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Small IAC 9323a206-e7c6-4213-8493-7870b51c6adf
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d19354f3-7ff2-495a-ad3f-064122e45602 Bernadette L. Koch, MD
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Head and Neck 590b726c-a68d-4f79-b2e1-e7bb638eaaa9 11 07/17/24 Small IAC Head and Neck, Differential Diagnosis, CPA-IAC and Posterior Fossa, Anatomically Based Differentials, Small IAC Small IAC | STATdx Small IAC DDX true
Head and Neck
Differential Diagnosis
CPA-IAC and Posterior Fossa
Anatomically Based Differentials
Small IAC

title: "Small IAC" docid: "9323a206-e7c6-4213-8493-7870b51c6adf" authors:

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ESSENTIAL INFORMATION

  • Key Differential Diagnosis Issues

    • Small internal auditory canal (IAC) ≤ 2****mm diameter - CT for bony anatomy; MR to evaluate CNVII & CNVIII components & brainstem anatomy - Occasional duplicated IAC: Partial or complete separation of IAC into 2 stenotic canals - Superior canal transmits CNVII ± superior vestibular nerve (VN); inferior canal transmits inferior ± superior VN ± hypoplastic cochlear nerve
    • Small IAC with cochlear nerve canal (CNC) stenosis/aplasia & hypoplastic/absent cochlear nerve - Unilateral finding in otherwise normal T-bone suggests nonsyndromic unilateral congenital sensorineural hearing loss (SNHL) - Bilateral finding with small horizontal semicircular canal (SCC) bone islands suggests trisomy 21 - Small vestibule & small/absent SCC suggests CHARGE syndrome
    • Severe inner ear anomaly: Cochlear aplasia, common cavity malformation, or cystic cochleovestibular anomaly - Look for coexistent pontine/brainstem anomaly - Occasional unusual origin & course of CNVII ± CNVIII hypoplasia/aplasia

  • Helpful Clues for Common Diagnoses

    • Trisomy 21 - Small bone island horizontal SCC or globular vestibule & horizontal SCC - Stenotic CNC, thickened modiolus ± small IAC, hypoplastic/absent cochlear nerve
    • Aplasia-Hypoplasia of Cochlear Nerve & Cochlear Nerve Canal - Common finding in unilateral congenital SNHL - Narrowed/absent CNC & thickened modiolus ± small IAC, hypoplastic/absent cochlear nerve

  • Helpful Clues for Less Common Diagnoses

    • CHARGE Syndrome - Small vestibule & hypoplastic/absent SCC - Variable cochlear segmentation deficiency - Narrowed/absent CNC, thickened modiolus, & small IAC - Hypoplasia/aplasia of some/all CNVIII components

  • Helpful Clues for Rare Diagnoses

    • Cystic Cochleovestibular Malformation (IP-I) - Cochlea lacks internal septation/modiolus - Globular vestibule & horizontal SCC ± stenotic/absent CNC ± small IAC
    • Common Cavity Malformation - Single primitive sac ± small IAC
    • Cochlear Hypoplasia - Small cochlea < 2 turns, CNC stenosis/atresia ± variable malformation SCC & vestibule - ± small IAC, obtuse angle anterior genu of CNVII canal
    • Cochlear Aplasia - Absent cochlea ± malformation of SCC & vestibule - Malformation of SCC & vestibule variable, mild to severe - ± small IAC, obtuse angle anterior genu of CNVII canal
    • T-Bone Fibrous Dysplasia - Progressive ground-glass fibroosseous thickening → IAC narrowing
    • Craniometaphyseal Dysplasia - Progressive osseous IAC narrowing

References

Selected References

  1. da Costa Monsanto R et al: Otopathologic abnormalities in CHARGE syndrome. Otolaryngol Head Neck Surg. 166(2):363-72, 2021
  2. Dewyer NA et al: Pediatric single-sided deafness: a review of prevalence, radiologic findings, and cochlear implant candidacy. Ann Otol Rhinol Laryngol. 131(3):233-8, 2021
  3. O'Brien WT , Sr et al: Nonsyndromic congenital causes of sensorineural hearing loss in children: an illustrative review. AJR Am J Roentgenol. 1-8, 2021
  4. Ginat DT: Imaging findings in syndromes with temporal bone abnormalities. Neuroimaging Clin N Am. 29(1):117-28, 2019
  5. Tahir E et al: Bony cochlear nerve canal and internal auditory canal measures predict cochlear nerve status. J Laryngol Otol. 131(8):676-83, 2017
  6. Kenna MA et al: Temporal bone abnormalities in children with GJB2 mutations. Laryngoscope. 121(3):630-5, 2011
  7. Morimoto AK et al: Absent semicircular canals in CHARGE syndrome: radiologic spectrum of findings. AJNR Am J Neuroradiol. 27(8):1663-71, 2006

Images

Selected Images

Axial bone CT in an infant with trisomy 21 and sensorineural hearing loss (SNHL) shows a small internal auditory canal (IAC) . The cochlear nerve canal (CNC) is absent. There is a mildly small horizontal semicircular canal (SCC) bone island . Trisomy 21 Axial bone CT in an infant with trisomy 21 and sensorineural hearing loss (SNHL) shows a small internal auditory canal (IAC) . The cochlear nerve canal (CNC) is absent. There is a mildly small horizontal semicircular canal (SCC) bone island .

