Cerebellopontine angle and Retromastoid Craniectomy

Retromastoid craniectomy is an important neurosurgical approach for operating lesions of the cerebellopontine angle ( CP angle).

Commonest CP angle tumor is acoustic schwannoma  Other common tumors of CP angle are CP angle meningioma and epidermoid. 

Retrosigmoid region is the part of the cranium situated just behind the sigmoid sinus and below the transverse sinus.

The lateral cerebellomedullary and cerebellopontine cisterns are encountered in this approach.

Three important arteries are encountered bilaterally in this region: 

Posterior inferior cerbellar artery (PICA),

Anterior inferior cerbellar artery (AICA), &

Superior cerebellar artery (SCA)

PICA arises from the vertebral artery and is seen in relation to the lower cranial nerves. Occasionally, it may reach high enough to come in contact with the lower limit of the VII/VIII complex.

AICA arises from the basilar artery and distal part is seen in relation to the VII/VIII complex. This nerve-related segment is divided into premeatal, meatal, and postmeatal parts. The meatal part forms a loop ( meatal loop) which travels along the VII/VIII complex for a variable distance toward the internal acoustic meatus. Several nerve-related branches of AICA exist: the internal auditory artery, recurrent perforating arteries, subarcuate artery , and cerebellosubarcuate artery. The internal auditory artery is the most constant branch of the AICA.

The SCA arises from the basilar artery quadrification courses back along the side of the mid brain and pons , and is seen in the cerebellopontine angle in front of the Vth cranial nerve.

The veins of the posterior fossa can be divided into superficial , deep , and bridging veins. Anterior hemispheric veins , veins of the cerebellopontine fissureand middle cerebellar peduncular veins, lateral medullary and lateral mesencephalic veins, and petrosal bridging veins.

Petrosal veins are divided into superior and inferior veins which drain into superior or inferior petrosal sinus.

The superior petrosal veins are the most prominent bridging veins of the posterior fossa. They drain blood from a large part of the cerebellar hemisphere and brainstem. Superior petrosal veins are divided into a lateral, intermediate or medial group based on their relation to the site of drainage into the superior petrosal sinus and IAM.

Cranial nerves V through XII are seen in this approach. CN V is a stout nerve arising from the upper and outer aspect of the anterior pons. Its motor component arises as a separate rootlet just anterior to the main nerve , and both quickly merge to form a single nerve. Loops of the SCA can be seen in front of the nerve at this level.The nerve courses anteriorly and outward to cross the petrous apex and enter into the Meckel,s cave.

Cranial nerve VI arise medially at the pontomedullary sulcus. It courses up the surface of the clivus exiting the posterior fossa through Dorello’s canal. It enters the posterior cavernous sinus on its way through the middle cranial fossa to the orbit.

The VII/VIII complex, with the nervus intermedius in between, arises from the pontomedullary junction surrounded by the same fascial sheath. They course backward, downward, and outward toward the internal acoustic meatus.

Cranial nerves IX, X, XI arise as a linear series of rootlets from just below the pontomedullary junction in the anterolateral medullary sulcus. After their origin, they converge outward and backward toward the jugular foramen. CN XII lies in close proximity to the vertebral artery-PICA junction. Its many rootlets exit though the hypoglossal canal.

Retromastoid craniectomy can be performed with the patient in sitting, park-bench, three-quarter prone position, lateral or supine position.
The description by Robert L. Martuza for surgery of acoustic neuroma in supine position is straight forward. The patient is positioned supine with one or two folded soft cotton blankets beneath the ipsilateral shoulder and padding beneath the knees and is secured with one or two belts to allow the table to be rolled if necessary during surgery. Too much turning of the body can place the shoulder in a position that interferes with surgery.
The head should be fixed in Mayfield or Sugita frame.
A retroauricular lazy 'S' or linear or C shaped incision is made in the scalp which is paramedian and about 5 mm medial to the mastoid notch.
Dr Robert L. Martuza suggests a linear incision approximately 3 cm behind the insertion of the pinna and going from approximately 1 cm above the tip of the pinna to 1 cm below the ear lobe. The muscles are then divided inferiorly and self retaining retractors are placed.
The sigmoid sinus lies beneath the mastoid groove.
The transverse sinus and sigmoid sinus generally curve around the asterion. Therefore, a burr hole is placed medial to the asterion.
Craniectomy is done after placing a burr hole.  After craniectomy the inferior edge of the transverse sinus and the medial edge of the sigmoid sinus should be defined with the help of rongeurs. The transverse sinus lies above the upper limit of craniectomy and sigmoid sinus is lateral to the craniectomy.typically a K-shaped incision or linear incision about 2 cm medial to the edge of the sigmoid sinus  is made in the dura with the edges being based on the sinus.
After retracting the cerebellar hemisphere medially and superiorly cisterna magna comes into the view. The arachnoid of this cistern is opened and CSF is drained and this maneuver relaxes the cerebellum, obviating the need for further retraction.

Middle neurovascular complex in CP angle includes AICA, pons and middle cerebellar peduncle. The AICA arises at the pontine level and courses in relationship to the abducens, facial & vestibulocochlear nerve to reach the surface of the middle cerebellar peduncle, where it courses along the cerebellopontine fissure and terminates by supplying the petrosal surface of the cerebellum. Common operations in this region are for removal of acoustic schwannomas and for the relief of hemifacial spasm.

Acoustic schwannomas, as they expand, may involve a majority of the cranial nerves, cerebellar arteries, and parts of the brain stem.

Lateral view of skull to show Asterion: Meeting point of Parietomastoid suture, Occipitomastoid suture and lambdoid suture.   

Sources:

1. Chapter: Suboccipital retrosigmoid surgical approach for vestibular schwannoma ( acoustic neuroma) by Robert L. Martuza in Schmidek & Sweet operative neurosurgical techniques- indications, methods & results. Alfredo Quinones- Hinojosa. 6^th ed. (Elsevier Saunders) 

2. Rhoton Cranial anatomy and surgical approaches, Neurosurgery, Albert L. Rhoton, Jr  

3. Oerative Neurosurgical Anatomy by Damirez T. Fosset and Anthony J. Caputy ( Thieme publication)
4, Wikipedia