Subfrontal

The oblique subfrontal operation is of key historical significance because it has been in continuous use since Charles Frazier first described it in 1913 (21). This route was adopted by Harvey Cushing, who used an extradural variant, an operation eloquently described by Henderson (7).

Despite the changing indications for pituitary surgery and various modifications to the flap, the arguments in favor of the oblique subfrontal approach remain entirely relevant in the 21st century. The approach gives excellent bilateral access to the optic nerves and the chiasm. The superior sagittal sinus is not divided, and the falx is left intact. Because the approach is off the midline, it is unnecessary to enter the frontal sinus if it is small. The unilateral nature of the operation limits the retraction that is required, thus minimizing the risk of injury to the olfactory tracts and the frontal lobes.

The patient is positioned supine, with a small degree of head up tilt to aid venous drainage. The head is in slight extension to encourage the brain to relax away from the skull base. The Mayfield pinholder provides rigid fixation and acts as a base for intrancranial retraction, but it is just as safe to use a padded horseshoe. A number of incisions can be used, but in the patient who has a normal hairline, a bicoronal incision is cosmetically acceptable. Midline forehead incisions (21) heal almost invisibly, and it is flap resorption and burr holes that are more often the cause of cosmetic problems if rigid flap fixation is not used. Whether titanium plates or other fixation devices improve the cosmetic results in the long-term, only time will tell.

With a high-speed drill, a small entry hole is made in the skull. This is made in the anterosuperior margin of the right temporalis muscle, just below the superior orbital ridge. In making the flap, it is important to take the anterior bone cut as close to the anterior fossa floor as possible. This can be carried to within a centimeter of the midline, before turning the craniotome superiorly.

We use an osteoplastic flap, which is more prevalent in UK practice. The flap is hinged on the temporalis muscle. If the frontal air sinus is entered, the mucosa is stripped from the flap and a formal repair is made with temporalis fascia and tissue glue at the end of the procedure. The dura is opened in a Y-shaped fashion. It should be noted that at the frontal tip, a large dural draining vein can easily be damaged if the incision is too bold.

Gentle retraction can now be applied under the frontal lobe. If the bone flap incision is made correctly, little elevation of the brain is required—and the less the better. It is most accurately done using the microscope, which is brought in at this stage. Care should be given to preserving the olfactory tract by mobilizing this as much as possible. If done without due care, a small artery bleeds persistently, which can obscure all further attempts at fine dissection, and the patient may well be rendered anosmic.

Space is created by gently advancing the brain retractor tip toward the right optic nerve. Once this structure is seen, its covering of arachnoid is sharply incised to allow the release of CSF. As CSF is drained away, the brain slackens and further gentle retraction can be used.

The characteristic bony anatomy of the anterior fossa floor and the sphenoid wing is extremely helpful in guiding the operator to the optic nerves. We have, therefore, found interactive image guidance of limited use during this particular operation.

Anterior Pituitary Optic Nerve

Fig. 1. The schematic view from the right anterior oblique approach of the skull and parasellar regions. The view includes olfactory tracts, optic nerves and chiasm, internal carotids, anterior and posterior margins of the sella, pituitary stalk, and top of gland. The corpus callosum, roof of the third ventricle with Monro's foramen, and the mouth of the aqueduct are also shown. Not that the right optic tract and part of the upper right chiasm have been removed at the broken line to show the stalk behind. Lesions at 1 are relatively straightforward and are approached either by the subfrontal or pterional routes. Lesions at 2 require more planning but can be approached from subfrontal (particularly if the chiasm is not prefixed) or infratemporal/orbito-zygomatic routes, or by extended transsphenoidal approach. Lesions at 3, in the third ventricle are probably best approached through the lamina terminals or even by the transcallosal route.

Fig. 1. The schematic view from the right anterior oblique approach of the skull and parasellar regions. The view includes olfactory tracts, optic nerves and chiasm, internal carotids, anterior and posterior margins of the sella, pituitary stalk, and top of gland. The corpus callosum, roof of the third ventricle with Monro's foramen, and the mouth of the aqueduct are also shown. Not that the right optic tract and part of the upper right chiasm have been removed at the broken line to show the stalk behind. Lesions at 1 are relatively straightforward and are approached either by the subfrontal or pterional routes. Lesions at 2 require more planning but can be approached from subfrontal (particularly if the chiasm is not prefixed) or infratemporal/orbito-zygomatic routes, or by extended transsphenoidal approach. Lesions at 3, in the third ventricle are probably best approached through the lamina terminals or even by the transcallosal route.

The lesion is next identified between the optic nerves, although the left nerve is often not visible at this stage. The capsule of the tumor is gently freed from the right optic nerve, and any further adhesions between it and the frontal lobe surface are freed with sharp and blunt dissection. As we have emphasized, every effort should be made to minimize manipulation of the optic nerve. Luckily, adenomas usually do separate easily, although meningiomas often have a close pial involvement. Caution is the watchword here.