Axial bone CT in an infant with trisomy 21 and sensorineural hearing loss (SNHL) shows a small internal auditory canal (IAC) . The cochlear nerve canal (CNC) is absent. There is a mildly small horizontal semicircular canal (SCC) bone island . Trisomy 21 Axial bone CT in an infant with trisomy 21 and sensorineural hearing loss (SNHL) shows a small internal auditory canal (IAC) . The cochlear nerve canal (CNC) is absent. There is a mildly small horizontal semicircular canal (SCC) bone island .

Sagittal oblique T2 MR images of the bilateral IACs (right on the left and left on the right) shows a significantly smaller right IAC compared to the left, and nonvisualization of the cochlear nerve . Notice the normal left cochlear nerve . Aplasia-Hypoplasia of Cochlear Nerve & Cochlear Nerve Canal Sagittal oblique T2 MR images of the bilateral IACs (right on the left and left on the right) shows a significantly smaller right IAC compared to the left, and nonvisualization of the cochlear nerve . Notice the normal left cochlear nerve .

Coronal bone CT reformat in a 7-year-old girl with CHD7 mutation shows a small IAC , diminutive vestibule , and absent SCC. Facial nerve canal overlies the atretic oval window with fusion to malformed stapes. There is an emissary vein indenting the tegmen tympani . CHARGE Syndrome Coronal bone CT reformat in a 7-year-old girl with CHD7 mutation shows a small IAC , diminutive vestibule , and absent SCC. Facial nerve canal overlies the atretic oval window with fusion to malformed stapes. There is an emissary vein indenting the tegmen tympani .

Axial bone CT in a 10-year-old girl with SNHL shows small IACs and a globular right vestibule and horizontal SCC . The right cochlea (not shown) lacked internal septation (IP-I). A hypoplastic, isolated left cochlea is also seen. Cystic Cochleovestibular Malformation (IP-I) Axial bone CT in a 10-year-old girl with SNHL shows small IACs and a globular right vestibule and horizontal SCC . The right cochlea (not shown) lacked internal septation (IP-I). A hypoplastic, isolated left cochlea is also seen.

Axial 3D T2 SPACE MR in a child with SNHL shows cochlear aplasia, a globular vestibule, and a horizontal SCC . There is a narrow, malformed IAC with a vestibular nerve noted posteriorly. Cochlear Aplasia Axial 3D T2 SPACE MR in a child with SNHL shows cochlear aplasia, a globular vestibule, and a horizontal SCC . There is a narrow, malformed IAC with a vestibular nerve noted posteriorly.

Oblique sagittal T2 SPACE MR in the same child shows a narrow IAC containing only a single normal-sized cranial nerve (CNVIII vestibular branch) and a possible hypoplastic CNVII anteriorly. Cochlear Aplasia Oblique sagittal T2 SPACE MR in the same child shows a narrow IAC containing only a single normal-sized cranial nerve (CNVIII vestibular branch) and a possible hypoplastic CNVII anteriorly.

Axial bone CT in a teenage boy with polyostotic fibrous dysplasia and precocious puberty (McCune-Albright syndrome) shows severe involvement of the skull base with ground-glass opacification. The middle ear spaces , IACs , and other foramina are small. Note relative otic capsule sparing. T-Bone Fibrous Dysplasia Axial bone CT in a teenage boy with polyostotic fibrous dysplasia and precocious puberty (McCune-Albright syndrome) shows severe involvement of the skull base with ground-glass opacification. The middle ear spaces , IACs , and other foramina are small. Note relative otic capsule sparing.

Axial bone CT in a young man with craniometaphyseal dysplasia shows bony overgrowth of the skull base with small middle ear spaces, ossicular fusion , and small inner ear structures and IACs . Craniometaphyseal Dysplasia Axial bone CT in a young man with craniometaphyseal dysplasia shows bony overgrowth of the skull base with small middle ear spaces, ossicular fusion , and small inner ear structures and IACs .

Additional Images

Axial 3D T2 SPACE MR in a teenager with SNHL shows small IACs . The right CNC is stenotic ; the left is absent. The modioli are thickened . The vestibular nerves and pons are hypoplastic. Aplasia-Hypoplasia of Cochlear Nerve & Cochlear Nerve Canal Axial 3D T2 SPACE MR in a teenager with SNHL shows small IACs . The right CNC is stenotic ; the left is absent. The modioli are thickened . The vestibular nerves and pons are hypoplastic.

Axial bone CT in a child with profound SNHL shows a small IAC and hypoplasia of the CNC . There is also a mildly large vestibular aqueduct . MR should be obtained in order to assess for aplasia or hypoplasia of the cranial nerve. Aplasia-Hypoplasia of Cochlear Nerve & Cochlear Nerve Canal Axial bone CT in a child with profound SNHL shows a small IAC and hypoplasia of the CNC . There is also a mildly large vestibular aqueduct . MR should be obtained in order to assess for aplasia or hypoplasia of the cranial nerve.

Axial T2WI MR in a teenage boy with polyostotic fibrous dysplasia and precocious puberty (McCune-Albright syndrome) shows severe involvement of the skull with fibrous dysplasia that appears hypointense on T2WI. The IACs are small due to progressive involvement of surrounding bone by fibrous dysplasia. T-Bone Fibrous Dysplasia Axial T2WI MR in a teenage boy with polyostotic fibrous dysplasia and precocious puberty (McCune-Albright syndrome) shows severe involvement of the skull with fibrous dysplasia that appears hypointense on T2WI. The IACs are small due to progressive involvement of surrounding bone by fibrous dysplasia.

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