Corpus: Tympanum

The term tympanic cavity refers to the cavity of the middle ear in which the ossicles are located. It begins directly behind the eardrum and is ventilated by the auditory tube, which enables pressure equalisation.

The tympanic cavity is approx. 12 to 15 mm long and 3 to 7 mm wide. Its internal volume is small and is only about 1 cm³. Simplified, it consists of 3 sections:

• Tympanic dome (epitympanum, recessus epitympanicus): cranial to the tympanic membrane
• Middle tympanic cavity (mesotympanum): Main part between the eardrum and labyrinth
• Tympanic cavity (hypotympanum): caudal to the tympanic membrane

Some authors also differentiate between

• Protympanum: anterior to the tympanic membrane
• Retrotympanum: posterior to the tympanic membrane.

Idealised, the tympanic cavity can be imagined as a rectangle, so that a total of 6 wall sections can be distinguished:

• Paries tegmentalis (tegmen tympani): superior
• Paries jugularis: inferior
• Paries labyrinthicus: medial
• Paries membranaceus: lateral
• Paries caroticus: anterior
• Paries mastoideus: posterior

Epitympanum[Bearbeiten]

The epitympanum is the uppermost section of the tympanic cavity, located above the eardrum. It is also known as the "recessus epitympanicus" or "atticus" and contains the paries tegmentalis.

Cranially, the tympanic cavity is separated from the middle cranial fossa by a thin plate of bone known as the tympanic roof (tegmen tympani). In up to 20% of people there are bone defects in the tegmen, particularly on the right side, which are covered by dura mater. The tegmen can also be pneumatised. It develops from the pars petrosa (medial) and pars squamosa (lateral) of the temporal bone. The section of the petrosquamous fissure localised in the roof ossifies except for a hole for the superior tympanic artery.

The hammerhead (caput mallei) and incus body (corpus incudis) are located in the epitympanum. The tendon of the tensor tympani muscle is usually located 5 mm below the tegmen and defines the boundary to the mesotympanum.

The 5 mm wide and long bony canal aditus ad antrum, which forms the access to the antrum mastoideum and the cellulae mastoideae, is also located here. A transverse mucosal fold, the plica mallearis superior, divides the epitympanum into an anterior and a posterior section.

Mesotympanum[Bearbeiten]

The mesotympanum is the middle section of the tympanic cavity. The anatomical relationship of the mesotympanum to the neighbouring structures is described by the surrounding 4 walls:

Paries caroticus[Bearbeiten]

The paries caroticus forms the anterior medial boundary of the tympanic cavity. In the lower half is the ascending section of the pars petrosa of the internal carotid artery (ACI) with a surrounding venous plexus. Dehiscences may be present in the bone wall, which is usually 2 mm thick, so that the ACI is directly covered by the mucous membrane of the tympanic cavity. The branches of the ACI (rami caroticotympanici) with sympathetic fibres (nervi caroticotympanici) branch off through fine canals (canaliculi caroticotympanici). Above the carotid canal lie the inner ostium of the auditory tube (ostium tympanicum tubae auditivae) and the tensor tympani muscle (canalis musculotubarius).

Paries membranaceus[Bearbeiten]

The paries membranaceus makes up the lateral lower wall of the tympanic cavity. It is mainly formed by the eardrum. Here there is a mucosal fold extending forwards and backwards, the plicae mallearis anterior and posterior. It starts from the malleus neck (collum mallei) and extends downwards along the malleus handle (manubrium mallei). Between the fold and the eardrum lie the anterior and posterior tympanic membranes (Troeltsch pockets). The latter extends through a slit between the pars flaccida of the tympanic membrane and the malleus collum (Prussak space or superior tympanic membrane recess).

The chorda tympani is located in the plica, which enters the tympanic cavity dorsal to the edge of the tympanic membrane and runs between the neck of the malleus and the long incus (crus longum incudis) forwards along the processus anterior mallei to the petrotympanic fissure.

Paries mastoideus[Bearbeiten]

The paries mastoideus forms the posterior wall section adjacent to the mastoid cells. The antrum mastoideum with the aditus ad antrum is located in the upper section. The short anvil limb with the posterior anvil ligament is attached to the entrance of the aditus. The antrum and the aditus are bounded anteriorly by the protrusion of the lateral semicircular canal (Prominentia canalis semicircularis lateralis). Below this lies the facial canal, which runs laterally at the base of the antrum and then descends behind the tympanic membrane to the stylomastoid foramen.

The eminentia pyramidalis contains the tendon of the stapedius muscle, which runs via a small bony opening at the tip to the posterior limb of the stapes. Between the fenestra vestibuli or stapes and the eminentia pyramidalis there is an opening in the tympanic cavity. This is usually divided by a small bone bridge, so that a distinction is made between the posterior sinus (cranial) and the tympanic sinus (caudal).^[1] Bone beams between the eminentia and promontorium are referred to as ponticuli. They run laterally from the posterior sinus.