Working between the optic nerves, the tumor can be entered. Gentle bipolar coagulation is applied, followed by a horizontal incision just above the planum.

Table 2

Transcranial Approaches to Pituitary Region Tumors

Approach

Selected references Features

Subfrontal

Oblique subfrontal

Midline subfrontal

Transglabellar

Lateral

Transcranial epidural

Frazier 1913 (21) Henderson 1939 Ray 1968 (23) Symon 1988 (22)

Choux and Lena

1998 (25) Fahlbusch 1999 (26)

Perneczky 1999 (27)

Pterional

Pterional or frontolateral

Dandy 1932 Van Alphen 1975 (20) Fahlbusch 1999 (26)

Dolenc 1997 (10)

Various skin incisions, including bicoronal. Low subfrontal approach via a small osteoplastic craniotomy. Osteoplastic flap hinged laterally. Frontal sinus not always opened. Superior sagittal sinus and falx not divided.

Extradural approach initially described by Frazier. Modified to the intradural route in 1919. Taken up by Cushing in 1929.

Bicoronal incision and bicoronal bone flap. Frontal sinus opened. Superior sagittal sinus and falx divided. Risks of bilateral frontal lobe retraction and olfactory tract retraction.

Spectacle incision within eyebrows. Anterior wall of frontal sinus removed as free flap. Access to intradural compartment via posterior wall of frontal sinus.

Frontolateral craniotomy.

Sylvian fissure split to expose retrochiasmal tumor. Adopted by Dandy in 1920. Limitations imposed by the intervening internal carotid and its branches.

Pterional craniotomy; extradural bone removal and reflection of the outer layer of the cavernous dura. Microsurgical dissection of cavernous sinus.

Table 2 (continued)

Approach

Selected references Features

Subtemporal

Transtemporal

Cushing 1914 (29)

Symon and Sprich 1985 (30)

Third ventricle

Trans-lamina terminalis

Transcallosal

King 1979 (33) Bhagwati 1990 (34) Suzuki 1998 (35)

Ture 1997 (36)

Temporal craniectomy.

Developed in dogs by Horsley to reach the gasserian ganglion. two-stage operation. High morbidity from temporal lobe retraction and cranial nerve deficits. Found impractical by Cushing. Now obsolete.

Frontotemporal craniotomy. 2-cm anterior temporal lobectomy. Access to interpeduncular fossa. Employed for craniopharyngiomas.

An addition to the subfrontal approach. Useful with a prefixed chiasm to access a third ventricular component

Parasagittal bone flap. Callosal incision.

The tumor is then gutted. With adenomas, this is often easy with a sucker and small tissue rongeurs. The Angell-James are favored by the authors but are sadly no longer made. Meningiomas may need more robust dissection, and craniophar-yngiomas can usually be emptied before the membrane is removed.

With the tumor gutted, it is often simple to peel the capsule out of the back of the cavity, preserving the pituitary stalk. The left optic nerve and both carotid arteries have been seen by this time and treated with due respect. Usually there is little bleeding; what there is can be controlled with patties and bipolar coagulation.

One source of potential difficulty during the subfrontal approach is the prefixed chiasm. When the optic nerves are short, the chiasm lies directly between the surgeon and the tumor. In this case, access below the chiasm can be improved by drilling through the tuberculum sellae to enter the sphenoid sinus (23). An alternative is to reach the intraventricular tumor above the chiasm via the lamina terminalis (see the following section). The sacrifice of an already-blind optic nerve to improve access, a maneuver described by Walter Dandy, is an absolute last resort and better consigned to the past (24).

Midline Subfrontal

A symmetric midline flap with division of the superior saggital sinus and the falx is an option, but for the reasons stated, the author prefers the unilateral approach, especially for pituitary adenoma. The operation has been applied to craniopharyngiomas (25,26) and is also useful for tumors sited more anteriorly, such as certain planum sphenoidale meningiomas. It may also be used if a translamina terminalis approach is planned. The view of the third ventricle is then obtained in the midline.

Transglabellar

In patients with a large frontal sinus, we have found the transglabellar route to be useful. This employs the frontal sinus as an entry point. A spectacle-type incision is sited within the eyebrow on each side, with a short horizontal component crossing the bridge of the nose; this incision heals without sign. The scalp flap is retracted upward to expose the glabella. A minicraniotomy free flap is fashioned from the anterior wall of the frontal sinus using the high-speed cran-iotome. The posterior wall of the sinus is then opened, and the superior sagittal sinus is coagulated or tied at its origin before being divided. On opening the dura, the operator can proceed along the floor of the anterior fossa toward the chiasm and the related tumor, which are dealt with as described in the preceding section (27). Again, care is required to protect the olfactory nerves.

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