Anatomy of the Middle Ear

Structures Crucial to the Function of Hearing

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The human ear is typically divided into three portions: the external ear, middle ear, and inner ear. The middle ear is also called the tympanic cavity or tympanum. The middle ear is separated from the external ear by the tympanic membrane (the eardrum) and from the inner ear by a lateral wall that contains the round and oval windows.

Anatomy of the middle ear

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Anatomy

Tympanic Membrane

The tympanic membrane is also called the eardrum. It separates the middle ear from the external ear. It is semitransparent and under normal circumstances surrounded by air on both sides.

The dimensions of the tympanic membrane are approximately 1 millimeter (mm) in thickness and 10 mm in diameter. It is normally slightly concave. The tympanic membrane has a tense portion (where it is stretched tight), called the pars tensa, and a loose section that is more flaccid, called the pars flaccida.

The structural makeup of the tympanic membrane can be succinctly described as a mixture of epithelial cells and collagen fibers.

The Tympanic Cavity

Medial to the tympanic membrane is the tympanic cavity, which essentially makes up the middle ear. A healthy middle ear is filled with air.

It is a rectangular space with four walls, a ceiling, and a floor. The lateral wall consists of the tympanic membrane. The roof separates the middle ear from the middle cranial fossa. The floor separates the middle ear from the jugular vein.

The medial wall separates the middle ear from the inner ear and is characterized by a distinct bulge created by the facial nerve. The anterior wall separates the middle ear from the internal carotid artery and has two openings—one for the auditory tube and one for the tensor tympani muscle.

The posterior wall is a bony partition that separates the middle ear and the mastoid air cells. There is a superior hole in the posterior wall (called aditus to the mastoid antrum), which allows communication between the middle ear and the mastoid air cells.

The Ossicles

The ossicles are three tiny bones contained in the middle ear that are essential in conducting sound. They are called the malleus (the hammer), incus (the anvil), and stapes (the stirrup). They are connected by synovial joints and ligaments.

Sometimes the three bones are referred to as the ossicular chain. The chain carries vibrations from the tympanic membrane to the oval window. The stapes is the smallest bone in the human body.

The Auditory (Eustachian) Tube

The auditory tube runs from the anterior wall of the middle ear to the nasopharynx (back of the throat). The auditory tube ventilates the middle ear and also clears it of mucus and unwanted debris.

The inside of the tube is lined with cilia, small hairs that sweep mucus out of the tube where it drains into the back of the throat. The auditory tube of a child is much smaller in diameter than that of an adult and lies more horizontally. An adult auditory tube is approximately 31 mm to 38 mm in length.

Function

The main function of the middle ear is to carry sound waves from the outer ear to the inner ear, which contains the cochlea and where sound input can be communicated to the brain. Sound waves are funneled into the outer ear and strike the tympanic membrane, causing it to vibrate.

These vibrations are carried through the three ossicles, and the stapes strikes the oval window, which separates the middle ear from the inner ear. When the oval window is hit, it causes waves in the fluid inside the inner ear and sets into motion a chain of events leading to the interpretation of sound as we know it.

Associated Conditions

The middle ear can be affected by several conditions in children and adults.

Ruptured Eardrum

A ruptured eardrum may more accurately be called a tympanic membrane perforation. It occurs when the tympanic membrane is torn or otherwise compromised. This can be caused by very loud noises such as a gunshot or explosion, barotrauma, or by an injury.

Common symptoms include hearing loss or tinnitus, ear drainage, and pain. Small perforations will heal on their own, but severely damaged eardrums sometimes must be surgically repaired.

Middle Ear Infections

Middle ear infections, also called otitis media, are common ailments, particularly in young children due to the small size of their auditory tubes. They can be caused by various germs, including bacteria and viruses, and can be accompanied by fluid in the middle ear space.

Symptoms include ear pain and fever, which may get worse at night. Middle ear infections sometimes require antibiotics, and pain can be managed using over-the-counter pain relievers such as acetaminophen.

Fluid in the Ear

Fluid in the ear is another common condition that frequently affects the middle ear. Like middle ear infections, it is more common in children than adults.

It is usually the result of auditory tube dysfunction, a failure of the auditory tube to drain and ventilate properly. The cause of auditory tube dysfunction is being clogged with mucus from an infection (such as a common cold) or another condition such as allergies.

Fluid in the ear can cause decreased hearing, a feeling of fullness in the ear, or even dizziness. It usually resolves on its own once the underlying cause is treated but sometimes must be treated by surgery.

Otosclerosis

Otosclerosis is a condition that affects the ossicular chain in the middle ear and leads to hearing loss. It is not always known what causes the condition, but it may be hereditary or associated with chronic untreated fluid in the ear.

Otosclerosis can sometimes be surgically treated by removing the diseased bone and replacing it will cadaver bone or a prosthesis.

Tests

Diagnostic tests used to determine the condition of the middle ear may include visualization of the eardrum using an otoscope (which can help diagnose ear infections or fluid in the ear), tympanometry testing for fluid in the ear, static acoustic impedance testing for fluid in the ear or a ruptured eardrum, or a variety of hearing tests, including standard audiometry testing.

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