Neuroradiology · Cerebellopontine angle tumours · Four-grade Koos system
Koos Grading of Vestibular Schwannoma
A four-grade anatomical classification of vestibular schwannoma based on internal auditory canal confinement, cerebellopontine angle extension and brainstem mass effect.
Purpose
To describe the anatomical extent of a vestibular schwannoma and its relationship to the brainstem on imaging.
| Grade | Tumour extent | Brainstem relationship |
|---|---|---|
| I | Small tumour confined to the internal auditory canal | No cerebellopontine angle component |
| II | Small tumour protruding into the cerebellopontine angle | No contact with the brainstem |
| III | Tumour occupying the cerebellopontine cistern | Contacts the brainstem but does not displace it |
| IV | Large cerebellopontine angle tumour | Displaces the brainstem and adjacent cranial nerves |
How to use it
- Grade I is entirely intracanalicular, whereas grades II–IV have an extracanalicular component.
- The key distinction between grades II and III is the absence or presence of brainstem contact.
- The key distinction between grades III and IV is brainstem contact without displacement versus definite displacement.
- The grade communicates anatomical extent and mass effect but does not independently determine management.
Common mistake
Do not assign Koos grade solely from tumour diameter. The defining features are extracanalicular extension and the relationship to the brainstem.
Exam pearl
Remember the progression: canal only → CPA without contact → brainstem contact → brainstem displacement.
Viva questions
- What is the Koos grading system used for?
- It grades vestibular schwannomas according to internal auditory canal confinement, extension into the cerebellopontine angle and their relationship to the brainstem.
- Enumerate the four Koos grades.
- Grade I is intracanalicular; grade II extends into the CPA without brainstem contact; grade III contacts but does not displace the brainstem; and grade IV displaces the brainstem.
- What defines a Koos grade I vestibular schwannoma?
- It is a small tumour confined entirely to the internal auditory canal, without an extracanalicular CPA component.
- How do Koos grades II and III differ?
- A grade II tumour extends into the CPA but does not contact the brainstem. A grade III tumour occupies the CPA cistern and contacts the brainstem without displacing it.
- What is the defining feature of Koos grade IV?
- Koos grade IV is defined by a large tumour producing displacement of the brainstem and adjacent cranial nerves.
- Can Koos grade be assigned using tumour diameter alone?
- No. Koos grading is primarily anatomical and depends on canal confinement, CPA extension, brainstem contact and brainstem displacement rather than diameter alone.
- Does the Koos grade alone determine treatment?
- No. It communicates tumour extent and mass effect, but treatment also depends on symptoms, hearing status, growth, patient factors and multidisciplinary assessment.