19 Skull Base Surgery

Question 1

What are the compartments of the skull base?

Answer 1

What are the compartments of the skull base?

The skull base is separated into three compartments, or fossae: anterior, middle, and posterior. The anterior compartment extends from the frontal sinus to the anterior clinoid process and planum sphenoidale (sphenoid roof). The middle cranial fossa extends from the greater wing of the sphenoid to the clivus, including the sella turcica. The posterior fossa consists of the occiput and begins from the basal aspect of the occipital bone (Figure 19-1).

Question 2

What are the most common pathologies in which skull base surgery (SBS) is performed?

Answer 2

What are the most common pathologies in which skull base surgery (SBS) is performed?

Benign sinonasal pathologies include inverted papilloma, juvenile nasal angiofibroma, fibrous dysplasia, and osteoma. Common intracranial skull base tumors include pituitary tumor, craniopharyngioma, and meningioma. Sinonasal malignancies include olfactory neuroblastoma (esthesioneuroblastoma), sinonasal undifferentiated carcinoma (SNUC), squamous cell carcinoma, adenocarcinoma, and melanoma.

Question 3

How can the skull base be accessed?

Answer 3

How can the skull base be accessed?

The skull base can be accessed using open, endoscopic, microscopic, or combined open-endoscopic approaches. The goal of any approach is to provide the best visualization and exposure to surrounding neurovascular structures and the tumors.

Question 4

What types of approaches are used to access the skull base?

Answer 4

What types of approaches are used to access the skull base?

See image.

Question 5

What incisions may be used to access the anterior skull base?

Answer 5

What incisions may be used to access the anterior skull base?

Open approaches to the anterior fossa may use the coronal, lid crease, brow, gullwing, Lynch (medial orbital), lateral rhinotomy, Weber-Ferguson, or midface degloving incisions (Figure 19-2).

Question 6

What tests and examinations are helpful prior to resecting a skull base malignancy?

Answer 6

What tests and examinations are helpful prior to resecting a skull base malignancy?

For surgical planning, basic laboratory studies including CBC, BMP, and coagulation studies should be obtained as well as imaging of the primary site (fine cut CT and MRI). Other labs may be obtained depending on tumor type (e.g., for pituitary tumor: ACTH, PRL, LH/FSH, GH, TRH) and location (arteriography). If indicated, a thorough evaluation for distant metastases should be performed.

Question 7

What imaging studies should be obtained?

Answer 7

What imaging studies should be obtained?

Fine cut CT scans (≈1mm section) should be obtained for evaluation of bony anatomy of the skull base; the use of contrast may be helpful in defining the lesion and nearby vasculature. MRI with and without contrast is useful to delineate the lesion from surrounding structures, help create a radiologic differential diagnosis, and evaluate for soft tissue invasion and perineural spread. CT, MR, or fused CT-MR images are helpful for intraoperative image guidance. PET scans can be used to rule out distant metastases.

Question 8

What is endoscopic skull base surgery (ESBS)?

Answer 8

What is endoscopic skull base surgery (ESBS)?

Endoscopic skull base surgery is an extension of endoscopic sinus surgery. The endoscope is used to improve visualization, illumination, and may obviate the need for facial incisions such as a lateral rhinotomy. Recent reports have demonstrated that endoscopic resections of skull base tumors may yield similar oncologic results to traditional open approaches in appropriately selected cases.

Question 9

When is an endoscopic skull base approach appropriate?

Answer 9

When is an endoscopic skull base approach appropriate?

Potential indications for endoscopic transnasal skull base surgery are increasing. The areas accessible via endoscopic endonasal approaches include the entirety of the anterior skull base, much of the middle cranial fossa, and portions of the posterior fossa. ESBS performed through small incisions is leading to minimally invasive lateral approaches and neurosurgical approaches as well.

Question 10

What are the advantages of an endoscopic skull base approach?

