167 lines
15 KiB
Markdown
167 lines
15 KiB
Markdown
---
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title: "Small IAC"
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docid: "9323a206-e7c6-4213-8493-7870b51c6adf"
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authors:
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- key: "d19354f3-7ff2-495a-ad3f-064122e45602"
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value: "Bernadette L. Koch, MD"
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breadcrumbs:
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-
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name: "Head and Neck"
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slug: "head-and-neck"
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treeNodeId: "ed24ed8c-5d57-4629-879b-447b82d2973d"
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-
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name: "Differential Diagnosis"
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slug: "differential-diagnosis"
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treeNodeId: "40d68862-8975-4dde-ac2b-ebc43ab0fb5c"
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name: "CPA-IAC and Posterior Fossa"
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slug: "cpa-iac-and-posterior-fossa"
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treeNodeId: "c590eedb-4a3b-4158-a04f-ad880564c992"
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-
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name: "Anatomically Based Differentials"
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slug: "anatomically-based-differentials"
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treeNodeId: "debfb06c-8656-4f5d-92c1-eaa468185d78"
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-
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name: "Small IAC"
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slug: "small-iac"
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treeNodeId: null
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category: "Head and Neck"
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documentVersionId: "590b726c-a68d-4f79-b2e1-e7bb638eaaa9"
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imageCount: 11
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lastUpdated: "07/17/24"
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pageDescription: "Small IAC"
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pageKeywords: "Head and Neck, Differential Diagnosis, CPA-IAC and Posterior Fossa, Anatomically Based Differentials, Small IAC"
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pageTitle: "Small IAC | STATdx"
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enhancedTitle: "Small IAC"
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type: "DDX"
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references: true
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breadcrumbs:
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- "Head and Neck"
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- "Differential Diagnosis"
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- "CPA-IAC and Posterior Fossa"
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- "Anatomically Based Differentials"
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- "Small IAC"
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---
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# ESSENTIAL INFORMATION
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- ## Key Differential Diagnosis Issues
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- Small internal auditory canal (IAC) **≤ 2****mm diameter**
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- CT for bony anatomy; MR to evaluate CNVII & CNVIII components & brainstem anatomy
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- Occasional duplicated IAC: Partial or complete separation of IAC into 2 stenotic canals
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- Superior canal transmits CNVII ± superior vestibular nerve (VN); inferior canal transmits inferior ± superior VN ± hypoplastic cochlear nerve
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- Small IAC with cochlear nerve canal (CNC) stenosis/aplasia & hypoplastic/absent cochlear nerve
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- Unilateral finding in otherwise normal T-bone suggests nonsyndromic unilateral congenital sensorineural hearing loss (SNHL)
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- Bilateral finding with small horizontal semicircular canal (SCC) bone islands suggests trisomy 21
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- Small vestibule & small/absent SCC suggests CHARGE syndrome
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- Severe inner ear anomaly: Cochlear aplasia, common cavity malformation, or cystic cochleovestibular anomaly
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- Look for coexistent pontine/brainstem anomaly
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- Occasional unusual origin & course of CNVII ± CNVIII hypoplasia/aplasia
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- ## Helpful Clues for Common Diagnoses
