Learning Objectives:After participating in this CME activity, the neurosurgeon should be better able to:
- Analyze the surgical anatomy of the extended lateral orbital (XLO) keyhole approach.
- Describe the benefits of XLO.
- Identify the limitations of XLO.
The pterional (frontotemporal) craniotomy provides a wide surgical exposure to peri- and intra-Sylvian structures and allows direct access to parasellar structures. Despite its widespread use, the pterional approach presents several limitations: (1) potential postoperative temporal atrophy due to complete temporal muscle dissection; (2) injury to the frontal branch of the facial nerve; and (3) large incision and craniotomy, which may need longer healing time.
Over the years, pterional craniotomy has seen many modifications, including extensions (intradural and extradural clinoidectomy, orbit unroofing, etc) and minimally invasive modifications as well. Among the minimally invasive pterional keyhole craniotomies, many have failed to address the limitations associated with the standard pterional craniotomy, and seldom were these approaches standardized. We present a minimally invasive keyhole craniotomy centered on the sphenoid ridge: the extended lateral orbital (XLO) approach (Figure 1). XLO is a novel approach developed to improve the limitations associated with the pterional craniotomy. This review illustrates the anatomic basis of the approach and its limitations and indications.
The incision of XLO starts at the middle of the eyebrow medial to the superior temporal line and then curves downward and laterally with the lateral orbital rim. The incision extends inferiorly to the zygoma (1.5–2 cm) in a zigzag fashion (Figure 2A). The frontal branch of cranial nerve (CN) VII (Figures 2A and 2B) is identified above the superficial temporal fascia, within the subgaleal fat pad. It is relatively safe during XLO incision and situated based on our cadaver studies 2.66 cm from the lateral canthus and 3.2 cm from the anterior zygoma on average. Still, one should be cautious during XLO incision, as aggressive retraction posteriorly may damage the frontal branch of CN VII.
The XLO incision requires skin incision and subcutaneous fat dissection, followed by an incision through the orbicularis muscle that is retracted away for exposure (Figure 2C). Medial to the superior temporal line, the area of muscle incised is at the orbicularis-frontalis fusion, whereas lateral to the line, the incision is made in the orbicularis. This muscle layer is in continuity throughout the incision. The next layer incised includes the subgaleal fat in the frontal area and the superficial (subglaeal) fat pad in the temporal area. The pericranium is encountered next in the frontal area and is preserved if a pericranial flap is needed. Over the temporal area, the deep temporal fascia is encountered and blunt spreading dissection is performed to elevate the superficial temporal fascia and subgaleal fat. The temporal line of fusion is taken down sharply and the deep temporal fascia is incised leaving a 0.5-cm cuff to sew to at the end. The temporalis muscle is then split and retracted. The surface area of the exposed skull is around 5 cm2. Depending on the cerebral exposure and degree of freedom needed, a surgeon can extend the burr hole from the blue zone to the red and green zones.
Durotomy and Craniotomy
Craniotomy is done using a high-speed drill. The sphenoid ridge, which appears at the center of the exposure, is further drilled up to the superior orbital fissure level. Durotomy is done through a C-shape incision and the dura is reflected anterolaterally (Figure 2D).
After dissection of the exposed anterior Sylvian fissure, one can identify intra-Sylvian structures and both the inferior frontal and superior temporal gyri (Figure 2E).
The middle cerebral artery (MCA) branches (M1 and M2) can be appreciated within the Sylvian fissure, making XLO a direct and straightforward approach to MCA aneurysms—without the need for retraction of the frontal lobe (Figure 2F).
After craniotomy extension (to red and green zones depicted in Figure 2C), one can have a deeper outlook of the skull base. Anteriorly, the optic chiasm, both optic nerves, and the anterior cerebral artery (A1) can be appreciated (Figure 2G).
With enhanced magnification superiorly, the ipsilateral and contralateral A1s, A2 segments of the anterior cerebral arteries, and anterior communicating artery (Acom) can be clearly identified (Figure 2H).
Inferiorly, one can expose the posterior circulation with adequate exposure of the basilar artery apex. The posterior cerebral, posterior communicating, and superior cerebellar arteries are also within the reach of the XLO approach (Figure 2I).
