Physiology, Eustachian Tube Function

Article Author:
Jarett Casale
Article Editor:
Jason Hatcher
Updated:
10/27/2018 12:31:34 PM
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Physiology, Eustachian Tube Function

Introduction

The Eustachian tube, also known as the pharyngotympanic tube, is a narrow fibrocartilaginous duct that connects the middle ear to the nasopharynx. The proper function of the Eustachian tube is critical for optimization of middle ear sound transmission and health maintenance. Normally a closed structure, the Eustachian tube opens in response to movement of the mandible and pharynx, such as chewing or swallowing. It serves many important roles in normal middle ear function including ventilation and the facilitation of secretion clearance, which serves to protect the middle ear from pathogens. Normal opening of the tube also helps to equalize pressure between the middle ear and atmospheric pressure present in the nasopharynx. Structurally, the Eustachian tube is comprised of osseous, junctional, and cartilaginous areas from lateral to medial.[1][2][3]

Cellular

The lumen of the Eustachian tube is lined with ciliated pseudostratified columnar epithelium. This ciliated epithelium plays an important role in the clearance of secretions into the nasopharynx.[3][2]

Development

The Eustachian tube develops as a connection between the lateral extension of the first pharyngeal pouch endoderm and first branchial cleft ectoderm. From the expansion of the pouch forms the tubotympanic recess, which will eventually develop into the middle ear. The surrounding fibrocartilaginous tissue and muscles associated with the Eustachian tube form from mesoderm. As the skull base extends caudally, the angle of the Eustachian tube changes from horizontal to oblique. This results in elongation and angle increase of the Eustachian tube with respect to the horizontal plane during normal growth and maturity into early adulthood. The increase in the volume of the Eustachian tube has been shown to be 20 cubic mm per year until 20 years of age. The fully developed Eustachian tube can be divided into thirds and consists of osseous, junctional, and cartilaginous areas. The osseous third is the most lateral and lies within the petrous temporal bone. The osseous transitions to the junctional zone and eventually to the cartilaginous zone present in the skull base in close proximity to the greater wing of the sphenoid. The torus tubarius comprises the area of the cartilaginous zone that connects to the nasopharynx through the anteromedial end of the tube.[1][3]

Function

The Eustachian tube has many functions, most notably to equalize the pressure between the middle ear and atmosphere and facilitate secretion drainage from the middle ear. Mucosal folds present in the Eustachian tube lumen are thought to work in conjunction with the surface cilia to promote clearance of secretions from the middle ear to the nasopharynx by functioning as microturbinates. Additionally, yawning and swallowing causes contraction of the muscles connected to the Eustachian tube, enabling the tube to open to small amounts of air. This allows for the equalization of pressure between the middle ear and atmospheric pressure and results in a “popping” sound in the ear. The “popping” sound results from small amounts of air entering the middle ear to balance the pressure differential with the environment. When the Eustachian tube becomes inflamed and cannot properly open, dysfunction occurs and manifests as distorted or reduced hearing, tinnitus, and a sense of fullness in the ear. Eustachian tube dysfunction can occur in the setting of otitis media or as a response to allergens that result in inflammation and failure of the tube to open.[4][2][3]

Mechanism

The Eustachian tube is closed at rest but interacts with two main muscles that help to facilitate its opening when contracted. These muscles include, most notably, the tensor veli palatini, but also the levator veli palatini. The tensor veli palatine contracts the anterolateral wall to cause dilation and opening of the distal Eustachian tube. Contraction of the levator veli palatine results in elevation of the soft palate and medial rotation of the cartilaginous lamina. When these muscles contract simultaneously through swallowing or yawning, air can pass through the tube to equilibrate pressure in the middle ear with atmospheric pressure. The pressure discrepancy between the middle ear and the atmosphere is due to the diffusion of atmospheric gases across venous capillary cell membranes in the middle ear. Carbon dioxide and oxygen readily pass through the venous capillary membranes, creating a net pressure vacuum in relation to atmospheric pressure. This explains why dilation of the Eustachian tube allows air at atmospheric pressure to equilibrate with the lower partial pressures of gas in the middle ear that develop. This mechanism also clarifies why exaggerated Eustachian tube dysfunction will result in hearing deficits if the pressure in the middle ear impacts the tympanic membrane to draw it more tautly due to a gas exchange deficit. Two other muscles associated with the Eustachian tube that have not been shown to have a significant role in opening the lumen include the tensor tympani and salpingopharyngeus muscles. Another mechanism involved in proper Eustachian tube function is the mucociliary clearance of middle ear secretions to the nasopharynx. This process is enhanced as the tube matures and becomes more oblique in angle, allowing gravity to play a role in the drainage process.[4][2][5]

Related Testing

The diagnosis of Eustachian tube dysfunction is poorly defined, and no comprehensive guidelines currently exist. Diagnosis is based on a clinical exam and symptoms specific to Eustachian tube dysfunction; however, various imaging studies can aid clinical examination in diagnosing dysfunction. One study using real-time MRI was able to visualize Eustachian tube opening with Valsalva maneuver in patients with documented dysfunction. Cases of unilateral dysfunction demonstrated normal opening on the unaffected side and failure to open on the affected side. Studies using CT imaging show a reduced cross-sectional area of the osseous portion of the Eustachian tube in patients with documented dysfunction. Another imaging study that shows promising results in diagnosing Eustachian tube dysfunction is scintigraphy, which uses radiolabeled albumin tracers introduced through the tympanic membrane to visualize the patency of drainage from the middle ear.