Answer 10

What are the advantages of an endoscopic skull base approach?

Compared to open surgery, the endoscopic approach allows for more direct visualization with less manipulation of the surrounding soft tissues. This may allow for a more precise resection of the lesion due to better visualization, and minimal manipulation of nearby neurovascular structures. Compared to the traditional microscopic view, endoscopes give a dynamic operative view with the added ability to see around corners using angled endoscopes. ESBS can avoid scars, decrease hospital stays, and cause less postoperative pain.

Question 11

What are the limitations of endoscopic skull base surgery?

Answer 11

What are the limitations of endoscopic skull base surgery?

Not all areas of the skull base can be visualized and safely instrumented via a transnasal endoscopic route. Malignancy involving the orbit and facial skin are considered contraindications to endoscopic skull base surgery. As a general rule, the endoscopic approach should not compromise the ability to achieve the appropriate oncologic resection, and crossing major neurovascular structures is not suggested.

Question 12

In what situations should adjuvant radiotherapy and/or chemotherapy be considered?

Answer 12

In what situations should adjuvant radiotherapy and/or chemotherapy be considered?

The primary modality of treatment for the majority of skull base malignancy is surgical resection. There are some exceptions to the rule (e.g., lymphoma and plasmacytoma). Adjuvant radiotherapy is considered when there is a high propensity of tumor recurrence, which includes presence of perineural invasion, high-grade tumor, and close or positive margins. Chemotherapy can be considered for induction therapy and for metastatic disease, or may be used as adjuvant therapy for chemosensitive histologies. Radiation therapy may have a role in the management of certain benign skull base pathologies, such as meningioma, schwannoma, vascular tumors, and chordoma.

Question 13

What is stereotactic radiosurgery?

Answer 13

What is stereotactic radiosurgery?

Stereotactic radiosurgery utilizes ionizing radiation to treat well-defined targets with high accuracy and precision. This can be applied to areas with adjacent critical neurovascular structures such as in the skull base. It is most often used for pituitary adenoma, schwannoma, meningioma, AV malformations, and metastases.

Question 14

Where is CSF produced and absorbed?

Answer 14

Where is CSF produced and absorbed?

CSF is produced by the choroid plexus in the lateral ventricles, and is reabsorbed into the dural venous sinuses through the arachnoid villi. Total CSF volume is approximately 150 ml. An adult produces approximately 20 ml/hr and 550 ml/day. Normal adult intracranial pressure ranges from 10 to 20 cm H20.

Question 15

What are the three layers of the meninges?

Answer 15

What are the three layers of the meninges?

The dura mater, arachnoid, and pia mater. The dura mater is separated into the superficial layer and the meningeal layers.

Question 16

Describe the segments of the carotid artery.

Answer 16

Describe the segments of the carotid artery.

The internal carotid artery can be separated into seven anatomic segments: C1 cervical, C2 petrous, C3 lacerum, C4 cavernous, C5 clinoid, C6 ophthalmic, and C7 communicating. A mnemonic for remembering branches in the skull is Please Let Children ConsumeOur Candy (Figure 19-3).

Question 17

How are approaches classified in endoscopic transnasal skull base surgery?

Answer 17

How are approaches classified in endoscopic transnasal skull base surgery?

Approaches to the ventral skull base are classified according to their location in the sagittal or coronal plane (Figures 19-4 and 19-5).

• Sagittal Plane
  
  • Transfrontal
  • Transcribriform
  • Transplanum/tuberculum
  • Transsellar
  • Transclival
  • Transodontoid
• Coronal
  
  • Anterior Coronal Plane
    
    • Supraorbital
    • Transorbital
  • Middle Coronal Plane
    
    • Medial petrous apex
    • Petroclival approach
    • Quadrangular space
    • Cavernous sinus
    • Transpterygoid/Infratemporal approach
  • Posterior Coronal Plane
    
    • Infrapetrous
    • Transcondylar
    • Transhypoglossal
    • Parapharyngeal space
      
      • Medial (Jugular foramen)
      • Lateral

Question 18

What is the crista ethmoidalis?

