Acoustic neuroma classification

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]Associate Editor(s)-in-Chief: Simrat Sarai, M.D. [2] Mohsen Basiri M.D.

Overview

Acoustic neuroma may be classified according to findings on magnetic resonance imaging (MRI) or classified based on microscopic histopathology, and whether or not they are associated with neurofibromatosis type 2. Based on microscopic histopathology, acoustic neuroma may be classified into four subtypes: conventional schwannoma, cellular schwannoma, plexiform schwannoma, and melanotic schwannoma. While acoustic neuromas are benign tumors, but there is no established system for the staging of acoustic neuromas. Koos grading scale provides four grades based on extrameatal extension and compression of the brain stem , a reliable method for tumor classification which is used in practice.

Classification

  • Acoustic neuroma may be classified into two subtypes, based on whether or not they are associated with neurofibromatosis type 2.
    • Sporadic
      • The vast majority are the sporadic form. Ninety five % of all cases of acoustic neuroma are sporadic. The cause of sporadic form is unclear
    • Acoustic neuroma associated with neurofibromatosis type II (NF2)[1]
      • NF2 is a rare disorder and it accounts for 5% of acoustic neuromas
      • Acoustic neuroma associated with neurofibromatosis type II are typically bilateral and cause gradually progressive hearing loss, tinnitus, and balance dysfunction
  • Based on the MRI scan, acoustic neuromas can be classified into three subtypes:
    • Entirely intracanalicular: The entire tumor is completely within the bony canal
    • Intracranial extension without brain stem distortion: Intracranial portion of the tumor is 1.5-2.5 cm. (Some references mentioned 1-2 cm)
    • Intracranial extension with brain stem distortion: Intracranial portion of the tumor is greater than 2.5 cm. (Some references mentioned more than 2 cm)
  • Based on microscopic histopathology, acoustic neuroma can be classified into four subtypes:[2][3]
    • Conventional schwannoma: It is the most common schwannoma
    • Cellular schwannoma: It may mimic malignant peripheral nerve sheath tumor
    • Plexiform schwannoma: It may mimic malignant peripheral nerve sheath tumor if cellular- especially in childhood
    • Melanotic schwannoma

Staging

Acoustic neuromas are benign tumors (WHO grade 1), but there is no established system for the staging of acoustic neuromas. Numerous stage grading systems have been reported according to tumor size, but also for the first step description of the tumor size is more important and have measured by the maximum diameter (means the one measured in cerebellopontine angle (CPA) along the long axis) of the tumor.[4][5][6]

According to the classification system known as the Koos grading scale there are 4 grades of acoustic neuroma based on findings on magnetic resonance imaging (MRI) and extrameatal extension and compression of the brain stem. The grades of acoustic neuroma are shown in the table below:[7]

Koos Classification for Acoustic Neuroma
Grade Definition
I Tumor involves only the internal auditory canal
II Tumor extend into the cerebellopontine angle, but do not encroach on the brain stem.
III Tumor fills the entire cerebellopontine angle
IV Tumor displaces the brain stem and adjacent cranial nerves

Below table summarizes current grading systems are used in practice:

Main grading systems for acoustic neuromas
Tumor size (CPA Maximum diameter) Sterker House Koos Samii Tumor Description
0

(intracanalicular)

Tube type intracanalicular Grade I T1 Confining to IAC
≤10 mm Small Grade 1

(Small)

Grade II T2 Superpassing IAC
≤15 mm Grade 2

(Medium)

T3a Tumor occupying CPA
≤20 mm Mild
≤3o mm Grade 3

(Moderately Large)

Grade III T3b Tumor occupying CPA and contacting

the brainstem without compression

≤40 mm Large Grade 4

(Large)

Grade IV T4a Tumor compressing the brainstem
>40 mm Huge Grade 5

(Giant)

T4b Sever brainstem displacement and deformation of fourth ventricle under tumor compression
Main grading systems for acoustic neuromas. The classifications on the left side (blue area) are mainly based on tumor size, while those on the right side (yellow area) are based on the anatomical relationship around the tumor. Koos classification (green area) combines the tumor size and anatomical relationship for larger tumors. IAC: Intracoustic canal

[8]

References

  1. D. Gareth R. Evans (2009). "Neurofibromatosis 2 [Bilateral acoustic neurofibromatosis, central neurofibromatosis, NF2, neurofibromatosis type II]". Genetics in medicine : official journal of the American College of Medical Genetics. 11 (9): 599–610. doi:10.1097/GIM.0b013e3181ac9a27. PMID 19652604. Unknown parameter |month= ignored (help)
  2. Kurtkaya-Yapicier O, Scheithauer B, Woodruff JM (2003). "The pathobiologic spectrum of Schwannomas". Histol Histopathol. 18 (3): 925–34. PMID 12792904.
  3. Sho Hashimoto (2003). "Classification of vestibular schwannoma". Springer Japan.
  4. Sterkers JM, Morrison GA, Sterkers O, El-Dine MM., JM (1994). "Preservation of facial, cochlear, and other nerve functions in acoustic neuroma treatment". Otolaryngol Head Neck Surg.
  5. Hitselberger WE, House WF (1966). "classification of acoustic neuromas". Arch Otolaryngol.
  6. Koos WT, Day JD, Matula C, Levy DI. "Neurotopographic considerations in the microsurgical treatment of small acoustic neurinomas". J Neurisurg.
  7. Nicholas J. Erickson, Philip G. R. Schmalz, Bonita S. Agee, Matthew Fort, Beverly C. Walters, Benjamin M. McGrew & Winfield S. 3rd Fisher (2018). "Koos Classification of Vestibular Schwannomas: A Reliability Study". Neurosurgery. doi:10.1093/neuros/nyy409. PMID 30169695. Unknown parameter |month= ignored (help)
  8. Hao Wu, Liwei Zhang, Dongyi Han, Ying Mao, Jun Yang, Zhaoyan Wang, Wang Jia, Ping Zhong, Huan Jia (2016). "Summary and consensus in 7th International Conference on acoustic neuroma: An update for the management of sporadic acoustic neuromas". World Journal of Otorhinolaryngology-Head and Neck Surgery.

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