pituitary tumor surgery
Pituitary adenomas may grow to considerable size and compress important neurologic and vascular structures at the skull base. When this occurs, surgery is indicated to decompress these structures, especially when vision is at risk. Tumor removal is usually accomplished through a transsphenoidal approach (Figure 1). When the tumor extends beyond the limits of the transsphenoidal operation, a more extensive operation requiring a craniotomy combined with skull base approaches may be needed. We will review these approaches, their indications and results. In addition, we describe our recent experience using intraoperative MRI (IMRI) to guide and document the amount of tumor removal during surgery.
Figure 1: The transsphenoidal approach to a large pituitary macroadenoma.
Anatomy of the pituitary region
Knowledge of the anatomy of the pituitary region is important to understand the indications and expectations involved with surgery in this area (Figure 2). The pituitary gland sits in a bony saddle called the sella turcica near the center of the head. This position provides excellent protection for the pituitary gland however, any surgical approach will require a relatively "long reach" along the course of normal structures. Several skull base approaches, which have been devised to minimize the involvement of important structures, allow optimal exposure of the tumor so it can be effectively removed.
Figure 2: Coronal cross-section of the head at the level of the pituitary gland depicting its relationship to the optic chiasm above, the sphenoid sinus below, and the cavernous sinuses laterally.
The sella turcica is located directly above the sphenoid sinus, a relationship that provides direct access to the pituitary gland via the nasal structures. On either side of the pituitary gland are the cavernous sinuses. Unlike the sphenoid sinus, which is an air filled, tissue-lined sinus that drains into the nose, the cavernous sinuses are actually a network of veins for the brain. Within the cavernous sinuses lie the carotid arteries, which are the main blood supply to the brain; and the third, fourth, and sixth cranial nerves, which control eye movement. The fifth cranial nerve, which provides facial and eye sensation, is in the outside wall of the cavernous sinus. All these structures and the veins of the cavernous sinuses are surrounded by the tough covering around the brain called the dura mater. The dura, forms the inside walls of the cavernous sinuses and separates these structures from the pituitary gland. Directly above the pituitary gland is the optic chiasm. The optic nerves, which are responsible for vision, leave the eyeballs and travel back to the optic chiasm where they partially cross before continuing to the back of the brain where vision is processed. Immediately above the optic chiasm is the hypothalamus, an area of the brain involved in many important regulatory functions (e.g. appetite, wakefulness, secretion of pituitary hormones).
Skull base approaches
When pituitary tumors enlarge they can compress the above mentioned structures and cause significant neurologic deficits. The tumor may extend down into the sphenoid sinus and even erode the sphenoid bone causing headache, bleeding from the nose, or leakage of spinal fluid. As the tumor expands, it can invade the cavernous sinuses and surround the carotid arteries, causing double vision (third, fourth and sixth nerves) or facial pain (fifth nerve). If the tumor grows up it can compress the optic nerves and chiasm, resulting in a loss of vision. The outside visual field in each eye is usually affected first (tunnel vision); this can progress to complete blindness if left unchecked. If the tumor is large enough to affect the hypothalamus or even block the flow of spinal fluid through the brain, hydrocephalus can result.
The downward growth of the tumor, and to a limited degree the upward extent, can usually be addressed via the transsphenoidal operation. However, there is limited accessibility in the other directions. Therefore, a transcranial, skull base approach may be better suited to ensure effective tumor removal.
The most common approach to the pituitary region, after the transsphenoidal route, is the subfrontal approach. When tumors extend far above the sella turcica, this approach allows improved visualization of the optic nerves and chiasm and surrounding brain structures to ensure an adequate removal. The most common indication for this approach is progressive visual loss. Most patients have already underwent the transsphenoidal route in an attempt to decompress the optic chiasm from below before this operation is needed. For the subfrontal approach a craniotomy removes the bone over the forehead to expose both frontal lobes of the brain to the level of the eyebrows (Figure 3).
Figure 3: (Left) Craniotomy outlined for subfrontal approach to large suprasellar pituitary tumor. (Right) Sagittal cross-section shows the line-of-sight obtained with the subfrontal approach.
The frontal lobes are gently elevated and separated to allow a corridor to the pituitary region. Tumor is then removed from beneath and around the optic nerves and chiasm, and from surrounding brain and blood vessels. The bone overlying the sella turcica is removed with a high-speed drill to allow further tumor removal from the sella and sphenoid sinus. The risks of this approach include possible injury to the frontal lobes with resultant memory difficulties, seizures, or a loss of sense of smell. There is also a risk of injury to the optic nerves and chiasm, pituitary stalk or the branches of the carotid arteries with resultant hemorrhage or stroke. Overall, the risk of a life-threatening complication is approximately 1%; the risk of other complications including cerebrospinal fluid leak or wound infection is not more than 5%. This approach does not allow access to the cavernous sinus but provides excellent decompression of the optic nerves.
Pituitary tumors may invade one or both cavernous sinuses adjacent to the pituitary gland and may involve the frontal and/or temporal lobes of the brain. A frontotemporal craniotomy, with further removal of the bone surrounding the optic nerve and next to the cavernous sinus, can allow access to these types of tumors (Figure 4).
Figure 4: Craniotomy outlined for the transcavernous approach.
