The pterion is located in the temporal fossa and defines the point of junction of the parietal bone, squamous part of the temporal bone, greater wing of the sphenoid bone, and the frontal bone [4]. These bones are joined by the sphenoparietal, coronal, and squamous sutures which meet at the pterion. The anterior Sylvian point, an ideal starting point for the opening of the Sylvian fissure due to its cisternal enlargement, lies just behind the pterion. The inferior frontal gyrus is located between the pterion and the superior temporal line.

A recent study found the pterion to be located within a 1 cm diameter, 2.6 cm behind and 1.3 cm above the posterolateral margin of the frontozygomatic suture in most adults [5]. As it is easily palpable, the frontozygomatic suture serves as a practical external landmark to define the pterion. The thickness of the bone at the pterion as well as the relationship to the underlying middle meningeal artery is reported to be highly variable [5]. The most important structures to be aware of upon soft tissue dissection are the temporal branch of the facial nerve and the superficial temporal artery (STA).

Before exiting through the fascia of the parotid gland, the temporal branch of the facial nerve divides into an anterior, middle (frontal), and posterior ramus. The anterior ramus mainly innervates orbicularis oculi muscle, the frontal ramus innervates the ipsilateral frontalis muscle, and the posterior ramus innervates the tragus and anterior and superior auricular muscles. Of note, the number, usually between one and four, of rami and their pattern of innervation are highly variable.

In most cases, the temporal branch of the facial nerve runs in the superficial musculoaponeurotic system (SMAS) over the zygomatic arch, continuing initially in a superficial layer of fat between the temporoparietal and the deep temporal fascia. The deep temporal fascia consists of one layer at the level of the superior temporal line where it blends with the periosteum. However, at the orbital level, it consists of two layers separated by an intermediate fat pad. A recent study suggests that because of this fat pad, the point where the fascia splits differs at the anterior, middle, and posterior portions of the zygomatic arch. There is significant variability regarding the course of the temporal branch between these layers of fascia and fat, and the optimal method of dissection to avoid neural injury remains controversial. Finally, to reach their target muscles, the terminal twigs of the temporal branch penetrate the galea. Since the temporal branch of the frontal nerve usually has no anastomotic connections, spontaneous functional recovery after injury is generally poor.

The STA originates from the external carotid artery (ECA) as a terminal branch. After its origin from the parotid gland, it courses superficially over the posterior root of the temporal bone zygomatic process and continues for 4–6 cm until it divides into a frontal and temporal branch. Injury to the STA can frequently lead to atrophy of the temporal muscle and in rare cases lead to the formation of an iatrogenic pseudoaneurysm.

The anterior part of the Sylvian fissure and the surrounding temporal and frontal lobes are the first relevant intracranial structures that are visualized after craniotomy and dural opening. The inferior frontal gyrus consists of the orbital, triangular, and opercular parts. In the dominant hemisphere, the opercular part is considered to be highly eloquent for it comprises Broca’s area, a region involved in language-relevant semantic tasks. The superior temporal gyrus (STG) borders the Sylvian fissure inferiorly. In its opercular part, the STG harbors the transverse gyri with their primary auditory cortex. This region remains unexposed when a mini-pterional approach is fashioned as opposed to a classic large pterional craniotomy.

Great variability exists regarding the pattern of the venous drainage for the Sylvian fissure. The superficial middle cerebral vein (SMCV) drains typically anteriorly and caudally toward the skull base. After turning medially, it joins the anterior portion of the cavernous sinus. Alternatively, it may also join the superior petrosal sinus or the basal vein of Rosenthal. Anastomotic relations to the superior sagittal sinus through the plexus of Trolard and the transverse sinus through the vein of Labbé are common, but also hypoplastic SMCVs are seen not infrequently. Some authors suggest that an approach-related compromise of the SMCV might be the single most important factor contributing to morbidity in patients undergoing pterional craniotomies for aneurysmal repair [6]. The mini-pterional approach reduces the surface of potential venous injury compared to conventional craniotomies.

