Otology & Neurotology 35:1140Y1144 Ó 2014, Otology & Neurotology, Inc.
Partitioned Versus Duplicated Internal Auditory Canal: When Appropriate Terminology Matters *Vincenzo Vincenti, †Francesca Ormitti, and †Elisa Ventura *Department of Clinical and Experimental Medicine, Unit of Audiology and Pediatric Otorhinolaryngology, University of Parma; and ÞDepartment of Neuroradiology, Azienda Ospedaliero-Universitaria di Parma, Parma, Italy
Objective: A double internal auditory canal (IAC) is an uncommon malformation among temporal bone anomalies related to sensorineural hearing loss. This anomaly has been described as an isolated finding in otherwise normal patients, as well as a frequent feature in pontine tegmental cap dysplasia (PTCD). In both circumstances, the term duplication is currently used to describe this malformation. We conducted a retrospective study on 3 cases of IAC duplication (2 isolated and 1 PTCD-associated form) and reviewed the literature to evaluate both imaging features and differences between the 2 forms. Intervention(s): All 3 patients underwent high-resolution computed tomography and magnetic resonance imaging. Results: Neuroimaging documented different radiologic features between isolated and PTCD-associated IAC duplication. In the isolated form, the IAC seemed to have normal position and size, but it was partitioned in 2 portions by a complete or incomplete
bony septum. Conversely, in the PTCD-associated form, neuroimaging revealed the presence of 2 separated and anomalously coursing canals for the facial and vestibulocochlear nerve. Conclusion: The aforementioned different radiologic findings probably reflect a more complex spectrum of etiopathogenetic mechanisms affecting the facioacoustic primordium and leading to overlapping anomalies of the IAC. We speculate on the appropriate terminology in describing this entity and propose that ‘‘partition’’ should be used to describe a double IAC otherwise normal in size and location; conversely, ‘‘duplication’’ should be reserved for IAC anomalies encountered in patients with PTCD or other hindbrain malformations. Key Words: ImagingVInternal auditory canal duplicationVSensorineural hearing lossVTemporal bone malformation.
A rare malformation of the temporal bone is represented by a duplication of the internal auditory canal (IAC). This anomaly has been described as an isolated finding in otherwise normal patients (1Y15) as well as a frequent finding in pontine tegmental cap dysplasia (PTCD), a rare congenital malformation of the brainstem and hindbrain (16,17). Here, we reported on 3 cases of IAC duplication (2 presenting SNHL without syndromic stigmata and 1 affected by PTCD), focusing on clinical and radiologic findings; in addition, we speculated on the appropriate use of the term ‘‘duplication’’ and theorized on the pathogenesis of this entity.
history of sensorineural hearing loss (SNHL). Examination of the head and neck was unremarkable, and the facial nerve (FN) function was normal and symmetric. Audiologic testing documented a profound SNHL on the right side and normal hearing thresholds in the left ear. A HRCT scan of the temporal bones demonstrated that the right IAC was partially divided in 2 canals by a horizontal bony septum occupying approximately almost half of the length of IAC (Fig. 1, A and B). The anterosuperior canal had a uniform diameter of 1.8 mm and continued into the labyrinthine segment of the FN; therefore, it was considered as the bony canal of the FN. The posteroinferior canal had a variable diameter ranging from 0.8 mm at the level of the porus acousticus to 4 mm at the level of the fundus of the IAC and pointed toward the vestibule and the cochlear aperture. Inner ear structures had a normal configuration bilaterally. On MRI, the right facial and vestibulocochlear nerves were both identified in the cerebellopontine angle cistern; however, only the FN was clearly demonstrated within the anteroinferior aspect of the IAC (Fig. 1, D and E). The stenotic inferior portion of the IAC contained thin nervous structures, probably
Otol Neurotol 35:1140Y1144, 2014.
PATIENT 1 A 9-year-old otherwise healthy girl was referred to our department for unilateral deafness. There was no family Address correspondence and reprint requests to Vincenzo Vincenti, M.D., Department of Clinical and Experimental Medicine, Unit of Audiology and Pediatric Otorhinolaryngology, University of Parma, Parma, Italy, Via Gramsci 14, 43126 Parma, Italy; E-mail: [email protected] The authors disclose no conflicts of interest.
