Tympanic membrane

Membrana tympanica

Also known as: eardrum, TM, pars tensa, pars flaccida, Shrapnell membrane, cone of light, umbo

A thin (≈0.1 mm), oval, cone-shaped membrane (≈9–10 mm across) separating the external acoustic meatus from the middle ear; it vibrates with sound and transmits motion to the malleus.

Show on body map Ear — Coronal Section

Overview

Diameter≈9–10 mm vertical × 8–9 mm horizontal

Thickness≈0.1 mm

Effective vibrating area≈55 mm² (total ≈70–85 mm²)

Inclination≈55° to the floor of the meatus

The tympanic membrane sits obliquely at the medial end of the meatus, facing downwards, forwards and laterally (≈55° to the floor of the canal). It is concave laterally: the handle of the malleus is attached to its inner surface and draws its centre inwards to form the umbo (tip of the handle). The lateral process of the malleus forms a small prominence superiorly, from which the anterior and posterior malleolar folds run to the margins.

The larger lower part, the pars tensa, is taut and is anchored by a thickened fibrocartilaginous annulus in the tympanic sulcus. The small triangular part above the malleolar folds, the pars flaccida (Shrapnell membrane), is lax because it lacks the middle fibrous layer. The pars tensa has three layers: outer skin (stratified squamous, continuous with the canal), middle fibrous layer (radial and circular collagen fibres), and inner mucous membrane (continuous with the middle-ear mucosa).

On otoscopy the normal drum is pearly grey and translucent. A bright triangular reflection, the cone of light, radiates anteroinferiorly from the umbo (at ≈5 o'clock in the right ear, ≈7 o'clock in the left). Its loss or displacement suggests a change in drum position (bulging or retraction). The drum is divided into four quadrants by a line along the malleus handle and a perpendicular through the umbo; the posterosuperior quadrant overlies the incus long process, stapes and chorda tympani, so myringotomy is performed in the anteroinferior or posteroinferior quadrant.

Location: Medial end of the external acoustic meatus; lateral wall of the tympanic cavity.

Parts & subdivisions

Pars tensa
Main, taut, three-layered portion; most of the area; site of most perforations.
Pars flaccida (Shrapnell membrane)
Small, lax, two-layered superior part above the lateral process of the malleus; site of attic retraction pockets and cholesteatoma.
Umbo
Point of maximal concavity at the tip of the malleus handle.
Handle (manubrium) and lateral process of malleus
Visible through the drum as a whitish streak and knob.
Cone of light
Light reflex in the anteroinferior quadrant.
Annulus
Fibrocartilaginous ring fixing the pars tensa in the tympanic sulcus; deficient superiorly (notch of Rivinus).

Anatomy

Innervation: Lateral surface: auriculotemporal nerve (V3) and auricular branch of the vagus (X), small contribution from the facial nerve. Medial surface: tympanic branch of the glossopharyngeal nerve (IX) via the tympanic plexus.
Arterial supply: Lateral surface: deep auricular artery (maxillary); medial surface: anterior tympanic artery (maxillary) and stylomastoid artery (posterior auricular).
Histology: Outer stratified squamous epithelium, middle lamina propria of radial and circular collagen fibres (absent in pars flaccida), inner simple cuboidal/squamous mucosa.
Development: All three germ layers: ectoderm (first cleft), mesoderm (fibrous layer) and endoderm (first pouch).

Function

Convert sound pressure waves into mechanical vibration
Transmit vibration to the ossicular chain (malleus)
Provide a large collecting area (≈55 mm² effective) for the area-ratio gain
Protect the middle ear from the external environment

Clinical significance

Perforation

Caused by infection (acute otitis media), trauma (cotton buds, slap injuries, blast, barotrauma) or chronic otitis media. Most traumatic perforations heal spontaneously within weeks; persistent ones are repaired by myringoplasty. A perforation causes conductive hearing loss (larger and more posterior → greater loss).

Cholesteatoma

Keratinising squamous epithelium in the middle ear, usually from a retraction pocket in the pars flaccida (attic). It erodes bone: ossicles (conductive loss), lateral semicircular canal (vertigo, fistula sign), facial canal (palsy) and tegmen (meningitis, abscess). Presents with foul-smelling chronic discharge; treated surgically.

Otoscopic signs

Bulging, red, opaque drum with lost cone of light → acute otitis media. Retracted, dull drum with fluid level or bubbles → otitis media with effusion. Chalky white plaques → tympanosclerosis. Blue drum → haemotympanum (basal skull fracture).

Test yourself

What is the main mechanism by which the middle ear increases sound pressure delivered to the cochlea?

The ≈17:1 area ratio of tympanic membrane to stapes footplate
Resonance of the external acoustic meatus
Contraction of the stapedius muscle
Air in the mastoid air cells amplifying vibration

Answer: The ≈17:1 area ratio of tympanic membrane to stapes footplate

Concentrating force from the ≈55 mm² effective drum area onto the ≈3.2 mm² footplate gives ≈17×; the ossicular lever adds ≈1.3×, for a total gain of ≈22× (≈25–30 dB). Stapedius contraction reduces transmission.

A young woman has progressive bilateral conductive hearing loss that worsened during pregnancy; the tympanic membranes are normal. The most likely cause is:

Otitis media with effusion
Otosclerosis
Presbycusis
Ménière disease

Answer: Otosclerosis

Otosclerosis fixes the stapes footplate in the oval window by abnormal otic-capsule bone remodelling; it affects young adults, women more than men, worsens with pregnancy and shows a Carhart notch at 2 kHz. Treatment: hearing aid or stapedotomy.