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Hearing Journal:
doi: 10.1097/01.HJ.0000403505.49971.84
Through the Otoscope

Anatomy of open ear

Rensink, Michael J. MD

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Michael J. Rensink, MD, is a member of ENT Associates of San Diego and has been a practicing ENT specialist for over 35 years.

Open ear occurs when a perforation of the tympanic membrane is extremely large, and when the perforation is greater than 80%, it allows easy viewing of many features of the ear that are usually obscured by the eardrum.

Figure. Michael J. R...
Figure. Michael J. R...
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When you study a normal eardrum with an otoscope, several landmarks are easy to see. The malleus provides two—the lateral process and the long arm (manubrium). The head of the malleus is obscured, hidden out of view in the epitympanic space of the middle ear. The end of the manubrium is shaped like a spatula, and helps form the center of the eardrum, called the umbo.

The lateral process of the malleus can be seen in all three figures as a small, bright white ball that protrudes out toward the ear canal. The second landmark, the manubrium, extends down from the lateral process. Notice how the eardrum is pulled back over the mid-portion of the manubrium. (Figure 2.) This bone also appears to be shorted, probably due to erosion.

Figure 1
Figure 1
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The third landmark, the umbo, is the point where the bottom end of the manubrium forms the center of the eardrum. A healthy, normal eardrum is cone-shaped. The edges attach the tympanic annulus, and the center (the top of the cone) is formed at the end of the manubrium in the middle of the drum.

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DISSECTING THE EARDRUM

In a normal ear, the eardrum can be divided into the major portion, called pars tensa, which has an almost perfect round shape, and the uppermost portion of the eardrum called pars flaccida, which is a small, half-moon-shaped extension of the eardrum. The pars tensa is the most familiar part of the drum to most practitioners.

Study the upper area of the eardrum—a normal eardrum—and note the arrow pointing to the pars flaccida. (Figure 3.) This zone is near the top and seen as a small white circle with red blood marking the center. (Figure 1.) This is an attic retraction pocket with a possible cholesteatoma. The bone around the pars flaccida has been eroded due to a long history of chronic infections and surgeries. The main portion of the eardrum almost seems separate from the pars flaccida.

Figure 2
Figure 2
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Figure 3
Figure 3
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Figure 2 shows the view through the perforation, looking at the inner wall of the middle ear. This surface, called the medial boundary or labyrinthine wall of the middle ear, is clearly seen in Figures 1 and 2. The wall bulging out in the center area is called the promontory, and is created by the basal turn of the cochlea.

To the left of the promontory is a depression leading to the oval window. In a normal ear, the stapes sits in the oval window, although the Stapes is not visible in this photograph. It has eroded along with the incus.

Below and to the right of the promontory is another large depression leading to the opening of the Eustachian tube. Directly below the promontory is another small dark area which is the round window niche. The membrane situated in this cochlear window provides pressure compensation for mechanical waves traveling through the cochlea, and is the terminus of the scala tympani.

While this anatomical overview is quite technical in nature, these landmarks are extremely important when it comes to assessing the proper hearing technologies for patients with open ear.

Next month: Critical considerations when fitting an open ear with hearing aids.

© 2011 Lippincott Williams & Wilkins, Inc.

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