------------------------------------------------------------------------------- TITLE: ANATOMY AND PHYSIOLOGY OF THE EUSTACHIAN TUBE SOURCE: Dept. of Otolaryngology, UTMB, Grand Rounds DATE: April 29, 1992 RESIDENT PHYSICIAN: R. Paul Fulmer, MD FACULTY: Chester L. Strunk, MD DATABASE ADMINISTRATOR: Melinda McCracken, M.S. ----------------------------------------------------------------------------- "This material was prepared by resident physicians in partial fulfillment of educational requirements established for the Postgraduate Training Program of the UTMB Department of Otolaryngology/Head and Neck Surgery and was not intended for clinical use in its present form. It was prepared for the purpose of stimulating group discussion in a conference setting. No warranties, either express or implied, are made with respect to its accuracy, completeness, or timeliness. The material does not necessarily reflect the current or past opinions of members of the UTMB faculty and should not be used for purposes of diagnosis or treatment without consulting appropriate literature sources and informed professional opinion." ANATOMY AND PHYSIOLOGY OF THE EUSTACHIAN TUBE I. EMBRYOLOGY The eustachian tube is of endodermal origin and formed from a dorsal pouch in the upper primitive gut, the first branchial pouch. The distal part of the pouch , the tubotympanic recess widens and gives rise to the primitive tympanic cavity; while the proximal remaining part gives rise to the eustachian tube. II. ANATOMY A. Protympanum (Bony Tube): - open cavity forming a direct continuation with the middle ear cavity. - approx. 10 mm long - lateral wall = separated from the mandibular fossa by a thin bony plate. - medial wall = thin plate of bone separates it from the carotid artery. - superior wall = formed by the semicanal of the tensor tympani muscle. - isthmus - narrowest aspect of the bony canal is at its attachment to the cartilaginous tube. oval shaped lumen which is approximately 2.5 by 1.5 mm. B. Cartilaginous Tube: - points in a medioinferior direction from the isthmus to the nasopharynx. - approximately 25 mm in length in an adult and lies at an angle of 45 degrees; whereas in a child the tube is shorter and lies at a more horizontal angle of approx. 10 degrees. This increase in the angle is formed when the hard palate drops away form the skull base during the increasing vertical development of skull during childhood. - the tube is lined with respiratory columnar epithelium with ciliated and goblet cells. A thin film of mucus is propelled by ciliary action from the middle ear through the eustachian tube to the nasopharynx, protecting the middle ear from ascending infection. - the cartilage is described as a 'crook-shaped' structure with the tubal cartilage larger on the medial side than the lateral side and these are connected by a thick layer of connective tissue. There is some evidence that a differential motion occurs between the medial and lateral lamina. - there is no open lumen within the cartilaginous tube, and like a valve this is felt to protect the middle ear from strong sounds and pressure variations evoked by phonation and respiration. - the cartilage protrudes somewhat into the nasopharynx at the torus tubarius. - the tube takes a gentle slow curving inverted S pathway to the nasopharynx. - the tubal cartilage fits into a sulcus at the skull base called the sphenoid sulcus(sulcus tubarius). C. Muscles: 1). Tensor Veli Palatini (TVP) - primary muscle responsible for active tubal opening. This muscle is divided into two bundles; the lateral TVP and the more medial dilatator tubae (DT). The TVP originates from an attachment on the cranial base at the scaphoid fossa, runs lateral to the tube, and inferiorly rounds the hamulus before its insertion to the anterior velum and onto the posterior margin of the hard palate. The fibers of the DT muscle are oriented more obliquely and attach medially to the lateral membranous tubal wall and laterally to the deep surface of the TVP. - the TVP muscle upon movement(ie. swallowing or yawning) contracts isometrically between two fixed points, the skull base and the hamulus and hard palate. This forms a rigid base from which the DT rapidly pulls the lateral membranous tubal wall causing opening of the eustachian tube. - the TVP/DT is innervated by the Trigeminal nerve via the otic ganglion. 