Knosp Classification System for Pituitary Adenoma Cavernous Sinus Invasion
Overview
The Knosp classification is a 5-grade MRI-based system (Grades 0-4) that predicts the likelihood of pituitary adenoma invasion into the cavernous sinus by using the internal carotid artery (ICA) as a radiological landmark on coronal MRI sequences. 1
The Five Grades Explained
Grade 0: No Parasellar Extension
- The tumor does not extend beyond the medial tangent of the ICA (an imaginary line drawn along the medial wall of the cavernous ICA). 1
- This represents normal anatomy with no cavernous sinus involvement. 1
- Surgical invasion rate: 0% 1, 2
Grade 1: Minimal Extension
- The tumor extends beyond the medial tangent but does not reach past the intercarotid line (an imaginary line connecting the centers of both ICAs). 1
- The adenoma pushes into the cavernous sinus space but remains medial to the center of the ICA. 1
- Surgical invasion rate: 1.5% with an 83% gross-total resection rate and 88% endocrinological remission rate. 2
Grade 2: Moderate Extension
- The tumor extends beyond the intercarotid line but does not pass the lateral tangent of the ICA (an imaginary line drawn along the lateral wall of the cavernous ICA). 1
- This is the critical transition zone where invasion becomes more likely. 1
- Surgical invasion rate: 9.9% with a 71% gross-total resection rate and 60% endocrinological remission rate. 2
- Important caveat: This grade has "very weak" interrater reliability (0.18) among physicians, meaning different doctors often disagree on Grade 2 classifications. 3
Grade 3: Significant Extension
- The tumor extends beyond the lateral tangent of the ICA but does not completely encircle it. 1
- The 2015 revision subdivided this into:
- Grade 3A: Tumor invades the superior-posterior compartment of the cavernous sinus with a 26.5% invasion rate, 85% gross-total resection rate, and 67% endocrinological remission rate. 2
- Grade 3B: Tumor invades the inferior-anterior compartment with a significantly higher 70.6% invasion rate, 64% gross-total resection rate, and 0% endocrinological remission rate. 2
- Overall Grade 3 surgical invasion rate: 37.9% 2
Grade 4: Complete Encasement
- The tumor completely encircles (encases) the intracavernous ICA. 1
- Surgical invasion rate: 100% with 0% gross-total resection rate and 0% endocrinological remission rate. 1, 2
- This is the only grade that consistently corresponds to true cavernous sinus invasion in all cases. 4
Clinical Application
When using the Knosp scale for surgical decision-making, dichotomize it into two clinically useful groups: Grades 0-2 (unlikely to have intraoperative cavernous sinus involvement) versus Grades 3-4 (likely to have cavernous sinus involvement), which improves reliability from "very weak" to "strong" (0.60) and increases physician agreement to 60%. 3
The endoscopic endonasal approach provides superior detection of actual cavernous sinus invasion compared to microscopic techniques, revealing that 43% of cases with Knosp grade >1 have no actual invasion on direct surgical inspection. 4 Endoscopic verification shows significantly lower invasion rates for Grade 2 (9.9% vs 88% microscopic) and Grade 3 (37.9% vs 86% microscopic) adenomas. 2
Key Imaging Requirements
Perform the Knosp grading on coronal MRI sequences with gadolinium contrast enhancement, as the ICA serves as the readily detectable radiological landmark. 1 High-resolution, focused field-of-view sequences targeted for sellar and parasellar assessment optimize visualization. 5
Critical Pitfalls
Grades 1 and 2 have poor interrater reliability, with only 5% agreement among all reviewers for middle scores, so consider obtaining a second opinion or using advanced imaging techniques like 3D multimodal imaging for borderline cases. 3 The distinction between Grade 3A and 3B is clinically crucial because Grade 3B adenomas have nearly three times the invasion rate and zero endocrinological remission compared to Grade 3A. 2