SIA WITH OR without Zygomatic Osteomy

The SIA is indicated for an M-type tumor with or without extracranial extension (Fig. 39-2). The zygomatic osteotomy is not always necessary but is effective to spare retraction damage to the temporal lobe by turnover of the temporal muscle inferiorly, especially for a large tumor extending higher than the tentorial fold. In the supine position, the patient’s head is rotated 60 degrees with a downward axis. A curved hemicoronal incision along the auricle is followed by exposure of the zygomatic arch, which is cut in two points by a surgical saw. The craniotomy is created along the middle fossa base to expose the foramen rotundum and ovale epidurally. The middle meningeal artery is coagulated and cut at the foramen spinosum. The periosteal side of the dura mater is incised above the foramen rotundum, and the incision is extended above the foramen ovale and toward the superior orbital fissure.

FIGURE 39-2 Parasellar type of trigeminal neurinoma. Note the compressed cavernous sinus medial to the tumor.

The lower part of the tumor appears by this point, and the upper part can be exposed by tacking the parasellar dura superiorly (Fig. 39-3). The tumor is commonly covered by a thin semitransparent membrane, the so-called inner reticular layer, showing the course of the trigeminal nerves through it (Fig. 39-3A and B).

FIGURE 39-3 A 53-year-old male patient. A, Magnetic resonance imaging (MRI) with T2 weighting (left) and gadolinium enhancement in a parasellar tumor with a small bulge into the posterior fossa. B, A surgical photo after tacking the parasellar meningeal dura. The tumor is covered with a transparent membrane, the so-called inner reticular layer. C, After removal of this inner layer, the trigeminal nerves become clear. D, Pons and basilar artery observing through a widened MC. E, Postoperative MRI showing the intact temporal lobe and the cavernous sinus.

The nerves are clearly identified and separated from the tumor by removal of the membrane (Fig. 39-3C). In large tumors, the nerve fascicule is lost and the tumor is commonly wrapped by numerous isolated trigeminal nerve fibers. The surgical windows for tumor removal are created by mobilizing the nerve fibers.

The cavernous sinus is separated from the tumor by the medial wall of the inner reticular layer, and it does not open by careful preservation of the membrane. After tumor removal, the widened orifice of MC can be confirmed by cerebrospinal fluid discharge. The anterolateral aspect of the pons can be seen through MC (Fig. 39-3C). The opened MC is packed by the fascial flap at closure. The zygomatic arch is replaced and fixed by titanium plates.

There are two options:

Anterior Petrosal Approach

The technical details of the APA were described earlier. This approach is indicated for dumbbell-shaped MP-type tumors (Fig. 39-4) or P-type tumors (Fig. 39-5). A fishhook-shaped scalp incision is followed by dissecting a fascial flap of the temporal muscle available for tight dural closure. The temporal muscle is reflected anteriorly without incision. The craniotomy size is almost the same as that of the squamous suture. After elevation of the middle fossa dura from the skull base, the middle meningeal artery is cauterized and cut. The petrous apex medial to the greater petrosal nerve is commonly eroded by the tumor; thus, the petrous resection is easier. In a P-type tumor, the petrous apex must be resected completely. The middle fossa dura is cut into a T shape along the superior petrosal sinus (SPS), which is ligated and cut with the tentorium thereafter.

FIGURE 39-4 Large dumbbell-shaped (MP) neurinoma in a 26-year-old male before (A) and after (B) by the APA.

FIGURE 39-5 A 61-year-old male patient with a P-type tumor and a history of partial resection by the suboccipital approach. A, Magnetic resonance imaging with gadolinium enhancement before surgery. B, A contrast CT scan after surgery by the APA.

The posterior fossa part of the tumor is observed later. The orifice of MC, which the tumor occupies, is a thick dural ring formed by the petroclival fold; it is incised anteriorly along the tumor. The trigeminal nerves caging the tumor are fragile and are not covered with “inner reticular layer” because they are in the subarachnoid space. The GG is not clear in a large tumor. The tumor is removed from among the nerve fibers, and the nerve of the tumor origin must be amputated. A larger posterior fossa tumor can be removed through the petrous keyhole by retraction of the tumor margin toward the tumor attachment using a hooked tumor retractor. An advantage of this approach is visualization of the entry zone of the trigeminal nerve to which the tumor adheres. The course of the abducens nerve must be cared for in the inferomedial aspect of the tumor. The superior cerebellar artery and trochlear nerve course along the superior aspect of the tumor.

The facial–auditory nerve complex is not seen before tumor removal because it is pushed back by the tumor. Injury to the seventh to eighth nerves is very rare by this approach because these nerves are separated from the tumor by the cerebellopontine angle arachnoid and protected by the fan-like extended trigeminal nerve.

The complex can be confirmed by neuroendoscope after tumor removal. Also after tumor removal, the medial wall of MC, anterolateral aspect of the pons, and basilar artery complex are seen. For closure, a piece of abdominal fat is placed on the air cells of the petrous apex and rim of the craniotomy and is fixed with fibrin glue. Moreover, the cells are wrapped with a temporal fascia, which is sutured with dura thereafter. The double-layer closure seldom causes complication of cerebrospinal fluid leakage or subcutaneous cerebrospinal fluid accumulation.

Zygomatic Petrosal Approach

The ZPA is a combination of SIA with zygomatic osteotomy and APA, indicated for a large MP-type tumor (Fig. 39-6) or a tumor extending into three fossae (middle, posterior, and infratemporal fossae); that is, a tumor of the MPE type. The operation has five steps. A question mark–shaped skin incision, zygomatic osteotomy, and basal craniotomy exposing the superior orbital fissure, foramen rotundum, foramen ovale, and petrous apex comprise the first step of operation. Skull base resection along the infratemporal tumor extension and resection of the petrous apex comprise the second step. The third step is removal of the tumor from the middle and infratemporal fossae. The fourth step consists of a dural incision along the SPS, a tentorial incision, and the opening of MC. The last step consists of removal of the posterior fossa tumor and closure. After tumor removal, the three fossae can be observed from a single craniotomy along the trigeminal nerve. Retraction damage to the temporal lobe is uncommon because exposure of the brain is minimal.

FIGURE 39-6 A 21-year-old male patient with a huge dumbbell-shaped (MP) tumor. A, Magnetic resonance imaging (MRI) with T2 weighting. The brain stem is severely compressed by the tumor. B, MRI with gadolinium enhancement after surgery. Note the intact temporal lobe and the cavernous sinus, operated by the ZPA. The brain stem is recovered to the normal configuration.