The Operation for Cerebellar Astrocytomas in Children

Patient Positioning

• Prone: The patient is placed lying face down on rolls to avoid abdominal compression.  Lateral decubitus positioning with the head rotated toward the floor and flexed is an alternative.
• Pin fixation: A head holder with pins is typically used to maintain the head in a flexed position and avoid pressure on the face.  Caution should be taken when placing pins in young children or in patients with long-standing hydrocephalus who may have a thin cranium.
• Horseshoe for young children and infants: Young children may be positioned in a padded horseshoe with caution taken to avoid pressure on the eyes and face.
• Head fixation: Head flexion is helpful for surgical exposure.  Breath sounds and airway pressures should be checked after positioning to ensure that the endotracheal tube has not been kinked or shifted into the right mainstem bronchus with flexion.

Surgical Approach

Midline suboccipital craniotomy is the most common surgical approach for children with infratentorial tumors.  The basic surgical steps include:

• Midline incision: A midline incision is made from the external occipital protuberance to below the spinous process of C2.
• Pericranial graft: Thought can be given to the need for such material. It can be harvested from above the suboccipital muscles at the beginning of the case, as duraplasty is often necessary.
• Dissection through ligamentum nuchae: This relatively avascular tissue lies between the suboccipital muscles along the midline. It is incised, and self-retaining retractors are placed to move the muscles laterally.
• Separation of muscle: The muscle is then dissected off the occipital bone, posterior arch of C1, and superior half of the posterior arch of C2.  Dissection of C1 should be subperiosteal with blunt instruments to avoid injury to the vertebral artery laterally.  In infants, dissection over C1 should be especially cautious, as the posterior arch may not be fully ossified.

Exposure after separation of muscle:

•  Craniotomy is preferred: Even when the tumor is located in one cerebellar hemisphere, the author prefers a large craniotomy crossing the midline for exposure, as it will minimize cerebellar herniation upon opening. The craniotomy will extend from just below the estimated location of the transverse sinus to the opisthion extended as wide as possible bilaterally. 
• Removal of the posterior arch of C1: This should be performed when the tumor has caused tonsillar herniation or it is large with inferior extension.
• Dural opening: The author typically uses a Y-shaped opening. This needs to be done with caution looking for the presence of enlarged occipital and circular venous sinuses.
• CSF drainage: The cisterna magna is then opened to allow drainage of CSF and relaxation and the introduction of the operating microscope.
• Identification of normal anatomy: Time is then taken for orientation by identifying structures including cerebellar tonsils, spinal cord, obex, arteries (PICA and vertebral artery), and cranial nerves bilaterally. The tonsils can be gently elevated to identify the floor of the fourth ventricle. A cottonoid paddy can be placed to protect the brainstem and spinal cord.

Intervention

• Intraoperative ultrasound: Prior to tumor resection ultrasound can be used to identify an echogenic tumor, and the image obtained then can be used for comparison at the conclusion of the surgery. For cystic tumors with a mural nodule, ultrasound may facilitate approach through the least amount of cerebellar tissue to reach the mural nodule.

Intraoperative ultrasound of cerebellar pilocytic astrocytoma: Apparent is a large cyst with a mural nodule on the far wall at midline in the image.

Intraoperative photograph of surgical field of ultrasound image shown above: Incision is being held open to show nodule on far wall.

Intraoperative ultrasound after resection: Mural nodule is no longer present, and cyst has partially collapsed.

• Tumor biopsied: A biopsy of the tumor for pathologic examination using frozen section technique can confirm the diagnosis of pilocytic astrocytoma.
• Tumor resected: The resection of solid tumors is typically performed using a combination of ultrasonic aspiration (e.g., an ultrasonic aspirator) and bipolar coagulation with suction.  Mural nodules and areas of thick enhancement on MRI scan are resected, but cyst walls with no enhancement or a thin rim of enhancement do not need to be removed.

Closure

• Dural closure:  Primary dural closure after tumor surgery in the posterior fossa is usually difficult because some coagulation of the dura is usually required during dural opening.  Duraplasty with pericranium or dural substitutes is usually performed.

Duraplasty with pericranium:

• Replacement of bone flap: Replacement of the bone flap may be done to improve the cosmesis of the healed surgical site. It may be performed with suture material, wires, or plating systems. Bone dust saved from the craniotomy opening is often placed to augment fusion.
• Muscle and fascial closure: Standard closure techniques for the suboccipital and cervical muscles, fascia, and skin are used.

Other Surgical Approaches

Surgical approaches to tumors involving the brainstem are reviewed separately in the chapter on Brainstem Tumors.