Three transcranial approaches were used in the surgical treatment of lesions of the pituitary gland and sellar region: subfrontal, pterional, and subtemporal. The subfrontal approach was devised at the turn of the century by Krause and later modified by Frazier. Cushing. in his later years, used only this approach, to the exclusion of transsphenoidal surgery.
The subfrontal approach had two variants. The midline subfrontal approach was popularized by Ray in the 1950s for the performance of hypophysectomy. The principal advantage of this approach along the falx is that the surgeon can work readily between the optic nerves and easily identify the pituitary stalk and carotid arteries. The frontal sinuses are easy to manage, and short optic nerves (a "prefixed chiasm") are not a major problem; the solution is to remove the tuberculum sellae or to work behind the chiasm between the optic tracts. Neither of these manoeuvres is difficult.
The main disadvantage of the midline subfrontal approach is that it required heavy retraction of the brain. This is because the most medial portion of the inferior frontal lobe (the gyrus rectus) had to be retracted upward out of the olfactory groove to expose the sellar region. The possible consequences of heavy retraction led Ray and Patters to advocate an oblique approach over the roof of the orbit for simple pituitary adenomas. In this approach, the gyrus rectus remains in place while retraction is placed on the orbital frontal gyri, which lie more laterally on the floor of the anterior cranial fossa. The retraction is less than in the midline approach, but the access to the pituitary fossa between the optic nerves is angled rather than straight.
The pterional approach had been widely used in the surgical treatment of aneurysms. In the sellar region, this approach is used for many lesions, including chordomas, pituitary adenomas, meningiomas, and aneurysms of the upper basilar artery. When used in the removal of pituitary tumors, the pterional approach places the optic nerves and carotid artery in the line of the tumor, leaving only a small space in which to operate. The opposite optic nerve and carotid artery are poorly seen, and the required rotation of the head puts the nerves and arteries at an odd angle that can confuse the surgeon.
The subtemporal approach, first suggested by Horsley, is sometimes useful for pituitary tumors with significant retrochiasmatic extension. However, this approach has serious disadvantages.
The transsphenoidal approach to the pituitary gland is more direct and less time-consuming than a transcranial operation and is the preferred approach for hypophysectomy and for the resection of most pituitary adenomas. However, it is unsuitable if the pituitary tumor has significant extension into the middle or anterior cranial fossa. In such cases, the transcranial approach allows better access to the tumor.
Suprasellar extension of a pituitary tumor, even when massive, does not contraindicate the transsphenoidal approach, since the suprasellar portion of the tumor usually descends into the sella turcica as the intrasellar portion is being removed. If it will not do so spontaneously, a Valsalva manoeuvre administered by the anesthesiologist will briefly raise the intracranial pressure and encourage descent of the tumor. Most dumbbell-shaped tumors may also be dealt with transsphenoidally; the aperture in the diaphragma sellae is rarely so narrow as to prevent the descent and removal of the suprasellar portion of the tumor.
A transcranial operation is indicated if residual tumor has been left during a transsphenoidal operation and total resection is still desired. A transcranial operation allows better access to the tumor in most cases of craniopharyngioma and parasellar meningioma. It is also suitable for treating chordoma and meningioma of the clivus, as well as carotid and basilar aneurysms.
Midline Subfrontal Approach
In positioning the head, the trick is to mentally visualize the roof of the orbit, which is the path to the pituitary. The orbital roof should be extended 30 degrees from vertical toward the surgeon. This allows the brain to fall back from the orbital roof, which lessens the need for retraction. The head should be positioned without lateral rotation, because most surgeons find orientation easier if the head is straight rather than turned. The optic nerves are more elusive and the carotid and anterior cerebral arteries assume unexpected locations when the head is rotated.
In most cases the approach will be from the right side, the more convenient for a right-handed surgeon. The incision is made in the hairline; it can be quite low for a patient with a low hairline but must be quite far back for a bald patient. It extends from the right temple at about the level of the lateral canthus of the eye to perhaps 5 to 8 mm to the left of the midline. The opening in the skull need not be more than 5 cm on a side, but it must reach the midline and must be flush with the roof of the orbit. The most important bone cut starts from a burr hole placed at the point of attachment of the zygoma to the orbital ridge and parallels the orbital ridge flush with the roof of the orbit to another opening made with a trephine over the superior sagittal sinus just above the nose. In the process, the frontal sinuses are usually opened. Another saw cut follows the sagittal sinus posteriorly for about 4 cm and curves laterally, eventually terminating at the first burr hole.
