What are the Knosp criteria in rhinology?

The Knosp Classification System for Pituitary Adenomas and Cavernous Sinus Invasion

The Knosp classification is a radiological grading system used to predict cavernous sinus invasion by pituitary adenomas based on MRI findings, which directly impacts surgical planning, approach, and expected outcomes in rhinology and neurosurgery. 1, 2

Overview of the Knosp Classification

The Knosp classification evaluates the lateral extension of pituitary adenomas in relation to the internal carotid artery (ICA) and cavernous sinus on coronal MRI images:

• Grade 0: Tumor does not extend to the medial tangent of the intra- and supracavernous ICA 2
• Grade 1: Tumor extends to the medial tangent of the intra- and supracavernous ICA, but does not cross it 2, 3
• Grade 2: Tumor extends beyond the medial tangent of the intra- and supracavernous ICA, but does not cross the intercarotid line 2, 3
• Grade 3: Tumor extends lateral to the intercarotid line
  • Grade 3A: Tumor extends beyond the intercarotid line but does not reach the lateral tangent of the ICA 1
  • Grade 3B: Tumor extends to or beyond the lateral tangent of the ICA 1
• Grade 4: Total encasement of the intracavernous carotid artery 2, 3

Clinical Significance and Predictive Value

• Grades 0-2 generally indicate low probability of cavernous sinus invasion, with surgical resection more likely to be complete 2, 4
• Grade 3A shows intermediate risk of invasion, with 56.3% of cases demonstrating cavernous sinus invasion intraoperatively 1
• Grades 3B and 4 indicate high probability of cavernous sinus invasion, with nearly 100% of grade 3B and 83-100% of grade 4 tumors showing invasion during surgery 1, 2

Imaging Modalities for Knosp Classification

• Constructive Interference in Steady State (CISS) MRI sequences provide superior evaluation of cavernous sinus invasion compared to T1-weighted sequences, with higher correlation to intraoperative findings (area under ROC curve: 0.97 vs 0.94) 1
• Multiplanar CT imaging offers excellent bone detail and soft-tissue imaging, particularly useful for evaluating bony landmarks and anatomical variants 5

Additional Predictive Factors for Cavernous Sinus Invasion

• Percentage of encasement of the intracavernous ICA:

  • <25% encasement strongly predicts absence of invasion (NPV 100%) 2

  • 45% encasement strongly predicts invasion 2

  • ≥30% encasement has an odds ratio of 49.25 for invasion 2
  • ≥28% encasement shows 84% sensitivity and 77% specificity 3

• Venous compartment obliteration:

  • Intact medial venous compartment strongly predicts absence of invasion (PPV 100%) 2
  • Obliteration of ≥3 venous compartments shows 65% sensitivity and 89% specificity for invasion 3

Recent Advances and Limitations

• 3D volumetric analysis may be superior to the traditional Knosp criteria for predicting extent of resection in complex cases 6
• Tumor volume alone has been shown to be independently associated with likelihood of gross total resection, with smaller tumors having better outcomes 6
• The Knosp classification alone may not always predict surgical outcomes, as some grade 4 tumors can still achieve complete resection 4

Board-Style Questions for Learning Assessment

Question 1

A 45-year-old patient presents with visual field defects. MRI shows a pituitary macroadenoma that extends to the medial tangent of the internal carotid artery but does not cross it. What is the Knosp grade of this tumor? A) Grade 0 B) Grade 1 C) Grade 2 D) Grade 3A

Question 2

According to recent studies, which MRI sequence provides superior evaluation of cavernous sinus invasion compared to T1-weighted sequences? A) T2-weighted B) FLAIR C) Constructive Interference in Steady State (CISS) D) Diffusion-weighted imaging

Question 3

A pituitary macroadenoma with Knosp grade 3B has what approximate likelihood of cavernous sinus invasion based on intraoperative findings? A) 25% B) 50% C) 75% D) 100%

Question 4

Which of the following MRI findings most strongly predicts the absence of cavernous sinus invasion? A) Percentage encasement of ICA <25% B) Knosp grade 2 C) Tumor volume <6 cm³ D) Absence of headache symptoms

Question 5

What is the most valuable criterion for predicting cavernous sinus invasion according to logistic regression analysis? A) Knosp grade B) Percentage of encasement of intracavernous ICA ≥30% C) Tumor size D) Patient age

Answer Key

1. B) Grade 1
2. C) Constructive Interference in Steady State (CISS)
3. D) 100%
4. A) Percentage encasement of ICA <25%
5. B) Percentage of encasement of intracavernous ICA ≥30%