25.1 Introduction

Acoustic neuromas, or vestibular schwannomas, are the most common masses found in the cerebellopontine angle (CPA), accounting for approximately 75% of lesions in this location and about 6% of all intracranial tumors combined. Meningiomas and epidermoid tumors are the next most common masses found in the CP angle and must be considered in the differential diagnosis of a newly discovered CP angle mass.1

Symptoms of patients who have acoustic neuromas closely correlate with the size of the tumor. The three most common presenting symptoms are hearing loss, tinnitus, and disequilibrium.2 Larger tumors may produce facial numbness, weakness, or twitching. If tumors become very large, then compression of the brainstem is possible, which can cause weakness or sensory changes of the extremities or hydrocephalus from compression of the fourth ventricle.

In a series of 131 patients, 66% had no abnormal physical findings except for hearing loss on the ipsilateral side of the acoustic neuroma.2 Patients will often complain of inability to use the telephone in the affected ear. Lateralization of Weber’s test to the normal ear and normal or indeterminate Rinne’s tests can be used to confirm sensorineural hearing loss. Facial nerve function should be graded on the House-Brackmann scale.3 Excluding hearing loss, the next three most common abnormal signs in patients who have acoustic neuromas are abnormal corneal reflex, nystagmus, and facial hypoesthesia.2

A pure tone audiogram is often the first screening test for patients when the diagnosis of acoustic neuroma is suspected. High-frequency hearing loss is the most common abnormality seen on pure tone audiometry in patients who have these lesions.4 Care must be taken in interpretation of pure tone audiogram results, however, as this is also the most common type of hearing loss with age and from noise exposure. In general, a hearing difference of greater than 10 to 15 dB from one ear to the other warrants further investigation. Notably, Johnson et al found that the likelihood of abnormal audiometry correlates with acoustic neuroma size.4

Another important aspect of the pure tone audiogram and clinical assessment is speech discrimination, which is not always related to the degree of pure tone hearing loss. Some patients may have exceptionally poor speech discrimination despite near-normal pure tone audiometry.5 Speech discrimination is an integral part of the clinical decision-making process, especially when determining the optimal surgical approach. When hearing in the contralateral ear is normal, residual hearing on the operated side is socially useful only if speech discrimination is good and the pure tone audiogram is within 30 dB of the normal side.5

MRI has been used to characterize the natural history of acoustic neuromas. In a series of patients treated conservatively and followed with serial scans, the average rate of tumor growth was 0.91 mm per year.6 Of these patients, 42% experienced no growth or reduction in tumor size. Overall postoperative growth rate for patients who underwent a subtotal resection was 0.35 mm per year; 68.5% of these patients demonstrated no growth or reduction in tumor size.6

25.2 Imaging

Tremendous improvements in both the quality and sensitivity of diagnostic imaging for acoustic neuromas have been made over the past several decades. Imaging techniques have evolved from plain films to CT scans. Currently, MRI is the imaging investigation of choice when evaluating and monitoring tumors of the CP angle. The increased sensitivity for detecting CP angle tumors is affecting the pattern of patient presentation and will likely influence management strategies and outcomes.5

25.2.2 Magnetic Resonance Imaging

MRI has become the best imaging modality for acoustic neuroma diagnosis and management. Most acoustic neuromas are hypo- or isointense on T1-weighted images (T1WI) when compared with normal brain parenchyma. These tumors demonstrate marked enhancement after administration of intravenous gadolinium (Fig. 25.1). On T2-weighted images (T2WI), acoustic neuromas are usually hyperintense when compared with normal brain. Large tumors may demonstrate a component of cystic degeneration (Fig. 25.2). Fluid attenuated inversion recovery sequences (FLAIR) may demonstrate peritumoral edema from tumor compression or transependymal edema suggestive of hydrocephalus. Compared with CT imaging, MRI offers superior resolution, a lack of beam-hardening artifacts, the ability to image the tumor in multiple planes, and the ability to identify adjacent vascular structures and possible vascular displacement or encasement.5

25.3 Surgical Anatomy

The anatomy of the CP angle was described in great detail several years ago by Rhoton.8 Briefly, the CP angle cistern is bound laterally by the petrous face, medially by the pons, and superiorly by the tentorium cerebelli.5 The cerebellopontine fissure opens medially and has superior and inferior limbs that meet at a lateral apex. Cranial nerves (CNs) IV through XI are located within the CP angle.8 The superior cerebellar artery and anteroinferior cerebellar artery (AICA) both arise medially from the basilar artery and course through the CP angle cistern.1 Veins from the pons, middle cerebellar peduncle, and cerebellopontine fissure unite near the trigeminal nerve and form the superior petrosal venous complex.9

The trochlear and trigeminal nerves are located in the superior CP angle, whereas the glossopharyngeal, vagus, and accessory nerves are located in the inferior CP angle. The abducens nerve is located near the base of the fissure.8 Although the facial and vestibulocochlear nerves may appear to pass as a single bundle from the pontomedullary junction to the internal auditory meatus, they are separate and distinct. Within the vestibulocochlear nerve complex, the superior and inferior vestibular nerves lie posteriorly and superiorly, whereas the cochlear nerve lies posteriorly and inferiorly. A shallow groove marks the boundary between them. The facial nerve normally lies anteriorly and slightly superiorly, separated from the vestibular nerves by the labyrinthine artery.5 Specific anatomical relationships, however, are often distorted in larger tumors.