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PARTITIONED VS DUPLICATED INTERNAL AUDITORY CANAL
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FIG. 1. Case 1: right incomplete double IAC. Reformatted coronal and parasagittal CT images (A and B) show a horizontal bony septum (black arrowhead) partially dividing the IAC in a superior portion (white arrow) coursing to the labyrinthine segment of the facial canal and an inferior portion (black arrow) pointing toward the cochlear aperture and the modiolus; note that the bony septum is absent at the lateral third of the IAC. A 3D volume rendering image (C) precisely depicts the incomplete partition of the right IAC. A reformatted parasagittal MR DRIVE image (D and E) demonstrates the presence of a thin nervous structure consistent with the facial nerve within the superior canal; conversely, no cochleovestibular nerve is identified in the inferior canal.
coursing toward the vestibule. Conversely, MRI showed a normal contralateral IAC containing cochlear, vestibular, and facial nerves. PATIENT 2 A 12-month-old female infant who failed newborn hearing screening on the right ear was referred to our department for hearing evaluation. There was no family history of SNHL and no syndromic deafness stigmata. Examination of the head and neck was unremarkable, and the FN function was bilaterally normal. Auditory brainstem response testing showed a profound SNHL on the right ear and a normal hearing on the left side. HRCT showed a right IAC divided in 2 compartments by a bony septum occupying the entire length of the IAC (Fig. 2, A and B). The anterosuperior portion had a uniform diameter of 1.6 mm and continued to the bony canal of the labyrinthine portion of the FN. The posteroinferior canal had a variable diameter ranging from 1.1 mm at the level of the porus acousticus to 2.3 mm at level of the fundus of the IAC and continued to the cochlear aperture. MRI failed to identify the right cochlear nerve (Fig. 2, CYE). PATIENT 3 A 6-month-old girl who failed newborn hearing screening was referred to our department for hearing evaluation. There was no family history of SNHL. Neurologic examination revealed truncal ataxia, hypotonic muscle tone, mild
motor delay, and impaired swallowing. A right FN palsy was also noted. Audiologic evaluation revealed a bilateral profound SNHL. HRCT of the temporal bone documented enlargement of vestibule and vestibular aqueduct in both ears; the cochlea was bilaterally dysplastic showing normal basal turn, cystic appearance of the apex, and modiolar deficiency. HRCT also revealed bilateral IAC duplication (Fig. 3, A and B). On MRI study, the cochleovestibular nerves could not be identified in the cerebellopontine angle nor could be recognized within the stenotic IAC (Fig. 3c); the patient presented pathognomonic imaging features of PTCD, consisting in prominent dorsal bulge of the pons together with flattening of its ventral aspect (Fig. 3D).
DISCUSSION An extremely rare anomaly of the IAC is characterized by the presence of 2 canals, divided by a complete or incomplete bony septum. This finding has been always identified as ‘‘duplication of the IAC’’ or ‘‘narrow duplicated IAC’’ and described either as an isolated finding in otherwise normal patients (isolated form) or as a characteristic finding in patients with PTCD (PTCD-associated form). While reviewing the images of our 3 patients, we had the impression that the term duplication has been used to describe 2 distinct radiologic entities. Thus, we reviewed the literature on this topic to verify our hypothesis. The first case of isolated form was reported in 1986 by Curtin and May (14). Since that time, other 16 cases were reported in the literature (Table 1). In all reported cases, the inferior Otology & Neurotology, Vol. 35, No. 7, 2014
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FIG. 2. Case 2: right complete double IAC. Reformatted coronal and parasagittal CT images (A and B) show a horizontal bony septum (black arrowhead) dividing the IAC in a superior portion (white arrow) coursing to the labyrinthine segment of the facial canal and an inferior portion (black arrow) pointing toward the cochlear aperture and the modiolus. A 3D volume rendering image (C) precisely depicts the complete partition of the right IAC. A reformatted parasagittal MR DRIVE image (D and E) demonstrates the presence of a thin nervous structure consistent with the facial nerve within the superior canal; conversely, no cochleovestibular nerve is identified in the inferior canal.
FIG. 3. Case 3: bilateral IAC duplication in PTCD. Axial CT images of the temporal bone (A and B) show bilateral enlargement of vestibule and vestibular aqueduct (black arrows). Severe narrowing, more prominent on the right side, and duplication of internal auditory canals are also noted: the superior canals (white arrows) coursing anteriorly in the petrous bone continue toward the labyrinthine portion of the facial canal, whereas the smaller inferiorly located canals (white arrowheads) course toward the cochlea (C). The cochleovestibular nerves could not be identified in the cerebellopontine angle cisterns or within the stenotic internal auditory canals. A sagittal T1-weighted image (D) demonstrates flattening of ventral pons together with a dorsal exophytic band partially effacing the IV ventricle, as typically encountered in PTCD. Otology & Neurotology, Vol. 35, No. 7, 2014
Copyright © 2014 Otology & Neurotology, Inc. Unauthorized reproduction of this article is prohibited.
PARTITIONED VS DUPLICATED INTERNAL AUDITORY CANAL TABLE 1.
