Tumors and Surgery of the Skull Base
By Hrayr K. Shahinian, M.D., FACS
Cranial base surgery involves treatment of the congenital, vascular, neoplastic, endocrine and traumatic lesions involving the basi cranium. This surgery is the brainchild of three specialties: Craniofacial surgery, Neurosurgery and neuro-otology and was championed by three pioneers: Tessier, Dandy and House.
Anatomically the skull base is divided into three subdivisions. The anterior skull base, the lateral skull base and the posterior or extreme lateral skull base (Fig. 1).
Anterior Skull Base
The anterior skull base is further subdivided into midline and paramedian regions. Midline lesions include congenital encephaloceles, craniofacial clefts (such as the Tessier 0-14 cleft), or tumors (such as pituitary lesions or craniopharyngiomas). Paramedian lesions include orbital tumors and locoregional extensions of paranasal sinus and head and neck tumors.
Lateral Skull Base
The lateral skull base involves lesions in the infratemporal fossa as described by Fisch. These consist of Type A, including glomus jugulare tumors, Type B, including lower clival tumors and petrous apex lesions such s cholesterol cysts. Type C and D lesions are further anterior extensions including juvenile angiofibromas and nasopharyngeal carcinomas. Lateral skull base surgery also includes transtemporal lesions (middle cranial fossa) including sphenoid wing meningiomas, neuromas of the trigeminal nerve and vascular lesions such as internal carotid artery aneurysms.
Posterior or Extreme Lateral Skull Base
Surgery of the extreme lateral skull base involves the cerebellopontine (CP) angle including acoustic neuromas, microvascular decompression of cranial nerves, meningiomas of the posterior fossa, and surgery of the craniocervical junction.
A (1). Anterior Skull Base Midline Lesions: Pituitary adenoma
A forty-eight year old white female with recent onset of visual problems and headaches. MRI confirmed the presence of a large tumor in the sella turcica. The patient underwent endocrinologic and neuro-ophthalmologic examinations. Some functioning tumors, such as prolactinomas, may be treated with bromocriptin. Non-functioning pituitary adenomas are, as in this case, treated with a transseptal, transsphenoidal hypophysectomy. An incision is make in the nasal septal mucosa and the upper buccal sulcus. Dissection is carried to the keel of the vomer after the perpendicular plate of the ethmoid is removed and the septal cartilage harvested for future use. The vomer is removed, the sphenoid sinus is entered and the mucosa stripped. The eggshell thin posterior wall of the sphenoid sinus is removed, the dura is incised and the tumor is evacuated. Structures at risk include the internal carotid artery and the optic nerves which are positioned on each side of the midline in the lateral wall of the sphenoid sinus. The septal cartilage is used to reconstruct the floor of the sella turcica.
A (2). Anterior Skull Base paramedian Lesions: Fronto orbital tumor
A 50 year old white male who on physical exam had minimal prominence of his left fronto-orbital area. Radiologic studies consisting of CT scans and plain x-rays revealed the presence of a lytic lesion over the left fronto-orbital area. The patient underwent metastatic workup which was negative. He underwent left frontal craniotomy and left fronto-orbital craniectomy with excision of the peri-orbita. Intraoperative frozen section was read as malignant melanoma. The area was reconstructed by repositioning the frontal bone flap and filling the smaller defects with methyl methacrylate. Postoperatively his course was unremarkable with good vision and movement of the left eye. His permanent pathology was reported as benign fibrous histiocytoma with reactive new bone formation.
B (1). Lateral Skull Base Lesions: sphenoid wing meningioma
A 46 year old OR nurse was found on physical exam to have right proptosis. CT scan confirmed the presence of a sphenoid wing meningioma with hyperostosis of the bone and circumferential compression of the optic canal. The patient was taken to the OR and underwent right pterional craniotomy. The meningioma infiltrated both the bone of the sphenoid wing and the dura. The tumor was excised, the bone of the sphenoid wing was drilled away and the involved dura was excised and patched with a liophilized dural graft. The optic nerve was decomposed circumferentially and all the hyperostotic bone responsible for the optic nerve compression was drilled away. Postoperatively her course was unremarkable. She maintained her preoperative baseline vision in the right eye.
B (2). Lateral Skull Base Lesion: recurrent trigeminal neuralgia
A 47 year old white female with a history of recurrent trigeminal neuralgia over a 17 year period. The patient had undergone a posterior fossa trigeminal nerve decompression, twice a stereotactic gasserian ganglion block, and had exhausted all forms of medical therapy. MR angiography confirmed the absence of a vascular loop compressing the nerve in the posterior fossa. The patient was advised to undergo an infratemporal fossa type D approach and neurectomy of the third and second branches of the trigeminal nerve. The patient was taken to the operating room and underwent a hemicoronal preauricular incision. The lateral and superior orbital rims and the zygomatic arch were osteotomized, the temporalis muscle was disinserted and retracted inferiorly and anteriorly. The infratemporal fossa retractor was positioned and the floor of the middle cranial fossa was drilled away. The foramen spinosum was identified and the middle meningeal artery was sacrificed. Continuing the drilling more medially the foramen ovale was identified, its lateral half was drilled away and the third branch of the trigeminal nerve was identified. The petrous portion of the internal carotid artery was identified posterior and medial to V3. Continuing the drilling anteriorly and medially to V3, the foramen rotundum and V2 were identified. V3 and V2 were divided. The inferior half of the gasserian ganglion was destroyed. The temporalis muscle was incised longitudinally and the posterior half was used to reconstruct the floor of the middle cranial fossa. The anterior half of the temporalis muscle was repositioned in its initial location. The orbitozygomatic complex disappearance of her facial pain.
C (1). Extreme Lateral Skull Base Lesion: acoustic neuroma (Fig. 6).
A 67 year old white female with a history of polio presented with progressive left hearing loss and tinnitus. MRI scan confirmed the presence of grade ll intracanalicular acoustic neuroma. She was taken to the operating room and underwent a retrosigmoid craniectomy. After release of CSF from the prepontine cistern and minimal retraction of the cerebellum, the left acoustico-facial bundle was identified and the tumor was noted to be extruding from the internal auditory canal. After drilling the posterior wall of the internal auditory canal and dissecting the tumor from the left facial nerve which was localized anteriorly, the tumor was resected completely from the cerebellopontine angle and the internal auditory canal. The patient's postoperative course was uneventful.
C (2). Microvascular Decompression of Cranial Nerves
A 39 year old white male presented with a twelve year history of spasmodic torticollis. The patient was treated by multiple injections of botulinum toxin, multiple oral medications including muscle relaxants and anxiolytics. MR angiography confirmed the presence of a vascular loop positioned on the left spinal accessory nerve in the cerebellopontine angle. The patient underwent decompression of the spinal accessory nerve through a suboccipital craniectomy. The vascular loop branch of the posterior/inferior cerebellar artery was identified, dissected off of the nerve and an avalon sponge was interposed between the vascular loop and the spinal accessory nerve. The torticollis did not return.
The recent advance in anesthesia, microsurgical instrumentation and operating microscopes have allowed new, aggressive yet safe approaches to areas of the skull base that were labeled for years as "no man's land".
The principles of shorter, more direct working distance and removing more bone while minimizing brain retraction have guided the skull base surgeon in tackling these challenging problems.
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