The internal auditory meatus houses the facial, cochlear, and inferior and superior vestibular nerves. The lateral portion of the meatus is divided into a superior and an inferior portion by a horizontal ridge called the transverse or falciform crest. The facial and the superior vestibular nerves are superior to the crest. The facial nerve is anterior to the superior vestibular nerve and is separated from it at the lateral end of the meatus by a vertical ridge of bone, called the vertical crest (Bill’s bar, named after William House).10 The cochlear and inferior vestibular nerves run below the transverse crest, with the cochlear nerve located anteriorly.

A consistent set of anatomical relationships at the brainstem facilitates the identification of the CNs on the medial side of an acoustic neuroma.11 The facial and vestibulocochlear nerves arise from the brainstem near the lateral end of the pontomedullary sulcus, anterosuperior to the choroid plexus protruding from the foramen of Luschka.8 In most cases, the AICA passes below the facial and vestibulocochlear nerves and would be displaced inferiorly by an acoustic neuroma. The labyrinthine, recurrent perforating, and subarcuate branches arising from the AICA are frequently stretched around a large acoustic neuroma.8 Venous structures that have a predictable relationship to the facial and vestibulocochlear nerves are the vein of the pontomedullary sulcus and the veins of the cerebellomedullary fissure, middle cerebellar peduncle, and cerebellopontine fissure.12 Identification of any of these veins during the tumor removal allows for easier identification of the CNs leaving the brainstem.

25.4 Treatment Considerations

Careful preoperative preparation and evaluation of a patient who has an acoustic neuroma incorporates clinical symptoms and signs, pure tone audiogram and speech discrimination testing results, and diagnostic imaging. Treatment options include observation, surgical intervention, and stereotactic radiosurgery. Surgical options consist of the hearing-preserving approaches of the retrosigmoid craniotomy and the middle fossa craniotomy as well as the hearing-sacrificing approach of the translabyrinthine craniotomy. A substantial amount of literature exists regarding diverse treatment paradigms of patients with acoustic neuromas.13 ^,​ 14 ^,​ 15

The general management scheme at our center has been refined over several decades.16 Patient age factors heavily into the decision-making process. Young patients (those 40 years old or younger) are generally advised to undergo surgical intervention with the goal of total excision. Middle-aged patients (those aged 41–70) in otherwise good health are typically offered surgery, although stereotactic radiosurgery is a viable option, especially in the case of small tumors with few symptoms. Gross total resection of the acoustic neuroma in these patients is the goal, but it is pursued less aggressively than in younger patients. In older patients (those older than 70 years of age), small tumors are generally followed using serial MRIs and treated using stereotactic radiosurgery if there is evidence of tumor growth. Older patients who have symptomatic, large tumors are offered surgery for tumor debulking followed by stereotactic radiosurgery.

The retrosigmoid approach is the workhorse of acoustic neuroma surgery. This hearing-preserving approach is selected for patients who have small tumors (< 2.5 cm) and good hearing (< 50 dB on pure tone average [PTA] and > 50% speech discrimination), as well as for patients who have large tumors (> 2.5 cm) on the side of their only hearing ear. Additional considerations are the amount of tumor within the CP angle and the amount of intracanalicular tumor (Fig. 25.3; Fig. 25.4).

In cases of tumor extension far laterally into the internal auditory canal, a large amount of drilling into the petrous temporal bone is required via a retrosigmoid approach, which increases the risk of injury to labyrinthine structures. For small tumors confined to the internal auditory canal that have no discernable CP angle component, a middle fossa craniotomy is the preferred hearing-preserving approach (Fig. 25.5). The translabyrinthine approach is a hearing-sacrificing approach that is preferred in patients who have poor hearing (> 50 dB on PTA and < 50% speech discrimination) and large tumors, especially when the tumor has a large canalicular component as well as a large CP angle component (Fig. 25.6; Fig. 25.7).

During the operation, constant facial nerve electromyography (EMG) is used to detect facial nerve irritation/injury and provide the ability to directly stimulate the facial nerve so as to test for function and aid in localization. Brainstem auditory evoked responses (BAERs) are also used to assess the function of the cochlear nerve. Early warning of prolongation of BAERs can alert the surgeon of overzealous cerebellar retraction and allow for correction prior to permanent cochlear nerve damage. Selective somatosensory evoked potentials (SSEPs) and motor evoked potentials (MEPs) are also used to monitor brainstem function.