Review article   Otolaryngologic Clinics of North America Volume 35 • Number 2 • April 2002

Acoustic neuroma:  Assessment and management

Steven Y. Ho, MD, John F. Kveton, MD

Section of Otolaryngology–Head and Neck Surgery Department of Surgery Yale University School of Medicine
New Haven, CT 06520, USA

Acoustic neuroma results from abnormal proliferation of Schwann cells. These tumors originate in the region of Scarpa's ganglion at the junction of peripheral and central myelin of the vestibular nerve located in the internal auditory canal (IAC)  The bony confine of the IAC houses cranial nerves VII and VIII. The presence of tumor mass compresses these structures. The growing tumor mass also may prolapse into the cerebellopontine angle (CPA). With continued growth, the tumor eventually compresses on the brainstem and cerebellum. Despite the benign nature of these tumors, the clinical course of this disease may be fraught with complications.

Classification

In the past, acoustic neuroma was thought to be mainly a disease of the superior vestibular nerve; however, a recent study shows that they arise with nearly equal frequency on both the superior and inferior vestibular nerves

Acoustic neuroma can be classified into two categories. One type is associated with neurofibromatosis type 2 (NF-2). In this rare, autosomal dominant disorder, the acoustic neuromas are bilateral. In addition to the bilateral acoustic neuromas, NF-2 is associated with other cranial nerve neuromas, meningiomas, and juvenile cataract . These patients usually present in their teens or early adulthood.

The more common type of acoustic neuroma occurs sporadically   This type is always unilateral. These patients tend to present later in life and do not have other associated tumors as in NF-2.

Epidemiology

Acoustic neuroma occurs in all parts of the world without ethnic predilection . No specific risk factor has been identified. The true incidence of this disease is unclear. The best estimate according to the NIH consensus places the incidence at 1 per 100,000 individuals per year in the United States  , which translates to 2000 to 3000 new cases diagnosed per year in the United States; however, three large autopsy series have identified these tumors in 0.8% to 1.7% of patients . Furthermore, Anderson  reported a 0.07% prevalence of acoustic neuromas in a review of 24,000 brain MR images. The incidence of NF-2, on the other hand, is well documented to be at 1 in 40,000 .

Presentation and physical findings

Hearing loss is the most common finding, occurring in more than 95% of patients over the course of this disease .The typical presentation is a slowly progressive, high-frequency sensorineural hearing loss (SNHL) on the affected side. More importantly, the reduced speech discrimination occurs out of proportion to the pure tone loss. Episodic and sometimes permanent, sudden hearing loss also has been reported as the presenting symptoms in as many as 25% of patients, as reported by the group at the University of California at San Francisco (UCSF) and the Otology Group of Nashville .

Normal hearing, as documented by audiogram, occurs in as many as 12% of patients with acoustic neuromas at the time of diagnosis. The incidence of normal hearing in these patients has dramatically increased over the past 2 decades, mostly because of the advent of MR imaging . With this technique, smaller tumors that would have otherwise escaped other means of surveillance are routinely detected. The mechanism of hearing loss is believed to be nerve compression and stretching in the cases of progressive hearing loss   Sudden hearing loss is believed to be caused by the occlusion of internal auditory artery, and end artery supplying the cochlear

Tinnitus also occurs frequently, being reported by 70% of patients in the Mayo Clinic group . It is typically continuous, high pitched, and unilateral. The onset of tinnitus commonly coincides with the hearing loss; both symptoms are consequences of cochlear nerve dysfunction. Of note is that patients with acoustic neuromas seldom present with complaints of tinnitus without concomitant hearing loss. Unilateral tinnitus, in the presence of hearing loss, warrants investigation. The mechanism of tinnitus is believed to be identical to that of hearing loss, namely neural or vascular insults.

Vestibular symptoms in the form of vertigo or dysequilibrium are present in more than 50% of patients. Earlier series report a much higher rate of vertigo  , whereas recent series quote only 19% of patients suffering from vertigo   Dysequilibrium is a more common symptom, occurring in 48% of the patients  . It is frequently unremitting but usually not disabling. The UCSF group reported that vertigo is a more common symptom in patients with smaller tumors, whereas dysequilibrium is more common in patients with larger tumors. These findings indicate that vertigo is likely to be present early in the disease, whereas dysequilibrium is a late finding; however, only a minority of patients with acoustic neuromas actually develop vertigo.

