Diagnosis: Cochlear Aplasia and Common Cavity
Congenital hearing loss has a somewhat long list of differential diagnosis. Some cases can be ruled in or out based on the patient's medical history, physical examination, or laboratory test and imaging results. The etiology of some cases may remain unknown. Congenital hearing loss can be attributed to very broad genetic or non-genetic causes. Genetic causes can be further broken down into syndromic or non-syndromic hearing loss. In turn, each subset of genetic hearing loss can be divided into various modes of inheritance, namely autosomal dominant, autosomal recessive, X-linked, or mitochondrial. Of all the genetic causes of congenital hearing loss, the most common is the autosomal recessive mutation in the connexin 26 that encodes for the gap junction beta 2 protein.
Non-genetic causes include prenatal or perinatal infections. The most common of these include syphilis, toxoplasma, rubella, cytomegalovirus, and herpes. Other non-genetic causes of hearing loss include anomalies of the inner ear, which occur very early in the embryogenesis process. The development of the inner ear starts in the third week of gestation until the end of the seventh week. An insult to the embryogenesis process at various times will cause different anomalies in the inner ear. Many of these anomalies are membranous, which can only be seen on the temporal bone histopathology and not on conventional imaging such as MRI or CT. Anomalies of the bony labyrinth are less common, but can be visualized on radiologic imaging. Complete labyrinthine aplasia or Michel's deformity can occur from an interruption to the development of the inner ear and likely occurs in the third week of gestation. Cochlear aplasia is the result of an insult to the cochlea early in the fourth week of embryogenesis. A dense bone is usually seen at the anatomical site of the cochlea. The cochlear nerve and its corresponding canal are absent. The usual bulging cochlear promontory that pushes into the middle ear is flattened. The vestibule and semicircular canals are typically somewhat dilated or malformed. The vestibular aqueduct that connects the vestibule to the posterior fossa is usually normal; however, the internal auditory canal is narrow and hypoplastic. Due to the absence of the cochlea, the facial nerve has an anomalous course with a much more obtuse angle around the geniculate ganglion. Since the middle ear structures are formed much later in the embryogenesis process, the ossicles are normally shaped. The oval window may appear normal, but may be atretic. Cochlear aplasia is rare, affecting only two to three percent of all ear anomalies.
The common cavity deformity occurs when the cochlea, vestibule, and horizontal canal form in a cystic cavity with no bony internal separation. The posterior and superior semicircular canals are usually absent or malformed. However, they may still be normal in some cases. The vestibulocochlear nerves are small and sometimes absent. Since the facial nerve takes its course around the cochlea and vestibular structures, the facial canal usually follows an anomalous course. The malleus and incus, which develop much later in embryogenesis, are generally normal. The stapes superstructure (arch and capitulum), which also develops much later, appears normal as well. The stapes footplate in the oval window derives from the otic capsule (inner ear bone), and tends to be anomalous or stenotic.
The next category of inner ear anomalies is under incomplete partition wherein the cochlea and the vestibule can be easily differentiated but the partitions within the cochlea (modiolus) are abnormal. There are two different types based on the degree of involvement in the cochlea and vestibule. Type two incomplete partition is the classic Mondini deformity, which has been described as the triad of minimally dilated vestibule, an enlarged vestibular aqueduct, and an absence of the apical portion of the modiolus leading to 1 1/2 turns of the cochlea instead of the usual 2 1/2 to 2 3/4 of turns.
The CT scan of the patient in this case showed that he was suffering from cochlear aplasia on the right side and common cavity deformity on the left side. The vestibule on the right side was somewhat enlarged, and the horizontal canal had a widened limb. A solid bone was present in the area where the cochlea is normally located. On the patient's left side, there was a common cavity but no discernible cochlea vestibule or horizontal canal.
Hearing rehabilitation for this patient could be difficult. Since the cochlea was absent in the right ear, it would be unlikely that hearing could be achieved with a cochlear implant. In a common cavity, it has been found that some perception of sound could be obtained with a cochlear implant. However, getting substantial open-set speech understanding (understanding words without having multiple choice answers) would be unlikely. Over the years, auditory brainstem implants have been done to help rehabilitate patients with substantial inner ear anomalies. In the United States, these procedures have not been approved by the Food and Drug Administration (FDA) for children. For adults, implants have to be done under an experimental protocol or under an FDA waiver not covered by insurance. Nonetheless, patients with severe deformities such as cochlear aplasia will rarely get significant open-set speech understanding. This child's family was given the option of getting cochlear implants in the left common cavity and auditory brainstem implant if the first procedure fails. In our experience, patients with a cochlear implant in a common cavity have similar outcomes as those who undergo auditory brainstem implant. The advantage is that they didn't have to go through the inherent risks of craniotomy for the auditory brainstem implant. Both technologies provide common cavity patients with sound awareness and some level of word differentiation but very limited speech development.
BONUS VIDEOS: VISUAL DIAGNOSIS
Read this month's Clinical Consultation case, then watch the accompanying videos from Hamid R. Djalilian, MD, to review the patient's imaging for yourself.
- Video 1. Axial (horizontal) CT of the temporal bone on the right shows the cochlear aplasia.
- Video 2. Axial (horizontal) CT of the left temporal bone shows the common cavity deformity.
- Video 3. Coronal (vertical parallel to the face) CT of the right temporal bone shows the relationship between the semicircular canals and dilated vestibule.
- Video 4. Sagittal (vertical, parallel to the ear) CT of the right temporal bone shows the internal auditory canal going toward the ear canal anatomy.
- Video 5. Coronal (vertical parallel to the face) CT of the left temporal bone shows the relationship between the common cavity and facial nerve.
- Video 6. Sagittal (vertical, parallel to the ear) CT of the left temporal bone shows the internal auditory canal going toward the ear canal anatomy.
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