Pterional approach (neurosurgery)

The pterional or fronto-temporo-sphenoidal approach is one of the most frequently performed neurocranial craniotomy/craniectomy approaches and allows access to numerous important supratentorial anatomical substrates of vascular and neoplastic pathology.

History

It was originally described and popularized by Yasargil in the 1970s to address the treatment of aneurysmal disease of the anterior communicating artery.  Techniques used before this were variously based on subfrontal, transcallosal, interhemispheric subfrontal, and frontoparietal approaches ¹. 

Since its original description, several variants have been described that allow additional indications to be addressed efficiently and with minimal disruption on surrounding tissue. These include the pretemporal, orbitozygomatic, minimally invasive and the so-called ‘keyhole’ approaches ^2-5.  

In general, extended pterional approaches can increase the operative corridor through additional bony resection, whereas minimally invasive approaches, such as the mini-pterional approach, reduce disruption of surrounding healthy tissue at the potential cost of reduced accessibility of a large lesion and with the assumed risk of difficulty handling intraoperative complications like massive hemorrhage.  

Anatomy

The pterion is defined as the anatomical region formed around the approximation of four cranial bones

• sphenoid bone (greater wing)
• parietal bone
• frontal bone
• temporal bone (squamous part)

or equally by the confluence of five cranial sutures

• sphenoparietal suture
• sphenofrontal suture
• sphenosquamosal suture
• coronal suture
• squamosal suture

The sphenofrontal and coronal sutures meet at the anterior aspect of sphenoparietal suture and the sphenosquamosal and squamosal sutures meet at posterior aspect of sphenoparietal suture.

The frontal (anterior) branch of the middle meningeal artery typically runs deep to the pterion within the dual layers of the dura mater. The pterional region is usually the thinnest part of the human calvarium making it susceptible to traumatic fracture with consequent rupture of the middle meningeal artery and then extradural hemorrhage. This is also relevant to keep in mind when performing burr hole craniotomies to avoid inadvertently ‘plunging’ the drill into the cranial cavity.

The pterional approach allows exposure to numerous important anatomical areas, including

• the orbit
• Sylvian fissure
• skull base of the anterior and middle cranial fossae
• frontal, temporal, parietal lobes
• insula, basal ganglia, mesial temporal lobes, hippocampus, upper brainstem (midbrain)
• supra- and parasellar
• basal cisterns
• the cavernous sinus
• anterior circulation (intracranial internal carotid artery, anterior and middle cerebral arteries, anterior communicating artery, posterior communicating artery)
• distal vertebrobasilar system of posterior circulation (top of basilar, proximal posterior cerebral artery, superior cerebellar artery)

Indications

Indications where pterional approaches are used, as permitted by anatomical accessibility, include:

• aneurysm clipping (ruptured or unruptured)
• tumor resection
• epilepsy surgery ⁶

Contraindications

The suitability of the pterional approach should be considered along with the assessment of appropriate imaging. Inaccessibility of a specific lesion, therefore, constitutes a strong contraindication, while significant risk to adjacent eloquent brain tissue suggests a relative contraindication although this must be balanced against the possibility of alternative approaches and the risk of not performing the procedure.  

Procedure

A broad outline of steps followed in performing a pterional craniotomy includes ⁷:

• induction and general anesthesia
• supine positioning
• head is immobilized in Mayfield or Sugita head frame, rotated contralaterally and extended somewhat (depending on access required, and to allow frontal lobe to naturally sink into the cranial vault reducing manual retraction)
• hair shaved, skin prepared, marked, and local anesthetic and epinephrine injected
• sterile field prepared
• skin incision:  curvilinear or arcuate semi-coronal frontotemporal incision performed, starting from superior margin of zygomatic arch, 1 cm anterior to the tragus, aiming towards the midline along, ideally posterior to the hairline (the pterion lies anterior to the incision)
• myocutaneous flap dissection: skin, connective tissue and galea mobilized from the underlying loose connective tissue and periosteum through either interfascial or subfascial approaches to preserve the frontal and parietal branches of the superficial temporal artery and the temporal branch of the facial nerve (to prevent frontalis and sometimes orbicularis oculi palsy)
• flap retracted anteriorly and secure with fishhooks
• periosteum raised
• dual burr holes placed: one at the MacCarty keyhole (7 mm superior and 5 mm posterior to the frontozygomatic suture) and one just above the zygomatic arch in the squamous part of the temporal bone, so just posterior to the temporozygomatic suture (or use single burr hole) ⁸
• burr hole edges sealed with bone wax and remaining bony slivers resected
• superficial periosteal dura mobilized from calvarium using Penfield dissector to prevent dural tear in following step
• craniotomy raised using craniotome, and carefully raised (the anatomical extent or size is determined by the surgical indication and exposure required)
• underlying dura revealed and hemostasis obtained if required
• additional osteotomy or bony resection of the lateral sphenoid ridge, orbital roof, or temporal bone can now be undertaken yielding the extended pterional approach, if this is required
• dural hitch or tack sutures may now be placed to prevent postoperative extradural collections
• dura can now be incised and reflected anteriorly, revealing the Sylvian fissure
• following the intradural part of the procedure, the dura is reapproximated and sutured closed
• bone flap is replaced with three-point cranial fixation or plates, with or without a central dura hitch suture, as required
• temporalis muscle is reattached
• subgaleal drain placed if difficult hemostasis suggests a risk of extracranial hematoma
• galea (aponeurosis) closed
• skin closed

Complications

In addition to anesthetic complications, general neurosurgical complications include:

• infection (wound, intracranial, extracranial, systemic)
• bleeding (including systemic hypovolemic shock, catastrophic intracranial, post-operative collections in various anatomical locations)
• CSF leak
• seizure
• stroke (due to local and systemic causes, including clot, vessel occlusion, hypoperfusion)
• neurological injury

Specific pterional craniotomy complications include ⁴:

• headache
• pain on mastication
• paralysis if the frontalis, orbicularis oculi
• visible scarring and other cosmetic defects
• temporal hollowing (superficial temporal fat pad atrophy)
• inadvertent breach of orbit or frontal sinus