Middle Ear Anatomy for Physicians

Restoration Hearing

Anatomy

Summary

The middle ear system is home to the three smallest bones in the body: the malleus, incus, and stapes.  These three bones are referred to as the middle ear ossicles and are suspended in space by a small network of ligaments and muscles.  Sound enters the middle ear space via that lateral entrance where the tympanic membrane is located and it exits the middle ear space at the medial entrance where the oval window is (entrance to the cochlea).  The middle ear space has a superior border known as the apex and its inferior or caudal border opens up into the eustachian tube.

Tympanic Membrane

The tympanic membrane (TM) is more commonly known as the ear drum.  It creates a border between the outer and middle ear space.  A healthy TM is slightly translucent and has a pearl-gray hue.

It is composed of 3 layers: the outer epithelial layer, the medial fibrous layer, and the inner membranous layer.  The outer epithelial layer is consistent with the epithelial skin layer of the ear drum.  The medial fibrous layer is composed of a few different fiber types and provides structure and stiffness for the TM. The membranous layer is consistent with the mucosal lining of the middle ear space. 

The tympanic membrane is home to several iconic landmarks that are useful during medical diagnosis: the umbo, the cone of light, the pars flaccida (shrapnell’s membrane), the pars tensa, and the anular ligament. In a healthy ear drum you can sometimes see the manubrium of the malleus and the long process of the incus.

The Umbo is located at the very center of the TM and is the point of maximal concavity.

The Cone of Light is one of the most commonly clinically referenced landmarks of the ear drum.  In a healthy ear the cone of light reflects back at the examiner at 7 o’clock in the left ear and at 5 o’clock in the right ear.  If the location of the cone of light does not reflect back in these positions or if it is hard to pin point a clear reflection then it is possible that the individual has fluid or a retracted TM from Eustachian Tube Dysfunction (ETD).  If an individual has fluid build up within the middle ear space then the ear drum will appear cloudy/gray in color.  You may even see fluid bubbles pressing up against the TM.  When an individual has a retracted ear drum you will see that the TM appears to be pulled tighter than normal, especially in the pars flaccida region, and the middle ear bones may be more visible.  These patients often have more TM vascularity/redness than normal.

 The Pars Flaccida is located that the peak of the eardrum.  At this location there is an absence of support fibers, which gives it a “flaccid” appearance.  It is important to obtain a visual of this region whenever examining an individuals ear due to its close proximity to the apex of the middle ear space.  The apex is a known hotspot for cholesteatoma formations.  Examiners can sometimes see early signs of cholesteatoma formation in the pars flaccida region (i.e. abnormal retraction, redness, and appearance of a growth/mass formation pressing against the pars flaccida.

The Pars Tensa is the stiff portion of the TM that provides most of the surface area coverage.

The Anular Ligament is the circular ligament that appears around the circumference of the TM.  It is the point of origin for the TM

Middle Ear Ossicles

The middle ear is home to the three smallest bones in the body: the malleus, incus, and stapes. In pop culture, these bones are often termed the hammer, anvil, and stirrup.  However, in the field of Audiology their appropriate terms are the formerly mentioned ones.  Despite being very small, no bigger than a penny, these bones play a crucial role in transforming sound from acoustic energy in the outside world into mechanical energy for the cochlea.

Malleus

The most lateral end of the ossicular chain is where the Malleus sits.  The Manubrium of the malleus is attached to the eardrum.  This bone is the longest of the three middle ear ossicles.

Incus

Next in line from lateral edge to medial edge is the Incus.  The incus is slightly smaller than the malleus, but still larger than the incus.  It connects the Malleus to the stapes.

Stapes

The stapes is the final middle ear ossicle and it is the smallest bone in the human body.  It is often called the stirrup because it looks much like the stirrup one would find attached to a horse’s saddle.  It has two crus (arms) and a footplate.  The footplate is embedded within the oval window of the cochlea.  It is through the oval window that sound, in the form of mechanical energy, passes into the cochlea.

Eustachian Tube

The Eustachian tube is located at the most inferior/caudal end of the middle ear space.  It connects the middle ear space to the back of the nasal cavity.  The ET’s primary purpose is to regulate middle ear pressure and it allows fluid to drain out of the ME space.

Physiology

Summary

The primary purpose of the middle ear is to convert acoustic energy from the outside world into mechanical energy and to overcome the impedance mismatch between the outside world and the cochlear fluids within the cochlea.  Two key terms for understanding this process are compliance and impedance.  The middle ear space has a natural resonant frequency of 1000 Hz.

