The middle ear

How the middle ear works

This page will discuss the anatomy of the middle ear, its functionality and the abnormalities that can afflict it. 

Anatomy of the middle ear

The middle ear consists of: tympanic membrane, tympanic cavity (tympanic case), auditory ossicles, mastoid wall and auditory tube or Eustachian tube.

  • The tympanic membrane encloses the bottom of the external auditory meatus, dividing it from the tympanic cavity, and has two surfaces: a lateral one facing the external auditory canal, and a medial one looking toward the tympanic cavity, against which the hammer handle is applied and adheres tenaciously. The complex system of fibers of which the tympanic membrane is composed enables it to vibrate when it receives sound waves. The connection between fibers and the hammer handle explains how the membrane's vibrations give rise to the movements of the ossicle chain.
  • The eardrum case is a bony cavity located between the outer ear and the inner ear. It houses the chain of ossicles and communicates through the auditory tube (also called the Eustachian tube) with the pharynx, and through the aditus ad antrum, with the mastoid apparatus. The eardrum case is a cavity that under normal conditions contains air; normal aeration of the eardrum case is provided by the proper functioning of the Eustachian tube, which is typically closed and opened by the action of muscular force (e.g., swallowing, chewing, sneezing, yawning). The auditory ossicles are a chain of three small bones: the hammer, anvil and stirrup
  • The ossicle chain is contained in the tympanic cavity and establishes a connection between the tympanic membrane and the oval window; in fact, the handle of the hammer is embedded in the thickness of the tympanic membrane, and, with the anvil in between, the stirrup sinks into the oval window (cochlea).The amplitude of the movements conferred on the chain by the vibrations of the tympanic membrane is regulated by two small muscles that act antagonistically: the tympanic tensor muscle, which by causing an increase in the tension of the tympanic membrane performs a protective action against too intense sound stimulation, and the stapedium, which has the task of preventing excessive movement of the stirrup.
  • The mastoid cavity (mastoid) consists of a mass of spongy bone tissue lined on the surface by a layer of compact bone tissue: within the spongy bone are housed numerous cavities (mastoid cells), the largest of which reaches about one centimeter in diameter and is called the tympanic antrum. The auditory tube or Eustachian tube is a 35-45 millimeter long duct that connects the anterior wall of the tympanic cord with the nasopharynx.

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What is the function of the middle ear?

From a functional point of view, the middle ear has the task of transmitting sound waves from the eardrum to the cochlea, draining secretions from the eardrum case into the pharynx, and allowing air from the pharynx to enter the eardrum cavity and mastoid cavity.

This last function allows the tympanic membrane to vibrate in optimal conditions: in fact, the air that from the pharynx penetrates, by means of the auditory tube, into the tympanic cavity, balances, on the medial face of the tympanic membrane, the pressure that the air contained in the external auditory canal exerts on its lateral face.

An alteration in tubal patency (blockage of the Eustachian tube) therefore causes alterations in the pressure normally existing in the tympanic cavity, resulting in altered functioning of the tympanic membrane and ossicle chain.

Diseases of the middle ear

Diseases of the middle ear can generally be traced to otitis. Otitis media is an inflammatory process of the tympanomastoid mucosa that can be caused by viruses or bacteria. Otitis, based on the exudate that comes to form, can be distinguished into catarrhal or purulent. Based on the clinical course, otitis is classified into acute and chronic. 

Acute otitis

In acute otitis the patient complains of ear pain (otalgia), fever, hearing loss, and tinnitus. In purulent forms, otorrhea (discharge of pus from the ear) may be associated with perforation of the tympanic membrane, which tends to close spontaneously once the inflammatory process is over.

Treatment of acute otitis relies on the use of antibiotic drugs both systemically and locally and the use of analgesic-anti-inflammatory drugs. Acute otitis is more likely to occur in pediatric age and can give, occasionally and if not adequately treated, complications such as meningitis, facial nerve palsy, petrosite and brain abscess. Because acute otitis media is often accompanied by inflammation of a viral nature of the upper airway, especially in young individuals, therapy with oral mucolytics or aerosols is useful.

Chronic otitis

Chronic catarrhal otitis is characterized by the prolonged presence of exudate in the tympanic cavity, usually mucous in children and serous in adults, resulting from the association of Eustachian tube dysfunction and middle ear inflammation. The main symptoms are given by stable or fluctuating hearing loss, occasionally by tinnitus, and rarely by pain. Medical therapy, in most cases resolving, relies on the use of oral and aerosol fluids and spa therapy. In more resistant cases, it is necessary to resort to the insertion of a ventilation tube into the tympanic membrane (tympanostomy) in order to promote drainage of exudate as well as aeration of the tympanic case.

Chronic purulent otitis often presents with perforation of the tympanic membrane. The patient complains of pain, hearing loss and recurrent otorrhea; dizziness and tinnitus may also be present. Chronic purulent otitis media can result in complications such as meningitis, brain abscess, and facial paralysis, which are now rare due to the advent of antibiotics. In chronic otitis media, a distinction is made between noncholesteatomatous and cholesteatomatous forms. Treatment of cholesteatomatous forms is surgical. The surgeries aim to reclaim the area of the inflammatory focus and are: 

Myringoplasty that is reconstruction of the tympanic membrane;

Tympanoplasty which is a myringoplasty with revision of the tympanomastoid structures.

These surgeries can be combined with an Ossiculoplasty in which the ossicular chain is reconstructed.

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