2). Tensor Tympani (TT) - a few of the fibers of the TVP do not attach to the skull base, but instead run above the cartilaginous tube and become tendinous again and then muscular to contribute to the formation of the tensor tympani muscle. TT attaches to the malleus and is innervated by the trigeminal nerve as well. 3). Levator Veli Palatini (LVP) - this muscle runs inferior and parallel to the cartilaginous tube from a superior skull base attachment to an inferior attachment into the nasal surface of the velum. - a motor branch of the pharyngeal plexus innervates this muscle. - its primary function is to elevate the soft palate in speech and deglutition and its function in eustachian tube opening is not completely clear. When the LVP contracts on its own, the tube does not open all the way to the isthmus. Its is believed that the LVP has a minor role in assisting in the dilation of the nasopharyngeal orifice of the eustachian tube. III. PHYSIOLOGY: A. Function 1). Ventilation - the ability of the eustachian tube to equalize negative pressure. Pressure equalization of positive pressure is usually done with out much effort by either passive pressure opening or active muscular opening of the tube. Negative pressure is more difficult to equalize and requires active dilation of the tube by muscular contraction. If negative pressure continues to develop without relief, the tube may become "locked" and require forceful inflation by either Valsalva or Frenzel maneuvers. Some individuals can relieve this negative pressure just with swallowing, however this is variable. 2). Protection - the eustachian tube has been proposed to play a role in diminishing the potential deleterious effects of a loud sound. In addition to the stapedius muscle contracting to reduce the displacement of the stapes footplate, the tensor tympani muscle may contract reducing the motion of the tympanic membrane. Moreover, the TVP/DT also may contract dilating the tube and creating one large cavity with the middle ear, ET and nasopharynx. 3). Drainage - a secondary purpose of the ET is to act as an active conduit for the movement of secretions and debris for the middle ear to the nasopharynx via the tube's mucociliary system. Also at rest the tube is closed and this prevents the retrograde passage of secretions from the nasopharynx to the middle ear. Likewise, the closed tube prevents the movement of sound created intraorally from entering the middle ear via the ET. B. Two Views of Eustachian Tube Function: 1). Classical View - the primary responsibility of the ET is to equalize the pressure between the atmosphere and the middle ear. Sound protection and drainage functions are also naturally assumed to be of importance as well. The tube must intermittently open to release negative pressure and allow new air into the middle ear. If this did not occur a negative pressure would develop from gas absorption and subsequently transudation of fluid would fill the ear cavity. This functional obstruction (very rarely anatomical obstruction) of the ET creating negative pressure and then fluid is the usual finding in diseased ears. 2). New Approach - Pressure-Regulating System - in the new approach the ET is not only seen as a pressure regulator, but also a bidirectional conduit for the diffusion of gases and fluid. The interplay between the liberation and absorption of gases and the production and elimination of fluid up and down the tubal passage constitutes the dynamic balance of the new hypothesis. The intratympanic pressure fluctuates close to the ambient pressure in a dynamic equilibrium. IV. PATHOPHYSIOLOGY A. Functional Eustachian Tube Obstruction - functional obstruction is a result of persistent collapse of the ET due to increased tubal compliance, an abnormal active opening mechanism, or both. This type is common in infants and younger children, because of their decreased amount of tubal cartilage stiffness and the horizontal projection of their ET causing a more ineffective TVP contraction for less effective active tubal opening. B. Mechanical Eustachian Tube Obstruction 1). Intrinsic - this is most commonly due to inflammation of the lining of the ET. Obstruction can also occur at the opening of the bony tube within the middle ear space by polyps or cholesteatoma within the middle ear. - inflammation of the lumen of the cartilaginous position of the ET often causes bouts of OM and /or atelectasis of the TM. 2). Extrinsic - may be due to extrinsic compression of the ET by nasopharyngeal tumors, adenoids or lesions of the