Facial nerve motor dysfunction is a rare initial presenting symptom, accounting for only 2% of patients. Both hyperfunction of the nerve in the form of twitching and hypofunction in the form of paresis can occur. Commonly, both forms are concurrently present . These dysfunctions tend to occur late in the course of disease and are rarely associated with small tumors. The presence of facial nerve dysfunction in a small tumor should lead one to suspect nonacoustic CPA tumor, such as facial neuroma. Facial nerve sensory dysfunction is far less meaningful because of difficulty in measuring and identifying the deficit; however, hypesthesia of the concha and external auditory canal as reported by Hitselberger in 1966 is present in as many as 95% of patients.

Trigeminal nerve dysfunction may be identified in as many as 50% of patients . Trigeminal nerve dysfunction most commonly manifests as an absent corneal reflex; however, patients rarely notice this deficit. The most common complaint associated with trigeminal nerve dysfunction is numbness or tingling of the malar eminence . Rarely does a patient present with facial pain. Trigeminal nerve symptoms correlate strongly with the tumor size. In patients with tumors of less than 1 cm in diameter, no trigeminal nerve symptom is identified, whereas up to 48% patients with tumors of more than 3 cm in diameter have these symptoms. In patients with extremely large tumors, weakness and even atrophy of muscles of mastication can be seen.

Patients with acoustic neuromas can present with symptoms of increased intracranial pressure. Headache, nausea, vomiting, and obtundation are associated with increased intracranial pressure. They manifest in the late stage of the disease and are gradual and persistent in nature. Headache secondary to acoustic neuroma is usually a sequela of hydrocephalus. There seems to be a predominance of frontal and occipital headaches on the ipsilateral side. The pain is usually worse in the morning and is variable in terms of location, duration, and quality  . The UCSF group reported that, among patients with tumors smaller than 1 cm in diameter, no patient reported headache. On the contrary, 48% of patients with tumors greater than 3 cm in diameter reported headache during their illness  .

Brainstem and cerebellar compressive symptoms present in the very late stage of the disease with massive tumor. Brainstem compressive symptoms manifest as ipsilateral upper or lower extremity dysfunction, whereas cerebellar symptoms include ataxia and gait disturbance. These findings are rare because of improved diagnostic modalities.

Lower cranial nerve palsies, such as dysarthria, hoarseness, aspiration, or dysphagia, are rarely associated with isolated acoustic neuromas. In the presence of these symptoms, jugular foramen tumors, such as schwannomas, must be sought, especially in patients with NF-2.

The clinical presentation of acoustic neuroma is quite variable. Most patients with small tumors complain of unilateral hearing loss, tinnitus, and vestibular symptoms. Patients with larger tumors may develop trigeminal nerve dysfunction, facial nerve dysfunction, and symptoms of increased intracranial pressure. Finally, with continued growth, brainstem and cerebellar compressive symptoms appear.

Imaging assessment

Plain radiography is no longer used for detecting acoustic neuroma. The deep recess in which the acoustic neuroma originates makes plain radiography difficult and confusing to interpret.

Computed tomography has a limited role in detecting acoustic neuroma. This technique clearly delineates the bony architecture of the petrous bone and labyrinth. Soft tissue differentiation and resolution are usually insufficient on CT to clearly delineate a small acoustic tumor. Calcification is rarely seen in an acoustic neuroma, but when present may allow for differentiation of the tumor from the surrounding nerves by CT scanning; however, the presence of calcification should raise suspicion of meningioma in this region. Contrast agent administered intravenously enhances virtually all acoustic tumors because of an altered blood–brain barrier caused by the tumor . Unfortunately, in small acoustic neuroma, this enhancement is usually obscured by the surrounding petrous bone, which is very dense and tends to overshadow the enhanced tissue. Tumors extending medial to the internal auditory canal enhance sufficiently to be delineated by CT scanning. Tumors enlarging the internal auditory canal can also be detected and delineated by CT scanning; however, these situations occur generally late in manifestation of the tumor growth.

MR imaging enhanced with gadolinium remains the most sensitive diagnostic tool available for detecting small acoustic neuroma . MR imaging demonstrates excellent soft tissue visualization but does not show bony details. Cortical bones are imaged as signal void and appear black on the constructed image. With gadolinium enhancement, a small acoustic neuroma surrounded by the petrous bone appears as a bright signal in the midst of a black background. This characteristic increases the sensitivity of this technique. Over the past decade, MR imaging technique has improved. Despite the appearance of other MR imaging techniques, such as the fast spine echo, enhanced MR imaging with gadolinium enhancement continues to be the gold standard diagnostic tool.

MR imaging with gadolinium enhancement remains the gold standard for the detection of acoustic tumors. CT scanning is limited by its ability to detect small tumor within the internal auditory canal, whereas plain radiography is no longer useful.

