Pterional Approach

R. Loch Macdonald

Keywords: aneurysm, cerebral aneurysm, craniotomy, frontotemporal, pterional

4.1 Indications and Alternatives

A frontotemporal or pterional craniotomy is the main route of approach for aneurysms located on the internal carotid artery, the precommunicating segment (A1) of the anterior cerebral artery up to and including the anterior communicating artery complex, and the sphenoidal (M1) segment of the middle cerebral artery. Proximal internal carotid artery aneurysms often require intra- or extradural removal of the anterior clinoid process. Exposure of the cervical carotid artery may be used to gain proximal control in these cases. The pterional approach also can be used for aneurysms of the upper basilar artery, the details of which depend mostly on the rostral caudal location of the aneurysm in relation to the posterior clinoid processes. The craniotomy can be expanded by orbital rim and zygomatic osteotomies. These maneuvers tend to be indicated when basilar artery bifurcation is high and to widen the field in order to allow more illumination and more room to manipulate instruments. The main alternative to a pterional route for upper basilar artery aneurysm is a subtemporal approach. An anterior interhemispheric approach can be used for anterior communicating artery aneurysms.

4.2 Anatomy

The pterion is the small area on the lateral surface of the skull where the frontal, parietal, greater wing of the sphenoid, and squamous part of the temporal bone meet. ¹ At this point, on the inner surface of the skull is the fused portion of the greater and lesser wings of the sphenoid. This ridge separates the anterior and middle cranial fossae and a portion generally is removed extradurally. The greater wing of the sphenoid forms the anterior wall of the middle fossa, whereas the lesser wing forms the posterior part of the floor of the anterior fossa. It juts posteriorly into the stem of the sylvian fissure between the basal surface of the frontal lobe and anteromedial temporal lobe.

4.3 Preoperative Preparation

Preoperative preparation includes an initial surgical checklist. The patient cannot be anesthetized until factors including ensuring the correct patient and side of approach and presence of the necessary neuroimaging and equipment including operating microscope equipped with indocyanine green fluorescence and mobile fluoroscopy for intraoperative angiography are taken into account. The patient is anesthetized and administered the preoperative antibiotics. Particular attention should be paid during induction of anesthesia and from here forward to preventing undue fluctuations in blood pressure, which might increase the chances of aneurysm rupture, especially when conducting surgery for a ruptured aneurysm. The initial positioning is done, consisting of flexing the operating table and reverse Trendelenburg tilting so that the head is up about 15 degrees in order to optimize venous drainage and brain relaxation. The head is fixed in a radiolucent head holder to permit intraoperative angiography.

The head position in the standard position for a pterional craniotomy is rotated about 30 degrees, raised up above the body and extended so the orbital rim and malar eminence are at the same level horizontally ( ▶ Fig. 4.1). More rotation is sometimes used for a middle cerebral artery aneurysm and less for an anterior communicating artery aneurysm. A shoulder roll under the ipsilateral shoulder may be needed to facilitate turning of the head. The degree of extension described is optimal when performing surgery with the diploscope attachment on the operating microscope because the surgeon and assistant stand on opposite sides of the head. When using a single binocular attachment with a side extension, the head can be extended more if desired but too much extension will make it difficult to see as far up the anterior fossa base. Another consideration is preparing the anterior neck and the carotid bifurcation area for proximal control for proximal internal carotid artery aneurysms or for possible bypass.

An option is to register the patient to use intraoperative neuronavigation. This is not absolutely necessary. It does have some value for teaching and showing trainees how to position the head to get a good trajectory toward the aneurysm and to facilitate insertion of an external ventricular drain (EVD) catheter, which often is required for ruptured aneurysm cases. Also, the frontal sinus can be delineated, and if it is large and the craniotomy is likely to enter it, a pericranial flap can be dissected from the skull from the superior temporal line up to the medial part of the incision.

It is adequate to only shave a line of hair along the incision ( ▶ Fig. 4.1). I often do an angiogram by retrograde injection through the superficial temporal artery so this can be palpated to locate it prior to marking the location of the incision. If the aneurysm is ruptured and insertion of an EVD is anticipated, then shave an additional swath of hair back from the incision at the level of the superior temporal line. An EVD may already have been inserted, although an advantage of waiting until craniotomy, if the patient’s neurological condition permits, is that there may be more cerebrospinal fluid to drain leading to better brain relaxation. The skin and hair are prepared with povidone iodine solution. This must be in contact with the skin for 3 or more minutes so it has time to dry.

