The tympanic membrane (eardrum, myringa) is a thin, ovoid structure that separates the outer from the middle ear.
Structure and Function
This structure has a role in sound transmission and amplification. Similar to the membrane on a drum, the tympanic membrane vibrates as it encounters sound. It then transmits these vibrations to the ossicles of the middle ear to be further passed on to the cochlea of the inner ear for transduction.
The tympanic membrane is derived from the invagination and meeting of the one pharyngeal groove with the one pharyngeal pouch, and as such, it is comprised of all three germ layers (ectoderm, mesoderm, endoderm). The lateral (external) epithelium is ectodermal in origin and is formed from canalization of an ectodermal plug involved in the formation of the external auditory meatus in the first pharyngeal groove. The medial aspect of the tympanic membrane is a continuation of the lining of the inner ear, which is endodermal in origin from the one pharyngeal pouch. The fibrous middle layer is mesodermal, of neural crest origin, and encases both the handle of the malleus and chorda tympani, of the same origin. The chorda tympani is the pre-trematic (coursing in the caudal aspect of the pharyngeal groove) nerve of the one pharyngeal arch, while the mandibular nerve is the post-trematic nerve of the one pharyngeal arch.
Blood Supply and Lymphatics
The lateral side of the tympanic membrane is supplied by the deep auricular branch of the maxillary artery. The medial side is irrigated by the auricular branch of the occipital artery and the anterior tympanic artery off the maxillary artery.
The lateral surface of the tympanic membrane receives sensory innervation from the auriculotemporal branch of the mandibular nerve, a branch of the trigeminal nerve (V3), the auricular branch of the facial nerve (CN VII), the auricular branch of the vagus nerve (CN X), and the glossopharyngeal nerve (CN IX). The medial surface of the tympanic membrane receives sensory innervation from the tympanic branch of the glossopharyngeal nerve (CN IX).
There are no direct muscle attachments to the tympanic membrane. However, the tensor tympani muscle can pull the malleus inward to increase the tension across the tympanic membrane, effectively stiffening it. This involuntary activity is part of the acoustic reflex, which protects the tympanic membrane and the cochlear apparatus from acoustic, vibrational trauma precipitated by very loud sounds including the sounds of chewing and talking. The stapedius muscle completes this reflex by retracting the stapes from the oval window to avoid damaging the cochlea with high-amplitude vibrations. The acoustic reflex requires approximately 40 milliseconds to take effect. Thus it is ineffective against very sudden loud sounds, for example, a gunshot.
Surface and Landmarks
The tympanic membrane is anchored circumferentially to the walls of the auditory canal in a slanted, posterosuperior to anteroinferior orientation. The handle (manubrium) of the malleus ossicle is embedded in the tympanic membrane and can be visualized as a ray. The tympanic membrane is ovoid in shape, conical in configuration, and comprised of two parts: the pars tensa antero-inferiorly and the pars flaccida superior posteriorly. The pars tensa is the sturdier of the two. Its fibrous framework is thickened at the edges to form an annulus, and it is covered by skin laterally (the external side) and mucosa medially (internal side). The pars tensa is under tension by a medial pull from the handle of the malleus at a central point of maximum depression called the umbo. The pars flaccida constitutes a small portion above the lateral process of the handle of the malleus and is delicate because it is devoid of a fibrous layer. When light is directed at the tympanic membrane during an otoscopic examination, a characteristic feature to be observed is the anterior cone of light, which radiates antero-inferiorly from the umbo. The tympanic membrane should be pearly-grey in color, translucent, shiny, and mobile on insufflation.
Rupture of the tympanic membrane may be caused by head trauma, loud blasts of sound, direct membrane trauma, barotrauma, and infection. The acoustic reflex provides some protection from loud sounds. Q-tips only should be used to clean the external ear and should not be inserted into the external auditory canal. Fliers and divers may avoid barotrauma by equalizing the pressure across the tympanic membrane. Equalization is done by allowing air entry into the Eustachian tube, which connects the middle ear to the nasopharynx; techniques include performing a Valsalva maneuver with pinched nostrils, yawning, and swallowing. In the case of tympanic membrane rupture, patients may complain of pain and bloody effusion from the external auditory canal and may experience some conductive hearing loss and tinnitus. If no infection persists, the damaged tympanic membrane heals on its own. Patients should be advised to minimize water entry into the ear while the membrane is perforated to avoid injury to middle ear structures. Interestingly, intentional rupture of the tympanic membrane has been found to have been a typical practice among aquatic hunters of the Bajau people in the Southeast Asian Pacific. This would have been done to allow diving to great depths as part of their hunts. As a result, many of these hunters experience hearing impairment.
Otitis media is a middle ear infection, which can cause an accumulation of pus behind the tympanic membrane. This may lead to pain or discomfort. Otoscopic examination typically reveals an erythematous and bulging tympanic membrane with obscured surface landmarks from distortion, possibly with a fluid layer or pus behind it. Recurrent otitis media infections may warrant tympanostomy tube placement to facilitate the drainage of pus and equalize the pressure across the tympanic membrane. The tympanostomy tubes are left in place for several months and are either removed later or fall out on their own.
Cholesteatoma is keratinization of squamous epithelium, often associated with the pars flaccida in the posterior and superior portion of the tympanic membrane. It is a destructive lesion that tends to expand, and it can engulf the ossicles and even erode the skull. Cholesteatoma must be fully excised to prevent further growth. Deafness, vertigo, abscess, and septicemia may result if left untreated.