Tympanogram is another test related to middle ear function that measures the compliance of the tympanic membrane. Abnormal tympanogram findings can result from middle ear or Eustachian tube pathology that results in increased pressure in the middle ear. Tympanogram also is used as a measure of improvement of middle ear function following interventions for Eustachian tube dysfunction.[4][6]

Pathophysiology

Conditions that cause the Eustachian tube to fail to open properly result in dysfunction. These conditions often are due to an inflammatory response of the epithelium within the lumen of the tube. Inflammation within the lumen has two main etiologies, irritant and infectious reactions. Irritant reactions include the response to allergens and chemicals such as smoke, while infectious reactions typically involve a viral or bacterial infection. Both processes result in the release of proinflammatory cytokines that promote edema of the epithelium in response to injury. Inflammation resulting in the inability of the Eustachian tube to open results in the failure of gas equilibrium between the middle ear and nasopharynx, leading to hearing impairment or a feeling of fullness in the ear. Whether cause or effect, inflammation of the Eustachian tube also leads to an inability to clear middle ear secretions, resulting in stasis and the potential of developing otitis media.[2][5]

Clinical Significance

Eustachian tube dysfunction often manifests in the setting of an inflammatory process such as otitis media or rhinosinusitis. Symptoms include tinnitus, distorted or diminished hearing, and pressure in the ear. These symptoms are typically mild and respond to actions that facilitate normal opening of the tube such as swallowing or yawning. More direct nonsurgical interventions for Eustachian tube dysfunction include the use of devices or techniques to increase nasopharyngeal pressure to facilitate air movement through the tube and into the middle ear. Medications aimed at reducing inflammation including corticosteroids, antihistamines, and antibiotics in the setting of an infectious process, may help to improve Eustachian tube function.

Eustachian tube dysfunction that is more severe or refractory may require more aggressive treatment that involves healthcare team-based approaches involving surgical intervention. The following techniques have been described and studied in regard to safety and efficacy for the treatment of the pathology related to the Eustachian tube.

Eustachian Tuboplasty

The most evidence related to Eustachian tube dysfunction interventional techniques involves Eustachian tuboplasty. One technique involves balloon dilatation of the Eustachian tube from the insertion of a balloon catheter into the Eustachian tube using transnasal endoscopic visualization. The balloon is pumped with saline and remains in the tube for about 2 minutes before it is removed. Results of the procedure were measured regarding the improvement of symptoms, including ear pressure, pain, fullness, and tolerance to pressure changes such as air travel. High levels of symptomatic improvement were noted in patients who had not undergone previous surgery; however, the intervention demonstrated mixed efficacy for symptoms of patients with chronic otitis media. Tympanometry was used as an additional objective measure of functional improvement in some studies, resulting in a high rate of conversion of abnormal tympanogram tracings back to type A (normal function) tympanogram tracings.

Myringotomy 

Myringotomy involves producing an opening in the tympanic membrane that is designed to relieve pressure buildup in the middle ear. This typically is done with a laser for increased accuracy and control and may involve ventilation tube placement. Studies regarding the efficacy of this technique in relief of middle ear symptoms appear promising; however, many are too short-term and poorly reported to appropriately draw conclusions about the value of the technique.

Tympanostomy 

Tympanostomy is a variation of myringotomy that involves the placement of a tube that traverses the tympanic membrane to ventilate the middle ear space. Tympanostomy tube placement is the most common ambulatory pediatric surgery in the United States. Indications for use include recurrent otitis media that is often refractory to antibiotic use. Otitis media is more common in children because the overall function of the Eustachian tube is worse due to it being more horizontal in children compared to that of adults. Tympanostomy tube placement is regarded as a safe and efficacious procedure providing relief of recurrent otitis media in most patients; however, otitis media still can occur following placement. Risks related to the procedure involve typical risks associated with anesthesia induction.[4][7][6]


References

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[2] Ars B,Dirckx J, Eustachian Tube Function. Otolaryngologic clinics of North America. 2016 Oct     [PubMed PMID: 27468632]
[3] Anthwal N,Thompson H, The development of the mammalian outer and middle ear. Journal of anatomy. 2016 Feb     [PubMed PMID: 26227955]
[4] Smith ME,Scoffings DJ,Tysome JR, Imaging of the Eustachian tube and its function: a systematic review. Neuroradiology. 2016 Jun     [PubMed PMID: 26922743]
[5] Tarabichi M,Najmi M, Site of eustachian tube obstruction in chronic ear disease. The Laryngoscope. 2015 Nov     [PubMed PMID: 25958818]
[6] Rosenfeld RM,Schwartz SR,Pynnonen MA,Tunkel DE,Hussey HM,Fichera JS,Grimes AM,Hackell JM,Harrison MF,Haskell H,Haynes DS,Kim TW,Lafreniere DC,LeBlanc K,Mackey WL,Netterville JL,Pipan ME,Raol NP,Schellhase KG, Clinical practice guideline: Tympanostomy tubes in children. Otolaryngology--head and neck surgery : official journal of American Academy of Otolaryngology-Head and Neck Surgery. 2013 Jul     [PubMed PMID: 23818543]
[7] Llewellyn A,Norman G,Harden M,Coatesworth A,Kimberling D,Schilder A,McDaid C, Interventions for adult Eustachian tube dysfunction: a systematic review. Health technology assessment (Winchester, England). 2014 Jul     [PubMed PMID: 25029951]