statdx/docs_md/articles/large-iac_d5405c3d-6941-4a2d-abeb-dd8ccf2b5d45.md

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Large IAC	d5405c3d-6941-4a2d-abeb-dd8ccf2b5d45	key value eef2f839-5706-47b9-89c3-60d8315b2b3a Nicholas A. Koontz, MD	name slug treeNodeId Head and Neck head-and-neck ed24ed8c-5d57-4629-879b-447b82d2973d name slug treeNodeId Differential Diagnosis differential-diagnosis 40d68862-8975-4dde-ac2b-ebc43ab0fb5c name slug treeNodeId CPA-IAC and Posterior Fossa cpa-iac-and-posterior-fossa c590eedb-4a3b-4158-a04f-ad880564c992 name slug treeNodeId Anatomically Based Differentials anatomically-based-differentials debfb06c-8656-4f5d-92c1-eaa468185d78 name slug treeNodeId Large IAC large-iac null	Head and Neck	fe254fbf-9caf-4b75-a18f-dd4c006eafc1	16	09/04/18	Large IAC	Head and Neck, Differential Diagnosis, CPA-IAC and Posterior Fossa, Anatomically Based Differentials, Large IAC	Large IAC | STATdx	Large IAC	DDX	true	Head and Neck Differential Diagnosis CPA-IAC and Posterior Fossa Anatomically Based Differentials Large IAC

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title: "Large IAC" docid: "d5405c3d-6941-4a2d-abeb-dd8ccf2b5d45" authors:

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ESSENTIAL INFORMATION

• Key Differential Diagnosis Issues

  • Bilateral large internal auditory canal (IAC) - Can be normal variant if imaging otherwise normal - Inner ear malformation - Cystic cochleovestibular malformation (CCVM) - X-linked stapes gusher - Risk of CSF gusher at surgery - Bilateral large IAC and tumors: Consider NF2
  • Unilateral large IAC - Ipsilateral cerebellar hypoplasia suggests PHACES - Inner ear malformation, e.g., CCVM - Cerebellopontine angle (CPA)-IAC tumor

• Helpful Clues for Common Diagnoses

  • Vestibular Schwannoma - Large IAC + enhancing CPA-IAC tumor with extension along vestibular nerve(s) ± cochlear nerve - Variable signal on T2, depends on cellularity - Presence of microhemorrhages on T2* GRE or SWI may help differentiate schwannoma from meningioma in this location

• Helpful Clues for Less Common Diagnoses

  • Neurofibromatosis Type 2, CPA-IAC - Bilateral vestibular schwannomas is hallmark lesion; effectively pathognomonic - Bilateral vestibular > > facial or cochlear schwannomas ± schwannomas &/or meningiomas of other cranial nerves/dura - Unilateral or bilateral large IAC with sharply marginated tumor in CPA-IAC - Variable signal on T2WI, depends on cellularity - Intense enhancement of solid component
  • Metastases, CPA-IAC - Uni- or bilateral large IAC due to IAC-CPA mass(es) - Hematogenous mets: Lytic/permeative bony destruction, hypointensity on T2WI, decreased diffusivity, and variable enhancement - CSF tumor dissemination of 1° intracranial tumor ± large IAC, variable MR appearance depending on tumor type
  • PHACES Association - Posterior fossa malformation, infantile craniofacial hemangioma, aortic and cerebral arterial anomalies, cardiac, eye and sternal/midline anomalies - Unilateral flared, large IAC - Ipsilateral cerebellar hypoplasia

• Helpful Clues for Rare Diagnoses

  • Schwannoma, Facial Nerve, CPA-IAC - Unilateral large IAC and facial nerve canal
  • Atypical Teratoid/Rhabdoid Tumor - Unilateral large IAC and IAC-CPA tumor with lytic bone destruction in infant - T2WI hypointensity, decreased diffusivity on DWI, and variable enhancement
  • Cystic Cochleovestibular Malformation (IP-I) - IAC most commonly enlarged; may be small or normal - Plump cochlea lacks internal septation/modiolus - Globular vestibule and lateral semicircular canal (SCC) or SCC anlage anomaly
  • X-Linked Stapes Gusher (DFNX2) - Lateral aspect IAC wide (bulbous) bilaterally - Corkscrew-shaped cochlea lacks normal interscalar septum and modiolus
  • Neurofibromatosis Type 1 - Common disorder, wide IACs uncommon - Symmetric large IACs from dural ectasia nottumor

References

Selected References

1. Plotkin SR et al: Neurofibromatosis and schwannomatosis. Semin Neurol. 38(1):73-85, 2018
2. Dağkıran M et al: Radiological imaging findings of patients with congenital totally hearing loss. J Int Adv Otol. 12(1):43-8, 2016
3. Meltzer DE et al: Enlargement of the internal auditory canal and associated posterior fossa anomalies in PHACES association. AJNR Am J Neuroradiol. 36(11):2159-62, 2015
4. Wang X et al: Atypical teratoid/rhabdoid tumor (AT/RT) arising from the acoustic nerve in a young adult: a case report and a review of literature. Medicine (Baltimore). 94(4):e439, 2015
5. Saylisoy S et al: Computed tomographic findings of X-linked deafness: a spectrum from child to mother, from young to old, from boy to girl, from mixed to sudden hearing loss. J Comput Assist Tomogr. 38(1):20-4, 2014
6. Thamburaj K et al: Intratumoral microhemorrhages on T2*-weighted gradient-echo imaging helps differentiate vestibular schwannoma from meningioma. AJNR Am J Neuroradiol. 29(3):552-7, 2008
7. Kumar G et al: X-linked stapes gusher: CT findings in one patient. AJNR Am J Neuroradiol. 24(6):1130-2, 2003

