Friday 12 August 2011

Acoustic Schwannoma "Ice cream on cone" appearance.

MRI Brain
T2w                          T1w                        Flair          Post contrast T1w
A well demarcated soft tissue signal intensity mass in left CP angle, centered to porus acousticus with obvious intra canalicular extension giving so called "ice cream on cone" appearance. 
Widening of corresponding IAC (Internal Auditory Canal). 
Signals are low on T1, hyperintense on T2 and isointense to brain parenchyma on FLAIR. 
Homogenous intense enhancement on post contrast T1w images.
Left half of Pons and middle cerebellar peduncle compressed. No obstructive hydrocephalus. 

Imaging diagnosis : Acoustic Schwannoma.


Syn : Acoustic schwannoma, Acoustic neuroma, Acoustic tumor, Vestibular Schwannoma

Imaging wise use of term Acoustic schwannoma in MRI Report is more accurate than Vestibular schwannoma, though most lesions arise from vestibular portion of 8th cranial nerve (CN) as its not possible imagingwise to comment whether it is arising form vestibular or choclear nerve.

A benign tumor arising from Schwann cells that wrap vestibulo cochlear nerve in Internal Auditary Canal (IAC)
Most common lesion in patients with unilateral SNHL (> 90%)
Most common CPA-lAC mass (85-90%)
Second most common extra-axial neoplasm in adults.
Genetics behind is inactivating mutations of NF2 tumor suppressor gene noted in 60% of sporadic AS. Multiple or bilateral schwannomas seen in NF2.

MRI is the investigation of choice. Axial FLAIR, T2, Post contrast T1 with FIESTA are the most recommended sequences.

Imagingwise well circumscribed extra axial avidly enhancing Cp angle solid tumor, centered to porus acousticus with intra canalicular extension giving so called ice cream on cone appearance.
Smaller lesions are Cylindrical when intra canalicular and confined IAC.
In 0.5% case an associated arachnoid cyst noted.

Histo pathology
Microscopic Features are differentiated neoplastic Schwann cells in a collagenous matrix.
Areas of compact, elongated cells = Antoni A. Most AS comprised mostly of Antoni A cells.
Areas less densely cellular with tumor loosely arranged, clusters of lipid-laden cells = Antoni B
Strong, diffuse expression of S-100 protein.
No necrosis but may have intramural cysts; rarely hemorrhagic.

Clinical Presentation
Adults with unilateral slowly progressive SNHL in most common.
If lesion is small may have Tinnitus (ringing in ear) and disequilibrium.
Larger lesion may present with Trigeminal neuralgia and/or facial neuropathy.
Peak age is 40-60 years, Rare in children unless NF2. No gender specificity.

Brainstem electric response audiometry (BERA) most sensitive pre-imaging test for AS, however unnecessary if early, screening MR is done.

Translabyrinthine resection if no hearing preservation possible.
Middle cranial fossa approach for intracanalicular AS, especially lateral lAC location.
Retrosigmoid approach when CPA or medial lAC component present.
Radiation therapy - Gamma knife: Low-dose, sharply collimated, focused cobalt-60 treatment when medical contraindications to surgery or residual post-operative lesion.

Reference: Diagnostic Imaging Osborn 

Closest DD
1. Meningioma
Intra canalicular meningioma may mimic ASch and difficult to differentiate imagingwise.
Cp angle Meningioma will be broad based to dura with an associated dural tailing on post contrast T1.

The preoperative axial T2WI shows solid signal intensity CP angle mass lesion centred to left IAC which is filled by a hypertintense mass with mild enhancement within the internal auditory canal on post contrast T1 weighted images. However left IAC is not dilated. Neither its signal intensity nor the intensity of enhancement of intracanalicular component is similar to that of the major extra axial component of tumor. There is a classical dural tailing on post contrast MRI.

The postoperative axial T2WI shows the absence of the preoperative mass in the left CPA but the IAC is still filled by abnormal signal intensity. The postoperative post-contrast axial T1WI reveals heterogeneously enhanced IAC.
Histopathology report mentions Meningioma.

2. Facial Nerve Schwannoma :
When confined to CPA - IAC may exactly mimic Asch.
Labyrinthine segment tail present.

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