Knosp classification of cavernous sinus invasion by pituitary macroadenomas

The Knosp classification is one of the more commonly used systems to determine the likelihood of cavernous sinus invasion by pituitary macroadenomas.

Usage

By stratifying the likelihood of cavernous sinus invasion, the Knosp classification is important in operative planning, and predicting residual tumor post-resection. Low Knosp grades are associated with a significantly greater chance of surgical remission after resection ³. 

Some concerns have been raised over its interobserver reliability ^4,5, leading to the development of alternative classification systems (e.g. Zurich pituitary score) ⁶.

Classification system

Three lines are drawn between the supraclinoid internal carotid artery and intracavernous internal carotid artery on coronal MR images ^1,2. 

1. medial tangent

2. intercarotid line

3. lateral tangent

These lines are used to define 4 grades of tumor invasion: 

• grade 0: tumor remains medial to the medial tangent

• grade 1: tumor extends to between the medial tangent and the intercarotid line

• grade 2: tumor extends to between the intercarotid line and the lateral tangent

• grade 3: tumor extends lateral to the lateral tangent

  • 3A: above the intracavernous internal carotid artery into the superior cavernous sinus compartment

  • 3B: below the intracavernous internal carotid artery into the inferior cavernous sinus compartment

• grade 4: complete encasement of intracavernous internal carotid artery

Interpretation

The likelihood of invasion varies across various studies. Generally, the following can be used pragmatically:

• grade 0 and 1: no invasion

• grade 2: possible invasion

• grade 3: probable invasion

• grade 4: definite invasion

More specifically ^1,2: 

• grade 0

  • surgical invasion: 0%

  • histological invasion: 0%

• grade 1

  • surgical invasion: 1.5%

  • histological invasion: 0%

• grade 2

  • surgical invasion: 9.9%

  • histological invasion: 88%

• grade 3 

  • 3A: surgical invasion: 26.5%

  • 3B: surgical invasion: 70.6%

  • combined histological invasion: 86%

• grade 4

  • surgical invasion: 100%

  • histological invasion: 100%

Similarly, these grades are predictive of gross total resection and endocrinological remission ². 

• grade 1

  • gross total resection: 83%

  • endocrinological remission: 88%

• grade 2

  • gross total resection: 71%

  • endocrinological remission: 60%

• grade 3A

  • gross total resection: 85%

  • endocrinological remission: 67%

• grade 3B

  • gross total resection: 64%

  • endocrinological remission: 0%

• grade 4

  • gross total resection: 0%

  • endocrinological remission: 0%

History and etymology

The classification system was originally published by Engelbert Knosp (an Austrian neurosurgeon) in 1993, as grades 1-4 ¹. The division of grade 3 into grade 3A and grade 3B was added in 2016, in a paper also by Knosp and colleagues ², and can be referred to as the modified Knosp classification.