- Seen in middle-aged adults (40–60 years, equal sex distribution) presenting with insidious progressive unilateral sensorineural hearing loss (SNHL), high-pitched tinnitus, and dysequilibrium/imbalance.
- Caused by benign WHO Grade 1 nerve sheath tumor arising from the Schwann cells of the superior or inferior vestibular branch of the vestibulocochlear nerve (CN VIII) within the internal auditory canal (IAC); bilateral tumors pathognomonic for Neurofibromatosis Type 2 (NF2).
Imaging Findings
- MRI Brain & Cerebellopontine Angle (Thin-Slice CISS/FIESTA & Contrast T1 - Gold Standard):
- –Classic 'Ice Cream Cone' Appearance: The intra-meatal component within the expanded internal auditory canal represents the 'cone', while the extra-meatal spherical component in the cerebellopontine angle (CPA) represents the 'ice cream'.
- –Trumpet Internal Auditory Canal: Funnel-shaped expansion and widening of the internal acoustic porus/canal with acute angles formed with adjacent petrous temporal bone.
- –Signal Characteristics & Enhancement: T1 hypointense, T2 heterogeneously hyperintense with microcystic changes (Antoni A compact vs Antoni B loose myxoid areas); intense contrast enhancement with cystic degeneration in large lesions (>2.5–3 cm).
- –High-Resolution 3D CISS / FIESTA (Sub-millimeter T2): Delineates tumor origin within fluid-filled IAC and its relationship to the facial nerve (CN VII, displaced anteriorly and superiorly) and cochlear nerve.
- –DWI / ADC: Shows no diffusion restriction (facilitated diffusion with high ADC values), which is the key sequence to differentiate a cystic CPA schwannoma from an epidermoid cyst (which shows intense, lightbulb-bright diffusion restriction on DWI).
- –**SWI / T2* GRE: Frequently demonstrates punctate intralesional blooming microhemorrhages** (a high-yield feature favoring vestibular schwannoma over CPA meningioma, which instead shows calcific blooming or dural tail).
- Koos Grading Scale for Vestibular Schwannoma:
- –Grade I: Intracanalicular only (strictly confined to IAC, <10 mm).
- –Grade II: Small CPA component (<20 mm) without brainstem contact.
- –Grade III: Moderate CPA mass (<30 mm) touching the brainstem without brainstem compression.
- –Grade IV: Large CPA mass (>30 mm) causing marked brainstem distortion, fourth ventricular displacement, and hydrocephalus.
Differential Diagnosis
| Differential Diagnosis | Key Distinguishing Features |
|---|---|
CPA Meningioma | Broad dural-based attachment centered along posterior petrous ridge; obtuse margins; prominent dural tail sign; petrous hyperostosis; does NOT widen internal auditory canal (eccentric to IAC); calcification common. |
Epidermoid Cyst (Congenital Cholesteatoma) | Non-enhancing, lobulated mass that insinuates and drapes around cranial nerves and vessels without expanding IAC; profoundly hyperintense on DWI with low ADC (distinguishing it from CSF / arachnoid cyst); 'flaky' heterogeneous on CISS. |
Arachnoid Cyst of CPA | Completely follows CSF on all sequences (T1 hypointense, T2 bright, suppresses fully on FLAIR); facilitated diffusion on DWI (black on DWI, bright on ADC); smooth rounded margins; non-enhancing. |
Facial Nerve Schwannoma | Located along the course of the facial canal (labyrinthine segment, geniculate ganglion, tympanic segment); manifests with early peripheral facial nerve palsy rather than isolated sensorineural hearing loss; expands fallopian canal. |
Important Points to Remember
- Diagnostic Pitfall: Forgetting to scrutinize the contralateral internal auditory canal! If bilateral vestibular schwannomas are present, the diagnosis is definitively Neurofibromatosis Type 2 (NF2). You must actively search for other NF2 hallmarks: multiple intracranial meningiomas, spinal ependymomas, and posterior subcapsular lens opacities (MISME syndrome).
- Crucial Review Areas:
- 1.Contralateral internal auditory canal to definitively rule out bilateral disease (NF2).
- 2.Fourth ventricle and cerebral aqueduct for compression and upstream obstructive hydrocephalus.
- 3.Facial nerve trajectory on thin-slice CISS/FIESTA: Facial nerve is typically displaced anterosuperiorly by vestibular schwannoma (critical surgical anatomy).
- 4.Fundus of the internal auditory canal for fluid cap (lack of lateral fluid cap in the fundus indicates complete canal filling and higher risk of postoperative hearing loss).
- Case-Specific Radiologist Management:
- 1.Immediate Verbal Alert: Direct telephone notification to neurotology / skull base surgical registrar if Koos Grade IV mass with acute brainstem compression or obstructive hydrocephalus is identified.
- 2.High-Resolution 3D Steady-State MRI Protocol: Recommend dedicated 3D CISS/FIESTA and thin-slice (1 mm) post-contrast fat-suppressed T1 weighted MRI through the temporal bones.
- 3.Audiological Correlation: Recommend comprehensive audiometry (pure tone audiogram and speech discrimination score) to assess functional serviceable hearing (Gardner-Robertson Class I/II).
- 4.Management Stratification (Wait-and-Scan vs Radiosurgery vs Resection): Koos Grade I/II in elderly or asymptomatic patients: 'Wait-and-Scan' active surveillance with annual volumetric MRI; small to medium growing tumors (<2.5–3 cm): Stereotactic Radiosurgery (SRS / Gamma Knife); large tumors (>3 cm, Koos Grade IV) causing brainstem displacement: Microsurgical resection via retrosigmoid or translabyrinthine approach.
- 5.Skull Base / Neurotology MDT Presentation: Present to the Joint Skull Base and Neurotology Multidisciplinary Team for treatment selection and counseling regarding hearing preservation and facial nerve integrity monitoring.