Diagnostic assessment

With a wide variety of clinical presentations, acoustic neuroma may be difficult to diagnose. During evaluation of the patient, certain clinical symptoms and findings should raise a clinician's suspicion of the presence of acoustic neuroma. The Otology Group has categorized the patients into three distinct risk groups based on the symptomatic presentation and audiogram findings.

Group 1 patients have limited symptoms, including isolated vertigo, historically explained unilateral hearing loss and tinnitus, or symmetric hearing loss. Patients in this category have a low risk for acoustic neuroma, calculated to be less than 5%. ABR is requested to rule out abnormality. With an abnormal ABR result, MR imaging with gadolinium enhancement is used to definitively exclude the presence of acoustic neuroma.

Group 2 patients present with sudden SNHL or unexplained persistent unilateral tinnitus. Patients in this category have intermediate risk for acoustic neuromas, calculated to be between 5% and 30%. In these cases, MR imaging with gadolinium should be the initial study to definitively rule out the presence of acoustic neuroma.

Group 3 patients present with unilateral asymmetric SNHL, tinnitus, and decreased discrimination. Patients in this category have a high risk for acoustic neuroma, calculated to be greater than 30%. Again, MR imaging with gadolinium is the initial study to definitively rule out the presence of acoustic neuroma. With a negative MR imaging result, the patient should be closely followed up with periodic ABR to rule out the development of acoustic neuroma.

Natural history of acoustic neuroma

The natural history of untreated acoustic neuroma remains unpredictable. No factors predictive of tumor growth have been identified. Small tumor size and advanced age are not predictors of slow tumor growth. Most of the recent studies demonstrate slow growth of acoustic neuroma ranging from 0.1 to 0.23 cm/y  . Less than 30% of untreated acoustic neuromas demonstrate growth greater than 0.2 cm/y on MR imaging; however, tumors larger than 2.0 cm are statistically more likely to grow .

The rate of tumor growth in a given patient is consistent over the course of disease. The rate of tumor growth at the start of the follow-up predicts the future tumor growth rate. The tumor growth pattern during the first 3 years after the diagnosis has been found to remain relatively stable over the course of the disease .

Acoustic neuroma growth is variable and unpredictable. The available clinical information provides no clue to the rate of tumor growth. Most patients demonstrate less than 0.2 cm/y of tumor growth, with less than 30% of patients demonstrating greater than 0.2 cm/y of tumor growth. The growth pattern of the tumor is fairly consistent with the pattern established during the first 3 years of the observation.

Management of acoustic neuroma

Nonsurgical treatments
Observation

With the slow growth of this tumor, close observation is an acceptable treatment option in a select group of patients. For elderly and infirm patients with small tumors, observation definitely is a preferred choice until the tumor has demonstrated significant growth. In young patients, this option is controversial. Even in the absence of obvious tumor growth, the risk for useful hearing loss is significant with noninvasive management . Furthermore, the enlarged tumor increases the risks associated with microsurgical resection. Given the lack of any predictor of tumor growth and lack of any specific clinical signs associated with tumor growth, periodic MR imaging is absolutely necessary in this setting. This routine imaging may become a cost issue over time.

Stereotactic radiosurgery

Stereotactic radiosurgery or gamma knife has been introduced as an alternative treatment modality to microsurgical resection. This technique uses a focused radiation beam targeting the tumor mass while attempting to preserve the surrounding normal tissue. The radiosurgery is usually performed in a single session on an outpatient basis. The expected outcome of radiosurgical therapy for acoustic neuroma is cessation of tumor growth, occurring in 43% to 74% of cases over 3-year follow-up . The prevalence of continued tumor growth after radiosurgery has been reported to be up to 12% in these studies . Regression of tumor after this therapy has been noted.

Complications of this therapy are identical to those of microsurgical resection. The most frequently reported complications associated with radiotherapy are facial palsy and trigeminal neuropathy. The reported prevalence of facial palsy is 20% to 34%. The mean onset of facial palsy is usually 6 months, and it is usually temporary. Trigeminal neuropathy has similar prevalence of 16% to 30% . Unlike the temporary facial palsy, most of the report stated persistent trigeminal neuropathy after treatment. In addition to facial neuropathy and trigeminal neuropathy, hydrocephalus has also been associated with radiotherapy. The reported rate varies but is believed to be approximately 5% . Furthermore, surgical removal of these tumors after radiation therapy has been noted to be more difficult and fraught with complications. Finally, the NIH consensus statement indicates that the microsurgical resection remains the treatment of choice.

