What are the key considerations for understanding the endoscopic anatomy of the anterior skull base?

Key Considerations for Understanding the Endoscopic Anatomy of the Anterior Skull Base

Thorough knowledge of endoscopic anatomy of the anterior skull base is essential for safe and effective surgical approaches, requiring specific training and understanding of anatomical landmarks to minimize morbidity and mortality during skull base procedures. 1

Anatomical Orientation and Compartmentalization

The anterior skull base can be divided into three key compartments for better surgical orientation:

1. Anterior compartment: Area between the posterior inferior border of the frontal sinus and the course of anterior ethmoidal artery
2. Middle compartment: Area between the anterior ethmoidal artery and posterior ethmoidal artery
3. Posterior compartment: Area between the posterior ethmoidal artery and the attachment point of the anterior border of the sphenoid sinus 2

Critical Anatomical Landmarks

• Ethmoidal arteries: The distance between posterior ethmoidal artery and anterior ethmoidal artery ranges from 10-17mm (mean 13mm)
• Optic canal relation: The distance between optic canal and posterior ethmoidal artery ranges from 8-16mm (mean 11.08mm) 3
• Safe dissection zone: The area between the anterior border of the sphenoid sinus and posterior ethmoidal artery is considered the safest first dissection zone 2

Surgical Approaches and Technical Considerations

Endonasal Endoscopic Approach

The extended endoscopic endonasal approach provides wide exposure of:

• Bony structures of the anterior skull base
• Extradural and intradural components
• Critical neurovascular structures including:
  • Olfactory nerves
  • Interhemispheric sulcus
  • Gyri recti
  • Anterior cerebral artery segments (A1 and A2)
  • Anterior communicating artery
  • Heubner arteries 3

Specialized Approaches

• Trans-Agger Nasi Approach: Provides direct access to the anterior skull base by removing the agger nasi, which is located anterior and inferior to the frontal ostium 4

Training Requirements and Learning Curve

Manual Training

• Bimanual technique: Essential to master handling the endoscope in one hand while operating instruments in the other
• Depth perception: Despite the two-dimensional nature of endoscopic images, depth perception can be achieved through:
  • Dynamic in-and-out movements of the endoscope
  • Tactile sensation from surgical instruments
  • High-definition imaging systems 1

Modular Training Approach

The Pittsburgh group has developed a five-stage training program that:

• Requires mastering simpler procedures before advancing to more complex ones
• Divides the ventral skull base into modular units with the sphenoid sinus as the starting point 1

Comparison with Traditional Approaches

• Endoscopic vs. Open Approaches: While craniofacial resections are still considered the gold standard for certain pathologies, growing evidence supports that endoscopic endonasal approaches yield equivalent oncologic outcomes with less morbidity in well-selected cases 5

• Complementary Approaches: Endoscopy and microscopy should be viewed as complementary rather than competing modalities:

  • Microscope provides straight-on view in line with trajectory
  • Endoscope offers clear visualization of deeper structures and ability to "look around corners" 1

Potential Pitfalls and Challenges

• Disorientation risk: Angled endoscopes (30°, 45°, and 70°) increase the view but also increase the risk of disorientation 1

• Anatomical variations: Recognition of variations (e.g., persistent trigeminal artery) is crucial for safe navigation 1

• Reconstruction limitations: Proper skull base reconstruction remains challenging after extensive resections 6

• Learning curve: Specific skills are required to achieve minimally invasive interventions and reduce postoperative complications 3

Practical Recommendations for Training

• Cadaveric dissection: Should be compulsory for all surgeons wishing to perform neuroendoscopic approaches to the skull base 1

• Simulation surgery: Allows practice of techniques before application in patients 1

• Preoperative planning: Overlapping endoscopic and radiologic anatomy helps create precise surgical plans and predict the endonasal surgical corridor anatomy 2