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- [Trisomy 21](/document/trisomy-21-down-syndrome/dea8cfda-4526-4373-bd25-2ca5c119d243)
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- Small bone island horizontal SCC or globular vestibule & horizontal SCC
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- Stenotic CNC, thickened modiolus ± small IAC, hypoplastic/absent cochlear nerve
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- [Aplasia-Hypoplasia of Cochlear Nerve & Cochlear Nerve Canal](/document/cochlear-nerve-and-cochlear-nerve--/a669349b-22f5-4b5a-9d6d-6b70a5717ecc)
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- Common finding in unilateral congenital SNHL
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- Narrowed/absent CNC & thickened modiolus ± small IAC, hypoplastic/absent cochlear nerve
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- ## Helpful Clues for Less Common Diagnoses
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- [CHARGE Syndrome](/document/semicircular-canal-hypoplasia-apla-/2112211e-f2fd-448d-9fe8-5352b2900cee)
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- Small vestibule & hypoplastic/absent SCC
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- Variable cochlear segmentation deficiency
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- Narrowed/absent CNC, thickened modiolus, & small IAC
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- Hypoplasia/aplasia of some/all CNVIII components
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- ## Helpful Clues for Rare Diagnoses
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- **Cystic Cochleovestibular Malformation (IP-I)**
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- Cochlea lacks internal septation/modiolus
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- Globular vestibule & horizontal SCC ± stenotic/absent CNC ± small IAC
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- [Common Cavity Malformation](/document/common-cavity-malformation/008305e1-2809-41b0-9e4e-730e30b05a66)
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- Single primitive sac ± small IAC
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- [Cochlear Hypoplasia](/document/cochlear-hypoplasia/58a34b9d-fbae-44a5-99cc-ee2319e0d13d)
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- Small cochlea < 2 turns, CNC stenosis/atresia ± variable malformation SCC & vestibule
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- ± small IAC, obtuse angle anterior genu of CNVII canal
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- [Cochlear Aplasia](/document/cochlear-aplasia/82e9634a-f9ed-407d-b611-110b7328f9fb)
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- Absent cochlea ± malformation of SCC & vestibule
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- Malformation of SCC & vestibule variable, mild to severe
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- ± small IAC, obtuse angle anterior genu of CNVII canal
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- [T-Bone Fibrous Dysplasia](/document/temporal-bone-fibrous-dysplasia/e5b44f77-f666-4f32-8eb0-6ed2da7d9898)
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- Progressive ground-glass fibroosseous thickening → IAC narrowing
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- **Craniometaphyseal Dysplasia**
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- Progressive osseous IAC narrowing
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## References
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# Selected References
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1. [da Costa Monsanto R et al: Otopathologic abnormalities in CHARGE syndrome. Otolaryngol Head Neck Surg. 166(2):363-72, 2021](http://www.ncbi.nlm.nih.gov/pubmed/?term=33874787%5Bpmid%5D)
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1. [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](http://www.ncbi.nlm.nih.gov/pubmed/?term=34036833%5Bpmid%5D)
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1. [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](http://www.ncbi.nlm.nih.gov/pubmed/?term=33502224%5Bpmid%5D)
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1. [Ginat DT: Imaging findings in syndromes with temporal bone abnormalities. Neuroimaging Clin N Am. 29(1):117-28, 2019](http://www.ncbi.nlm.nih.gov/pubmed/?term=30466636%5Bpmid%5D)
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1. [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](http://www.ncbi.nlm.nih.gov/pubmed/?term=28566097%5Bpmid%5D)
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1. [Kenna MA et al: Temporal bone abnormalities in children with GJB2 mutations. Laryngoscope. 121(3):630-5, 2011](http://www.ncbi.nlm.nih.gov/pubmed/?term=21298644%5Bpmid%5D)
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1. [Morimoto AK et al: Absent semicircular canals in CHARGE syndrome: radiologic spectrum of findings. AJNR Am J Neuroradiol. 27(8):1663-71, 2006](http://www.ncbi.nlm.nih.gov/pubmed/?term=16971610%5Bpmid%5D)
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## Images
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### Selected Images