Compliance

Compliance is the rate at which energy admitted into the middle ear space.  In a healthy ear the normal compliance range is between -180 daPa and 150 daPa.  Ideally the peak compliance point occurs right around 0 daPa, meaning that the peak admittance of energy into the middle ear space occurs when the pressure inside the middle ear space is equal to the pressure outside the middle ear.  When the point of maximum compliance is more (-), then this means the pressure inside the middle ear is less than the pressure outside the ear.  When the point of maximum compliance is more (+), then this means that the pressure inside the middle ear is more than the pressure outside the ear.  Both instances of too much positive pressure and too much negative pressure are indicative of middle ear pathologies.

Impedance

Impedance is the opposition or resistance of energy admitted into the middle ear space.  It is affected by mass and stiffness of the middle ear space.  Generally speaking, low stiffness and high mass systems are better for transferring low-frequency information.  While high stiffness and low mass systems are better for transferring high-frequency information.

Impedance Mismatch

Although sound travels through the middle ear space quite well, there is a strong dampening effect that takes place when mechanical energy passes from the stapes footplate to the fluid filled cochlea.  As we know, sound does not travel as well in water as it does in air.  This is why it is important for the middle ear space to overcome this impedance mismatch by amplifying the sound passing through.  It does this through three methods: lever affects, areal transformation, and buckling effects.  Through these three methods, the middle ear space can add an additional 33 dB of gain to the signal.

Lever Effects occur due to the relative size of the malleus, incus, and stapes.  The malleus is the largest, then the incus, and then the stapes.  As we know from personal experience, we can apply a lot more force to an object when we have a longer arm (think prying something loose with a flat head screw driver vs a long crow bar; the crow bar will require much less effort from us and apply a greater force to the object).  This same principle applies to the middle ear space.  Energy passes much more easily from the malleus to the incus to the stapes because of the differences in size.

Areal Transformation occurs because the area of the TM is smaller than the area of the stapes footplate.  Pressure equals force / area. If we have a larger area, then that means our pressure will be smaller.  Therefore the pressure at the TM is less than the pressure at the stapes footplate.  To provide you with a real world example that will help you better understand this, imagine two people walking down a hallway; one is wearing high heels and the other is wearing a flat shoe.  Which one will make more noise upon impact?  The high heel always makes more noise.  This is because there is a lot more pressure trying to escape through the small tip of the shoe.

Buckling Effects take place because of the slight concavity and fixation of the TM to the manubrium.  At the point of fixation, the TM is stiffest.  Generally speaking, the further you go away from this point the looser or more movement the surface of the TM is.  Imagine taking a jump rope and fixing one end.  Then, with your arm move the jump rope up and down as fast as you can.  The point of least movement, the fixed end, will have the most force.  The same thing happens in the ear.  This factor doubles the ratio of energy gained through these three methods from 23:1 to 46:1.

Eustachian Tube

The Eustachian Tube plays an important role in maintaining auditory system integrity.  This tube opens and closes in order to keep middle ear pressure at the optimal level.  When this tube becomes swollen/closed off or patulous it can affect the ability of sound to pass through the ear and it can also cause complications.

The ET can become swollen, inflamed, or closed off from things such as the common cold/fever, auto-immune disorders, anatomical abnormalities, and excess weight. 

Inflamation commonly occurs in the ear when an individual has a cold/fever due to the fact that the ET connects to the back of the nasal cavity.  If the nasal cavity is infected or inflamed, then there is a very good chance the ET will also become inflamed and infected.  Once the infection clears, the ET usually returns to normal.  This is why nasal sprays and antibiotics are very effective in returning the ET back to to normal. 

Infants are more prone to ear infections because their ET is more flaccid, horizontal, and straight when compared to an adult’s ET.  However, by late childhood years their ETs tend to be more adult like and have a steeper anterior slope.  Another risk group for chronic ear infections are those with down syndrome because their ETs typically remain more horizontal for their lifetime. 

Weight loss/gain can also affect ET health.  In an individual who is overweight, excess fat cells may put pressure on their ET and close it off.  If an individual undergoes a dramatic weight loss in a short period of time, it is possible that their ET may become permanently open due to the sudden changes in the density of the area surrounding the ET.  When individuals have a patulous ET they may complain their own voice sounds hollow (termed Darth Vader syndrome).  Patulous ET can be tested for using clinical audiometry equipment.  In some cases patulous ETs self-correct after a few weeks/months following weight loss. 

FOR MORE INFORMATION REGARDING MIDDLE EAR DISORDERS AND DIAGNOSTIC TESTING PLEASE REFER TO RESOURCE CENTER