Middle Ear Space (Tympanic Cavity)

Definition and Overview

1. The tympanic cavity (the middle ear space proper)
2. The Eustachian (pharyngotympanic) tube
3. The mastoid air cell system

Subdivisions

• Tympanic cavity proper - immediately adjacent to the tympanic membrane (mesotympanum)
• Epitympanic recess (attic) - the space superior to the level of the tympanic membrane, housing the heads of malleus and incus

• Protympanum - anterior to the tympanic membrane, near the Eustachian tube orifice
• Hypotympanum - below the level of the tympanic annulus
• Posterior epitympanum and anterior epitympanic space, separated by the cog (a bony crest projecting from the tegmen tympani anteriorly to the malleus head) - surgically important as a site for residual cholesteatoma

Walls of the Tympanic Cavity

Roof - Tegmental Wall

Floor - Jugular Wall

Lateral Wall - Membranous Wall

• Pars tensa - the larger inferior portion with a tympanic annulus, three layers (epidermis, lamina propria with radial and circular fibres, inner mucosa)
• Pars flaccida - superior triangular portion above the anterior and posterior malleolar folds, lacking a firm annulus, within the notch of Rivinus

Medial Wall - Labyrinthine Wall

• Promontory - a rounded elevation over the basal turn of the cochlea, carrying the tympanic plexus (from CN IX tympanic branch + caroticotympanic sympathetic fibres)
• Oval window (fenestra vestibuli) - posterosuperior to the promontory, kidney-shaped, closed by the stapes footplate and annular ligament; connects to the vestibule
• Round window (fenestra cochleae) - posteroinferior to the promontory, closed by the secondary tympanic membrane; allows pressure release of cochlear fluid
• Prominence of the facial canal - ridge produced by CN VII passing through the temporal bone, posterosuperior to the oval window
• Prominence of the lateral semicircular canal - broader ridge above and posterior to the facial canal
• Processus cochleariformis - a bony shelf at which the tensor tympani tendon turns laterally to attach to the malleus

Anterior Wall

• Lower third: thin bone covering the internal carotid artery (can be dehiscent in ~2%), perforated by caroticotympanic nerves (sympathetic)
• Middle third: tympanic orifice of the Eustachian tube (~5 × 4 mm)
• Just above: canal for the tensor tympani muscle
• Upper third: may contain the supratubal recess, separated from the anterior epitympanic space by the tensor fold
• Exit foramen for the chorda tympani nerve

Posterior Wall - Mastoid Wall

• Aditus ad antrum - large irregular opening from the posterior epitympanum leading to the mastoid antrum
• Fossa incudis - houses the short process of the incus and its suspensory ligament
• Pyramid (pyramidal eminence) - hollow conical projection housing the stapedius muscle; its tendon inserts into the head of the stapes
• Facial recess - groove between the pyramid/facial nerve and the tympanic annulus; important surgical corridor
• Sinus tympani - medial to the facial nerve, between the ponticulus and subiculum; a common site for residual disease

Contents of the Middle Ear

Ossicular Chain

Muscles

• Tensor tympani - in a bony canal along the medial wall; tendon hooks around the processus cochleariformis and attaches to the malleus handle. Supplied by CN V3 (medial pterygoid nerve). Stiffens the ossicular chain.
• Stapedius - smallest skeletal muscle in the body; housed in the pyramid, inserts into the stapes head. Supplied by CN VII. The stapedial (acoustic) reflex protects against loud sounds.

Nerves

• Chorda tympani (branch of CN VII) - enters through the posterior wall, runs between the handle of the malleus and the long process of the incus across the medial surface of the tympanic membrane, and exits via the anterior wall into the infratemporal fossa. Carries taste from anterior 2/3 of tongue and preganglionic parasympathetics to submandibular ganglion.
• Tympanic plexus - on the promontory; formed mainly by the tympanic branch of CN IX (Jacobson's nerve). Gives the lesser petrosal nerve (preganglionic parasympathetics to parotid gland via otic ganglion).

Communications

Lining

Clinical Relevance

• Otitis media - inflammation commonly spreading from the nasopharynx via the Eustachian tube; most frequent in young children
• Cholesteatoma - keratinizing squamous epithelium in the middle ear; commonly forms in the pars flaccida region or the posterior epitympanic recess; can erode ossicles, tegmen, and facial canal
• Mastoiditis - infection spreading from the middle ear through the aditus; can progress to intracranial complications given the thin tegmen tympani
• Jugular bulb anomaly - high-riding or dehiscent jugular bulb risks injury during myringotomy or tympanomeatal flap elevation
• Facial nerve dehiscence - the facial canal may be dehiscent over the oval window niche in a significant minority, risking injury during middle ear surgery
• Carotid artery dehiscence - ~2% of cases; the internal carotid is exposed in the protympanum, presenting as a pulsatile mass