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.


Acoustic neuroma may be classified according to the findings on magnetic resonance imaging (MRI) or it can also be 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, 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 based on the association with neurofibromatosis type 2:

Not associated/Sporadic

  • The vast majority are the sporadic form. 95% of all the cases of acoustic neuroma are sporadic. The cause of sporadic form is unclear

Associated with Neurofibromatosis type II (NF2)[1]

Classification based on the MRI scan:

Classification based on Microscopic Histopathology:[2][3]


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. Tumor size is more important and can be measured by measuring the maximum diameter of the tumor.[4][5][6]

According to the Koos grading scale, there are 4 grades of acoustic neuroma based on the findings on magnetic resonance imaging (MRI), extrameatal extension and compression of the brain stem:[7]

Koos Classification for Acoustic Neuroma
Grade Definition
I Tumor involves only the internal auditory canal
II Tumor extends into the cerebellopontine angle, but does 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 the current grading systems used in practice:

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


Tube type Intracanalicular Grade I T1 Confining to internal acoustic canal
≤ 10 mm Small Grade 1


Grade II T2 Superpassing internal acoustic canal
≤ 15 mm Grade 2


T3a Tumor occupying CPA
≤ 20 mm Mild
≤ 30 mm Grade 3

(Moderately Large)

Grade III T3b Tumor occupying CPA and contacting

the brainstem without compression

≤ 40 mm Large Grade 4


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


T4b Severe 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.



  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/nyy40. 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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