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Neuroradiology Case of the Week
Case 407
May 2009
Gurshawn Singh MS2, Rajiv Mangla MD, and P-L Westesson, MD, PhD, DDS
Clinical
Presentation: A 6-year-old girl presented with bilateral conductive hearing loss.
Imaging Findings: The high resolution axial and coronal images of bilateral temporal bones reveal non-visualization of foot plate of stapes and obliteration of oval window on both sides. The horizontal portions of bilateral facial nerve canals were abnormal in position situated inferior to the expected normal. Also the bony canals were absent. The external auditory canal were normal. The malleus and incus were normally visualized. The bilateral internal auditory canals, semicircular canals and cochlea were also normal.
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| Figure 1: Right middle ear, coronal reconstruction at the level of oval window shows non-visualization of foot plate of stapes, obliteration of oval window and the facial nerve canal being situated inferior to the normal location. The bony wall of the facial nerve canal is absent.
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Figure 2: Right middle ear axial image also shows non-visualization of foot plate of stapes, obliteration of oval window.
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Figure 3: Left middle ear coronal reconstruction shows the same findings as seen on right side. Also the bony canal for horizontal portion of facial nerve is absent (arrow).
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Figure 4: Coronal image of right middle ear in a normal two-year-old child shows normal anatomy of foot plate of stapes, oval window and the facial nerve canal.
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| Figure 5: Axial image of right middle ear shows normal anatomy of foot plate of stapes, oval window and the facial nerve canal.
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Diagnosis: Bilateral congenital absence of oval windows with abnormal facial nerve canal
Discussion: Conductive hearing loss is an obstruction to the external auditory canal or middle ear. Most commonly, a conductive hearing loss is associated with an inflammatory process in the middle ear. Congenital abnormalities resulting in conductive hearing deficits that do not have external ear malformations are very rare. This case had a presentation of a pediatric patient with non-progressive hearing loss. CT examination revealed the absence of the oval windows, stapes, and an abnormal pathway for the facial canal. The oval window is necessary in transmitting sound waves to the cochlea, and thus, its absence will result in hearing loss.
Bilateral congenital anomalies are most likely attributed to autosomal dominant genetics. The second branchial arch is the origin for both the oval window and the main structures of the stapes. In this case, the stapes fails to fuse with the vestibule so therefore, the oval window cannot develop and is absent. The dysplasia or absence of stapes can sometimes be also associated with dysplasia of incus. Also related to the development of the oval window and stapes is the facial nerve canal, originating from the second branchial arch as well. The horizontal facial nerve canal can be abnormally low and may be located inferior to the oval window on coronal images and even bony canal may be dehiscent in many cases. The abnormal course of facial nerve becomes more clinically relevant when surgery is contemplated. The horizontal facial nerve canal is also seen medial than normal when the oval window is absent. The labyrinthine and geniculate portions of the facial nerves are usually normal in position.
The absence of the oval window can be best seen on coronal and axial CT scans. The developmental arrest of the stapedial anlage results in abnormal crura of the stapes and is very well defined on CT. Coronal images are better for viewing the size and shape of the oval window along with the course of the facial canal. The axial images also show the oval window but are most useful in examining the stapes superstructure.
References:
- Zeifer B, Sabini P, Sonne J. Congenital absence of the oval window: radiologic diagnosis and associated anomalies. AJNR Am J Neuroradiol. 2000 Feb;21(2):322-7. [PubMed]
- Swartz JD, Harsnberger HR. Imaging of the Temporal Bone. 3rd ed. New York: Thieme, 1998:140-142.
- Kamal SA, Palkar V. Oval window agenesis. Ann Saudi Med. 1997 Jul;17(4):468-70. [PubMed]
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