Aeration Pathways of the Middle Ear

The Epitympanic Diaphragm

Pathway 1 - The Tympanic Isthmus (Primary Pathway)

1. Starts at the Eustachian tube (protympanum / bony ET opening)
2. Proceeds posteriorly through the mesotympanum, staying medial to the manubrium of the malleus and the long process of the incus
3. Passes medial to the lateral incudomalleal fold
4. Continues superiorly into the posterior epitympanum (superior lateral attic)
5. From there communicates posteriorly via the aditus ad antrum into the mastoid antrum and mastoid air cell system

Pathway 2 - Anterior Epitympanic Space (via the Tensor Fold)

• When the tensor fold is absent or incomplete, air can pass from the supratubal recess directly into the anterior epitympanum - this is the "supratubal recess pathway."
• When the tensor fold is present and intact, it acts as a barrier, blocking direct ventilation of the anterior epitympanum from below. In this situation, the AES can only be aerated via a round-about route through the posterior epitympanum.
• Surgically, opening (incising) the tensor fold during chronic ear surgery provides an alternate ventilation route for the anterior epitympanum, which is exploited in procedures for cholesteatoma to prevent recurrence.

Pathway 3 - Mastoid and Petrous Apex Air Cell Tracts

• Mastoid air cells - the largest pneumatized reservoir (normal combined middle ear + mastoid volume ~6 cc)
• Petrous apex air cells - via pneumatization tracts including the subcochlear canaliculi (passing inferior to the cochlea through the retrotympanum/hypotympanum)
• Peritubal cells and perilabyrinthine cells - smaller accessory tracts

Why Aeration Matters - Clinical Consequences

• Normal middle ear + mastoid volume: ~6 cc
• Volume <0.5 cc: progressive conductive hearing loss
• Non-aerated ears: 40-60 dB air-bone gap due to:
  • Reduced ossicular coupling
  • Restricted round window membrane movement (round window cannot move if middle ear is gas-free/fluid-filled, since cochlear fluids are incompressible)
• Negative middle ear pressure (ET dysfunction): disproportionate hearing loss at frequencies <1,000 Hz, with worse effect when middle ear pressure is negative relative to the ear canal

Summary Table

Spread of Cholesteatoma

Mechanism of Spread - Bone Erosion

1. Enzymatic osteolysis - The cholesteatoma matrix and perimatrix produce collagenases, metalloproteinases (MMPs), and other proteolytic enzymes that actively digest bone. Children's cholesteatomas show increased expression of these markers compared to adults, explaining the faster spread often seen in paediatric disease.
2. Pressure necrosis - The accumulating keratin debris expands the sac, generating pressure against surrounding bony walls, leading to ischaemic necrosis of bone.
3. Osteoclastic resorption - Osteoclast activation by cytokines (IL-1, TNF-α) released from the perimatrix further amplifies bone destruction.

Origin and Initial Spread

Pars Flaccida (Attic/Epitympanic) Cholesteatoma - Most Common (~80%)

Prussak's Space (lateral epitympanic space)
        ↓
Erosion of SCUTUM (earliest radiological sign)
        ↓
Medial displacement of ossicular chain
        ↓
Posterior epitympanum → Aditus ad antrum
        ↓
Mastoid antrum → Mastoid air cells

Pars Tensa Cholesteatoma - Less Common (~20%)

Posterosuperior retraction pocket
        ↓
Medial to ossicular chain (posterior mesotympanum)
        ↓
Sinus tympani / posterior tympanic sinus / hypotympanum
        ↓
Round window niche → Cochlea (sensorineural loss)
        ↓
Retrotympanum → Petrous apex (via subcochlear canaliculi)

• Pars tensa cholesteatomas tend to lie medial to the ossicular chain from the outset, so they erode the long process of the incus and stapes superstructure early.
• They are more likely to invade the inner ear structures and round window, causing SNHL earlier than pars flaccida types.

Ossicular Erosion - Pattern and Frequency

Spread to Critical Structures

Facial Nerve Canal

• Cholesteatoma erodes the horizontal (tympanic) segment of the facial canal most commonly (up to 19% of cases at surgery).
• Spread to the supratubal recess threatens the geniculate ganglion and first genu.
• CT detects dehiscence in only ~66-88% of cases; surgeons must always assume dehiscence.

Axial CT scan - right-sided cholesteatoma with middle ear opacification and bone erosion

Labyrinthine Fistula

• Occurs in 10-15% of cholesteatoma cases.
• The lateral semicircular canal is the most commonly affected (>90% of fistulas).
• Presents with vertigo, positive fistula test (pressure-induced nystagmus).
• CT sensitivity for fistula detection is unreliable (38-90% in reported series).

Intracranial Spread

1. Tegmen tympani/antri erosion → extradural abscess, subdural empyema, meningitis
2. Posterior fossa plate/sigmoid sinus plate erosion → sigmoid sinus thrombosis, cerebellar abscess
3. Petrous apex via subcochlear canaliculi → petrous apicitis (Gradenigo's syndrome)

Petrous Apex

• Pars tensa cholesteatomas in the retrotympanum can track through the subcochlear canaliculi (inferior to the cochlea) into petrous apex air cells.
• Presents as Gradenigo's triad: otorrhoea + ipsilateral abducens palsy (CN VI) + retro-orbital pain (CN V).

Anterior Spread

• Into the supratubal recess and protympanum - threatens the geniculate ganglion of the facial nerve and the Eustachian tube orifice.
• May rarely extend anteriorly to the carotid canal.

Summary - Routes at a Glance

                    ┌─ Tegmen → Intracranial (middle fossa)
                    │
Prussak's space ────┼─ Posterior epitympanum → Antrum → Mastoid
(attic chol.)       │   ├─ Lateral SCC (fistula)
                    │   ├─ Facial canal (horizontal segment)
                    │   ├─ Mastoid cortex (subperiosteal abscess)
                    │   └─ Posterior fossa plate → Sigmoid sinus / cerebellum
                    │
                    └─ Inferior mastoid tip → Bezold's abscess

Posterosuperior
retraction pocket ──── Medial to ossicles → Sinus tympani
(pars tensa chol.)      └─ Round window → Inner ear (SNHL)
                          └─ Retrotympanum → Petrous apex (Gradenigo)