Cerebellopontine Angle Tumors

History of Cerebellopontine Angle Surgery

The cerebellopontine angle (CPA) is the most common site for posterior fossa neoplasms. Tumors occupying this region account for approximately 10% of all intracranial neoplasms, with vestibular schwannomas accounting for 80% of these CPA tumors. Other tumors involving this region include meningiomas, dermoid tumors, arachnoid cysts, lipomas, and metastases. The cerebellopontine angle is densely inhabited by vital neurologic tissue. As such, tumor growth in this region may give rise to significant neurologic dysfunction and, if left untreated, can ultimately lead to death.

As early as 1777, Dutch physician and anatomist Eduard Sandifort provided the first documented postmortem description of a CPA tumor. However, no clinical correlation with the mass was made at the time. In 1810, Leveque-Lesource correlated symptoms of a CPA mass with postmortem findings. His clinical description was that of a 38-year-old woman with vomiting, headache, decreased vision, numbness of the extremities, dysarthria, and deviation of the tongue. At autopsy, Lesource documented the deceased patient to have a tumor adherent to the eighth cranial nerve.

As the 19th century ended, the clinical manifestations of a CPA mass, at least in its advanced stages, were becoming increasingly well known and recognized among clinicians. Sir Charles Ballance is credited with performing the first successful complete surgical removal of a CPA tumor in November 1894. Patient symptomatology, which incited suspicion for a CPA mass and spurred the operation, included a 1-year history of vertigo, headache, instability, and unilateral loss of vision. Ballance performed the operation in two stages, set 1 week apart. In the first stage, a right posterior fossa craniectomy was performed. One week later, Ballance removed the tumor by inserting an ungloved finger between the pons and petrous bone. The patient, 46 at the time of the operation, went on to live an additional 18 years, unfortunately suffering from facial anesthesia, hemifacial paralysis, and delayed corneal ulceration requiring removal of her right eye.

In 1903 Krause of Berlin set forth the unilateral suboccipital approach for the removal of vestibular schwannomas. Like Ballance, Krause utilized his finger as the instrument for tumor dissection and removal. Krause's mortality rate was close to 85%, a result of the uncontrollable bleeding he encountered with this approach. In 1904 Panse attempted the first translabyrinthine approach for removal of a vestibular schwannoma. With limited instrumentation, the approach did not permit sufficient exposure and was similarly associated with high mortality rates. In 1905 Victor Horsley performed a complete removal of a vestibular schwannoma at National Hospital in London. The patient survived for only a few years after the operation and suffered from presumed severe brainstem ischemia. In the same year, Borchardt performed the first transsigmoid removal of a vestibular schwannoma, but fatal hemorrhage from the lateral sigmoid sinus led to the rapid abandoning of this approach. Owing to the extensive bleeding complications experienced by surgeons attempting to operate on CPA masses, Harvey Cushing would later refer to the anatomic region as “the bloody angle.”

Early attempts at CPA tumor resection were complicated by a lack of diagnostic capabilities or refined surgical instruments coupled with unreliable anesthesia, limited possibilities for hemostasis, and an incomplete appreciation of the CPA anatomy. As such, early surgeries were fraught with intraoperative difficulty and extremely high mortality rates nearing 70% to 85% even among the most practiced surgeons. Harvey Cushing, finding these mortality rates unacceptable, would be the next pioneer to significantly advance the field of CPA surgery.

In 1917 Cushing published his famous Tumors of the Nervus Acusticus and the Syndrome of the Cerebellopontine Angle in which he documented his operative and perioperative experiences with 30 vestibular schwannoma patient cases. Cushing reported markedly lower mortality rates than those of his predecessors, with an initial rate of 20% that he would later reduce to as little as 4%. Though his predecessors broadly recognized the symptomatology, it was during that time that Cushing first coined the persisting diagnostic entity CP angle syndrome and first accurately professed the early development of tinnitus followed by ipsilateral hearing loss in patients afflicted with CPA tumors.

Cushing's vastly improved outcomes were made possible by a number of key factors, starting with his belief that the complete removal of a vestibular schwannoma was simply unattainable. Instead, he focused primarily on achieving decompression of the brainstem and avoiding medullary compression during his intracapsular subtotal tumor resections. To control intracranial pressure in a patient population that was still universally plagued by elevated intracranial pressures at the time of diagnosis, Cushing utilized a ventricular tap. Cushing performed a bilateral suboccipital craniectomy, permitting him to explore both CPAs and achieve an osseous decompression. Early access to the cerebellomedullary cistern allowed Cushing to drain cerebrospinal fluid (CSF) and decompress the cerebellar tonsils. In addition to his new approach, Cushing's meticulous operative technique included close attention to patient vital signs and persistence in achieving intraoperative hemostasis with the implementation of vessel clipping and electrocautery.

Despite Cushing's comparatively stellar immediate postoperative mortality rates, his intracapsular, partial resection approach was associated with high recurrence rates and an overall 5-year mortality rate approaching 54%. Cushing's student protégé, Walter E. Dandy, found this recurrence rate intolerable and pursued complete tumor capsule removal after employing Cushing's bilateral suboccipital craniectomy and thorough intracapsular decompression. Dandy also followed Cushing's example with respect to careful hemostatic control, clipping all vessels around the tumor to avoid bleeding complications, and became the first surgeon to achieve complete tumor removal with low operative mortality.

