3/19/2023 0 Comments Internal auditory canal mriOur search for the most cost-effective technique to study patients with retrocochlear (central) sensorineural hearing loss (SNHL) and/or vestibular disturbances of unknown origin and to screen for vestibular schwannomas has led us to a multiform approach. According to these same authors, this approach has become the most cost-effective method to screen for retrocochlear pathology. Recently, fast spin-echo (FSE) magnetic resonance imaging (MRI) without contrast has been shown by several authors to be “as sensitive and effective in the detection of vestibular schwannomas as contrast enhanced MRI” (3–8). TECHNIQUEĬontrast-enhanced magnetic resonance (MR) has become the gold standard for diagnosis of vestibular schwannomas and other space-occupying lesions within the IAC. The amount of fluid within the canal is proportional to the size of the canal. The IAC is lined by dura and is bathed by cerebrospinal fluid (CSF). The remaining portion of the inferior vestibular nerve passes through the cribriform plate to reach the macula of the saccule. At the junction between the inferior and posterior canal walls is the opening of the singular canal, which transmits a branch of the inferior vestibular nerve (singular nerve) to the ampulla of the posterior semicircular canal. Below the falciform crest, 2–3 mm medial to the fundus of the canal, is a spiral foraminous tract, a series of small foramina in the base of the modiolus for the cochlear nerve. Above the falciform crest in the anterior wall of the canal laterally is the opening of the fallopian canal for the facial nerve and nervus intermedius. At the fundus, the superior vestibular nerve passes through small openings in the cribriform plate to reach the ampullae of the horizontal and superior semicircular canals as well as the macula of the utricle. There are several small openings within the canal. The IAC contains the acoustic nerve, which splits within the canal in its cochlear and vestibular divisions, the facial nerve, the nervus intermedius, the labyrinthine artery, and, in 20% to 40% of the cases, a loop of the anterior inferior cerebellar artery (AICA). Although the IAC in different individuals may differ greatly in size, the two canals of any person are identical or vary by no more than 1 mm. It measures 15 to 20 mm in length and has a diameter as small as 2 mm and as wide as 12 mm (1,2). The internal auditory canal (IAC) extends from its opening in the posteromedial surface of the petrous pyramid to the cribriform plate, which closes the canal laterally and separates the canal from the vestibule. In this article, we will discuss our approach to assessment of the IAC in patients with retrocochlear SNHL or vestibular symptoms of central origin, review the pathological processes involving the IAC walls or arising within the canal, emphasizing the appropriate MRI sequences used for diagnosis. Auditory brain stem response has been widely used as a screening procedure, but this test fails to recognize small lesions and cannot be used whenever hearing loss is severe. Unfortunately, the high cost of MR has been a limiting factor in its use as a screening test for patients with sensorineural hearing loss (SNHL) of unknown origin. The possibility of demonstrating masses as small as 2 mm has propelled MRI into the leading role for diagnosis of vestibular schwannoma. Pathological processes arising within the IAC are well visualized by various MR sequences. MRI provides excellent assessment of the IAC and the bony changes occurring in the canal walls, and it provides excellent demonstration of the content of the canal. Magnetic resonance imaging (MRI) is presently the study of choice for assessment of the internal auditory canal (IAC).
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