The most common symptom is an asymmetrical visual loss starting with a unilateral decrease in central visual acuity or blurring in the visual field, followed by progression to bilateral involvement. The pattern of visual loss may be acute, gradual, or fluctuating. On examination there is almost always a reduction of visual acuity in at least one eye, and most patients have bilateral field defects. Incongruity and asymmetry of the field defects is the common finding; a symmetrical bitemporal field loss is the exception. Optic atrophy is frequently found and correlates better with the visual acuity loss than with the peripheral field impairment. MRI outlines the tumor and its relationship to the optic nerves, chiasm, and each internal carotid artery and its branches. In most patients angiography is not needed and there is no indication for embolization. The indication for surgical treatment is usually worsening vision. Surgical removal of the tumor gives the best chance for relief of symptoms and the best possibility of curing the patient. Surgery should also be considered in an asymptomatic patient because of the probability of future visual symptoms. When large tumors involve the optic apparatus or the internal carotid or anterior cerebral arteries with dense adherence, it may be wise to leave a small amount of tumor (radical subtotal removal). Radiation therapy is recommended when there has been a subtotal removal with an inadequate decompression or there is evidence of recurrence on MRI after radical subtotal removal. It is tended to wait on this recommendation for evidence of regrowth unless the operation has been done for recurrence.
In planning the operation, several important points should be kept in mind:
I. The blood supply usually comes through the tuberculum, and this should be interrupted systematically.
2. Internal decompression of the tumor is essential before the surgeon tries to dissect the tumor off important visual and arterial structures.
3. As the tumor grows, it elevates and displaces the optic nerves laterally and may surround the nerves. Each internal carotid artery is also displaced laterally and may be engulfed by tumor. Growth may extend posteriorly above the chiasm to involve the anterior cerebral arteries and in large tumors compress the hypothalamus.
4. The tumor may grow into the optic foramen and/or involve the dura under the optic nerve.
5. The pituitary stalk is behind the tumor and is usually covered by arachnoid.
6. The dura over the tuberculum should be removed.
In general, some prefer a right subfrontal exposure, elevating the frontal lobe just in front of the sphenoid wing. A left subfrontal exposure is utilized when the tumor bulk is greater on that side, and occasionally a bifrontal exposure is used for large tumors. Most neurosurgeons have also used the right subfrontal approach unless the visual loss is greater on the left side. Others use a unilateral right subfrontal exposure but approach the tumor along the midline. Another uses a unilateral supraorbital exposure. For large tumors, a bifrontal craniotomy is advised.
The patient is placed carefully in the supine position, with the head elevated, held with the three-point skeletal fixation headrest, and rotated about 60 degrees to the left so that the anterior zygoma is uppermost. An incision is made beginning just above the zygoma a few millimeters anterior to the ear and then, staying behind the hairline, extending medially to end in the midline of the forehead. The skin. underlying temporal is muscle, and pericranial tissue are turned down together, exposing the inferior lateral frontal and anterior temporal bones.
The most important burr hole is the one placed just below the anterior end of the superior temporal line just behind the zygomatic process of the frontal bone. It is important that this hole be placed properly so that the exposure will be on the floor of the anterior fossa. Two or three other burr holes are placed as shown and the bone flap is cut. This can be modified as you need by using the electrical craniotome. The lateral portion of the sphenoid wing is removed, as is bone over the anterior superior temporal region. The dura is opened over the inferior frontal and anterior temporal regions. Draining veins from the anterior temporal lobe along the sphenoid wing are divided. The frontal lobe is elevated carefully along the sphenoid wing, revealing the posterior part of the olfactory tract. This normally will lead the surgeon to the optic nerve unless there is significant displacement.
The dura anterior to the optic nerve may be reddish and have increased vascularity. Slightly more exposure reveals the anterior clinoid process, the internal carotid artery, and a varying portion of the right optic nerve, depending on the size of the tumor. In patients with smaller tumors, the arachnoid over the lateral aspect of the optic nerve and over the internal carotid artery is opened and CSF is aspirated to give further decompression. In some patients with larger tumors, both the optic nerve and internal carotid artery may be surrounded by tumor. The frontal lobe tissue is carefully freed from the surface of the tumor. and self-retaining retractors are placed.
The tumor capsule is opened. Attachments of the tumor along the tuberculum are divided to interrupt the blood supply as it comes into this area. Internal decompression of the tumor is done, using the bipolar coagulator and/or the ultrasonic aspirator. The tumor is then reflected carefully from the right optic nerve.
When a large tumor projects beneath or surrounds a portion of the right optic nerve. it is usually possible to free the tumor from above and below the nerve, remove it from its loose attachment to the internal carotid artery, and roll it out from beneath the right optic nerve and carotid artery. In smaller tumors, the left optic nerve may be seen as the anterior capsule is depressed into the area of decompression. In large tumors it is sometimes best to identify the chiasm and the left optic nerve by dissection of the posterior capsule, taking great care to visualize directly any attachment to the anterior cerebral arteries or their branches. It is sometimes easier to remove tumor from under the left optic nerve because of direct vision into this area.
The A1 segment of the anterior cerebral artery and the anterior communicating artery complex may be surrounded by tumor. Usually the tumor can be removed from these arteries. In some of these patients a small portion of the tumor will need to be left if it is densely adherent. In some patients the arterial supply to the tumor will come off the A1 or A2 segments of the anterior cerebral artery, and great care must be taken to coagulate and divide this attachment and not avulse it from the artery.
As the tumor is removed from between the optic nerves and in front of the chiasm, arachnoid is encountered which may be thickened. Just beneath this is the pituitary stalk, which may have been displaced by the tumor. This structure can usually be preserved.
After tumor removal has been completed, the dura over the tuberculum and adjacent area is excised. Usually, this is all that is required, but on occasion there may be hyperostosis, which needs to be removed with a diamond burr.
After operation, vision improves in 60%, the same in 30%, and worse in 10%. 10% of patients has transient changes in mental function that recover. When diabetes insipidus occurs it is transient. The patients with radical subtotal removal have a small area of tumor left adherent to the internal carotid or anterior cerebral artery or under the optic nerve. Subtotal removal could be in 10% due to various causes.
Radiation therapy in use in 30% of patients, with subtotal removal and beginning regrowth of tumor and in whom operation had been done for recurrence. In 20% of cases undergoing postoperative radiation without effect. A long history of visual impairment does not preclude good recovery but improvement is more likely when the history is of relatively short duration.