--- title: "Small IAC" docid: "9323a206-e7c6-4213-8493-7870b51c6adf" authors: - key: "d19354f3-7ff2-495a-ad3f-064122e45602" value: "Bernadette L. Koch, MD" breadcrumbs: - name: "Head and Neck" slug: "head-and-neck" treeNodeId: "ed24ed8c-5d57-4629-879b-447b82d2973d" - name: "Differential Diagnosis" slug: "differential-diagnosis" treeNodeId: "40d68862-8975-4dde-ac2b-ebc43ab0fb5c" - name: "CPA-IAC and Posterior Fossa" slug: "cpa-iac-and-posterior-fossa" treeNodeId: "c590eedb-4a3b-4158-a04f-ad880564c992" - name: "Anatomically Based Differentials" slug: "anatomically-based-differentials" treeNodeId: "debfb06c-8656-4f5d-92c1-eaa468185d78" - name: "Small IAC" slug: "small-iac" treeNodeId: null category: "Head and Neck" documentVersionId: "590b726c-a68d-4f79-b2e1-e7bb638eaaa9" imageCount: 11 lastUpdated: "07/17/24" pageDescription: "Small IAC" pageKeywords: "Head and Neck, Differential Diagnosis, CPA-IAC and Posterior Fossa, Anatomically Based Differentials, Small IAC" pageTitle: "Small IAC | STATdx" enhancedTitle: "Small IAC" type: "DDX" references: true breadcrumbs: - "Head and Neck" - "Differential Diagnosis" - "CPA-IAC and Posterior Fossa" - "Anatomically Based Differentials" - "Small IAC" --- # 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](/document/trisomy-21-down-syndrome/dea8cfda-4526-4373-bd25-2ca5c119d243) - 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](/document/cochlear-nerve-and-cochlear-nerve--/a669349b-22f5-4b5a-9d6d-6b70a5717ecc) - 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](/document/semicircular-canal-hypoplasia-apla-/2112211e-f2fd-448d-9fe8-5352b2900cee) - 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](/document/common-cavity-malformation/008305e1-2809-41b0-9e4e-730e30b05a66) - Single primitive sac ± small IAC - [Cochlear Hypoplasia](/document/cochlear-hypoplasia/58a34b9d-fbae-44a5-99cc-ee2319e0d13d) - Small cochlea < 2 turns, CNC stenosis/atresia ± variable malformation SCC & vestibule - ± small IAC, obtuse angle anterior genu of CNVII canal - [Cochlear Aplasia](/document/cochlear-aplasia/82e9634a-f9ed-407d-b611-110b7328f9fb) - 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](/document/temporal-bone-fibrous-dysplasia/e5b44f77-f666-4f32-8eb0-6ed2da7d9898) - 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](http://www.ncbi.nlm.nih.gov/pubmed/?term=33874787%5Bpmid%5D) 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) 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) 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) 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) 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) 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) ## 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 .](images/app.statdx.com_image_thumbnail_69c10678-064c-4a14-b9bc-fc104bad6011_annotated_true_size_900_quality_90_022530d12bb1df8515bb78c38493c84592b1d804.jpg) **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 .](images/app.statdx.com_image_thumbnail_69c10678-064c-4a14-b9bc-fc104bad6011_size_174_quality_85_8c339968.jpg) **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 .](fd6e92f8-4c55-4db1-b686-e31ac3f16b65) **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 .](images/app.statdx.com_image_thumbnail_cf3d36b2-cc2e-461a-8a7b-3ce9c7d8bf3d_annotated_true_size_900_quality_90_836f5c50a7f11ba292db1a36494295b2a13fa8e9.jpg) **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.](images/app.statdx.com_image_thumbnail_cd4d3d47-6080-4b07-9e17-af75691687db_annotated_true_size_900_quality_90_3637bb2fa0293dabb75788331fc7d81113251cc9.jpg) **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.](images/app.statdx.com_image_thumbnail_3f1345b3-b83f-4a8e-875c-0ed7b0a536c0_annotated_true_size_900_quality_90_71a061daa65df0a6a73c582aa536c83b0bd648d9.jpg) **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.](images/app.statdx.com_image_thumbnail_48395920-1458-4f3f-a03b-06a26b1d091d_annotated_true_size_900_quality_90_55f1bab5280371efdb7a3798df854ea4ebf9f8a4.jpg) **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.](images/app.statdx.com_image_thumbnail_508c5499-4769-4a9b-bcfa-8339b4718289_annotated_true_size_900_quality_90_404847f7f6e9c105072852f2620d70be0149f05d.jpg) **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 .](03aefa37-9bb2-4212-8199-c8f8c485669b) **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.](3049cbef-1768-457f-9cee-3bb3957ae3f5) **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.](ba6e5c92-e87c-4c7a-b830-79c1c2a00d4a) **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.](images/app.statdx.com_image_thumbnail_7c51af12-70f6-4050-b56b-8dd6ecd8347c_annotated_true_size_900_quality_90_5ea958b39d3b9fbf4641c8a6d59302b6f740e4f1.jpg) **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.*