In 1934 Dandy abandoned the bilateral approach in favor of the unilateral suboccipital approach similar to that originally performed by Krause. In addition to his profound surgical skill, Dandy had the advantage of operating on generally smaller tumors than those addressed by previous surgeons. Cushing's popularized observance of tinnitus preceding ipsilateral hearing loss coupled with Dandy's invention and utilization of pneumoencephalography made earlier and more accurate diagnosis of CPA masses feasible. Dandy also had the opportunity to take advantage of other technologic advancements in the surgical realm, including improved cauterization technology, more reliable anesthesia, and the availability of blood transfusions. Dandy progressively improved upon his technique by utilizing ventricular and cisterna magna taps to lower CSF pressure and achieved adequate CPA access by resecting the lateral cerebellar hemisphere. In 1941 Dandy reported on 46 vestibular schwannoma complete resections with an operative mortality of just 10.8%.

The modern era of CPA surgery was ushered in by otologist William House. In 1961 House took full advantage of the newly minted instrumentation available to him, including the surgical microscope, otologic drill, and irrigation suction. Alongside neurosurgeon John B. Doyle, House pursued surgical resection of small acoustic neuromas via the middle fossa approach. With advancements in audiometric testing and improvements in x-ray techniques of the temporal bone, House and Doyle were able to identify acoustic tumors before the masses were large enough to incite hydrocephalus and papilledema. In cases where the tumor was confined to the internal auditory canal, tumor extraction and high rates of facial nerve preservation were feasible via the microsurgical middle fossa approach. However, in cases where the tumor extended into the CPA, the middle fossa approach did not permit adequate exposure for resection. Hence, House went on to partner with neurosurgeon William E. Hitselberger, and the two pioneered a microsurgical approach to the formerly abandoned translabyrinthine technique.

By 1968 House had documented 200 cases of microsurgical neuroma resection with a markedly low mortality rate of 7% and an astonishing facial nerve preservation of 88%. Neurosurgeons of the time, including Rand and Kurze, also aimed to perfect the microsurgical suboccipital approach. By 1995 collective efforts at technique improvement and the advent of new diagnostic imaging, including the computed tomography (CT) and magnetic resonance imaging (MRI), yielded an overall reduction in the mortality associated with acoustic neuroma surgery of less than 0.5% and facial nerve preservation nearing 90%.

Over time, it became clear that acoustic neuromas are neither tumors of the cochlear nerve nor have the neuron as the cell of origin. Rather, the tumor arises from Schwann cells of the vestibular nerve. To be more accurate, an acoustic neuroma is now referred to as a vestibular schwannoma.

Cerebellopontine Angle Anatomy

The CPA in the posterior fossa is the region adjoining the cerebellum, brainstem, and adjacent skull base. The neural elements of the CPA exit the brainstem and pass through the cerebellopontine cistern as they course toward the skull base.

The trigeminal, facial, and vestibulocochlear nerves arise between the superior and inferior limbs of the cerebellopontine fissure, the sulcus between the pons, middle cerebellar peduncle, and the cerebellum. The trigeminal nerve courses anterior toward Meckel cave, where it divides into its three branches. The sensory rootlets of the trigeminal nerve, termed portia major, are lateral and inferior to the motor rootlets, termed portia minor . The facial and vestibulocochlear nerves exit the brainstem adjacent to one another in the pontomedullary fissure with the facial nerve anterior and medial to the vestibulocochlear nerve. These two then pass anterolaterally toward the internal auditory canal. The glossopharyngeal and vagal nerves are found in the caudal region of the CPA as they course toward the jugular foramen.

The vascular elements of the cerebellopontine (CP) angle originate from the basilar artery, which runs along the ventral aspect of the brainstem. The anterior inferior cerebellar artery (AICA) is the main artery of the CP angle, though occasionally the caudal trunk of the superior cerebellar artery (SCA) can descend into the CP angle. The AICA bifurcates in the cerebellopontine cistern to form a rostral and caudal trunk; the rostral trunk supplies the middle cerebellar peduncle and superior surface of the cerebellopontine fissure, whereas the caudal trunk supplies the inferior ventral surface of the cerebellum. The superior petrosal vein, also known as the Dandy vein, is the largest vein in the CP angle and is located rostrally inferior to the tentorium. This vein drains into the superior petrosal sinus, which courses along the petrous ridge.

The internal auditory canal, located in the temporal bone, is the conduit from the cerebellopontine cistern to the temporal bone. The porus acousticus is the opening to the internal auditory canal (IAC) from the CP angle cistern. There are five nerves in the IAC: the facial nerve, the cochlear nerve, the superior vestibular nerve, the inferior vestibular nerve, and the nervus intermedius. The vascular portion of the IAC consists of the labyrinthine artery, a branch of the AICA, which supplies the cochlea and inner ear. Occasionally, AICA itself can loop into the IAC.

The location of the nerves within the IAC is constant and organized by quadrants. The facial nerve is located anterosuperiorly in the IAC with the cochlear nerve in the anteroinferior quadrant. Posteriorly, the superior vestibular nerve is rostral to the inferior vestibular nerve. The transverse crest delineates the superior and inferior IAC, whereas Bill's bar separates the facial nerve in the anterosuperior quadrant from the superior vestibular nerve in the posterosuperior quadrant. The nervus intermedius runs in close association with the facial nerve in the anterosuperior quadrant, and it exits the IAC alongside the facial nerve through the facial canal.

Surgical Approaches to the Cerebellopontine Angle