Pterional Approach

25 Pterional Approach

Jean G. de Oliveira, Paulo A. S. Kadri, Miguel Giudicissi Filho, José Carlos Esteves Veiga, and Luis A. B. Borba

Abstract

In the 1970s, Yasargil described and eternalized the interfascial pterional (frontotemporosphenoidal) craniotomy, which became the most common approach in the neurosurgical world. For this approach, the pterion is the center of the craniotomy, and the drilling of the sphenoid wing, in addition to the wide splitting of Sylvian fissure, is paramount to minimize brain retraction. The pterional transsylvian approach allows neurosurgeons to access the frontotemporal lobes, Sylvian fissure, and basal cisterns to achieve the microsurgical treatment of cerebrovascular and tumoral lesions. This chapter summarizes the historical landmarks of the frontotemporal approach evolution and provides stepwise description of this fundamental neurosurgical procedure and its surgical applications.

Keywords: craniotomy, frontotemporal approach, pterional approach, Sylvian fissure, microsurgery

25.1 Introduction

In the 1970s, Yasargil published the classic description of the interfascial pterional (frontotemporosphenoidal) craniotomy, which was popularized simply as pterional approach by several publications that emphasize its indications and advantages, becoming the most common craniotomy performed in neurosurgery.1 ^, ² ^, ³ ^, ⁴ ^, ⁵ ^, ⁶ ^, ⁷ ^, ⁸ The term pterion is derived from the Greek root pteron, meaning “wing,” and refers to the wings attached to the head of Hermes, the messenger of the Greek gods.⁹ Anatomically, pterion is the intersection area of the frontal, temporal, parietal, and sphenoid bones (Fig. 25.1). Having the pterion as its central craniometric point, this craniotomy requires drilling the sphenoid wing up to the superior orbital fissure, with or without the removal of the anterior clinoid process and the orbital roof. This bone removal allows a broad exposition and opening of the Sylvian fissure and basal cisterns, providing a wide range of routes to treat cerebrovascular and tumoral lesions.

Fig. 25.1Illustrative representation of the pterion (red circle), the intersection area of the frontal, temporal, parietal, and sphenoid bones.

25.2 Historical Landmarks

Heuer¹⁰ and Dandy¹¹ performed the pioneering descriptions of frontotemporal craniotomy. In 1964, Hamby was the one who introduced the term “pterional” approach for decompression or tumor removal in the orbit.¹² Hayes, one of Dandy’s students, published a frontotemporal approach referred to as a “small Dandy pituitary type of flap” to operate an anterior communicating artery aneurysm.¹³ Other Dandy’s fellows reported on the results of the surgical treatment of intracranial aneurysms using a frontotemporal craniotomy.¹⁴ Kempe and co-workers described a modification of the pterional flap for anterior communicating artery aneurysms.¹⁵ ^, ¹⁶

Although other authors have published their early experience using a microscope in neurosurgery, Yasargil was the one who introduced the surgical microscope as a standard procedure into the neurosurgery,¹ ^, ² ^, ³ ^, ¹⁷ becoming the father of modern neurosurgery.¹⁸ The higher magnifications and luminosity provided by neurosurgical microscopes, associated with the development of microinstruments, bipolar microcoagulation, and microdrills, allowed the microdissections through the cisterns, which are natural spaces, avoiding transgression of the pial surface into the brain parenchyma.⁸

25.3 Indications

Tumors and cerebrovascular lesions arising from the sellar/parasellar area, anterior and anterolateral circle of Willis, middle cerebral artery, anterosuperior aspect of the brainstem, upper basilar artery, insula, basal ganglia, mesial temporal region, anterior cranial fossa, orbit, and optic nerve can be surgically treated using the pterional approach.¹⁹ ^, ²⁰ ^, ²¹ ^, ²² ^, ²³ ^, ²⁴ ^, ²⁵ ^, ²⁶ ^, ²⁷ ^, ²⁸ ^, ²⁹ ^, ³⁰

25.4 Step-by-Step Procedure

25.4.1 Positioning

Under general anesthesia and after the neurophysiological monitoring setup, the patient is placed in a supine position with the shoulders at the operating table’s limit. The head should be positioned above the level of the thorax to turn the venous drainage physiological. The fixation of the head may be performed using a head holder device. We prefer the Mayfield head holder with three-point fixation to the skull, where one pin should be placed in the ipsilateral mastoid while the remaining pair of pins should be placed in the contralateral side, avoiding the temporal muscle. The head is slightly elevated, and the malar eminence should be placed in the superior point of the operating field (Fig. 25.2). The rotation and extension of the head should be tailored to each patient, according to the area to be reached and the lesion to be treated. Lesions around the clinoid do not require a great extension since this movement may leave this region far from the shallow. Care must be taken to avoid extreme positions that may cause compression on the trachea, jugular veins, carotid, or vertebral arteries.³

Fig. 25.2 (a, b) Fixation and head positioning. Mayfield head holder with three-point fixation to the skull. The malar eminence should be placed in the superior point of the operating field. The rotation and extension of the head are tailored to each patient.

