Tympanic cavity

Tympanic cavity refers to the middle ear cavity. It is an air-filled space, which is lined by mucous membrane, and is situated within the petrous part of the temporal bone.

The tympanic cavity (or the middle ear cavity) comprises of three sub-parts:

• Mesotympanum–referring to the middle ear space that lies adjacent to the tympanic membrane

• An epitympanic recess–that lies above the level of tympanic membrane

• A hypotympanum–that lies below the level of tympanic membrane

The tympanic cavity communicates with other spaces within the ear, nose and throat region. For instance, it connects anteriorly with nasopharynx (via eustachian tube), while extends posteriorly into the mastoid air cells through the epitympanic recess. Hence, in case of nasopharyngitis, the infection can spread to the tympanic cavity leading to middle ear infection or otitis media. This can further spread into the mastoid air cells, causing mastoiditis.

There are three small interconnected bones, called the auditory ossicles–the malleus, incus and stapes–situated within the tympanic cavity. These connect the tympanic membrane with the oval window of the inner ear. This enables them to transmit the sound vibrations from the tympanic membrane to the inner ear. The ossicles receive attachments from two important muscles, called the tensor tympani and stapedius (innervated by trigeminal and facial nerves, respectively). Both these muscles contract to limit the movements of the ossicles. Hence, trigeminal and facial nerve palsies could lead to hyperacusis, wherein patient hears louder than usual, due to an uninhibited unusually enlarged amplitude of vibration of these ossicles.

Sensory innervation from the mucous lining of the tympanic cavity comes off via the tympanic plexus, that is formed primarily by the tympanic branch of the glossopharyngeal nerve and branches from internal carotid artery plexus. An important nerve, called the chorda tympani, traverses through the middle ear cavity, hence, infection of middle ear cavity (otitis media) could possibly lead to an impaired taste sensation.

The tympanic cavity resembles a box-like space which has anterior, posterior, medial, lateral, superior and inferior walls to it:

• The anterior wall is incomplete above due to the opening for pharyngotympanic (eustachian) tube, while below it separates middle ear cavity from the internal carotid artery in front.

• The posterior or the mastoid wall is also incomplete above due to communication of the epitympanic recess with the mastoid antrum.

• The lateral or the membranous wall is related to the tympanic membrane below, while the upper lateral wall is bony which bounds the epitympanic recess.

• The inferior wall (floor) or the jugular wall is a thin bony layer which separates the tympanic cavity from the internal jugular vein.

• The roof or the tegmental wall refers to a thin bony layer which is called tegmen tympani. It separates tympanic cavity from the middle cranial fossa.

• The medial or the labyrinthine wall of the tympanic cavity, is in fact, the lateral wall of the inner ear or the labyrinth. It is characterized by a rounded bulge called the promontory, created by the basal coil of the cochlea. It has tympanic plexus lying on it, that receives contributions from tympanic branch of glossopharyngeal nerve and internal carotid artery plexus, while gives off a lesser petrosal nerve that is destined to reach the otic ganglion. Other important structures within the medial wall include the oval and round windows. The oval window serves as the site of attachment of stapes. In addition, posterosuperior to the oval window, we can also see prominences of facial and lateral semicircular canals.