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Reported cases of isolated double internal auditory canal
Study
Sex
Age
Side
Bony septum
VIIIth CN
VIIth CN
Cochlea
Hearing
Kew and Abdullah (6) Coelho et al. (4) Kesser et al. (7) Lee et al. (15) Case 1 Lee et al. (15) Case 2 Lee et al. (15) Case 3 Kono et al. (3) Baik et al. (8) Goktas Bakar et al. (2) Weon et al. (9) Demir et al. (1) Ferreira et al. (5) Cho et al. (10) Vilain et al. (11) Casselman et al. (12) Present Case 1 Present Case 2
M M F F M F F F F M M F F M M F F
34 yr 14 yr 5 mo 2 mo 9 yr 7 yr 14 yr 6 yr 9 yr 28 mo 7 yr 50 yr 18 yr 13 yr 42 yr 9 yr 12 mo
Right Bilateral Bilateral Right Right Right Left Right Bilateral Bilateral Right Right Right Right Left Right Right
Incomplete Complete Complete Complete Incomplete Incomplete Complete Incomplete Complete Complete Complete Complete Complete Incomplete NS Incomplete Complete
Absent Absent Absent No MRI No MRI Absent Hypoplastic Absent No MRI Absent Absent Absent Absent Present Absent Hypoplastic Absent
Absent Present Present No MRI No MRI Present Present Present No MRI Present Present Present Present Present Present Present Present
Normal Unilateral dysplasia Normal Normal Normal Normal Normal Normal Mondini dysplasia Normal Normal Bilateral dysplasia Normal Normal Normal Normal Normal
Profound SNHL Bilateral profound SNHL Bilateral profound SNHL Severe SNHL Profound SNHL Severe SNHL NS SNHL Total deafness Bilateral profound SNHL Bilateral profound SNHL Bilateral CHL Profound SNHL Profound SNHL Total deafness Total deafness Profound SNHL Total deafness
MRI indicates magnetic resonance imaging; SNHL, sensorineural hearing loss; CHL, conductive hearing loss; NS, not specified.
located canal continued to the cochlea and was regarded as the bony canal of the vestibulocochlear nerve; the superior located canal, usually slightly wider than the inferior one, continued to the bony canal of the labyrinthine segment of the FN and was considered as the bony canal of the FN. The bony septum partitioning the IAC was reported to be complete in 9 cases and incomplete in 6 patients; in the remaining 2 cases, this finding was not reported. The pathogenesis of this anomaly is still unclear. It has been suggested that the primary defect is a bony stenosis with consequent aplasia or hypoplasia of the VIIIth nerve; however, this hypothesis seems to be less than likely (18). Current medical literature frequently describes a narrow IAC as a result of aplasia or hypoplasia of the VIIIth cranial nerve (19). Thus, because the vestibulocochlear nerve has a trophic influence on the growth of the IAC, the latter does not develop normally and becomes stenotic. However, this theory does not explain why in some cases, the IAC consists of a single and stenotic canal, whereas in other patients, it is divided into 2 portions by a bony septum. To find a plausible answer, we need to start looking at IAC embryogenesis. The IAC develops by ossification of the mesoderm surrounding the facial and vestibulocochlear nerves, which arise together from the facioacoustic primordium and then separate into distinct nerves. The mesoderm surrounding the VIIIth nerve chondrifies initially and afterward ossifies at 4 ossification centers to form the IAC during the fifth and sixth months of gestation (11). In accordance with Weon et al. (9), we speculate that a deficient cochlear nerve determines the presence of a variable gap between the facial and the vestibulocochlear nerve; subsequently, in some cases, an anomalous and precocious activation of one or more centers of ossification would lead to the formation of a complete or incomplete bony septum determining the presence of 2 separate canals. In fact, on reviewing both the published images and those ones of our Patients 1 and 2, the IAC seemed normal in its orientation and overall size but partitioned in 2 canals by a horizontal complete or incomplete bony septum. Thus, we speculate
that this anomaly is not a true ‘‘duplication’’ but rather a partition within an IAC with normal position and diameter. For this reason, we suggest using the term partition as more appropriate than ‘‘duplication.’’ Conversely, ‘‘duplication’’ should be reserved for IAC anomalies as described in our patient with PTCD (Patient 3). Desai et al. (16), first, introduced the finding of duplicated IACs in 3 children affected by PTCD. All their patients demonstrated pathognomonic imaging findings of PTCD, together with total absence of VIIIth cranial nerve, bilateral duplication of the IACs, and variable deficiency of the FNs. In 2012, Leiva-Salinas et al. (17) reported an additional case with similar radiologic features. However, we observed that radiologic features in the PTCD-associated form were quite different from those one described in the isolated form. In fact, in PTCD patients, the IAC consisted of 2 narrow canals coursing independently in the temporal bone with no evidence of a clearly defined bony septum dividing them. Furthermore, ‘‘duplication’’ was typically bilateral and complete, and the FNs demonstrated an abnormal elongated cisternal segment and a more vertical course through the duplicated IAC. The embryologic premise of PTCD is likely to be related to a complex neuronal migration derangement that affects the brainstem and influences also the facioacoustic primordium; consequently, it has been supposed that the presence of an aberrant interplay between various guidance cues from the malformed brainstem and the otic vesicle could result in abnormalities of the VIIth and VIIIth cranial nerves (16). In agreement with Desai et al. (16), we think that in patients with an isolated double IAC, the same embryologic premise is less realistic; in such cases, a more focal migrational defect could be hypothesized. In conclusion, we postulate that the different radiologic features of IAC malformation in both isolated and PTCDassociated forms probably reflect a more complex spectrum of etiopathogenetic mechanisms. Therefore, we speculate on the appropriate terminology in describing this entity and propose that ‘‘partition’’ should be used to describe a Otology & Neurotology, Vol. 35, No. 7, 2014
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double IAC otherwise normal in size and location; conversely, ‘‘duplication’’ should be reserved for IAC anomalies encountered in patients with PTCD or other hindbrain malformations. REFERENCES 1. Demir OI, Cakmakci H, Erdag TK, Men S. Narrow duplicated internal auditory canal: radiological findings and review of the literature. Pediatr Radiol 2005;35:1220Y3. 2. Goktas Bakar T, Karadag D, Calisir C, Adapinar B. Bilateral narrow duplicated internal auditory canal. Eur Arch Otorhinolaryngol 2008;265:999Y1001. 3. Kono T, Kuwashima S, Arakawa H, et al. Narrow duplicated internal auditory canal: a rare inner ear malformation with sensorineural hearing loss. Arch Otolaryngol Head Neck Surg 2009; 135:1048Y51. 4. Coelho LOM, Ono SE, Neto AC, Polanski JF, Buschle M. Bilateral narrow duplication of the internal auditory canal. J Laryngol Otol 2010;124:1003Y6. 5. Ferreira T, Shayestehfar B, Lufkin R. Narrow, duplicated internal auditory canal. Neuroradiology 2003;45:308Y10. 6. Kew TY, Abdullah A. Duplicate internal auditory canals with facial and vestibulocochlear nerve dysfunction. J Laryngol Otol 2012; 126:66Y71. 7. Kesser BW, Raghavan P, Mukherjee S, Carfrae M, Essig G, Hashisaki GT. Duplication of the internal auditory canal: radiographic imaging case of the month. Otol Neurotol 2010;31:1352Y3. 8. Baik HW, Yu H, Kim KS, Kim GH. A Narrow Internal Auditory Canal with Duplication in a Patient with Congenital Sensorineural Hearing Loss. Korean J Radiol 2008;9(Suppl):S22Y5.
9. Weon YC, Kim JH, Choi SK, Koo J-W. Bilateral duplication of the internal auditory canal. Pediatr Radiol 2007;37:1047Y9. 10. Cho YS, Na DG, Jung JY, Hong SH. Narrow internal auditory canal syndrome: parasagittal reconstruction. J Laryngol Otol 2000; 114:392Y4. 11. Vilain J, Pigeolet Y, Casselman JW. Narrow and vacant internal auditory canal. Acta Otorhinolaryngol Belg 1999;53:67Y71. 12. Casselman JW, Offeciers FE, Govaerts PJ, et al. Aplasia and hypoplasia of the vestibulocochlear nerve: diagnosis with MR imaging. Radiology 1997;202:773Y81. 13. Weissman JL, Arriaga M, Curtin HD, Hirsch B. Duplication anomaly of the internal auditory canal. AJNR Am J Neuroradiol 1991;12:867Y9. 14. Curtin H, May M. Double internal auditory canal associated with progressive facial weakness. Am J Otol 1986;7:275Y81. 15. Lee SY, Cha S-H, Jeon MH, et al. Narrow duplicated or triplicated internal auditory canal (3 cases and review of literature): can we regard the separated narrow internal auditory canal as the presence of vestibulocochlear nerve fibers? J Comput Assist Tomogr 2009; 33:565Y70. 16. Desai NK, Young L, Miranda MA, Kutz JW, Roland PS, Booth TN. Pontine tegmental cap dysplasia: the neurotologic perspective. Otolaryngol Head Neck Surg 2011;145:992Y8. 17. Leiva-Salinas C, Mukherjee S, Kesser BW, Deib G, Flors L, Raghavan P. Imaging case of the month: Abnormalities of the cochlear nerves and internal auditory canals in pontine tegmental cap dysplasia. Otol Neurotol 2012;33:e73Y4. 18. Yates JA, Patel PC, Millman B, Gibson WS. Isolated congenital internal auditory canal atresia with normal facial nerve function. Int J Pediatr Otorhinolaryngol 1997;41:1Y8. 19. Sakina MS, Goh BS, Abdullah A, Zulfiqar MA, Saim L. Internal auditory canal stenosis in congenital sensorineural hearing loss. Int J Pediatr Otorhinolaryngol 2006;70:2093Y7.
Otology & Neurotology, Vol. 35, No. 7, 2014
Copyright © 2014 Otology & Neurotology, Inc. Unauthorized reproduction of this article is prohibited.
