Anatomy

The trigeminal nerve courses in three compartments: middle cranial fossa, posterior fossa, and extracranial space. Therefore, tumors extending into multiple fossae are common (58%)¹ (Table 39-1). The most common origin is around the Gasserian ganglion (GG), showing a dumbbell-shaped tumor extending both middle and posterior fossae (MP type, 38.6%) through Meckel’s cave (MC). MC is a subarachnoid space depressed from the posterior fossa, separated from the cavernous sinus by a thin meningeal dura (dura propria), and opened freely to the posterior fossa containing loose trigeminal nerve bundles without perineural covering.² However, the trigeminal nerves peripheral to the GG course in the interdural space, are wrapped with a so-called inner reticular layer (perineurium), and are easily separated from the lateral wall of the cavernous sinus by a cleavage plane³ (Fig. 39-1A).

Table 39-1 Location and Type of Tumor (MPE Classification)

FIGURE 39-1 A, Parasellar trigeminal neurinoma based on the meningeal anatomy. The tumor is located between the meningeal dura (break line) and the cavernous sinus, wrapped with inner reticular layer (dotted line). Incision of the periosteal dura (broad line) on the foramen rotundum and ovale (arrow) offers tumor exposure without exposing the temporal lobe. B, An illustrative anatomy of MC. The orifice is covered by two meningeal folds: the petroclinoid fold (tentorium) and the petroclival fold. The inferolateral margin is surrounded by the petrous apex.

Therefore, the dumbbell-shaped tumor is spreading over the two anatomically different spaces: the interdural space anteriorly and the subarachnoid space posteriorly. The anterior part of the tumor can be removed through the interdural space by Dolenc’s approach.⁴ A small posterior fossa tumor can be removed through the orifice of MC; however, resection of the petrous apex, which obstructs the surgical field, is commonly necessary to observe the posterior fossa sufficiently. If the posterior fossa tumor is larger than 1 cm, the bone resection of the petrous apex plus detachment of the tentorium by the anterior petrosal approach (APA) is necessary.

Resection of the petrous apex may offer a surgical effect to widen the surgical space, as well as to decrease the tension of the trigeminal bundle bent by the petrous apex (Fig. 39-1B). The posterior fossa (P) type of tumor commonly attaches on the trigeminal nerve bundle alone, and it can be removed by the APA,⁵ even when the tumor is large.

Selection of Surgical Approaches

Every type of trigeminal neurinoma can be removed safely by the following two approaches, plus their combination. The authors do not use the suboccipital approach to spare injury to the facial and auditory nerves.

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

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