Answer 18

What is the crista ethmoidalis?

The crista ethmoidalis is a bony landmark located just anterior to the sphenopalatine foramen.

Question 19

How can the lateral aspect of the sphenoid sinus be accessed?

Answer 19

How can the lateral aspect of the sphenoid sinus be accessed?

The lateral recess of a pneumatized sphenoid sinus can be accessed by the use of angled instruments following a wide sphenoidotomy, or through the transpterygoid approach through the posterior wall of the maxillary sinus and pterygopalatine fossa.

Question 20

What is the vidian canal?

Answer 20

What is the vidian canal?

The vidian canal runs through the inferolateral aspect of the sphenoid sinus, transmitting the vidian nerve and an arterial branch from the internal carotid artery. It can be used as a surgical landmark for finding the internal carotid artery (Figure 19-6).

Question 21

What structures are present in the cavernous sinus?

Answer 21

What structures are present in the cavernous sinus?

Within the cavernous sinus lie cranial nerves III, IV, and VI as well as the ophthalmic/maxillary branch of the trigeminal nerve (CN V1 + V2) and the carotid artery. CN VI is the most medial and therefore the most commonly injured (Figure 19-7).

Question 22

How is a skull base defect reconstructed?

Answer 22

How is a skull base defect reconstructed?

The skull base defect can be repaired in layers using autologous or synthetic materials. The dural layer can be reconstructed using temporalis fascia or fascia lata, or allogeneic materials. The bony defect may be reconstructed with cartilage or bone from the septum, turbinate, or split calvarial grafts, if desired. The mucosal surface may be repaired with free mucosal grafts, fascia, autologous fat, or pedicled rotational flaps (i.e., nasoseptal flap). If the need arises, external approaches utilizing the pericranium may also be considered. Packing material, tissue glues or direct suture techniques may be used to fix graft materials in place.

Question 23

Describe the nasoseptal flap.

Answer 23

Describe the nasoseptal flap.

The nasoseptal flap is the workhorse pedicled mucosal flap based off of the posterior septal branch of the sphenopalatine artery. Its size can be customized, and its wide arc of rotation makes it versatile.

Question 24

What is a free mucosal transfer?

Answer 24

What is a free mucosal transfer?

A free mucosal transfer utilizes a mucosal graft harvested from another area in the nasal cavity, similar to a skin graft. Common sites of graft harvest include the inferior or middle turbinates, septum, or nasal floor.

Question 25

What free flaps are preferred for reconstruction of larger defects?

Answer 25

What free flaps are preferred for reconstruction of larger defects?

Common flap options include anterolateral thigh, radial forearm, latissimus dorsi, and rectus myocutaneous flaps. If an osseocutaneous flap is required, options include a free fibula flap and scapular flap.

Question 26

What are the complications of skull base surgery?

Answer 26

What are the complications of skull base surgery?

Common complications from open skull base surgery include anosmia and associated taste dysfunction, poor aesthetic results, and neurologic complications such as cranial nerve injury or those secondary to brain retraction. Major skull base surgery complications include CSF rhinorrhea, meningitis, intracranial hemorrhage, orbital complications such as diplopia or vision loss, vascular injury, stroke, and death.

Complications from endoscopic approaches are similar to those from the open approach, however there is improved cosmesis and less brain retraction. The most common complications are hyposmia and associated taste disturbance, epistaxis, and prolonged local wound healing.

Question 27

What are the rates of complications of open and endoscopic skull base surgery?

Answer 27

What are the rates of complications of open and endoscopic skull base surgery?

There is a 5% to 20% rate of surgical complications and 8% to 40% rate of medical complications. Several studies now demonstrate that both medical and surgical complications are more frequent and severe with open approaches compared to endoscopic approaches.

