Microanatomical variations in the cerebellopontine angle.pdf

Neurosurg Focus 5 (3): Article 1, 1998

Microanatomical variations in the cerebellopontine angle associated

with vestibular schwannomas (acoustic neuromas)

Prakash Sampath, M.D., David Rini, M.F.A., and Donlin M. Long, M.D., Ph.D.

Departments of Neurological Surgery and Art as Applied to Medicine, Johns Hopkins School of

Medicine, Baltimore, Maryland

Great advances in neuroimaging, intraoperative cranial nerve monitoring, and microsurgical technique

have shifted the focus of acoustic neuroma surgery from prolonging life to preserving cranial nerve

function in patients. An appreciation of the vascular and cranial nerve microanatomy and the intimate

relationship between neurovascular structures and the tumor is essential to achieve optimum results. In

this paper the authors analyze the microanatomical variations in location of the facial and cochlear nerves

in the cerebellopontine angle (CPA) associated with acoustic neuromas and, additionally, describe the

frequency of involvement of surrounding neural and vascular structures with acoustic tumors of varying

size. The authors base their findings on their experience treating 1006 consecutive patients who

underwent surgery via a retrosigmoid or translabyrinthine approach.

Between July 1969 and January 1998, the senior author (D.M.L.) performed surgery in 1022 patients for

acoustic neuroma: 705 (69%) via the retrosigmoid (suboccipital); 301 (29%) via translabyrinthine; and

16 (2%) via middle fossa approach. Patients undergoing the middle fossa approach were excluded from

the study. Patients were subdivided into three groups based on tumor size: Group 1 tumors (609 patients

[61%]) were smaller than 2.5 cm; Group 2 tumors (244 patients [24%]) were between 2.5 and 4 cm; and

Group 3 tumors (153 patients [15%]) were larger than 4 cm. Operative notes were analyzed for each

patient. Relevant cranial nerve and vascular "involvement" as well as anatomical location with respect to

the tumor in the CPA were noted. "Involvement" was defined as intimate contact between neurovascular

structure and tumor (or capsule), where surgical dissection was required to free the structure. Seventh

and eighth cranial nerve involvement was divided into anterior, posterior, and polar (around the upper or

lower pole) locations. Anterior and posterior locations were further subdivided into upper, middle, or

lower thirds of the tumor.

The most common location of the seventh cranial nerve (facial) was the anterior middle third of the

tumor for Groups 1, 2, and 3, although a significant number were found on the anterior superior portion.

The posterior location was exceedingly rare (< 1%). Interestingly, patients with smaller tumors (Group 1)

had an incidence (3.4%) of the seventh cranial nerve passing through the tumor itself equal to that of

patients with larger tumors. The most common location of the seventh cranial nerve complex was the

anterior inferior portion of the tumor. Not surprisingly, larger tumors (Group 3) had a higher incidence of

involvement of sixth cranial nerve (41%), fifth cranial nerve (100%), ninth-11th cranial nerve complex

(99%), 12th cranial nerve (31%), as well as superior cerebellar artery (79%), anterior inferior cerebellar

artery (AICA) trunk (91.5%), AICA branches (100%), posterior inferior cerebellar artery (PICA) trunk

(59.5%), PICA branches (79%), and the vertebral artery (93.5%). A small number of patients in Group 3

also had AICA (3.3%), PICA (3.3%), or vertebral artery (1.3%) vessels within the tumor itself.

In this study, the authors show the great variation in anatomical location and involvement of

neurovascular structures in the CPA. With this knowledge, they present certain technical lessons that

may be useful in preserving function during surgery and, in doing so, hope to provide neurosurgeons and

neurootologists with valuable information that may help to achieve optimum cranial nerve outcomes in

patients.

Key Words * vestibular schwannoma * acoustic neuroma * anatomy * microsurgical *

cerebellopontine angle

Since Sir Charles Ballance first described the successful removal of an acoustic tumor in 1894,[4]

surgeons have been challenged by lesions of the cerebellopontine angle (CPA) and have striven to

improve outcomes in patients. In recent years, great advances in neuroimaging, cranial nerve monitoring,

and microsurgery have shifted the focus of acoustic neuroma surgery from prolongation of patient life to

preservation of cranial nerve function,[37] which has led to extraordinary improvements in patient

outcomes, with reports from modern series of up to 90% postoperative normal or near-normal facial

nerve function[1,3,19,22,25,33,36] and 40% hearing preservation.[2,7,12,15,16,21,23,27,34] A

significant number of patients, however, still develop postoperative cranial nerve dysfunction.