Partitioned Versus Duplicated Internal Auditory Canal: When Appropriate Terminology Matters *Vincenzo Vincenti, †Francesca Ormitti, and †Elisa Ventura *Department of Clinical and Experimental Medicine, Unit of Audiology and Pediatric Otorhinolaryngology, University of Parma; and ÞDepartment of Neuroradiology, Azienda Ospedaliero-Universitaria di Parma, Parma, Italy
Objective: A double internal auditory canal (IAC) is an uncommon malformation among temporal bone anomalies related to sensorineural hearing loss. This anomaly has been described as an isolated finding in otherwise normal patients, as well as a frequent feature in pontine tegmental cap dysplasia (PTCD). In both circumstances, the term duplication is currently used to describe this malformation. We conducted a retrospective study on 3 cases of IAC duplication (2 isolated and 1 PTCD-associated form) and reviewed the literature to evaluate both imaging features and differences between the 2 forms. Intervention(s): All 3 patients underwent high-resolution computed tomography and magnetic resonance imaging. Results: Neuroimaging documented different radiologic features between isolated and PTCD-associated IAC duplication. In the isolated form, the IAC seemed to have normal position and size, but it was partitioned in 2 portions by a complete or incomplete
bony septum. Conversely, in the PTCD-associated form, neuroimaging revealed the presence of 2 separated and anomalously coursing canals for the facial and vestibulocochlear nerve. Conclusion: The aforementioned different radiologic findings probably reflect a more complex spectrum of etiopathogenetic mechanisms affecting the facioacoustic primordium and leading to overlapping anomalies of the IAC. We speculate on the appropriate terminology in describing this entity and propose that ‘‘partition’’ should be used to describe a double IAC otherwise normal in size and location; conversely, ‘‘duplication’’ should be reserved for IAC anomalies encountered in patients with PTCD or other hindbrain malformations. Key Words: ImagingVInternal auditory canal duplicationVSensorineural hearing lossVTemporal bone malformation.
A rare malformation of the temporal bone is represented by a duplication of the internal auditory canal (IAC). This anomaly has been described as an isolated finding in otherwise normal patients (1Y15) as well as a frequent finding in pontine tegmental cap dysplasia (PTCD), a rare congenital malformation of the brainstem and hindbrain (16,17). Here, we reported on 3 cases of IAC duplication (2 presenting SNHL without syndromic stigmata and 1 affected by PTCD), focusing on clinical and radiologic findings; in addition, we speculated on the appropriate use of the term ‘‘duplication’’ and theorized on the pathogenesis of this entity.
history of sensorineural hearing loss (SNHL). Examination of the head and neck was unremarkable, and the facial nerve (FN) function was normal and symmetric. Audiologic testing documented a profound SNHL on the right side and normal hearing thresholds in the left ear. A HRCT scan of the temporal bones demonstrated that the right IAC was partially divided in 2 canals by a horizontal bony septum occupying approximately almost half of the length of IAC (Fig. 1, A and B). The anterosuperior canal had a uniform diameter of 1.8 mm and continued into the labyrinthine segment of the FN; therefore, it was considered as the bony canal of the FN. The posteroinferior canal had a variable diameter ranging from 0.8 mm at the level of the porus acousticus to 4 mm at the level of the fundus of the IAC and pointed toward the vestibule and the cochlear aperture. Inner ear structures had a normal configuration bilaterally. On MRI, the right facial and vestibulocochlear nerves were both identified in the cerebellopontine angle cistern; however, only the FN was clearly demonstrated within the anteroinferior aspect of the IAC (Fig. 1, D and E). The stenotic inferior portion of the IAC contained thin nervous structures, probably
Otol Neurotol 35:1140Y1144, 2014.
PATIENT 1 A 9-year-old otherwise healthy girl was referred to our department for unilateral deafness. There was no family Address correspondence and reprint requests to Vincenzo Vincenti, M.D., Department of Clinical and Experimental Medicine, Unit of Audiology and Pediatric Otorhinolaryngology, University of Parma, Parma, Italy, Via Gramsci 14, 43126 Parma, Italy; E-mail: [email protected] The authors disclose no conflicts of interest.
