Introduction of eustachian tube
Eustachian tube is also known as pharyngotympanic tube. It connects the middle ear cavity to the nasopharynx.
The Eustachian tube removes the mucus from the middle ear into nasopharynx by the mucociliary action. Mucociliary action prevents the infection from entering the middle ear. The Eustachian tube also aerates the middle ear. The opening of the Eustachian tube helps in equalizing the atmospheric pressure in the middle ear. Closing of the Eustachian tube helps in protecting the middle ear from unnecessary pressure fluctuations and loud sounds.
Anatomy of eustachian tube
The Eustachian tube is 36 mm long. It mainly consists of two portions, explained as follows.1
Lateral Third Portion
This is approximately 12 mm in length. This is the bony portion, which is widest at its tympanic end, passes through the squamous and petrous portions of the temporal bone, and is always open. This gradually narrows to the isthmus. Isthmus is the narrowest part of the Eustachian tube. This bony part arises from the anterior wall of the tympanic cavity.
Medial Two-Third Portion
This is approximately 24 mm long and is a fibro-cartilaginous part, which is closed at rest and opens only on swallowing, yawning, or forceful inflation. This opens into the nasopharynx. The fibro-cartilaginous portion consists of a plate of cartilage posteromedially and bends forward to form a short projection. The fibrous tissue forms the remaining of the anterolateral wall.
A tubal elevation called torus tubarius is formed by the wider medial end, which lies under the mucosa of the nasopharynx and raises the mucosa. Behind the elevation is the fossa of Rosenmüller.
This is the common site of origin for nasopharyngeal carcinoma and occult primary tumors. The cartilaginous Eustachian tube is attached to the bottom of the skull in a groove, which is present between the petrous part of the temporal bone and the greater wing of the sphenoid. The lumen of the Eustachian tube is approximately 2–3 mm vertically and 3–4 mm horizontally.2
The muscles of the Eustachian canal include:
Tensor Veli Palatini (TVP): This muscle is responsible for the opening of the Eustachian tube.
Tensor Tympani (TT): These muscles are formed when a few fibers of the TVP are not attached to the base of the skull but become tendinous and muscular.
Salpingopharyngeus: This muscle is attached to the pharyngeal end of the Eustachian tube and blends with the palatopharyngeus muscle downward.
Levator Veli Palatini (LVP): This muscle runs downwards and parallel to the cartilaginous tube from the upper part of the skull base attachment to the lower attachment into the nasal surfaces.
Physiology of eustachian tube
The physiological function of the Eustachian tube involves the following.3
The ventilatory functions include:
- Pressure regulation especially the negative pressure.
- Opening through passive pressure or active opening of the muscular tube equalizes positive pressure with a little effort.
- Equalizing negative pressure requires more effort and is done through active dilation of the tube by muscular contraction.4
The protective functions involve:
- Reducing the harmful effects of loud sound.
- The tympani muscle contracts lowering the motion of tympanic membrane thereby reducing the displacement of the stapes footplate.
- Contracting the TVP muscle helps in dilation of the Eustachian tube.
Clearance or Drainage
The clearance functions include:
- Eustachian tube’s mucociliary system acts as a channel for the movement of secretions and waste from the middle ear to the nasophrynx.
- The Eustachian tube is closed at rest and hence the backward movement of secretions from the nasopharyx to the middle ear is prevented.
- The Eustachian tube closes and thereby prevents the sound created interiorly from entering the middle ear.
1.Cummings CW, et al. Anatomy and Physiology of the Eustachian Tube. Otolaryngology - Head and Neck Surgery. C.V. Mosby Co. 1986.
2.Alper CM, Tabari R, Seroky JT, Doyle WJ. Magnetic resonance imaging of the development of otitis media with effusion caused by functional obstruction of the eustachian tube. Ann Otol Rhinol Laryngol. 1997; 106(5): 422–431.
3.Bluestone CD, Paradise JL, Beery QC. Physiology of the eustachian tube in the pathogenesis and management of middle ear effusions. Laryngoscope. 1972; 82(9): 1654–1670.
4.Jahn AF, Santos-Sacchi J. Physiology of the eustachian tube and middle ear pressure regulation. Physiol Ear. Raven Press. 1988.