Surgical management

Three surgical approaches to the acoustic neuroma are routinely used: (1) the middle cranial fossa approach, (2) the suboccipital approach, and (3) the translabyrinthine approach. The first two approaches permit hearing preservation, whereas the last approach results in loss of hearing in the affected ear. With the advent of MR imaging, smaller acoustic tumors are routinely detected. This phenomenon results in an increasing number of patients presenting with mild hearing loss  . Consequently, hearing preservation surgery is increasingly used. In addition to tumor removal, facial nerve preservation is a major goal in any of these surgical approaches.

The benefit of the intraoperative facial nerve monitoring in facial nerve preservation is clear. It is now the standard of care as recommended by the NIH Consensus Conference ; however, for monitoring to be useful, it must be of high quality and the surgeon must be willing to modify his or her technique based on the intraoperative changes. With hearing preservation approaches, multiple intraoperative monitoring techniques have been reported. ABR and cochlear nerve action potential are the primary techniques used. With intact waveforms, specifically wave I and V, at the end of procedure, the likelihood of hearing preservation is excellent; however, exceptions have been reported.

Middle fossa approach is a technically difficult approach because of the paucity of landmarks and limited field of view. This approach is applicable in patients with small tumors—less than 5 mm extension into the CPA. The hearing conservation nature of the approach requires good preoperative hearing. The suggested criteria include a speech reception threshold of better than 30 dB and a speech discrimination score of better than 70% . With more hearing loss, the translabyrinthine approach should be considered as the technique of choice. Hearing preservation, as defined by preservation within 15 dB of the preoperative level, is reported at 52%, with 68% retaining serviceable hearing. Ninety-five percent of patients retain facial nerve functions at House-Brackmann grade I and II levels

The suboccipital approach, also known as the retrosigmoid approach, is a versatile approach to the internal auditory canal and the CPA. The approach provides an excellent view of posterior fossa; however, the lateral aspect of the internal auditory canal cannot be adequately visualized using this approach. The neurosurgical team performs the craniotomy and exposes the tumor and the posterior surface of the petrous bone. The CPA portion of the tumor is dissected off of the brainstem. The neuro-otologist then opens the internal auditory canal and removes the remainder of the tumor. Following the removal, the neurosurgical team completes the closure. This approach can be applied to tumors of any size. The potential hearing conservation dictates serviceable preoperative hearing status. The suggested audiometric requirements are identical to those listed for middle fossa approach. The reported hearing preservation ranges from 25% to 58%. The rate of hearing preservation seems to be independent of the tumor size and continues to hover around 50%. Facial nerve preservation, on the other hand, seems to correlate with the tumor size. Most series report that well over 96% of patients retained facial nerve function at House-Brackmann grade I and II levels with small tumors. With tumors larger than 4 cm in diameter, 38% of the patients retain those levels of facial nerve function

Translabyrinthine approach is the most direct surgical approach to the CPA. With this approach, minimal cerebellar retraction is needed for exposure. Furthermore, the lateral extent of the internal auditory canal can be completely exposed with this approach. Acoustic neuromas ranging in size from 0.5 to 6.5 cm have been successfully removed with this approach. Because no hearing preservation is possible, patients with nonserviceable hearing are primary candidates for this approach, regardless of tumor size. Large tumor with significant intracanalicular extension should also be approached with this technique. Regardless of the size of tumors removed, 83% of patients retained facial nerve function at House-Brackmann grade I and II levels at 1-year follow-up . With tumor greater than 3 cm, 52.6% of the patients still maintain those levels of facial nerve function at 1-year follow-up.

Most acoustic tumors are approached by one of the following three techniques: (1) middle fossa, (2) suboccipital, or (3) translabyrinthine. In patients with serviceable hearing, hearing conservation approaches are considered. In patients with small tumors, the middle fossa approach is an option if the surgeon is experienced in this technique. Similarly, all patients with serviceable hearing are candidates for the suboccipital approach in an attempt to preserve hearing . In patients with nonserviceable hearing, the translabyrinthine approach offers the most direct route to the resection of acoustic tumors, regardless of the size . The reported hearing preservation in middle fossa approach is 35%, whereas that of the suboccipital approach is approximately 50%. Facial nerve function in all three approaches depends on the size of the tumor, with the reported rate of more than 80% in all patients. Intraoperative facial nerve monitoring has demonstrated its benefits in facial nerve preservation, whereas intraoperative nerve VIII monitoring has not accurately predicted the postoperative hearing preservation.