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PARTITIONED VS DUPLICATED INTERNAL AUDITORY CANAL
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FIG. 1. Case 1: right incomplete double IAC. Reformatted coronal and parasagittal CT images (A and B) show a horizontal bony septum (black arrowhead) partially dividing the IAC in a superior portion (white arrow) coursing to the labyrinthine segment of the facial canal and an inferior portion (black arrow) pointing toward the cochlear aperture and the modiolus; note that the bony septum is absent at the lateral third of the IAC. A 3D volume rendering image (C) precisely depicts the incomplete partition of the right IAC. A reformatted parasagittal MR DRIVE image (D and E) demonstrates the presence of a thin nervous structure consistent with the facial nerve within the superior canal; conversely, no cochleovestibular nerve is identified in the inferior canal.
coursing toward the vestibule. Conversely, MRI showed a normal contralateral IAC containing cochlear, vestibular, and facial nerves. PATIENT 2 A 12-month-old female infant who failed newborn hearing screening on the right ear was referred to our department for hearing evaluation. There was no family history of SNHL and no syndromic deafness stigmata. Examination of the head and neck was unremarkable, and the FN function was bilaterally normal. Auditory brainstem response testing showed a profound SNHL on the right ear and a normal hearing on the left side. HRCT showed a right IAC divided in 2 compartments by a bony septum occupying the entire length of the IAC (Fig. 2, A and B). The anterosuperior portion had a uniform diameter of 1.6 mm and continued to the bony canal of the labyrinthine portion of the FN. The posteroinferior canal had a variable diameter ranging from 1.1 mm at the level of the porus acousticus to 2.3 mm at level of the fundus of the IAC and continued to the cochlear aperture. MRI failed to identify the right cochlear nerve (Fig. 2, CYE). PATIENT 3 A 6-month-old girl who failed newborn hearing screening was referred to our department for hearing evaluation. There was no family history of SNHL. Neurologic examination revealed truncal ataxia, hypotonic muscle tone, mild
motor delay, and impaired swallowing. A right FN palsy was also noted. Audiologic evaluation revealed a bilateral profound SNHL. HRCT of the temporal bone documented enlargement of vestibule and vestibular aqueduct in both ears; the cochlea was bilaterally dysplastic showing normal basal turn, cystic appearance of the apex, and modiolar deficiency. HRCT also revealed bilateral IAC duplication (Fig. 3, A and B). On MRI study, the cochleovestibular nerves could not be identified in the cerebellopontine angle nor could be recognized within the stenotic IAC (Fig. 3c); the patient presented pathognomonic imaging features of PTCD, consisting in prominent dorsal bulge of the pons together with flattening of its ventral aspect (Fig. 3D).
DISCUSSION An extremely rare anomaly of the IAC is characterized by the presence of 2 canals, divided by a complete or incomplete bony septum. This finding has been always identified as ‘‘duplication of the IAC’’ or ‘‘narrow duplicated IAC’’ and described either as an isolated finding in otherwise normal patients (isolated form) or as a characteristic finding in patients with PTCD (PTCD-associated form). While reviewing the images of our 3 patients, we had the impression that the term duplication has been used to describe 2 distinct radiologic entities. Thus, we reviewed the literature on this topic to verify our hypothesis. The first case of isolated form was reported in 1986 by Curtin and May (14). Since that time, other 16 cases were reported in the literature (Table 1). In all reported cases, the inferior Otology & Neurotology, Vol. 35, No. 7, 2014
Copyright © 2014 Otology & Neurotology, Inc. Unauthorized reproduction of this article is prohibited.
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FIG. 2. Case 2: right complete double IAC. Reformatted coronal and parasagittal CT images (A and B) show a horizontal bony septum (black arrowhead) dividing the IAC in a superior portion (white arrow) coursing to the labyrinthine segment of the facial canal and an inferior portion (black arrow) pointing toward the cochlear aperture and the modiolus. A 3D volume rendering image (C) precisely depicts the complete partition of the right IAC. A reformatted parasagittal MR DRIVE image (D and E) demonstrates the presence of a thin nervous structure consistent with the facial nerve within the superior canal; conversely, no cochleovestibular nerve is identified in the inferior canal.
FIG. 3. Case 3: bilateral IAC duplication in PTCD. Axial CT images of the temporal bone (A and B) show bilateral enlargement of vestibule and vestibular aqueduct (black arrows). Severe narrowing, more prominent on the right side, and duplication of internal auditory canals are also noted: the superior canals (white arrows) coursing anteriorly in the petrous bone continue toward the labyrinthine portion of the facial canal, whereas the smaller inferiorly located canals (white arrowheads) course toward the cochlea (C). The cochleovestibular nerves could not be identified in the cerebellopontine angle cisterns or within the stenotic internal auditory canals. A sagittal T1-weighted image (D) demonstrates flattening of ventral pons together with a dorsal exophytic band partially effacing the IV ventricle, as typically encountered in PTCD. Otology & Neurotology, Vol. 35, No. 7, 2014
Copyright © 2014 Otology & Neurotology, Inc. Unauthorized reproduction of this article is prohibited.
PARTITIONED VS DUPLICATED INTERNAL AUDITORY CANAL TABLE 1.
