Eustachian Tube (Anatomy, Physiology and Diagnostic Tests)

Anatomy of Eustachian Tube

Eustachian tube (ET)  also called auditory tube or pharyngo tympanic tube. This tube connects the lateral part of nasopharynx to the anterior wall of the tympanic cavity (middle ear).  ET is approximately 36mm in length. The lateral 1/3 of ET is bony. The medial 2/3 is cartilaginous. The narrowest part of ET is known as Isthmus. Isthmus is located on a bony cartilaginous junction. The cartilaginous end of the eustachian tube protrudes in a nasopharynx called Torus Tubarius.  ET is an angle of 45 degrees to the horizontal plane. Incase of neonates, this ET is always horizontal and tube length is always half of it. It is almost 16-18mm. Since the ET is shorter and horizontal in children, the children are more prone to middle ear infection.   ET has middle ear opening (5mmX2mm), has broader  opening in the pharynx (8mmX3mm). The narrowest part (junction of bone and cartilage)is approximately 3mm 

Ostmans Pad of Fat

Keeps the eustachian tube in a closed position. The eustachian tube  is closed at rest and that is because of Ostmans Pad of Fat. The median fibres of tensor veli palatini is the main opener or dilator of the eustachian tube. Also known as dilator tube. When tensor veli palatini contracts, pulls the cartilage of the eustachian tube anteriorly, that leads to opening of the eustachian tube. Secondary function of the levator veli palatini is opening of the eustachian tube. 

Tensor and Levator Vili Palatini

Eustachian tube opens in the lateral wall of the nasopharynx, just behind the nasal cavity. Levator pili palatini and tensor veli palatini attached to the anterior surface. Tensor veli palatini contracts and opens the eustachian tube. Tensor veli palatini goes down , takes a turn in pterygoid hamulus and is attached in the centre (soft palate). Patients of cleft palate have ET dysfunction that lead to middle ear disorders because in cleft palate the muscles won't be able to meet in the centre.

Functions of Eustachian tube 

• It maintains middle ear air pressure equal to outside atmospheric pressure. Normally it is closed at rest. At yawning or swallowing, the soft palate moves, which leads to the contraction of tensor veli palatini , which opens the eustachian tube, the air enters the middle ear and air pressure is restored.  The advantage of closing the eustachian tube at rest is that it prevents food  and infection going into the middle ear. 
• It drains middle ear secretions. The middle ear is lined by mucosa and it's almost a closed cavity. It keeps on receiving air from the ET. Mucosa has a property that it absorbs oxygen from the surface, which means the middle ear has more carbon dioxide and less oxygen as compared to atmospheric air. The advantage of low oxygen is that it prevents the growth of aerobic bacteria in the middle ear. Therefore the middle ear is maintained as a sterile environment. If there is tympanic membrane perforation or hole, the chance of infection and bacteria will increase. 

Euctashian Tube Function Test 

Passive test- Also known as non physiological test. 

Valsalva Maneuver

 Ask the patient to close the nose and blow the cheeks. Positive pressure is generated in the naso pharynx. It forcefully opens the eustachian tube. The air will go in the middle ear and the tympanic membrane will bulge out. That shows that it's anatomically normal. 

Politizer test

Done in children who are unable to perform the valsalva test. It consists of a bag. Insert the tip of the bag in the nose, close the nose and ask the child to keep the mouth close and press the bag. By pressing it will create pressure in the nose and eustachian tube, which opens the eustachian tube and tympanic membrane bulges out. 

Eustachian Tube Catheterization

This catheter has a soft rounded ending with a bulge. On the other side the ring is present . The ring helps to identify the side of the turn.  Insert the ET catheter straight into the nasal cavity, pass it along into the nasopharynx and then rotate it medially and pull it back. The turn will engage behind the nasal septum. Rotate it by 180 degrees and enter into the eustachian tube. Attach the politizer bag and blow open the ET. 

Active Test - Also known as physiological test

Toynbee test

The patient is asked to drink water against the closed nasal cavity. Swallowing generates the negative pressure against the closed nose.,which will lead to contraction of tensor veli palatini and that opens the eustachian tube.  The negative pressure of the nasopharynx and oropharynx will pull the air out of the middle ear, which causes retraction of the tympanic membrane. It’s a physiological test because the ET is being opened by the negative pressure. 

Tympanometry

Tympanometry is also called inflation, deflation test. Insert probe in the ear. On the probe there is a sound source, microphone (that receives how much sound is reflected back  after striking the tympanic membrane and pressure meter (in which  we can increase or decrease the pressure in the EAC). Ask the patient to do valsalva, the ET will open up, air will enter and tympanic membrane bulges out. Then ask the patient to do the Toynbee test, the patient swallows against the closed nose. It will generate negative pressure and all the air will be sucked out and the tympanic membrane will retract. The tympanometry test can also be done even with a perforated membrane. 

Radiological Test

Radiological Test generally done in perforated tympanic membrane. Instill radio opaque dye in the middle ear, take an Xray, observe ET and check for obstruction.

A perforated tympanic membrane instead of dye can also instill saccharine . Saccharine has a sweet taste, it will come in the throat. Can also use dye ( methylene blue) 

Patulous Eustachian tube

When the eustachian tube is open at rest. The patulous eustachian tube will lead to autophony. The patient hears his only voice very loud. Patulous eustachian tubes can happen incase of rapid weight loss or in pregnancy. Treatment- Scar the eustachian tube or inject fat in the eustachian tube.

 SONOTUBOMETRY

Sono means sound, tonometry is to check the eustachian tube function. This is the latest investigation. Sound is given in the nose, which reaches in the middle ear and the sound source can be picked up by a probe in the EAC. 

Chronic Eustachian Tube Disorders

The most disorders of the eustachian tube are because of the obstruction of the eustachian tube.  Obstruction can be due to mass, polyp, malignancy. In these conditions, treat the cause. However sometimes, because of the repeated infections, eustachian tubes may undergo scarring, fibrosis. Because of that eustachian tube may undergo stenosis. In these chronic eustachian tube disorders, nowadays eustachian tube balloon dilation is done.

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