25.4.2 Trichotomy

Although wider trichotomies (Fig. 25.2) will be solved in few weeks after hair growth, some patients prefer to have the minimum area shaved on their heads. In this regarwith the temporoparietal fasciad, we perform the trichotomy only to the extent of skin incision, which means an arcuate area from the tragus to the median pupillary line with 3 cm in width (Fig. 25.3). If the lesion to be reached is located close to the midline, the shaved area should reach the midline, or even cross the midline, up to the contralateral pupillary line or contralateral superior temporal line. If the lesion extends more posteriorly, the arcuate area should be more posterior over the ear.

Fig. 25.3 (a–c) Minimal shaved area and the arcuate curved frontotemporal skin incision (red dashed line) in three patients who underwent a pterional approach.

25.4.3 Skin Incision

The skin incision depends on some variables such as the anatomical area of interest, the cranial bone and sinus location and size, and the hairline implantation. Usually, the skin incision starts at the zygomatic arch level, just 1 cm anterior to the tragus as a straight line up to the superior border of the ear, where it becomes arcuate superiorly and anteriorly up to the hair implantation at the level of the pupillary line (Fig. 25.3). It is recommendable to dissect the superficial temporal artery (STA) at the early stage of the skin incision to preserve at least one of its branches, especially when a bypass (STA-MCA) may be required during the neurosurgical procedure (i.e., moyamoya, giant aneurysms) (Fig. 25.4). In addition to preserving STA, care must be taken to avoid injury to the facial nerve branches. The hemostasis of the skin flap may be performed using bipolar coagulation and Raney clips. The cutaneous flap is separated from the muscle through a sharp dissection and placed anteriorly using traction hooks. Alternatively, if the anatomical exposition is required, the skin incision may reach the midline or even cross it, up to the contralateral superior temporal line.

Fig. 25.4Cadaver specimen dissection. (a) Skin incision starting at the tragus. (b–d) Subcutaneous dissection and superficial temporal artery (STA) preservation from its trunk to its branches. (e–h) Cutaneous flap separation from the muscle.

25.4.4 Interfascial Dissection

There are five soft tissue layers from the surface to the depth: skin, subcutaneous tissue, galea aponeurotica, loose areolar tissue, and pericranium (Fig. 25.5). Below the superior temporal line, the galea aponeurotica is in continuity with the frontalis muscle anteriorly, with the occipitalis muscle posteriorly, and merges laterally with the temporoparietal fascia. At the superior temporal line level, the loose areolar tissue separates the galea aponeurotica from the pericranium, splitting the temporoparietal fascia from the temporal fascia covering the temporal muscle below the superior temporal line. At this point in the superior temporal line, the pericranium joins the temporal fascia above the temporal muscle. Then, the temporal muscle splits into superficial and deep layers, with the interfascial fat pad in between.³¹

The temporalis muscle is protected by a deep fascia that contains its vascularization and innervation, and a superficial fascia, which splits into an external sheet and an internal sheet, only in its most anterior portion.32 There are three identifiable fatty layers, which may help in preventing damage to the frontotemporal branch of the facial nerve: the supratemporal layer, which is located above of the outer sheet of the superficial temporal fascia; the interfascial fatty layer, situated between the two layers of the superficial temporal fascia; and the fat layer subfascial that is located below the inner leaf of the superficial temporal fascia, between this fascia and the temporal muscle.

The frontal branch of the STA runs through the supratemporal fat, as well as the frontal branch of the facial nerve. An innominate vein runs through the interfascial fat layer, which is usually coagulated and cut when performing an interfascial dissection. It is essential to keep away from the facial nerve branches, which are positioned approximately 1 cm from the frontal branch of the STA. The temporal branch of the facial nerve originates three main components at the level above the zygomatic arch: the auricular (posterior), the frontal (medium), and the orbicular (anterior). The latter two branches innervate the frontalis and orbicularis muscles, respectively, having clinical importance. Consequently, this step is paramount to avoid lesion on the frontotemporal branch of the facial nerve and bad cosmetic results.

