleolar joint
• Attached by the anterior malleolar ligament (through petrotympanic fissure), lateral malleolar ligament, and superior malleolar ligament

Incus

• Middle ossicle; consists of body, short process, long process, and lenticular process
• Body articulates with the head of malleus in the epitympanum
• Long process runs parallel to the manubrium and ends in the lenticular process, which articulates with the stapes
• The long process of the incus is the most vulnerable ossicle to avascular necrosis and erosion in chronic otitis media - it has a single nutrient vessel with no collateral circulation; this is why incus erosion is the most common ossicular problem in COM
• Short process projects posteriorly and rests in the fossa incudis

Stapes

• Smallest ossicle (and smallest bone in the body); consists of head, neck, two crura (anterior and posterior), and footplate
• The footplate (3 mm × 1.4 mm) sits in the oval window secured by the annular ligament
• The stapedius tendon inserts into the neck/posterior crus of the stapes
• In otosclerosis, the annular ligament becomes fixed by abnormal bone → conductive hearing loss → stapedectomy/stapedotomy is the surgical treatment

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PART 3: MIDDLE EAR MUSCLES

Stapedius Muscle

• Arises from within the pyramidal eminence (cavity within the pyramid)
• Tendon emerges from the apex of the pyramid and inserts into the stapes
• Supplied by a branch of the facial nerve (CN VII)
• Contracts reflexly (stapedial reflex) in response to loud sounds, stiffening the ossicular chain and protecting the inner ear
• Facial nerve palsy distal to pyramidal eminence causes hyperacusis (loss of stapedial reflex)

Tensor Tympani

• Arises from walls of bony canal above the Eustachian tube and from the cartilaginous Eustachian tube and greater wing of sphenoid
• Runs posteriorly, turns 90° at the cochleariform process, and inserts into the medial surface of the malleus handle
• Supplied by the medial pterygoid nerve (branch of the mandibular nerve, CN V3)

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PART 4: IMPORTANT NERVES IN THE TYMPANIC CAVITY

Facial Nerve (CN VII)

• The tympanic (horizontal) segment runs from the geniculate ganglion to the second genu, above the promontory and oval window
• Dehiscence of the facial canal is common (25-55%); facial nerve may prolapse into the oval window niche
• The descending (mastoid) segment runs vertically from the second genu to the stylomastoid foramen

Chorda Tympani

• Enters the tympanic cavity via the posterior canaliculus at the junction of the lateral and posterior walls
• Runs medial to the upper part of the malleus handle, between the mucosal and fibrous layers of the tympanic membrane
• Exits anteriorly via the anterior canaliculus into the petrotympanic fissure
• Carries taste from the anterior 2/3 of the tongue and preganglionic parasympathetics to the submandibular/sublingual glands
• Injury during ear surgery causes ipsilateral metallic taste and taste disturbance

Tympanic Plexus (Jacobson's nerve)

• Formed on the promontory by Jacobson's nerve (branch of CN IX), sympathetic fibres from the carotid plexus, and the caroticotympanic nerves
• Provides sensory supply to the mucosa of the middle ear, Eustachian tube, and mastoid
• Continues as the lesser petrosal nerve → parotid gland parasympathetics
• Glomus tympanicum arises from paraganglionic cells along this nerve - the most common middle ear tumour

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PART 5: THE EUSTACHIAN TUBE

• Connects the anterior wall of the tympanic cavity (at the protympanum) to the nasopharynx
• Length approximately 35-36 mm; angled ~45° downward from middle ear to nasopharynx
• Bony (tympanic) third - lateral, opens into the protympanum; always open
• Fibrocartilaginous two-thirds - medial; normally closed at rest, opens during swallowing and yawning
  • The tube is shaped like an inverted-J in cross section; collapsed laterally by a fibrous membrane
  • Tensor veli palatini (innervated by CN V3) opens the tube on contraction
  • Levator veli palatini supports the tube
• Internal carotid artery is closely related to the medial wall of the bony Eustachian tube - at risk during petrous apex surgery
• Bony-cartilaginous junction (isthmus) is the narrowest point; site where secretions accumulate

• Eustachian tube dysfunction → negative middle ear pressure → retraction pockets → cholesteatoma formation
• Failure of Eustachian tube function is the fundamental pathogenesis of secretory otitis media (glue ear); treatment is ventilation tubes (grommets) which bypass the tube's pressure equalization function
• Cleft palate patients have Eustachian tube dysfunction due to abnormal tensor veli palatini insertion → chronic otitis media with effusion
• Balloon Eustachian tuboplasty is a minimally invasive procedure for chronic Eustachian tube dysfunction

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PART 6: THE MASTOID ANTRUM AND MASTOID AIR CELL SYSTEM

Mastoid Antrum

• The mastoid antrum is the largest and most constant air cell in the mastoid, always present regardless of mastoid pneumatization pattern
• Located posterosuperior to the epitympanum, communicating with it via the aditus ad antrum
• The antrum lies approximately 15 mm deep to the surface of the mastoid cortex in adults (less in children)
• Related to: tegmen (roof - middle fossa dura above), sigmoid sinus (posteriorly), posterior semicircular canal and lateral semicircular canal (medially), facial nerve (anteromedially)