Question 28

What are signs of failure in reconstruction?

Answer 28

What are signs of failure in reconstruction?

Symptoms include clear rhinorrhea and constant postnasal drip. Other signs may include meningitis, severe headaches, seizures, and worsening pneumocephalus.

Question 29

If a CSF leak is identified, what nonoperative management options exist?

Answer 29

If a CSF leak is identified, what nonoperative management options exist?

If the defect is small and there is confidence in the original reconstruction, conservative management with bed rest, stool softeners, and lumbar drainage can be considered. Antibiotics are sometimes administered for meningitis prophylaxis. Medications may be prescribed to decrease CSF production (acetazolamide, furosemide, digoxin) or decrease ICP (corticosteroids).

Question 30

What causes postoperative pneumocephalus?

Answer 30

What causes postoperative pneumocephalus?

Nearly all patients undergoing skull base surgery will have some degree of postoperative pneumocephalus on initial imaging. Significant pneumocephalus is reported in 5% to 10% of patients when a ball valve action occurs along the reconstruction, either from increased negative pressure from the intracranial side (excessive CSF drainage from lumbar drain) or increased extracranial pressure (coughing, nose-blowing, CPAP use).

Question 31

What are the consequences of a pneumocephalus?

Answer 31

What are the consequences of a pneumocephalus?

Patients with pneumocephalus may present with headaches, dizziness, nausea and vomiting, seizures, depressed neurologic status, or neurologic symptoms from mass effects on nearby structures.

Question 32

What is the treatment of symptomatic pneumocephalus?

Answer 32

What is the treatment of symptomatic pneumocephalus?

Nasal packing should be removed and the lumbar drain should be clamped. If severe, emergent drainage with needle aspiration should be performed if the area is safely accessible. The reconstruction may need to undergo revision, or in refractory cases airway diversion (tracheotomy) can be considered.

Question 33

Oncologic outcomes for endoscopic versus open surgery for malignancies.

Answer 33

Oncologic outcomes for endoscopic versus open surgery for malignancies. Controversy

Endoscopic management of skull base tumors appears to have similar outcomes to open approaches in properly selected cases. True en bloc resection does not appear necessary to achieve good oncologic outcomes. The surgeon must evaluate whether complete tumor resection with negative margins can be accomplished through an endoscopic approach.

Question 34

What is the role of lumbar drains?

Answer 34

What is the role of lumbar drains? Controversy

Lumbar drains can decrease intracranial pressure and thereby reduce the pressure applied to the skull base reconstruction; however, they may be associated with significant morbidity and potential for complications. The use of lumbar drain primarily following a repair varies from different surgeons, and should not be universally utilized in a routine fashion. When used, the duration of drainage is also up to surgeon discretion.

Question 35

Best type of reconstruction.

Answer 35

Best type of reconstruction.

Most reconstructive methods appear to have similar efficacy, and therefore there is no universal “best type of reconstruction.” In general, small defects (<1cm) can be closed in a single layer, and multilayer repair is preferred for larger defects. Some surgeons prefer to use a rigid layer of bone or cartilage to reconstruct the skull base, although this is not required. Vascularized mucosal tissue (e.g., nasoseptal flap) has been demonstrated to improve repair results for larger defects; however, single layer nonvascularized tissue can also be successful in this setting.

Question 36

Use of perioperative antibiotics.

Answer 36

Use of perioperative antibiotics.

Postoperative antibiotics are an important consideration for skull base surgery because of the temporary connection between the intracranial space and external world. Rates of postoperative wound infection following ESBS are approximately 2%, and appear to be higher in open skull base surgery. Broad coverage with IV cephalosporins with or without vancomycin (or oral amoxicillin/clavulanate) is most often recommended. Studies are lacking to support the use of prolonged postoperative antibiotics, although most surgeons prefer to use systemic or topical antibiotics in some form after surgery.