The skull base is the bottom part of the skull. While the sides, front, back, and top of the skull are essentially smooth, thin walls of bone, the skull base is dramatically more complex. The skull base is complex because every nerve in the body that carries signals to and from the brain crosses the skull base. Additionally, the large vessels that carry blood to and from the brain run through the skull base.
Types of Diseases Treated
A variety of benign tumors are treated with skull base surgery. These include acoustic neuroma, meningioma, schwannoma, glomus jugulare and vagale, epidermoid, pituitary tumors, and many others. Malignant tumors managed with skull base surgery include squamous cell carcinoma, esthesioneuroblastoma, chondrosarcoma, chordoma, and endolymphatic sac tumors. Occasionally, particularly in the pediatric population, a skull base biopsy may be required in order to diagnose other diseases such as leukemia, lymphoma, plasmacytoma, Langerhans' dell histiocytosis, rhabdomyosarcoma, osteosarcoma, fibrosarcoma, hemangiopericytoma, malignant fibrous histiocytoma, or metastatic disease. Skull base surgical techniques are also sometimes employed to repair fractures of the skull base, to treat cerebral aneurysms, as well as to remove deep-seated infections of the skull base bone.
Skull Base Surgery
Since tumors of the skull base sit underneath the brain, it can be difficult during surgery to get to the tumor in order to remove it. Traditional techniques used to approach tumors of the brain itself can be used to approach skull base tumors, but these often require significant force to retract the patient's brain out of the way. This may lead to unwanted injury to otherwise normal brain tissue.
The basic concept of skull base surgery is to approach the tumor from underneath or from the side by removing specific areas of skull base bone. Thus, the tumor can be exposed with little to no brain retraction. Skull base procedures, for example, may be designed to traverse the bone containing the ear (transtemporal/transpetrosal/retrosigmoid approaches), low on the temple beneath the brain (middle fossa approach), around the eye (transorbital/orbital-zygomatic/craniofacial approaches), through the nose or paranasal sinuses (transsphenoidal/transethmoidal/transfacial approaches), or from the neck (transcervical). Fundamentally, these are minimally-invasive techniques designed to maximize tumor removal while preserving neurological function to the greatest extent possible.
Tumors arising within the skull base are rare and usually cause few symptoms until they grow to a size where they begin to affect neurologic function. These symptoms may manifest as double vision, facial pain or twitching, hearing loss, loss of balance or dizziness, hoarseness, or tongue weakness. Occasionally, an asymptomatic tumor may be diagnosed when a middle ear mass is noted during routine examination of the ears.
An MRI of the brain and/or skull base with and without gadolinium contrast is extremely sensitive at diagnosing these rare tumors with little risk to the patient. A CT scan of the skull base or temporal bone may also be needed to evaluate the condition of the surrounding skull base bone. Occasionally, an MRA or MRV (magnetic resonance angiogram and magnetic resonance venogram) are useful to study how the blood vessels to the brain are affected by the tumor. For large or very vascular tumors, an angiogram with embolization may be performed prior to surgical resection in order to plug the arteries that feed the tumor. This reduces blood loss during surgery.
Microsurgery of skull base tumors is a technologically intensive endeavor. A high-power microscope with a stable, anti-vibratory base mount is important for proper visualization of vital structures. A high powered drill with a wide assortment of burrs is essential, as it is critically important to be able to navigate around each nerve and blood vessel. Delicate microsurgical instruments in a wide variety of sizes and angles, such as hooks, scoops, scissors, and probes, are required. These permit atraumatic microdissection of the tumor off the vital structures.
Cranial nerve monitoring is often used in skull base surgery. A neurophysiologist is present in the operating room throughout the surgery, and tracks the various nerves' health on a computer system. This optimizes the preservation of cranial nerves by facilitating gentle microdissection of the tumor off of the nerve.
A three-dimensional image guidance navigation system may be required for large tumors that envelop major vascular or neurologic structures. This computer system is programmed to project images of an individual patient's tumor on a TV for use in the operating room. Using a probe referenced to the patient on the operating table, the surgeon can verify exactly where each vital structure is in relation to the tumor.
Success Rate and Other Treatment Options
The goal when managing skull base tumors is to improve patient survival and quality-of-life. Obviously, the best way to treat a skull base tumor is a decision that can only be made after close collaboration between the patient and the physician team. Proper patient selection is critical to good outcomes.
Not all skull base tumors require intervention. Some are so slow growing that they pose only minor risk of more serious problems,
especially to older individuals. In such cases, the tumor may simply be monitored by periodic imaging studies. In this situation, because the tumor is predicted not to grow to a size where it will cause disabling symptoms prior to the patient dying of other reasons. Thus, these patients can be treated with the philosophy that they will "take the tumor with them."
Another treatment option that is at times preferable to microsurgery in selected skull base tumors is stereotactic radiation. The goal is to use a computer-guided delivery system to focus the radiation directly on the skull base tumor, while minimizing radiation exposure to the sensitive surrounding structures. While this technique does not remove the tumor, it can reduce or stop the tumor growth in certain situations. We recently purchased the Novalis radiosurgery system to use specifically for patients with skull base tumors, particularly acoustic neuromas. It is the most advanced form of stereotactic radiosurgery technology available today, and offers many improvements over its predecessor, the Gamma Knife.
Success rates of skull base surgery have continued to improve. This is because of the evolution of modern approaches designed to reduce the requirement of brain retraction and sacrifice of normal structures in order to get to the tumor. Our surgical philosophy is based on the concept of removing as much tumor as possible, while maintaining preservation of function. This has led to excellent rates of tumor control, long-term patient survival, as well as in numerous, important quality-of-life measures.