Consequently, further efforts to refine operative techniques and improve intraoperative monitoring to

preserve maximum cranial nerve function continue to be made. This need is underscored by the

challenge to the traditional treatment of acoustic tumors, which is gross-total microsurgical resection, by

newer treatment strategies such as subcapsular partial removal,[20] conservative observation with serial

imaging,[5] and radiosurgery.[8,9,14,26,39]

The difficulty in microsurgical removal of acoustic tumors can be attributed, in part, to the great

anatomical variation in the location of cranial nerve and vascular structures associated with the tumor

capsule in the CPA.[31] To date, attention has generally been directed to the location of cranial nerves

distal to the tumor in the lateral aspect of the internal auditory canal (IAC) after removal of the posterior

meatal wall (porus acusticus).[24,31,32,38] At this site, there is a consistent set of relationships of facial

and vestibulocochlear nerves. The facial nerve can be identified anterior to the vertical crestae, separating

the facial nerve from the superior vestibular nerve, and above the transverse plate, which separates the

cochlear nerve and inferior vestibular nerve from the facial and superior vestibular nerves. Less attention

has been given to the microanatomy of the brainstem surface facing an acoustic tumor.[28-30] Here

again, consistent landmarks on the pons, medulla, and cerebellum, such as the pontomedullary sulcus, the

foramen of Luschka, the flocculus, the cerebellopontine and cerebellomedullary fissures and nerve root

entry zones of the glossopharyngeal, vagus, and accessory nerves, can be very helpful in identifying

displaced cranial nerves on the medial aspect of the tumor capsule. It is in the CPA itself, however, that

the greatest variation in cranial nerve displacement can be found, especially for the facial and cochlear

nerves. Moreover, the paucity of consistent landmarks in this area can lead to inadvertent injury of these

and other important neurovascular structures.[17,18]

The purpose of this paper is to identify the different anatomical locations of the facial and cochlear

nerves associated with the tumor capsule of acoustic tumors in the CPA. In addition, we describe the

frequency of involvement of surrounding neural and vascular structures with acoustic tumors of varying

size. This in-depth analysis is based on careful documentation of cranial nerve and vascular anatomy

during microsurgical removal of acoustic tumors in 1006 patients over a 29-year period. With this

knowledge, we outline a number of intraoperative techniques that can help surgeons avoid injuring

cranial nerves. We hope to provide neurosurgeons and neurootologists with information that may be

valuable in the surgical handling of these complex lesions and may allow for the best possible cranial

nerve outcomes in patients.

CLINICAL MATERIAL AND METHODS

Between July 1969 and January 1998, the senior author (D.M.L.) performed microsurgical resection of

acoustic neuromas in 1022 consecutive patients. The operative approach was described in detail for each

patient and was dictated by the size of the tumor and the patient's preoperative hearing status. In patients

with absent hearing and tumors of small-to-moderate size, a translabyrinthine approach was used. In

patients with preserved hearing or moderate-to-large tumor size, a retrosigmoid or suboccipital approach

was used. In selected patients with small intracanalicular tumors and intact hearing, a middle fossa

approach was used. For this report, patients who underwent middle fossa approaches were not studied

further because their tumors were small and located outside the CPA.

The remaining 1006 patients were divided in three groups according to tumor size. Group 1 included

patients with tumors that were smaller than 2 cm in maximum dimension; Group 2 combined patients

with intermediate-sized tumors from 2.5 to 4 cm; and Group 3 included patients with tumors that were

larger than 4 cm in maximum dimension.