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**Trisomy 21**
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*Axial bone CT in an infant with trisomy 21 and sensorineural hearing loss (SNHL) shows a small internal auditory canal (IAC) <img src='img/arrows/WS.png'/>. The cochlear nerve canal (CNC) is absent. There is a mildly small horizontal semicircular canal (SCC) bone island <img src='img/arrows/WO.png'/>.*
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**Trisomy 21**
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*Axial bone CT in an infant with trisomy 21 and sensorineural hearing loss (SNHL) shows a small internal auditory canal (IAC) <img src='img/arrows/WS.png'/>. The cochlear nerve canal (CNC) is absent. There is a mildly small horizontal semicircular canal (SCC) bone island <img src='img/arrows/WO.png'/>.*
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**Aplasia-Hypoplasia of Cochlear Nerve & Cochlear Nerve Canal**
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*Sagittal oblique T2 MR images of the bilateral IACs (right on the left and left on the right) shows a significantly smaller right IAC <img src='img/arrows/WS.png'/> compared to the left, and nonvisualization of the cochlear nerve <img src='img/arrows/WO.png'/>. Notice the normal left cochlear nerve <img src='img/arrows/WC.png'/>.*
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**CHARGE Syndrome**
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*Coronal bone CT reformat in a 7-year-old girl with CHD7 mutation shows a small IAC <img src='img/arrows/WS.png'/>, diminutive vestibule <img src='img/arrows/WO.png'/>, and absent SCC. Facial nerve canal <img src='img/arrows/WC.png'/> overlies the atretic oval window with fusion to malformed stapes. There is an emissary vein indenting the tegmen tympani <img src='img/arrows/BS.png'/>.*
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**Cystic Cochleovestibular Malformation (IP-I)**
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*Axial bone CT in a 10-year-old girl with SNHL shows small IACs <img src='img/arrows/WS.png'/> and a globular right vestibule and horizontal SCC <img src='img/arrows/WO.png'/>. The right cochlea (not shown) lacked internal septation (IP-I). A hypoplastic, isolated left cochlea <img src='img/arrows/WC.png'/> is also seen.*
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**Cochlear Aplasia**
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*Axial 3D T2 SPACE MR in a child with SNHL shows cochlear aplasia, a globular vestibule, and a horizontal SCC <img src='img/arrows/WS.png'/>. There is a narrow, malformed IAC <img src='img/arrows/WO.png'/> with a vestibular nerve noted posteriorly.*
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**Cochlear Aplasia**
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*Oblique sagittal T2 SPACE MR in the same child shows a narrow IAC <img src='img/arrows/WS.png'/> containing only a single normal-sized cranial nerve (CNVIII vestibular branch) <img src='img/arrows/WO.png'/> and a possible hypoplastic CNVII anteriorly.*
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**T-Bone Fibrous Dysplasia**
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*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 <img src='img/arrows/WS.png'/>, IACs <img src='img/arrows/WO.png'/>, and other foramina are small. Note relative otic capsule sparing.*
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**Craniometaphyseal Dysplasia**
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*Axial bone CT in a young man with craniometaphyseal dysplasia shows bony overgrowth of the skull base with small middle ear spaces, ossicular fusion <img src='img/arrows/WS.png'/>, and small inner ear structures and IACs <img src='img/arrows/WO.png'/>.*
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### Additional Images
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**Aplasia-Hypoplasia of Cochlear Nerve & Cochlear Nerve Canal**
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*Axial 3D T2 SPACE MR in a teenager with SNHL shows small IACs <img src='img/arrows/WS.png'/>. The right CNC is stenotic <img src='img/arrows/WO.png'/>; the left is absent. The modioli are thickened <img src='img/arrows/WC.png'/>. The vestibular nerves and pons <img src='img/arrows/BS.png'/> are hypoplastic.*
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**Aplasia-Hypoplasia of Cochlear Nerve & Cochlear Nerve Canal**
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*Axial bone CT in a child with profound SNHL shows a small IAC <img src='img/arrows/WS.png'/> and hypoplasia of the CNC <img src='img/arrows/WO.png'/>. There is also a mildly large vestibular aqueduct <img src='img/arrows/WC.png'/>. MR should be obtained in order to assess for aplasia or hypoplasia of the cranial nerve.*
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**T-Bone Fibrous Dysplasia**
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*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 <img src='img/arrows/WS.png'/> on T2WI. The IACs are small <img src='img/arrows/WO.png'/> due to progressive involvement of surrounding bone by fibrous dysplasia.*
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