Neural

The posterior cranial fossa contains several vital neural structures that are relatively intolerant to excessive manipulation and trauma. Handling of the cerebellum should be performed gently to avoid contusion of the cerebellar hemisphere, which can result in uncontrolled swelling. Appropriate patient positioning, as described later, decreases venous congestion in the cerebellum and brainstem, thus aiding in retraction.

Knowledge of the course of the cranial nerves through the posterior fossa is important in predicting their location during surgery. Tumor resection within the posterior fossa often requires piecemeal removal of the target lesion while working in between the cranial nerves. The trigeminal nerve exits the pons on the anterolateral surface and moves rostrally toward Meckel’s cave. The abducens nerve exits the brainstem at the pontomedullary sulcus and courses rostrally to Dorello’s canal. The facial nerve, nervus intermedius, and vestibulocochlear nerve exit the ventrolateral brainstem adjacent to each other approximately 2 to 3 mm rostral to the glossopharyngeal rootlets and course laterally into the internal auditory canal. The glossopharyngeal, vagus, and spinal accessory nerves exit the medulla lateral to the inferior olive and course anteriorly toward the jugular foramen. It is necessary to perform dissection and tumor removal in between these nerves to prevent unintended cranial nerve palsies.

Obtaining appropriate brain relaxation is key to reducing morbidity in posterior fossa surgery. Historically, many surgeons advocated for preoperative placement of a lumbar subarachnoid drain to achieve adequate cerebrospinal fluid drainage; this practice has been shown to slightly increase the risk of drain-associated complications without decreasing other operative risks.

Advancements in inhalational and intravenous anesthetics, hyperosmolar therapy with mannitol and hypertonic saline, and moderate hyperventilation have all resulted in improved control of brain relaxation. Intraoperatively, piercing the arachnoid membrane covering the foramen magnum on the suboccipital surface of the cerebellum as a first step after dural opening facilitates significant cerebrospinal fluid egress and results in excellent brain relaxation.

Arterial

The vertebral arteries join in the posterior fossa anterior to the brainstem to form the basilar artery. This junction occurs caudal to the level of the pontomedullary sulcus. The last major branch of the vertebral artery is the posterior inferior cerebellar artery (PICA), which has an extradural origin in up to 20% of cases. Rhoton identified that the extradural PICA, unlike cases of intradural PICA origin, remained lateral and posterior to the medulla without providing a perforator branch to the anterior brainstem.

The basilar artery runs along the ventral surface of the brainstem in the basilar sulcus before bifurcating into the posterior cerebral arteries at the level of the rostral mesencephalon. The major named arterial branches of the basilar artery are the anterior inferior cerebellar artery (AICA) and the superior cerebellar artery (SCA). Either artery can have a duplicated origin, and the SCA may receive contribution from the posterior cerebral artery.

Anatomic studies of cadaveric specimens have identified wide variability in the number and caliber of small perforating arteries that arise from the basilar artery, AICA, and SCA ( Fig. 18.1 ). Perforators off the basilar artery range in number from 5 to 20 with diameters from 80 to 940 mcm. These arteries are divided into short paramedian arteries that supply the medial brainstem and long circumferential arteries that supply the lateral brainstem. Importantly, all of these arteries arise from the dorsal surface of the basilar artery. The variability among brainstem perforators and their parent vessels is matched by the finding that perforator arteries tend to penetrate the brainstem itself at relatively constant locations along the anterolateral surface.

Ventral view of the posterior fossa after anatomic dissection removing all of the bony anatomy. The basilar artery is anterior to the brainstem, with multiple small feeder vessels seen coursing toward the brainstem itself. Starting from rostral, cranial nerves III through XII are viewed as they exit the brainstem.

Ventral or ventrolateral displacement of the basilar artery from the basilar sulcus into the prepontine cistern is occasionally seen in cases of posterior fossa tumors extending to the midline. In these situations, the small perforator arteries emanating from the dorsal side of the basilar artery are stretched, and they can be encased within the tumor itself. Occasionally a meningioma will encase the basilar artery with much displacement. The presence of an arachnoidal plane separating the perforators from the meningioma is variable, but its presence does aid in careful dissection and preservation of these small arteries.

Venous

Venous drainage of the posterior cranial fossa consists of several anastomotic pathways, and sacrifice of small cerebellar draining veins to aid in retraction or visualization is generally considered to be a safe maneuver. The superior petrosal vein, also known as Dandy’s vein, is found in the rostral aspect of the cerebellopontine angle as it courses toward its junction with the superior petrosal sinus. The superior and inferior petrosal sinuses are found within the dura along the petrous ridge and temporal bone, respectively, and as such do not usually pose as risks to surgery within the posterior fossa.

Ensuring adequate venous drainage during patient positioning is critical to lower venous pressure during surgery. Elevation of the head above the level of the heart using reverse Trendelenburg is useful for encouraging cerebral outflow and maintaining adequate venous drainage to the heart. Prevention of excessive flexion or rotation of the head and neck ensures patency of both jugular veins and also reduces venous congestion.

Red Flags

• •
  

  History of prior surgery

  
  
• •
  

  History of prior radiation therapy

  
  
• •
  

  Displacement of the basilar artery out of the basilar sulcus

  
  
• •
  

  Peritumoral edema (T2/FLAIR hyperintensity in the adjacent brainstem and cerebellum)

  
  
• •
  

  Cystic component of the tumor adjacent to the brainstem

  
  
• •
  

  Circumferential encasement of posterior circulation arteries and their branches

Only gold members can continue reading. Log In or Register to continue

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)

Related

Related posts:

Medical Complications in Neurosurgery Cerebral Vasospasm: Complications and Avoidance Complications in Vestibular Schwannoma Patients Procedure-Related Complications: AVMs Complications of Surgery for Peripheral Nerve Injuries and Tumors Complications of Surgery for Intrinsic Spinal Cord Tumors

Stay updated, free articles. Join our Telegram channel

Join