Paries labyrinthicus[Bearbeiten]

The paries labyrinthicus corresponds to the medial wall of the tympanic cavity. It separates the tympanic cavity from the inner ear or cochlea. The wall runs almost parallel to the eardrum. The majority is formed by the promontorium tympani, caused by the basal cochlear coil.

Above the promontorium lies the fenestra vestibuli (oval window), which is filled by the footplate of the stapes. Below the promontorium lies the fenestra cochleae (round window), which leads to the scala tympani of the cochlea. The fenestra vestibuli is closed by the membrana tympanica secundaria. The subiculum promontorii forms a bony bridge to the prominentia styloidea.

Above the fenestra vestibuli is the prominentia canalis facialis, in which the facial canal runs. In front of and above the oval window is the cochleariform process, around which the vision of the tensor tympani muscle is directed laterally. The tympanic end of the semicanalis tensoris tympani muscle is also located here and below it is the inner tube opening. A groove for the tympanic nerve and the inferior tympanic artery runs from below to above the promontorium. This groove is known as the sulcus promontorii. The tympanic nerve enters the tympanic cavity from below from the foramen jugulare through the canaliculus tympanicus and leaves it above the promontorium through the hiatus canalis nervi petrosi minoris.

Hypotympanum or paries jugularis[Bearbeiten]

The posterior lower section of the tympanic cavity lies below the floor of the external acoustic meatus and is largely part of the pars tympanica ossis temporalis. A thin bone plate forms the dividing wall to the bulb superior to the internal jugular vein (paries jugularis tympani). If the bulb is only slightly developed, the floor can also be thick and pneumatised by cellulae tympanicae. Occasionally bone dehiscences are also found.

The space between the lower edge of the tympanic membrane and the paries jugularis defines the hypotympanum. The styloid prominence of the styloid process protrudes from the inside.

The three auditory ossicles (auditory ossicles) are located in the tympanic cavity:

• Malleus (hammer)
• Anvil (incus)
• Stapes (stapes)

The malleus is attached to the tympanic membrane and articulates with the incus via a saddle joint (articulatio incudomallearis). However, the tight joint capsule and small bony protrusions ensure that the joint functionally corresponds to an amphiarthrosis. The incus and stapes are connected by a flat ball-and-socket joint with a similarly small range of motion (incudostapedial articulation). The stapes is embedded in the fenestra vestibuli.

See main article: Ossicles

Delicate collagenous ligament structures, the auditory ossicular ligaments, run through the tympanic cavity. They serve to suspend the auditory ossicles. These include:

• Ligamentum mallei superius
• Ligamentum mallei anterius
• Ligamentum mallei laterale
• Ligamentum incudis posterius
• Ligamentum incudis superius
• Ligamentum anulare stapedis

Sensory and vasomotor nerve fibres of the mucous membranes and blood vessels run in the tympanic plexus on the promontorium tympani. It is formed from preganglionic, parasympathetic fibres from the tympanic nerve of the glossopharyngeal nerve and sympathetic carotid tympanic nerves. Ganglion cells (ganglion tympani) and the glomus tympanicum are regularly located in the plexus.

The arterial vascular supply of the tympanic cavity is via branches of the external carotid artery (ascending pharyngeal artery, maxillary artery, posterior auricular artery) and internal carotid artery (carotidotympanic rami):

• Rami caroticotympanici: Run through the paries caroticus to the anterior sections of the promontorium and to the bony tube section.
• Posterior tympanic artery: Is given off in the facial canal by the stylomastoid artery from the posterior auricular artery. Runs in company with the chorda tympani to the malleus, incus and tympanic membrane.
• Superior tympanic artery from the middle meningeal artery of the maxillary artery: enters the epitympanum and the promontorium through the hiatus canalis petrosi minoris or directly through the tegmen (hole in the ossified petrosquamous fissure).
• Inferior tympanic artery from the ascending pharyngeal artery: Branches off in the jugular foramen through the tympanic canal into the hypotympanum and reaches the lower promontorium and the stapes.
• Anterior tympanic artery from the maxillary artery: Runs through the petrotympanic fissure into the epitympanum and from there to the malleus, incus and antrum mastoideum.
• Rami mastoidei and rami musculi stapedii from the stylomastoid artery: Branches originate in the facial canal.
• Arteria subarcuata from the arteria labyrinthi: Runs through the fossa subarcuata and petromastoid canal to the cellulae mastoideae.

The veins follow the arteries and drain into the pterygoid and pharyngeal venous plexus and the retromandibular vein.

The lymph of the tympanic cavity flows with the blood vessels to the nodi lymphatici parotidei, jugulares interni and retropharyngeales.

The tympanic cavity is lined by a thin layer of mucous membrane consisting of a single-layered isoprismatic (cubic) epithelium in which goblet cells are interspersed. Small glands, the glandulae tympanicae, are found in the lamina propria in the anterior region of the tympanic cavity. In the area of the auditory ossicles, the isoprismatic epithelium is replaced by a multi-layered squamous epithelium.

A frequent pathological change in the area of the tympanic cavity is tympanic effusion (seromucotympanum).

• Image source podcast: ©Magda Ehlers / Pexels