skull base. C. Abnormal Patency of the Eustachian Tube -a patulous ET usually permits airflow readily into the middle ear, but can also have an tendency to cause 'reflux otitis media'. During active tubal opening, nasopharyngeal secretions may also more easily be introduced to the middle ear along with the positive pressure. D. Allergy and Eustachian Tube Function - instead of the allergic rhinitis patient having an increased incidence of OM from reflux of nasal secretions into the middle ear , it is believed that the primary cause of OM in these patients is due to allergic inflammation of the ET causing a functional obstruction. E. Eustachian Tube Function related to Cleft Palate - the eustachian tube in patients with both repaired and unrepaired cleft palates suffer from a functional obstruction of the ET. This is felt to be do the their inability to actively open the ET. Patients with bifid uvula and submucous clefts have similar functional obstructions of their ET's. These abnormalities presumably are due to the abnormal relationships between the crainiofacial defects and the tensor veli palatini muscle. V. TESTING TUBAL PATENCY A. Valsalva - the patient makes an effort to exhale while pinching the nose and keeping the mouth closed. If the middle ear is inflated the subject will experience a popping sound and feel a fullness in the ears. This can be checked by otoscopy showing the TM bulging outward. B. Toynbee Test - the patient pinches the nose while swallowing and a biphasic pressure change occurs in the nasopharynx as the soft palate moves up and then down. This is often transmitted to the middle ear and seen on tympanogram as a small positive peak pressure followed by a negative pressure peak. C. Frenzel Maneuver - this is opposite of the Valsalva inflation. the patient takes a mouthful of air and while keeping his mandible open, he closes his lips and pinches his nose. The air in the oral cavity is then compressed by raising the tongue and pressing the cheeks. This can be hard to learn, but is very effective in inflating the ears. D. Politzer Test - forcing air up the ET by having the patient close off their soft palate by either saying "k-k-k" or swallowing water; then blowing air into one nostril while the other one is pinched. This can be unpleasant and should be done with a light touch. (Dr. Bradfield's favorite procedure). F. Inflation-Deflation Test 1). Nonintact Tympanic Membrane - when there is a perforation of the TM or a tube in place then the ET ventilatory function can be tested. Positive pressure is applied to the middle ear until the ET tube opens. The remaining pressure after passive opening and closing is the closing pressure. 2). Intact Tympanic Membrane - ( Nine-Step Test) a. tympanogram records the resting middle ear pressure b. +200mm H2O to EAC deflecting the TM medially and increases middle ear pressure. Subject swallows to equilibrate excess pressure. c. Refrain from swallowing and EAC pressure is returned to normal creating a relative negative pressure in the middle ear and an outward deflection of the TM. Tympanogram is preformed. d. Subject swallows attempting to equilibrate the negative pressure. e. Tympanogram is recorded. f. EAC has -200mm H2O applied causing a lateral deflection of the TM. Subject swallows to equilibrate. g. Refrain from swallowing and EAC pressure is returned to normal creating a slight positive pressure in the middle ear. Tympanogram is recorded. h. Subject swallows to equilibrate the over pressure in the middle ear. i. Tympanogram is recorded. This test allows evaluation of the ET quantitatively with an intact TM. ------------------------------------------------------------------------------ BIBLIOGRAPHY: 1. Bluestone, CD and Stool, SE. "Otitis Media, Atelectasis, and Eustachian Tube Dysfunction." Pediatric Otolaryngology. W.B. Saunders Co. 1990. 2. Cummings, CW, et. al. "Anatomy and Physiology of the Eustachian Tube." Otolaryngology - Head and Neck Surgery. C.V. Mosby Co. 1986. 3. Doyle, WJ, et. al. "Effect of palatoplasty on the Function of the Eustachian Tube in Children with Cleft Palate." Stool & Bluestone, OMRC Progress Report - 1998. 4. Jahn, AF and Santos-Sacchi, J. "Physiology of the Eustachian Tube and Middle Ear Pressure Regulation." Physiology of the Ear. Raven Press. 1988. 5. Sadler, TW. "Ear." Langman's Medical Embryology. Williams & Wilkins. 1985. ---------------------------------END------------------------------------------