Images

Selected Images

Vestibular Schwannoma Axial T1 C+ FS MR shows a large, avidly enhancing left CPA-IAC vestibular schwannoma that markedly enlarges the IAC as well as exerts mass effect upon the adjacent pons, brachium pontis, and cerebellum.

Vestibular Schwannoma Axial T1 C+ FS MR shows a large, avidly enhancing left CPA-IAC vestibular schwannoma that markedly enlarges the IAC as well as exerts mass effect upon the adjacent pons, brachium pontis, and cerebellum.

Neurofibromatosis Type 2, CPA-IAC Axial T1 C+ FS MR shows large CPA-IAC masses with heterogeneous, but avid enhancement that enlarge the IACs bilaterally . When present, the finding of bilateral vestibular schwannomas is effectively pathognomonic for the diagnosis of NF2.

Metastases, CPA-IAC Axial bone NECT shows a large, destructive left petrous apex metastasis that has eroded into and widened the left IAC . Note the caliber of the normal right IAC for comparison.

PHACES Association Axial T2 FS MR in an infant with a facial hemangioma (not shown) demonstrates left cerebellar hypoplasia with large retrocerebellar CSF space and enlarged ipsilateral IAC , a constellation of findings indicative of PHACES.

Schwannoma, Facial Nerve, CPA-IAC Axial 3D T2-SPACE MR shows a CPA-IAC mass extending along the posterior wall of the IAC to the fundus, then along the labyrinthine segment of CNVII. Although CNVII sits anteriorly in the IAC, this schwannoma was displaced by a large cystic component that is slightly hypointense to CSF .

Atypical Teratoid/Rhabdoid Tumor Axial bone CT in an infant with a CPA-IAC mass shows a large irregular IAC and facial nerve canal due to atypical teratoid/rhabdoid tumor, diagnosed following resection.

Cystic Cochleovestibular Malformation (IP-I) Axial T2 MR shows a malformed, featureless vestibule and cochlea . The wide IAC houses CNVIII and CNVII , which are splayed apart. Note a hypoplastic pons and a malformed cerebellum. The mastoid and middle ear fluid could be serous (common) or conceivably result from a perilymph fistula (rare).

X-Linked Stapes Gusher (DFNX2) Axial bone CT in a boy with X-linked mixed hearing loss shows a wide lateral IAC . The corkscrew-shaped cochlea lacks internal septation or a modiolus. The wide lateral SCC is partially ossified .

Additional Images

Vestibular Schwannoma Axial T1 C+ MR in an ataxic teenager shows an avidly enhancing mass widening the CPA cistern , distorting the pons, and expanding the IAC , consistent with schwannoma, which was confirmed after resection.

Neurofibromatosis Type 2, CPA-IAC Axial T1 C+ FS MR in a patient with NF2 shows bilateral, avidly enhancing IAC tumors extending into the left cochlear canal and along the vestibular nerves . A V3 schwannoma is also seen .

Neurofibromatosis Type 2, CPA-IAC Axial 3D T2 SPACE MR in a teenager with NF2 shows bilateral IAC-CPA schwannomas in widened IACs . There is also a V3 schwannoma in the foramen ovale .

Metastases, CPA-IAC Axial T1WI C+ FS MR in an infant with otalgia and CNVII palsy shows an enhancing tumor expanding the IAC and CPA cistern, invading the petrous apex , CNVII , and the middle ear space. Bony destruction was seen on CT. The differential diagnosis included rhabdomyosarcoma, ATRT, or metastases. A final diagnosis of metastatic neuroblastoma was made after middle ear biopsy.

Metastases, CPA-IAC Coronal T2 FSE MR shows a patient with CSF dissemination of glioneuronal neoplasm. The hyperintense tumor filling the widened IACs is hard to distinguish from CSF on this image but was more evident on FLAIR images. The patient had large IACs at the time of presentation. As CSF tumor dissemination/metastatic disease occurred, the IACs showed progressive massive enlargement over time on successive MR studies.

PHACES Association Axial T2 MR in an infant with a facial hemangioma shows left cerebellar hypoplasia with a prominent adjacent CSF space, large flared left IAC , and enlarged Meckel cave .

Cystic Cochleovestibular Malformation (IP-I) Axial bone CT in a patient with profound SNHL shows an enlarged vestibule and lateral SCC with an enlarged IAC . Extensive opacification of the mastoid air cells and middle ear space is also seen.

Neurofibromatosis Type 1 Axial T2 MR in a teenager with NF1 shows large IACs bilaterally with no evidence of inner ear anomaly or IAC mass lesion. This is a feature of mild dural ectasia in NF1.