The arterial contents of the Sylvian fissure from lateral to medial are distal branches of the middle cerebral artery (MCA), the M1–2 bifurcation of the MCA, and the carotid bifurcation with the optic-carotid triangle and the carotid-oculomotor triangle as its neurovascular landmarks.

Meningiomas of the sphenoid ridge frequently show “en plaque” growth and prominent dural involvement. In some meningiomas, extensive bone invasion (hyperostosis) of the orbital roof, lateral orbital wall, and the orbit itself occurs. In these cases, loss of visual acuity and exophthalmos can be presenting symptoms.

Tumors located on the outer sphenoid ridge can for practical reasons be considered to behave essentially like meningiomas of the convexity. Their clinical presentation and surgical management does not differ from that of convexity meningiomas. In such cases, a mini-pterional craniotomy is usually not feasible as these tumors can grow quite large before they create symptoms, and a more extensive craniotomy is often required to completely resect the dural base of the tumor.

The vascular supply of lateral tumors mainly consists of feeding vessels arising from the STA and middle meningeal artery but sometimes includes anterior meningeal and branches of ethmoidal arteries.

Tumors located more medially on the sphenoid ridge may compress and encase the carotid artery and its branches, optic apparatus, or pituitary stalk. These tumors may also grow into the dural layers overlying the cavernous sinus and in the cavernous sinus itself and, therefore, typically become symptomatic at an earlier stage of the disease.

Neurologic compromise, especially cranial nerve deficits, after removal of intracavernous meningiomas is prohibitively high [7, 8]. There is growing agreement that these tumors should be resected less aggressively and any remnants that are left behind closely followed with serial imaging or treated with radiosurgery. These medially located meningiomas are mostly fed by direct branches of the ICA or ascending pharyngeal artery and sometimes by a recurrent branch of the ophthalmic artery through the superior orbital fissure.

Any useful classification should help the surgeon in choosing an appropriate treatment strategy for the lesion. The most basic approach is to differentiate between globular tumors and en plaque (sphenoorbital) tumors.

Originally, the first classification of sphenoid meningiomas was proposed by Eisenhard and Cushing [9]. Later classifications reflected the involvement of the cavernous sinus, the extent of hyperostosis, and en plaque growth. The classification (Table 20.1) by Al-Mefty further categorizes medial tumors regarding their spatial relation toward the carotid cistern and their point of origin on the clinoid process [10]. This illustrates that a strict differentiation between medial sphenoid meningiomas, clinoid meningiomas, and meningiomas of the optic foramen and sheath is often not possible.

Our strategy regarding the choice of approach is guided by the size of the tumor, its location on the sphenoid ridge, arterial relations, bone invasion with or without orbital and middle fossae involvement, and finally involvement of the cavernous sinus beyond the infiltration of the outer wall (Table 20.2).

Effective keyhole surgery depends on exquisite attention to details, meticulous microsurgical technique, and ample preoperative planning and evaluation of imaging to select the optimal approach. Keyhole surgery utilizes simple but maximally effective approaches that minimize extensive skull base resection and unnecessary static brain retraction while utilizing the endoscope to assist with improved visualization. Simplifying the steps associated with a procedure decreases the opportunities for mistake, saves operative time, facilitates closure, and provides the patient with a low-morbidity alternative to classic skull base approaches.

The pterional craniotomy, championed by Yasargil [11, 12], is one of the classic approaches used for pathology involving the anterior and middle cranial fossae. Although the pterional craniotomy provides wide exposure to the skull base, the full exposure afforded by this approach is rarely needed or used. The approach involves a large C-shaped incision, temporalis muscle elevation (commonly resulting in devascularization, denervation, and subsequent atrophy of the muscle), extensive skull base drilling, and potential for injury to the frontal and temporal lobes [13–16].