Using a blade, a straight or slightly curved incision is made on the superficial fascia of the temporal muscle from the superior temporal line, 2 cm posterior to the orbital rim, and should be directed toward the superior border of the zygomatic arch. Using a Metzenbaum scissor, the incision is penetrated by cutting the outer sheet of the superficial temporal fascia and the interfascial fatty layer, until reaching the leaf of the internal superficial temporal fascia, keeping forward on this plane. Using a cinzel, this fascia is detached from the bone by a subperiosteal dissection until the complete exposure of the superior orbital rim. The hooks are repositioned under this plane to expose the whole temporal muscle (Fig. 25.6 and Fig. 25.7). Alternatively, a subfascial dissection may be performed, especially when the layers of superficial temporal fascia are thin, providing an extra protection for the frontotemporal branches of the facial nerve (Fig. 25.8).

Fig. 25.5Schematic drawing illustrating the layers from the skin to the bone.

Fig. 25.6Interfascial dissection on a cadaver. (a–c) Superficial fascia of the temporal muscle. (d–f) Dissection of the interfascial fatty layer. (g–i) Dissection of the fatty layer from the internal superficial temporal fascia where usually there is an innominate vein.

Fig. 25.7 (a–f) Interfascial dissection on a live patient.

Fig. 25.8 (a–c) Subfascial dissection on a live patient. Note the exposure of muscular fibers once both layers of the superficial temporal fascia of the temporal muscle are opened.

25.4.5 Temporal Muscle Dissection

One of the essential steps of pterional craniotomy is the temporal muscle disinsertion, not only for the correct bone exposition but also for avoiding muscle atrophy and achieving excellent cosmetic results. There are at least four different risk factors for postoperative atrophy of the temporal muscle: (1) direct damage to muscle fibers (inappropriate dissection, excessive retraction, or the use of a large muscle cuff for reinsertion); (2) muscle ischemia (interruption of the arterial supply or prolonged retraction); (3) inadequate tension in reinserted muscle; and (4) muscle denervation (caused by direct damage to the nerve branches or indirect damage by interruption of nerve supply).³²

Knowing these risk factors for muscle atrophy, we avoid cutting the temporal muscle fibers, making a cut along the superior temporal line, and proceeding to the muscle disinsertion. Subperiosteal dissection is then performed, following a direction from inferior to superior and posterior to anterior fashion, to prevent muscle fiber damage. Care must be taken to preserve the deep temporal fascia, which protects the temporal muscle’s vascularization and innervation. Subsequently, the muscle flap is retracted downwards, avoiding excessive traction and, therefore, some muscle ischemia. The exposition of the frontotemporal bony area must be centered in the pterion and it is mandatory to expose the squamous suture, which is the bony reference of the Sylvian fissure (Fig. 25.9).

Fig. 25.9 (a–f) Step-by-step technique for detachment of the temporal muscle on cadaver specimen.

Fig. 25.10Classic pterional craniotomy performed on a cadaver specimen, with four burr holes: keyhole, stephanion, temporal, and frontal (a–d).

25.4.6 Craniotomy

By performing a pterional approach, the neurosurgeon seeks to achieve enough exposure of the lateral surface of the frontal and temporal lobes, usually having the Sylvian fissure at the center portion of the surgical field. Bone removal should expose more frontal lobe (middle and lower frontal gyrus) and less temporal lobe (superior temporal gyrus only). Thus, the exhibition is two-thirds of the frontal lobe and one-third of the temporal lobe.

Classically, the pterional craniotomy is performed with four burr holes.1 ^, ² ^, ³ ^, ³³ We prefer to avoid the frontal burr hole, since it may become visible under the skin at the forehead. Therefore, we usually perform three burr holes: (1) the classic keyhole (at the point just superior to the frontozygomatic suture under the superior temporal line); (2) at stephanion (the point where the coronal suture crosses the superior temporal line); and (3) at temporal region (the point in the squamous temporal bone behind the sphenotemporal suture, about 4 cm inferior to the second hole and 3 cm posterior to the first). Using a small curved periosteal elevator into the burr holes, a careful detachment of the dura mater can be achieved. After the dural dissection, the holes can be connected using a high-speed electric drill with footed attachments and a tapered tool to perform the bone flap’s detachment. The sphenoid bone’s superficial area may be drilled using a match head tool (Fig. 25.10).

25.4.7 Sphenoid Wing Drilling

After the elevation and removal of the bone flap, the lesser wing of sphenoidal bone and the orbital roof should be flattened using the drill. Flattening the orbital and sphenoidal surfaces provides wide access to the anterolateral skull base, minimizing the brain manipulation. The inferior limit of the bony drilling is the meningo-orbital fold at the superior orbital fissure level (Fig. 25.11).