Imaging the internal auditory canals

In this video, Dr. Katie Bailey describes the internal auditory canal including the anatomy and some of the common pathology that you may encounter.

Review of internal auditory canal (IAC) anatomy. The IAC includes arteries and nerves. The most easily seen structures are the facial (cranial nerve VII) and vestibulocochlear (cranial nerve VIII) nerves. On a sagittal view, the anterior portion of the canal contains the facial nerve (superior) and cochlear nerve (inferior). This can be remembered by the mnemonic “7up, Coke down”. The oropharynx includes the tonsils (both lingual and palatine), the squamous mucosa of the pharynx, the uvula, and the vallecula.   oral cavity includes the lips, teeth, hard and soft palate, gingiva, retromolar trigone, the buccal mucosa, and anterior 2/3 of the tongue. Masticator space. Contains the muscles of mastication, the mandible, branches of the trigeminal nerve, lymph nodes, and minor salivary glands. 

Vascular loop. Sometimes an arterial branch can compress the nerves as they enter the IAC. If you see mass effect, this is particularly possible. Smaller loops of the anterior inferior cerebellar artery are more controversial but has been previously described as associated with hemifacial spasm. If you see it, you can be descriptive about which portion of the nerve is involved.

Vascular malformations are a rare cause of symptoms, but if you see an unusual tangle or cluster of vessels in the region you should

Vestibular schwannomas are the most common tumor affecting the IAC. They usually arise from the inferior vestibular nerve, and have previously been referred to as “acoustic neuromas”. With these tumors, you will see an enhancing mass with pretty homogeneous enhancement centered in the IAC with extension into the porus acousticus. They can sometimes be quite small but still symptomatic.

Meningiomas are the second most common solid mass. They are also solid enhancing masses near the IAC. Key clues that they are not schwannomas are a center outside of the IAC. Dural tails, or linear areas of tumor tracking along the dura, can be helpful, but schwannomas can also have them.

Epidermoids are relatively uncommon non-enhancing masses of the CP angle and IAC. They look cystic on T2 and can be close to CSF, but their key distinguishing feature is hyperintensity on DWI.

Bell’s Palsy is an idiopathic facial paralysis on one side. Imaging can often be normal, but if you see linear enhancement in the distal auditory canal this can be a sign of Bell’s palsy. The geniculate and mastoid portions of the facial nerve can enhance in normal cases, so in those cases you must consider the symmetry.

Red flags in the IAC include nodular enhancement and multiple cranial nerves enhancing. This should make you think of unusual pathology such as lymphoma, sarcoidosis, metastatic disease, perineural spread of tumor, and Lyme disease.

Thanks for checking out this quick video on the internal auditory canal. Be sure to check out the additional videos on other head and neck topics.

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