Benefits and Indications
The XLO incision is an extension of the traditional eyebrow incision down to the zygomatic arch and provides excellent exposure of the anterior Sylvian fissure (∼3 cm of exposure) and anterior temporal lobe with minimal to no lobar retraction needed. The XLO approach (Figure 3, blue) overlaps with the lateral supraorbital approach (Figure 3, pink) and directly targets the anterior and middle fossas. Using XLO, MCA aneurysms can be very easily accessed without the need for brain manipulation. As seen in Figure 4, an MCA aneurysm is centered on the field of exposure merely after Sylvian fissure dissection and cistern opening, enabling the ease of MCA aneurysm clipping. XLO can also be used for select aneurysms in other locations (view its extensive access, as seen in Figure 2) and other temporal and skull base pathologies (cavernomas, gliomas, meningiomas, etc). The field of XLO exposure is wide and can extend to the posterior circulation.
Although the XLO incision extends caudally to the face, its zigzag configuration is designed to be cosmetically appealing, as it breaks the scar and runs along the natural crow's feet skin lines, therefore minimizing the visibility of the scar. The cosmetic results can be appreciated in a 65-year-old man who was treated surgically for an incidental MCA aneurysm using the XLO approach (Figure 5). At 1-year follow-up, his scar is barely noticeable (Figure 5B). Furthermore, XLO poses minimal to no risk to the frontal branch of the facial nerve, as the nerve is more than 2 cm away from the orbital rim. Other studies evaluating the position of the frontal branch of the facial nerve in the setting of the pterional craniotomy highlights a distance of 1.5 to 1.8 cm, depending on the anatomical landmark used as reference.
XLO has limited posterior Sylvian fissure and posterior temporal lobe access and is therefore not suitable for pathologies that present posteriorly. Moreover, its small craniotomy size restricts instrumentation access and angle of approach. Hence, it is not suitable for large and complex aneurysms and does not provide enough freedom for a bypass procedure.
A 78-year-old woman had a 5.6 × 4.1 mm, unruptured irregular wide-necked (3.7 mm) right MCA bifurcation aneurysm (Figure 6A). The aneurysm was found incidentally on a CT scan (Figure 6B) during investigation of syncope. Based on the morphology of the aneurysm, it was felt to be high risk for rupture and the patient opted to receive surgical treatment for her aneurysm. During the procedure, an XLO incision and craniotomy were performed with the patient's head turned 35 degrees. XLO provided a direct corridor to the patient's aneurysm (Figure 6C) and a straight clip was placed across the neck of the aneurysm (Figure 6D). Indocyanine green angiogram (Figure 6E) showed a small amount of filling from a small gap in the proximal aneurysm due to the differential wall thicknesses. A second shorter clip (tandem clip) resulted in complete aneurysm occlusion (Figure 6F). The patient had no postoperative complication and had no residual aneurysm on 4-month catheter angiogram follow-up (Figure 6G) and 1-year CT angiography evaluation (Figure 6H). She had excellent cosmetic outcome as seen on her 4-month follow-up photograph (Figure 6I).
The XLO approach is a novel minimally invasive pterional approach that is centered on the anterior Sylvian fissure and sphenoid ridge. XLO is cosmetically appealing, presents minimal risk to the facial nerve, and provides a direct angle of attack to the anterior temporal lobe, MCA aneurysms, and anterior and middle cranial fossa pathologies.
- XLO craniotomy is an optimal pterional keyhole craniotomy that is centered on the sphenoid ridge and provides adequate intracranial exposure, accessibility, and maneuverability to the middle fossa and anterior temporal lobe pathologies.
- The XLO incision comprises 2 segments: a curvilinear segment over the outer lateral aspect of the eyebrow parallel to the lateral orbital rim and a caudal zigzag extension over the crow's feet wrinkles.
- The XLO approach optimizes facial cosmetic outcomes and may decrease the incidence of temporal hallowing.
- XLO approach provides a limited-sized craniotomy that is not suitable for large lesions or complex procedures such as bypass.
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