Corpus: Eardrum

The tympanic membrane is part of the sound conduction system and separates the middle ear or tympanic cavity (cavum tympani) from the external auditory canal (meatus acusticus externus).

The eardrum is of entodermal origin on the inside and ectodermal origin on the outside. It forms from the entoderm of the 1st pharyngeal arch and the ectoderm of the 1st gill furrow.

The tympanic membrane has a round to longitudinal oval shape and is slightly oblique in relation to the external auditory canal. It is offset medially at the bottom front and laterally at the top back. The angle of inclination between the floor of the auditory canal and the lower eardrum is 36 to 46°. The angle to the median plane (declination) is 32 to 38°. The dimensions of the eardrum are manageable: it is 10 to 11 mm high and approx. 8 to 9 mm wide. Its thickness is about 0.03 to 0.09 mm, the surface area 70 to 80 mm².

The eardrum is held in place by a fibrocartilaginous ring, the annulus fibrocartilagineus (annulus tympanicus). The eardrum is tightly clamped in this ring - like a percussion instrument, hence the name. During otoscopy, this ring can be seen as a white rim. The fibrocartilage ring itself is fused to the surrounding bone and is embedded in a notch (sulcus tympanicus) through tight collagenous connective tissue, which is interrupted at one point at the top (incisura tympanica). In addition to the tightly stretched lower section of the tympanic membrane within the fibrocartilaginous ring (pars tensa), there is also an upper, looser and thinner section of the tympanic membrane in the area of the incisura tympanica, where the tympanic sulcus and annulus fibrocartilagineus are missing (pars flaccida or Shrapnell membrane).

The tympanic membrane is not flat, but is retracted in the centre in an umbilical shape, as the malleus (manubrium mallei) protrudes in the upper half of the tympanic membrane (so-called stria mallearis). The short malleus process causes a protrusion at the border of the pars tensa and pars flaccida, the prominentia mallearis. The free end of the manubrium is tilted medially and pulls the eardrum inwards. This creates the tympanic umbo membranae tympani.

Classification[Bearbeiten]

In addition to the division into pars tensa and pars flaccida, the tympanic membrane is clinically divided into 4 quadrants for easier localisation of pathological findings, with the umbo representing the centre:

Innervation[Bearbeiten]

Sensory innervation of the outside of the tympanic membrane is provided by the auriculotemporal nerve and the auricular ramus of the vagus nerve. The vagus branches are accompanied by sensory fibres from the glossopharyngeal nerve and the facial nerve. The inner side is innervated by the tympanic plexus of the glossopharyngeal nerve.

Vascular supply[Bearbeiten]

The eardrum is supplied by the stylomastoid artery of the posterior auricular artery (from the outside) and by the anterior tympanic artery (from the inside). These vessels form a vascular ring in the periphery of the fibrocartilage ring. From there, vessels spread along the malleus handle to the umbo.

Lymphatic drainage[Bearbeiten]

Lymphatic drainage of the tympanic membrane has not been described.

The eardrum consists of several layers of tissue. Viewed from the outside in, the organ is organised in two layers as follows:

• Stratum cutaneum: Continuation of the external auditory canal
  • Lamina epithelialis: typical multi-layered keratinised squamous epithelium of the epidermis
  • Lamina propria with blood vessels and nerve fibres
• Stratum mucosum: Continuation of the tympanic mucosa
  • Lamina epithelialis: simple cubic to flattened epithelium
  • Lamina propria with blood vessels and nerve fibres

The two laminae propriae unite to form a common collagen-rich middle layer, which is sometimes also referred to as the stratum fibrosum. Radial collagen fibres (stratum radiatum) run on the outside and circular fibres (stratum circulare) on the inside. There are also transverse fibres in the lower sections of the eardrum and oblique bundles in the upper sections.

The eardrum can rupture or perforate as a result of trauma or inflammation (e.g. otitis media). This is referred to as an eardrum rupture or eardrum perforation. Inflammation of the eardrum is known as myringitis.

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