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Reported cases of isolated double internal auditory canal
Study
Sex
Age
Side
Bony septum
VIIIth CN
VIIth CN
Cochlea
Hearing
Kew and Abdullah (6) Coelho et al. (4) Kesser et al. (7) Lee et al. (15) Case 1 Lee et al. (15) Case 2 Lee et al. (15) Case 3 Kono et al. (3) Baik et al. (8) Goktas Bakar et al. (2) Weon et al. (9) Demir et al. (1) Ferreira et al. (5) Cho et al. (10) Vilain et al. (11) Casselman et al. (12) Present Case 1 Present Case 2
M M F F M F F F F M M F F M M F F
34 yr 14 yr 5 mo 2 mo 9 yr 7 yr 14 yr 6 yr 9 yr 28 mo 7 yr 50 yr 18 yr 13 yr 42 yr 9 yr 12 mo
Right Bilateral Bilateral Right Right Right Left Right Bilateral Bilateral Right Right Right Right Left Right Right
Incomplete Complete Complete Complete Incomplete Incomplete Complete Incomplete Complete Complete Complete Complete Complete Incomplete NS Incomplete Complete
Absent Absent Absent No MRI No MRI Absent Hypoplastic Absent No MRI Absent Absent Absent Absent Present Absent Hypoplastic Absent
Absent Present Present No MRI No MRI Present Present Present No MRI Present Present Present Present Present Present Present Present
Normal Unilateral dysplasia Normal Normal Normal Normal Normal Normal Mondini dysplasia Normal Normal Bilateral dysplasia Normal Normal Normal Normal Normal
Profound SNHL Bilateral profound SNHL Bilateral profound SNHL Severe SNHL Profound SNHL Severe SNHL NS SNHL Total deafness Bilateral profound SNHL Bilateral profound SNHL Bilateral CHL Profound SNHL Profound SNHL Total deafness Total deafness Profound SNHL Total deafness
MRI indicates magnetic resonance imaging; SNHL, sensorineural hearing loss; CHL, conductive hearing loss; NS, not specified.
located canal continued to the cochlea and was regarded as the bony canal of the vestibulocochlear nerve; the superior located canal, usually slightly wider than the inferior one, continued to the bony canal of the labyrinthine segment of the FN and was considered as the bony canal of the FN. The bony septum partitioning the IAC was reported to be complete in 9 cases and incomplete in 6 patients; in the remaining 2 cases, this finding was not reported. The pathogenesis of this anomaly is still unclear. It has been suggested that the primary defect is a bony stenosis with consequent aplasia or hypoplasia of the VIIIth nerve; however, this hypothesis seems to be less than likely (18). Current medical literature frequently describes a narrow IAC as a result of aplasia or hypoplasia of the VIIIth cranial nerve (19). Thus, because the vestibulocochlear nerve has a trophic influence on the growth of the IAC, the latter does not develop normally and becomes stenotic. However, this theory does not explain why in some cases, the IAC consists of a single and stenotic canal, whereas in other patients, it is divided into 2 portions by a bony septum. To find a plausible answer, we need to start looking at IAC embryogenesis. The IAC develops by ossification of the mesoderm surrounding the facial and vestibulocochlear nerves, which arise together from the facioacoustic primordium and then separate into distinct nerves. The mesoderm surrounding the VIIIth nerve chondrifies initially and afterward ossifies at 4 ossification centers to form the IAC during the fifth and sixth months of gestation (11). In accordance with Weon et al. (9), we speculate that a deficient cochlear nerve determines the presence of a variable gap between the facial and the vestibulocochlear nerve; subsequently, in some cases, an anomalous and precocious activation of one or more centers of ossification would lead to the formation of a complete or incomplete bony septum determining the presence of 2 separate canals. In fact, on reviewing both the published images and those ones of our Patients 1 and 2, the IAC seemed normal in its orientation and overall size but partitioned in 2 canals by a horizontal complete or incomplete bony septum. Thus, we speculate
that this anomaly is not a true ‘‘duplication’’ but rather a partition within an IAC with normal position and diameter. For this reason, we suggest using the term partition as more appropriate than ‘‘duplication.’’ Conversely, ‘‘duplication’’ should be reserved for IAC anomalies as described in our patient with PTCD (Patient 3). Desai et al. (16), first, introduced the finding of duplicated IACs in 3 children affected by PTCD. All their patients demonstrated pathognomonic imaging findings of PTCD, together with total absence of VIIIth cranial nerve, bilateral duplication of the IACs, and variable deficiency of the FNs. In 2012, Leiva-Salinas et al. (17) reported an additional case with similar radiologic features. However, we observed that radiologic features in the PTCD-associated form were quite different from those one described in the isolated form. In fact, in PTCD patients, the IAC consisted of 2 narrow canals coursing independently in the temporal bone with no evidence of a clearly defined bony septum dividing them. Furthermore, ‘‘duplication’’ was typically bilateral and complete, and the FNs demonstrated an abnormal elongated cisternal segment and a more vertical course through the duplicated IAC. The embryologic premise of PTCD is likely to be related to a complex neuronal migration derangement that affects the brainstem and influences also the facioacoustic primordium; consequently, it has been supposed that the presence of an aberrant interplay between various guidance cues from the malformed brainstem and the otic vesicle could result in abnormalities of the VIIth and VIIIth cranial nerves (16). In agreement with Desai et al. (16), we think that in patients with an isolated double IAC, the same embryologic premise is less realistic; in such cases, a more focal migrational defect could be hypothesized. In conclusion, we postulate that the different radiologic features of IAC malformation in both isolated and PTCDassociated forms probably reflect a more complex spectrum of etiopathogenetic mechanisms. Therefore, we speculate on the appropriate terminology in describing this entity and propose that ‘‘partition’’ should be used to describe a Otology & Neurotology, Vol. 35, No. 7, 2014
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double IAC otherwise normal in size and location; conversely, ‘‘duplication’’ should be reserved for IAC anomalies encountered in patients with PTCD or other hindbrain malformations. REFERENCES 1. Demir OI, Cakmakci H, Erdag TK, Men S. Narrow duplicated internal auditory canal: radiological findings and review of the literature. Pediatr Radiol 2005;35:1220Y3. 2. Goktas Bakar T, Karadag D, Calisir C, Adapinar B. Bilateral narrow duplicated internal auditory canal. Eur Arch Otorhinolaryngol 2008;265:999Y1001. 3. Kono T, Kuwashima S, Arakawa H, et al. Narrow duplicated internal auditory canal: a rare inner ear malformation with sensorineural hearing loss. Arch Otolaryngol Head Neck Surg 2009; 135:1048Y51. 4. Coelho LOM, Ono SE, Neto AC, Polanski JF, Buschle M. Bilateral narrow duplication of the internal auditory canal. J Laryngol Otol 2010;124:1003Y6. 5. Ferreira T, Shayestehfar B, Lufkin R. Narrow, duplicated internal auditory canal. Neuroradiology 2003;45:308Y10. 6. Kew TY, Abdullah A. Duplicate internal auditory canals with facial and vestibulocochlear nerve dysfunction. J Laryngol Otol 2012; 126:66Y71. 7. Kesser BW, Raghavan P, Mukherjee S, Carfrae M, Essig G, Hashisaki GT. Duplication of the internal auditory canal: radiographic imaging case of the month. Otol Neurotol 2010;31:1352Y3. 8. Baik HW, Yu H, Kim KS, Kim GH. A Narrow Internal Auditory Canal with Duplication in a Patient with Congenital Sensorineural Hearing Loss. Korean J Radiol 2008;9(Suppl):S22Y5.
9. Weon YC, Kim JH, Choi SK, Koo J-W. Bilateral duplication of the internal auditory canal. Pediatr Radiol 2007;37:1047Y9. 10. Cho YS, Na DG, Jung JY, Hong SH. Narrow internal auditory canal syndrome: parasagittal reconstruction. J Laryngol Otol 2000; 114:392Y4. 11. Vilain J, Pigeolet Y, Casselman JW. Narrow and vacant internal auditory canal. Acta Otorhinolaryngol Belg 1999;53:67Y71. 12. Casselman JW, Offeciers FE, Govaerts PJ, et al. Aplasia and hypoplasia of the vestibulocochlear nerve: diagnosis with MR imaging. Radiology 1997;202:773Y81. 13. Weissman JL, Arriaga M, Curtin HD, Hirsch B. Duplication anomaly of the internal auditory canal. AJNR Am J Neuroradiol 1991;12:867Y9. 14. Curtin H, May M. Double internal auditory canal associated with progressive facial weakness. Am J Otol 1986;7:275Y81. 15. Lee SY, Cha S-H, Jeon MH, et al. Narrow duplicated or triplicated internal auditory canal (3 cases and review of literature): can we regard the separated narrow internal auditory canal as the presence of vestibulocochlear nerve fibers? J Comput Assist Tomogr 2009; 33:565Y70. 16. Desai NK, Young L, Miranda MA, Kutz JW, Roland PS, Booth TN. Pontine tegmental cap dysplasia: the neurotologic perspective. Otolaryngol Head Neck Surg 2011;145:992Y8. 17. Leiva-Salinas C, Mukherjee S, Kesser BW, Deib G, Flors L, Raghavan P. Imaging case of the month: Abnormalities of the cochlear nerves and internal auditory canals in pontine tegmental cap dysplasia. Otol Neurotol 2012;33:e73Y4. 18. Yates JA, Patel PC, Millman B, Gibson WS. Isolated congenital internal auditory canal atresia with normal facial nerve function. Int J Pediatr Otorhinolaryngol 1997;41:1Y8. 19. Sakina MS, Goh BS, Abdullah A, Zulfiqar MA, Saim L. Internal auditory canal stenosis in congenital sensorineural hearing loss. Int J Pediatr Otorhinolaryngol 2006;70:2093Y7.
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