Lesions were resected by the senior author by using standard microsurgical technique. All patients

undergoing suboccipital (retrosigmoid) approaches to their tumor underwent operation in the lateral

(park-bench) position as described previously.[6,22,30,35] Translabyrinthine approaches were performed

in conjunction with a neurootological team, as described elsewhere.[13]

At the time of operation, the frequency of adjacent cranial nerve and arterial and venous structure

involvement with the tumor capsule was identified and documented. In addition, the exact anatomical

location of the facial and cochlear nerves on the tumor capsule in the CPA was noted. Operative notes

were analyzed for each patient when available. Involvement was defined as intimate contact between

cranial nerve or vascular structure and the tumor capsule, where surgical manipulation or dissection was

required to free the structure; neurovascular structures without actual adherence to the tumor were not

documented. The cranial nerve type was confirmed by intraoperative monitoring (that is, facial nerve

electromyography or brainstem auditory potential [BAEP] monitoring) whenever possible. Otherwise,

identification of the cranial nerve was based, after careful inspection, on the impression of the surgeon.

Cochlear and facial nerve involvement was divided into anterior, posterior, and polar (around the upper

or lower pole) locations. In addition, anterior and posterior locations were further subdivided into upper,

middle, or lower thirds of the tumor capsule. When possible, the origin of the tumor from either the

superior or inferior vestibular nerve was noted.

Occasionally, a cranial nerve or vessel was seen to pass through the tumor itself or to become completely

enfolded by the tumor. In a minority of cases, the tumor also was seen to infiltrate into cranial nerve

sheaths.

RESULTS

Of the 1006 patients studied over a 29-year period, 705 (70%) were treated via the suboccipital

(retrosigmoid) approach, and 301 (30%) via the translabyrinthine approach. When tumor size was

considered, 609 patients (61%) had small-sized tumors (Group 1); 244 (24%) had intermediate-sized

tumors (Group 2); and 153 (15%) had large-sized tumors (Group 3). More tumors appeared to arise from

the superior vestibular nerve (410 patients [40.75%]) than from the inferior vestibular nerve (372 patients

[37%]), although this could not be determined in 224 patients (22.26%).

Frequency of Neural Involvement

Facial Nerve. The most common location of the facial nerve was on the anterior middle third of the

tumor capsule, regardless of the tumor size (Fig 1).

Fig. 1. Artist's illustratings depicting the location of the facial nerve in the CPA in Group 1

(upper), Group 2 (center), and Group 3 (lower) tumors.

In Group 1 patients, the facial nerve was found on the anterior middle portion of the tumor in 244 cases

(40%); the anterior superior third in 204 (33.5%); the anterior inferior third in 30 (4.9%); the superior

pole in 85 (14%); and the inferior pole in 15 (2.5%). The posterior location was rare and was found in

only 10 patients (1.6%): five facial nerves (0.8%) were located on the posterior superior, two (0.3%) on

the posterior middle, and three (0.5%) on the posterior inferior portion of the tumor. The facial nerve

passed through the tumor itself in 21 patients (3.4%): 15 (2.5%) had tumor infiltrating the nerve sheath,

whereas six (0.9%) had the tumor enfolding the nerve completely. In Group 2 patients, the facial nerve

was found on the anterior middle portion of the tumor in 98 cases (40.2%); the anterior superior third in

81 (33.2%); the anterior inferior third in 12 (4.9%); the superior pole in 34 (13.9%); and the inferior pole

in six (2.5%). Posterior location was found in six patients (2.5%): two facial nerves (0.8%) were located

on the posterior superior, two (0.8%) on the posterior middle, and two (0.8%) on the posterior inferior

portion of the tumor. The facial nerve passed through the tumor itself in seven patients (2.9%): three

(1.2%) had tumor infiltrating the nerve sheath, whereas four (1.6%) had the tumor enfolding the nerve

completely. In Group 3 patients, the facial nerve was found on the anterior middle portion of the tumor in

61 cases (39.8%); the anterior superior third in 50 patients (32.7%); the anterior inferior third in eight

(5.2%); the superior pole in 21 (13.7%); and the inferior pole in four (2.6%). Posterior location was

found in four patients (2.6%): two facial nerves (1.3%) were located on the posterior superior, one

(0.65%) on the posterior middle, and one (0.65%) on the posterior inferior location. The facial nerve

passed through the tumor itself in five patients (3.3%): four (2.6%) had tumor infiltrating the nerve

sheath, whereas only one (0.65%) had the tumor enfolding the nerve completely.

Cochlear Nerve. The cochlear nerve, not surprisingly, was found on the anterior inferior portion of the

tumor in the majority of cases (Fig. 2).

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