Applied anatomy of ear drum
Also known as Tympanic membrane / Myringa
Introduction
Anatomically it is considered to be part of the external auditory canal since it is attached to the medial terminal end of the bony external meatus. Functionally ofcourse it is part of the middle ear. It is roughly oval in shape or egg shaped. It is concave laterally (towards external auditory canal) and is slightly drawn inwards at its centre. This indrawn portion is known as the umbo. It is at the umbo the handle of malleus is attached.
Size:
Length:9-10 mm
Width: 8-9 mm
Thickness: 0.1 mm
The broad portion of the ear drum lies superiorly.
Appearance:
Normal ear drum is thin semitransparent and pearly white in color. When viewed under illumination a triangular light shadow (reflected light) is seen extending from the centre of the drum downwards in an antero inferior direction. This feature is caused due to the sectional shape of the drum (it is set obliquely at an angle of 55 degrees to the floor of the external auditory canal). The umbo forms the apex of the cone of light. From the umbo extending upwards the handle of malleus is seen as an ivory colored extension. The base of the cone of light is at the antero inferior quadrant of the ear drum.
Importance of cone of light in diagnosis:
Cone of light is present in all normal ear drums.
It is distorted in Retracted ear drum (atelectatic ear)
It is broken into numerous segments in (bulging ear drum AOM)
Totally absent when the middle fibrous layer of the ear drum is lost (Tympanosclerosis). Healed perforation is known to commonly cause this.
Structure of ear drum:
The ear drum is composed of 3 layers. The outer layer is formed by stratified squamous epithelium, and is continuous with that of the external auditory canal. Any condition affecting the skin of the external canal will also affect the outer layer of the ear drum. Common conditions like dermatitis involving the skin of the external canal can also involve the outer layer of the ear drum. Embryologically outer layer of the ear drum developed from the ectoderm. Myringitis granulosa a common condition affecting the ear drum affects only the outer layer of the tympanic membrane. The middle and inner layers are not involved in this condition. It is commonly caused by infections arising from the external canal. Constant irritation of the ear drum due to presence of wax may also predispose to this condition. Another condition which involves the outer layer of the ear drum is Bullous myringitis. In this condition blebs may be seen in the outer layer of the ear drum. It is commonly caused by viral infections, or mycoplasma pneumonia. It may also be associated with middle ear effusion.
Middle layer is fibrous is present in pars tensa portion of ear drum. It is absent in pars flaccida (attic portion of the ear drum). This layer contains 3 types of fibers.
Radial Circular Parabolic
These fibers blend with the fibrocartilaginous tympanic ring that is present in the tympanic sulcus in the medial portion of the external auditory canal.
Inner layer is mucosal and is formed by a single layer of epithelium continous with the lining of the middle ear cavity and eustachean tube.
Notch of Rivinus:
This is a small defect in the superior portion of the bony annulus (tympanic ring). This defect is located just superior to the tympano mastoid suture line in the posterior ear canal. This defect is named after Agustus Q Rivinus a German anatomist. The portion of ear drum that bridges this defect is known as the Pars flaccida.
The ear drum is divided into two portions:
Pars flaccida: Also known as the Sharpnell membrane / Rivinus membrane. This portion of the ear drum bridges the defect over the notch of Rivinus. It is relatively lax when compared to that of the pars tensa because of the lack of middle fibous layer. Another important anatomical fact of this portion of the ear drum is that it lies above the level of anterior and posterior malleolar folds.
Pars tensa: This is relatively taut portion of the ear drum. It forms the remaining portion of the ear drum. It contains all the three layers.
Annulus: This is the thick rolled periphery of pars tensa
Embryology:
Middle ear begins to form by the 3rd week of intrauterine life. It completes its development by the 30th week. The tympanic membrane forms by joining of expanding first pharyngeal pouch which is endoderm lined and groove which is ectoderm line. These two structures are separated by a thin layer of splanchnic mesoderm. The three layers of ear drum are derived from the three germinal layers.
The outer epithelial layer is formed from ectoderm Middle fibrous layer is formed from mesoderm Inner mucosal layer is formed from Endoderm
Blood supply:
Outer surface - By the deep auricular branch of maxillar artery. This small artery leaves the first part of the maxillary artery behind the neck of the mandible and gains access into the external canal by piercing the anterior wall behind the mandibular joint. It sends small branches into the membrane from the whole circumference of the pars tensa and one or more manubrial branches that descend on the handle of mandible from above.
Internal surface - Anterior tympanic branch of maxillary artery in the front and stylomastoid branch of posterior auricular artery from behind.
Venous supply:
The superficial veins open into the extenal jugular vein and veins from the internal surface drain into the transverse sinus and veins of the duramater. Some of the veins partly drain in to the venous plexus around the eustachean tube.
Nerve supply:
Posterior half of the ear drum is innervated by the auricular branch of 10th cranial nerve and the anterior half is supplied by the auriculotemporal branch of 5th nerve. The inner surface of the ear drum is supplied by the tympanic branch of the 9th cranial nerve.
Applied anatomy:
Since the vasularity reaches from the periphery towards the centre the umbo region of the ear drum has poor blood supply when compared to that of other areas of ear drum. It is this area that perforates commonly when arteries at the periphery are thrombosed due to inflammation.
Formation of retraction pockets: Because of lack of middle fibrous layer, pars flaccida commonly retracts first in response to negative middle ear presssure.
Myringitis granulosa: Affects only the outer epithelial layer due to infection of external ear canal or sometimes constant irritation by cerumen could also cause this problem.
Myringitis bullosa: Blebs in the outer layer of ear drum dur to viral infections, mycoplasma pneumoniae, or middle ear effusion.
Red drum: Is seen in acute otitis media, glomus jugulare. The red drum in glomus jugulare is also known as the rising sun appearance.
Blue drum: Is caused by secretory otitis media or the presence of high jugular bulb
Schwartz sign: Also known as Flemingo's sign or the pink sign is usually seen through intact ear drum, close to the area of oval window. The presence of this sign indicates the presence of active otosclerosis. This usually occurs during pregnancy.
Light house sign: Extruding pulsatile discharge from a small pin hole perforation in ear drum is seen in acute suppurative otitis media.