Additional bone removed at the skull base is depicted by cross-hatching.
For a long time surgery within the cavernous sinus was considered too risky because of concern about bleeding from the veins and injury to the delicate neurovascular structures within it. With improved knowledge of the surgical anatomy and new microsurgical techniques the cavernous sinus can be opened and the tumor removed. There remains a 20% chance of temporary or permanent injury to one of the cranial nerves and a 2% risk of injury to the carotid artery. While this approach allows excellent access to the cavernous sinus and region above the sella turcica, it is limited to only one side and does not provide access to the region below the pituitary gland.
Le Fort maxillotomy
Pituitary tumors rarely grow through the sphenoid sinus being predominately in the sphenoid bone and below (the clivus). However, when this does occur, a more extensive approach from in front and below (Le Fort maxillotomy), can then be considered. The operation is similar to the transsphenoidal route with a much greater exposure of the skull base. This increased exposure is obtained by separating the bone containing the upper teeth (maxilla) from the cheekbones and nose. Once the separated maxilla is down-fractured, the tumor removal is performed through the back of the mouth and nasal area. At the end of the surgery, the maxilla bone is reattached using small titanium plates and screws. Potential complications of this approach include meningitis, spinal fluid leak, trauma to the sixth cranial nerves (double vision), and malocclusion of the teeth.
Intraoperative MRI (IMRI)
One of the major challenges of using either the transsphenoidal or the transcranial approach to pituitary tumors is the difficulty in precisely determining how much tumor has been removed at the time of surgery. Sometimes this difficulty arises because the surgeon's view of the tumor is incomplete using any single approach. In addition, tumor can hide behind membrane folds, nerves, and arteries. If tumor is missed, the patient may need to undergo additional surgery or receive some form of radiation treatment. In some cases, the type of radiation given to the patient is determined by how much tumor remains and how close it is to the optic nerves. If the tumor is far enough away from the optic nerves, then it may be possible to treat the residual tumor with stereotactic radiosurgery, which is a powerful single dose of radiation that has proven to be more effective than multiple, smaller doses of radiation (i.e., fractionated radiation therapy). Stereotactic radiosurgery provides excellent long-term control rates.
Recently, we have addressed the problem of determining precisely the amount of tumor removal by developing a magnetic resonance operating room (MROR). Our MROR contains an open MRI system that has been specifically modified to acquire high-resolution images of the brain during the surgical procedure (Figure 5). The patient is placed on the MRI scanner table and general anesthesia is administered.
Figure 5: Superior view of the Magnetic Resonance Operating Room (MROR) showing surgery using the Hitachi Intraoperative MRI (left). Photo of Hitachi Intraoperative MRI (right).
Positioning of the patient is similar to obtaining a routine, awake outpatient MRI scan. Surgery proceeds in a routine fashion except that when the neurosurgeon believes that the tumor has been completely or optimally removed, the motorized scanner table moves the patient into the center of the open MRI magnet so that images can be obtained to verify the exact amount of tumor removed. This is done while the patient remains in surgical position and is anesthetized. The neurosurgeon and neuroradiologist carefully evaluate the intraoperative images during the surgery, and if any residual tumor is still present, the neurosurgeon can immediately perform additional tumor removal. By using IMRI, we have been able to achieve the planned amount of tumor removal for all patients who underwent surgery in the MROR, thus avoiding unnecessary second operations. In our experience, approximately half of the time additional tumor has been detected on IMRI that the surgeon then immediately removed.
Results of surgery
Most pituitary tumors can be controlled with medication, transsphenoidal surgery, radiation, or a combination of these modalities. Various surgical approaches using conventional craniotomies with extended removal of skull base structures to provide access to the pituitary region can be used to treat pituitary tumors that have not responded to, or are not suitable for, conventional treatment. Pituitary tumors, although benign, can invade critical nerves and blood vessels at the skull base, resulting in major neurologic deficits such as blindness, double vision, or hydrocephalus. When this occurs, surgery is indicated to remove the tumor from around these structures. We have reviewed the surgical approaches that can be used, as well as our more recent use of IMRI to guide and confirm the amount of pituitary tumor removal during the operation. The extent and direction of tumor growth determines which approach can be used. However, it must be emphasized that even with the extensive operations described, some pituitary tumors remain surgically incurable due to invasion of the cavernous sinuses and other important structures. However, stereotactic radiosurgery can be used to treat unresectable tumor remnants with very good long-term control rates. As our understanding of pituitary tumors grows so will our ability to achieve effective treatment. Skull base approaches for direct surgical removal will likely continue to play an important role in that treatment. In addition, we believe that the use of IMRI will increase the effectiveness of initial surgical removal and enhance the effectiveness of postoperative treatment when needed.
Sources and links
If you have more questions, please contact the Mayfield Clinic at 800-325-7787 or 513-221-1100.
Pituitary Network Association (PNA) www.pituitary.com
National Brain Tumor Foundation (NBTF) at 1-800-394-CURE
American Brain Tumor Association (ABTA) at 1-847-827-9910 or www.abta.org
updated > 2.2013
reviewed by > John Tew, MD, Mayfield Clinic / University of Cincinnati Department of Neurosurgery, Ohio
Originally published in The Pituitary Patient Resource Guide, 2nd edition, pp. 115-118, 1997.