• A surgical surface landmark for locating the mastoid antrum
• Bounded by the posterior root of the zygoma (temporal line) above, posterosuperior canal wall anteriorly, and a tangent line from these two points posteriorly
• The antrum lies ~1.5 cm deep to the center of this triangle

Mastoid Pneumatization

• The degree of pneumatization varies widely:
  • Pneumatic type: well-aerated, with multiple large air cells extending to the tip, zygomatic root, and perisinus regions
  • Diploic type: small cells, diploic bone predominates
  • Sclerotic type: dense bone, minimal or no air cells (associated with chronic otitis media in childhood)
• Primary regions of pneumatization: mastoid, perilabyrinthine, petrous apex, accessory (zygomatic, squamous, occipital, styloid)
• Air cell tracts serve as pathways for spread of infection to intracranial, infralabyrinthine, and petrosal compartments

Surgical landmarks on the mastoid surface:

• Spine of Henle (suprameatal spine) - small bony projection just above and behind the posterior wall of the EAC; the antrum lies just medial and slightly superior to this
• Temporal line (linea temporalis) - marks the approximate level of the tegmen; ~5 mm below the middle fossa floor
• Sigmoid sinus - the posterior limit of mastoid dissection; its anterior wall (Trautmann's triangle area) is exposed to access the posterior fossa
• Digastric ridge - inferior landmark for the mastoid tip; the stylomastoid foramen (where the facial nerve exits) is at its anterior limit
• Sinodural angle - the angle between the tegmen and sigmoid sinus; Citelli's angle; important landmark in mastoid surgery

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PART 7: SURGICAL IMPORTANCE - COMPREHENSIVE SUMMARY

1. Cholesteatoma Surgery

• Cholesteatoma typically originates from pars flaccida retraction (attic cholesteatoma) or pars tensa retraction (posterosuperior retraction pocket)
• It erodes the scutum, invades the epitympanum, traverses the aditus ad antrum, and colonizes the mastoid
• Key surgical principles: remove all keratinizing epithelium from the sinus tympani, facial recess, oval and round window niches, and aditus
• Sinus tympani (medial to the pyramidal eminence) is the most common site of residual disease; endoscopic assistance (0° or 45° endoscope) is needed to visualize it fully
• The tegmen and sigmoid sinus plate must be identified to define the limits of dissection

2. Tympanoplasty and Myringoplasty

• Knowledge of the three-layer structure of the tympanic membrane guides graft placement (underlay vs. overlay technique)
• The middle fibrous layer is responsible for the stiffness needed for sound transmission; myringoplasty grafts (typically temporalis fascia) aim to replicate this function
• Chorda tympani identification and preservation is essential to avoid taste disturbance

3. Stapedectomy / Stapedotomy (for Otosclerosis)

• The facial nerve is the primary structure at risk during stapes surgery; its horizontal segment runs directly above the oval window; dehiscence must be assessed preoperatively on HRCT
• The long process of incus must be identified and preserved; necrosis here necessitates prosthesis work-around
• The annular ligament is divided and the stapes footplate removed (stapedectomy) or a small fenestration created (stapedotomy) to place a prosthesis
• A high jugular bulb may extend into the hypotympanum and be at risk if the surgeon works inferiorly

4. Mastoidectomy

• Cortical mastoidectomy (Schwartze operation): standard approach for acute coalescent mastoiditis; the antrum is entered after identifying MacEwen's triangle and drilling through the cortex
• Modified radical mastoidectomy: posterior canal wall removed, creating a common cavity; used for extensive cholesteatoma
• Radical mastoidectomy: all middle ear contents and posterior canal wall removed; the Eustachian tube is obliterated
• Canal wall-up (combined approach tympanoplasty): preserves posterior canal wall; risk of residual disease mandates second-look surgery
• Key surgical steps: identify the tegmen (roof), sigmoid sinus (posterior), lateral semicircular canal and facial nerve (medial limits) - these are the four corners of safe mastoid dissection

5. Cochlear Implant Surgery

• The posterior tympanotomy (facial recess approach) provides access to the round window for electrode insertion without disturbing the posterior canal wall
• The facial recess is bounded by the facial nerve medially, chorda tympani laterally, and fossa incudis superiorly
• The cochleariform process and pyramidal eminence are key landmarks in this approach

6. Facial Nerve Surgery

• The tympanic (horizontal) segment of the facial nerve is exposed via a transmastoid approach combined with a middle fossa or transcanal route
• The cochleariform process is the landmark for the geniculate ganglion (located just deep to and anterolateral to it)
• Dehiscent facial nerve must be recognized on preoperative HRCT to avoid inadvertent injury during any middle ear procedure

7. Glomus Tympanicum

• Most common middle ear tumor; arises from paraganglionic cells along Jacobson's nerve on the promontory
• Appears as a reddish pulsatile mass behind an intact tympanic membrane ("rising sun" sign)
• Surgery involves identification and ligation of the blood supply before resection from the promontory

8. Complications of Otitis Media - Anatomical Basis

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EMBRYOLOGICAL BASIS (Brief)

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SUMMARY TABLE: KEY SURGICAL LANDMARKS

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CLINICAL IMAGES FOR REFERENCE

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MARK ALLOCATION GUIDE (for 20-mark question)

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