An enhancing mass centred on the internal auditory canal (IAC), extending into the cerebellopontine angle as an "ice-cream cone" — measured in its largest CPA dimension and staged by its relation to the brainstem.
Orient first
- The commonest CPA mass; meningioma is the main differential (broad dural base, dural tail, calcification, eccentric to the IAC).
- Asymmetric sensorineural hearing loss is the usual indication; a heavily T2-weighted screen can exclude a lesion.
- Koos grade or similar describes the relation to the brainstem (verify the system used by your neurosurgeons).
Acquire the study
- High-resolution 3D heavily T2-weighted (CISS/FIESTA/DRIVE) through both IACs, and post-gadolinium 3D T1; whole-brain axial FLAIR and DWI.
The manoeuvre
- Heavily T2-weighted axial and oblique sagittal through the IAC: the four nerves in the canal and any filling defect.
- Post-contrast T1: enhancement, intracanalicular vs CPA component; measure the largest extracanalicular diameter in mm parallel to the petrous ridge.
- Fundus: CSF cap between tumour and fundus (relevant to hearing preservation).
- Brainstem and fourth ventricle: contact, compression, hydrocephalus.
- Contralateral IAC: bilateral schwannomas mean NF2 (schwannomatosis) — look at the whole neuraxis.
What confirms it
- Enhancing mass within the IAC ± CPA extension, acute angle with the petrous bone, no dural tail.
What licenses you to exclude it
- Normal nerves and CSF signal filling the IAC to the fundus on a heavily T2-weighted 3D sequence exclude a schwannoma of meaningful size.
The classic misread
- Calling a meningioma with IAC extension a schwannoma — check the dural base and tail.
- Missing a small intracanalicular tumour on routine thick-slice brain MRI.
- Measuring the intracanalicular part in the CPA size.