Opening the dura mater and retracting the frontal lobe are facilitated by draining spinal fluid through a previously placed lumbar needle. This relaxes the brain, reduces the need for retraction, and prevents spinal fluid from welling up into the operative field. If the head is tilted back sufficiently, the brain tends to fall away from the roof of the orbit. Additional room can be provided by using osmotic diuretics and hyperventilation. An effort should be made to preserve draining veins from the frontal lobe to the superior sagittal sinus, even if they must be put on stretch to achieve adequate exposure.
The midline approach proceeds along the falx. The olfactory nerve was routinely coagulated and divided. If the brain was particularly full, if the operation was expected to be long, or if extensive exposure was required, it was prudent to resect a few grams of the undersurface of the frontal lobe in order to gain early exposure of the sellar region. The biggest danger of the surgery was postoperative swelling in the frontal lobe. The degree of swelling is directly related to the force and duration of retraction.
The olfactory tract leads to the optic nerves as the brain is gradually retracted. Once the ipsilateral optic nerve is located, the arachnoid that binds the frontal lobes to the nerves is divided with microscissors or a sharp dissecting instrument, thereby exposing both optic nerves and the chiasm. In dealing with pituitary adenomas and tuberculum sellae meningiomas, the surgeon should completely expose both optic nerves and the chiasm early in the operation to avoid unintentionally injuring an unseen nerve. Often, exposure of the chiasm can be facilitated by dividing the arachnoid over the carotid artery and out the sylvian fissure. This separates the frontal and temporal lobes, so that the frontal lobe can fall back farther.
Access to the sella turcica may be blocked by a prefixed optic chiasm and a large tuberculum sellae, particularly in cases of craniopharyngioma.
undergoing transfrontal hypophysectomy, the tuberculum had to be removed to gain adequate-working space. To do this, a flap of dura is peeled off the tuberculum and a chisel is used to knock a hole through the thin bone into the sphenoid sinus. The sphenoid mucosa is fairly thick and often can be pushed away without being perforated. Long, delicate bone instruments or a diamond drill are used to remove the tuberculum and anterior wall of the sella. Care must be taken in separating the dura from the bone to avoid entering the venous sinuses. After the bone is removed. it is possible to coagulate the circular sinus and divide the dura. thus providing a good view of the contents of the sella. The pituitary stalk is identifiable by its shape and orientation and also by the portal veins. which impart a longitudinally striped appearance to the stalk.
In the now rarely performed operation of subfrontal hypophysectomy. the sella is entered by dilating the hole in the diaphragma sellae after dividing the stalk flush with the diaphragm. The contents of the sella are then removed with an assortment of ring curettes. Particular attention is paid to the region under the right optic nerve, where fragments of gland are most likely to be left behind. The cavity of the sella can be treated with alcohol or some other fixative after the gland is removed.
For large subfrontal meningiomas, it may be necessary to remove a small portion of frontal lobe to "uncap" the tumor. It has never been found necessary to perform a bifrontal exposure of these tumors, even those as large as 200 g. There always seems to be enough space to remove all the tumor through a unilateral approach. If the surgeon feels more comfortable with a bifrontal exposure, then the falx is cut above the crista galli. The frontal lobes can both be retracted or can even be retracted separately. The tumor is evacuated with a cutting loop, a laser, or an ultrasonic aspirator. An effort is made early to interrupt the tumor's blood supply, which springs from the floor of the anterior cranial fossa.
In the removal of subfrontal meningiomas. the greatest risk to the patient comes from injury to the anterior and middle cerebral arteries. Sometimes the mass greatly stretches these arteries, with the result that they are mistaken for inconsequential feeders of the tumor. The surgeon should not cut or divide any arteries attached to the tumor capsule unless it is absolutely certain that they do not supply an important area of the brain.
It is sometimes possible to remove craniopharyngiomas between the optic nerves without any further exposure. However, in many cases a prefixed optic chiasm will block access to the tumor. In such cases, the tuberculum sellae should be removed so that fragments of tumor can be pushed down into the sphenoid sinus rather than being pulled between short optic nerves, with resulting trauma to the nerves. A second trick that facilitates removal is to expose the tumor between the optic tracts behind the optic chiasm and through the lamina terminalis. The tumor then can be pushed down, away from the chiasm and hypothalamus, and out through the sphenoid sinus. The main problem comes with cystic tumors, because tugging on a thin cyst wall is likely to result in a tear that leaves a fragment behind, attached to the hypothalamus.