For unruptured cases, I usually infiltrate the skin and subcutaneous tissue with about 8 mL of 1% xylocaine without epinephrine. This is avoided in patients with ruptured aneurysms because of the risk of affecting the blood pressure by an inadvertent intravascular injection. Nimodipine is commenced after diagnosis in patients with subarachnoid hemorrhage (SAH). The usual oral dose is 60 mg every 4 hours. Maintenance of normovolemia is imperative in the perioperative phase. There is no uniform agreement on choice of fluid replacement, but our practice is to use 0.9% normal saline with close monitoring of electrolytes and sodium and potassium supplementation as necessary. Anticonvulsants such as phenytoin may be administered to patients who have had a seizure, who are at high risk of having one (intracerebral hematoma), or who would be potentially harmed by one (poor-grade patients, those with already increased intracranial pressure). There is no evidence to support giving anticonvulsants to all craniotomy patients.

Smooth induction of anesthesia without altering the blood pressure is important so as to reduce the risk of aneurysm rupture or rerupture. The head pins should only be applied when it can be assured that the patient is adequately anesthetized. We do not use lumbar drainage but place an EVD preoperatively in patients with neurological symptoms and signs due to hydrocephalus or intraoperatively through the craniotomy when additional brain relaxation is needed. ² An arterial line and urinary catheter are used in all patients. Central venous lines are optional. Always be prepared for massive uncontrolled bleeding. Intraoperative hypothermia was not beneficial in a large, randomized study of good-grade patients with SAH, so despite experimental data to the contrary, its use has to be considered questionable at this time.

For complex or giant aneurysms where prolonged temporary clipping or bypass procedures may be necessary, we use electroencephalography monitoring so we can induce burst suppression if necessary.

Also, we insert a ventricular drain into the frontal horn of the lateral ventricle once the dura is open in almost all patients with SAH. This is done by aiming perpendicularly to the brain from the top of a triangle 2.5 cm back along the sylvian fissure and 2.5 cm superiorly. ²

4.4 Operative Procedure

4.4.1 Scalp and Temporalis Muscle

The incision starts temporally 0.5 to 1.0 cm anterior to the tragus and not more than 1.0 cm below the zygomatic arch. It curves up to or across the midline at the hairline. An angiogram will usually be done through the superficial temporal artery, so care should be taken not to injure it during the initial incision. Once a short incision is made, isolate the superficial temporal artery using Metzenbaum scissors. Once a segment is freed, tie a 3–0 absorbable suture loosely around it to mark it so that it can be located once the scalp flap is reflected. The rest of the incision is completed in stages. It is optimal to avoid incising the temporalis muscle fascia initially. The pericranium above the superior temporal line can be incised so that it can be reflected with the scalp flap unless entrance into the frontal sinus is anticipated. The skin is incised with a scalpel and the deeper layers with a scalpel or electrocautery. Scalp clips, such as Raney clips, are placed starting at the end of the incision so they can be stacked up close together with no spaces in between. We only make short segments of incision before applying clips in order to minimize blood loss. If stray hairs get in the incision, do not pull them since this will pull in more and more hair. Simply cut them with scissors.

Once the scalp is open down to the pericranium, the temporalis muscle needs to be incised and reflected, a procedure for which there are numerous variations ( ▶ Fig. 4.2). The author prefers to reflect the scalp and muscle as one unit. The advantage is that this minimizes the risk of injuring the frontalis branch of the facial nerve. The disadvantage is there is more muscle anteriorly that may hinder visualization along the sphenoid ridge. Initially, incise the temporalis fascia and pericranium, and start reflecting it with a flair or similar instrument. If one starts temporally and sweeps up, it is easier to preserve the deep layer of fascia under the muscle. It is also helpful to leave a cuff of muscle along the superior temporal line to sew the muscle back up to. The scalp and temporalis muscle are reflected forward as one layer. There are many other options including an interfascial approach and addition of bone removal of the orbital rim and zygoma. If an interfascial approach is chosen, the scalp is reflected before incising the temporalis fascia. As the scalp dissection proceeds to the lateral rim of the orbit, dissection should stop where the temporalis fascia splits into two layers and encases a fat pad. The outer fascial layer is incised here longitudinally and the fat dissected forward along with the scalp. This method allows the temporalis muscle to be reflected inferiorly, providing somewhat better exposure along the sphenoid ridge.

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