Definition and Overview
Skull base surgery is a highly specialized set of surgical procedures performed to treat various conditions that affect the base of the skull. This form of surgery is very challenging, dealing with complex cases and lesions that lie in close proximity to vital neurologic structures.
The base of the skull is divided into several compartments, namely the anterior, middle and posterior. Diseases of the anterior fossa typically involve the paranasal sinuses, the orbit, and the nasal cavities. Nasopharyngeal carcinomas (NPCA) are the most frequently encountered tumors in the anterior compartment of the skull base. Masses of the anterior fossa grow slowly; thus, they usually present when they have grown large already, resulting in considerable mass effect and symptoms. Diseases of the middle fossa are usually benign, and include diseases of the pituitary gland and cavernous sinus. Diseases in this area can present with symptoms of cranial nerve involvement, making surgery difficult. Diseases of the posterior fossa involve the cerebellopontine angle and the foramen magnum. Symptoms of posterior fossa diseases include cranial nerve deficits, extremity weakness and gait problems, to name a few.
A multidisciplinary approach is usually necessary when dealing with diseases of the skull base. Neurosurgeons, along with specialists in otorhinolaryngology, ophthalmology and head and neck surgery, primarily manage skull base diseases. Interventional radiologists and oncologists are also involved when dealing with malignant conditions of the skull base. A combination of therapies, including surgery, chemotherapy, radiotherapy and gamma knife treatment, may be needed in the management of various skull base diseases.
Who Should Undergo and Expected Results
Patients with diseases of the skull base are candidates for skull base surgery. These diseases may either be benign or malignant. Benign conditions include congenital abnormalities, vascular malformations, bone diseases, aneurysmal bone cysts and benign tumors such as fibromas and giant cell tumors. Malignant conditions include tumors such as schwannomas (nerve sheath tumors), chondrosarcomas and meningiomas, to name a few.
Imaging studies are essential in the management of skull base diseases. Options include cranial computed tomographic (CT) scan, magnetic resonance imaging (MRI), and cerebral angiography. These studies can help determine the extent of the lesion and can aid in the decision-making regarding surgery and adjuvant therapy. A biopsy of mass lesions may have to be taken to confirm the diagnosis.
If at all possible, the goal of skull base surgery is complete resection of lesions, resulting in resolution of symptoms and functional outcomes. However, not all diseases, particularly tumors in the advanced stages, can be removed entirely. There are several factors to consider prior to subjecting a patient to skull base surgery. These include the patient’s condition and ability to tolerate major surgery, the natural history of the disease, the patient’s symptoms, and the structures that are involved, among others. Patient prognosis depends on the kind and extent of the lesion. Benign lesions can generally be removed with minimal mortality and morbidity.
How Does the Procedure Work?
The goal of skull base surgery is to remove as much of the lesion or mass as possible, with minimal morbidity. The approach to the skull base is mainly dependent on the location of the tumor, and usually involves either a craniotomy or an endoscopic approach. For example, lesions of the anterior skull base may involve an intracranial approach, an extracranial approach, or a combination of both. Middle fossa lesions can be accessed by going through the temporal bone, and diseases of the lateral skull base may be approached via a preauricular or postauricular incision. Posterior approaches, meanwhile, include suboccipital and retromastoid craniotomies. Open approaches involve incisions on the face and scalp, and may necessitate movement of bones. Endoscopic techniques, including endoscopic-assisted operations, allow minimally invasive access to lesions in the brain, and are especially useful when working in areas that are difficult to reach. The endoscope is inserted through the nose, and the disease is approached from below. In recent years, indications for using the endoscopic approach are increasing.
Once access is gained, the mass or the lesion is then removed as completely as possible. The procedure is performed with utmost care using a microscope, in order to preserve the vital structures. The need for reconstruction, such as a free flap, depends on the extent of resection involved.
After the surgery, the patient is transferred to the intensive care unit (ICU). The patient is closely monitored for several days.
Skull base surgery continues to evolve. Significant improvements in this field have been seen, with developments in high-resolution imaging technology, the advent of endoscopic surgery and increasing experience in reconstructive techniques.
Possible Complications and Risk
Skull base surgery is a complex procedure, and various complications can occur during the operation. Neurologic complications are unique to this kind of surgery. Injury to the cranial nerves is not uncommon, and can be caused by electrocautery or traction injury. Direct involvement of the cranial nerves may necessitate complete transection of the cranial nerves. Damage to several cranial nerves can result in devastating consequences, such as difficulty in swallowing, hearing loss, or inability to close the eyes.
Aside from cranial nerve injuries, cerebrospinal fluid (CSF) leaks can also occur. This happens when the dura is invaded by the mass or is opened on purpose. CSF leaks can lead to other complications, such as meningitis. Holes in the dura may be tricky to repair, and may need flaps for adequate coverage.
Other neurologic complications include hydrocephalus, contusion, brain edema, strokes and seizures, to name a few. Significant bleeding is a risk with this kind of operation, as the skull and the meninges are vascular structures. Skull base surgery may also result in scars that are not cosmetically pleasant. It can also produce deformities, such as distorted facial contours, depressions on the scalp, or uneven eyes. Reconstructions using flaps, bone grafts and plates may be necessary to avoid these problems. Complications related to the wound, such as infection, cellulitis or flap necrosis, may also occur.
Kassam A B, Gardner P, Snyderman C H, Mintz A, Carrau R. Expanded endonasal approach: fully endoscopic, completely transnasal approach to the middle third of the clivus, petrous bone, middle cranial fossa, and infratemporal fossa
Snyderman C, Kassam A, Carrau R, Mintz A, Gardner P, Prevedello D M. Acquisition of surgical skills for endonasal skull base surgery: a training program. Laryngoscope. 2007;117:699–705. [PubMed]