In closing the wound, the principal challenge is presented by the open sinuses. Fat can be packed into the sphenoid sinus and a few stitches placed in the dural flap to hold the fat in place. The frontal sinus can be closed with a plug of fat or gelatin foam, and then a flap of periosteum can be turned down from the underside of the scalp flap and sutured to the dura. It is not necessary to remove the mucosa from the sinus; the surgeon need only push it down into the sinus and then place the packing on top of the mucosa. Spinal fluid that has been kept in sterile syringes is reinjected through the spinal needle at the end of the procedure to flush out air and any blood that may have run down into the basilar cisterns.
Oblique Subfrontal Approach
The incision and flap are exactly the same as described above, except that the approach is over the roof of the orbit instead of along the midline. This requires less retraction of the frontal lobe and possibly carries less risk of stretching the opposite olfactory tract with consequent anosmia. The disadvantage is that the cavity of the sella is less accessible, so the surgeon must work over the right optic nerve. In most pituitary adenomas this is not a major problem, and the approach could be recommended.
In dealing with a pituitary adenoma. it is important to make a large opening in the stretched diaphragma sellae, which comprises the capsule of the tumor. This will prevent any postoperative bleeding in the tumor cavity from reinflating the tumor capsule and causing sudden loss of vision. A large opening allows any blood or exudate to leak into the subarachnoid spaces rather than becoming loculated in the tumor capsule. The tumor itself is evacuated with suction, laser, and ring curettes.
In many cases the capsule of the tumor will fall away from the optic nerves, but if it does not, it is wise to leave the capsule adhering to the nerves. Sometimes stripping the capsule from the nerves will strip away their blood supply, thereby damaging vision. Normal optic nerves can stand a fair amount of manipulation, but this is not true of nerves stretched by tumor. The slightest trauma may result in visual loss.
In the pterional approach, the sphenoid ridge is drilled off until the landmark of the orbitomeningeal artery is reached. After the dura mater is opened, the sylvian fissure is split, either from the medial to the lateral aspect or from the lateral to the medial. Splitting the fissure allows retraction of the frontal lobe with less distortion of the brain. The medial side of the temporal lobe is dissected free off the third nerve and the edge of the tentorium. and the carotid artery is retracted medially, using a narrow self-retaining retractor. For adequate exposure, the sylvian fissure must be split much more widely than is necessary for aneurysms of the anterior circulation. Unless the fissure is widely split, the posterior clinoid process may appear to block the exposure. Most of the clivus can be seen, and chordomas and meningiomas, which occur in this location, can be removed totally or in part. This has been found to be the safest approach to aneurysms of the upper basilar artery .
In the subtemporal approach, a temporal craniotomy is performed and a craniectomy is carried to the floor of the middle cranial fossa. The dura mater is opened, and the temporal lobe is retracted superiorly to provide access to the sellar region.
Although this approach may be useful for lesions with retrochiasmatic extension. its disadvantages are considerable. Heavy retraction of the temporal lobe is required, and structures in the prepontine cistern are poorly seen. The surgeon can see the ipsilateral posterior cerebral and superior cerebellar arteries well enough, but their counterparts on the opposite side and the opposite third nerve are difficult or impossible to see. Also. the ipsilateral third and fourth nerves are in the way and are therefore prone to injury.
The principal operative complication of the subfrontal approach was postoperative edema of the frontal lobe, which was caused by excessive brain retraction. This must be avoided at all costs. If mannitol and spinal drainage do not provide sufficient exposure, then it is safer to resect a few grams of frontal lobe than to apply heavy retractor pressure for an hour or two.
Other preventable complications include injury to the optic nerve, the third nerve, and the cerebral arteries. As emphasized above, a stretched or impaired optic nerve should never be manipulated; to do so is to risk further visual loss. The damage to the arteries occurs because the surgeon fails to recognize them for what they are. The anterior cerebral and other arteries may be embedded in a meningioma or may be so stretched and distorted that they are taken for tumor vessels. Any artery on or in the tumor must be preserved until it is confirmed as an artery feeding only the tumor.
Anosmia was a distinct risk in subfrontal surgery. The olfactory tract on the right often was divided, and if osmotic diuretics, spinal drainage, and old age cause the left frontal lobe to fall back, the other olfactory nerve may suffer avulsion.