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As discussed below, the therapeutic options for acoustic neuromas include observation, surgery and radiosurgery. The optimal treatment varies according to whether the tumor is large or small, whether it has caused neurologic damage prior to treatment and on patient factors.
Because acoustic neuromas are usually slow-growing, immediate intervention is not always necessary. For patients with very small, asymptomatic tumors, elderly patients, and patients with serious medical problems, a conservative approach with observation including serial MRI studies may be reasonable.
The options for the treatment of the acoustic neuromas include surgery and radiosurgery. Our approach to surgery for acoustic neuromas is based on the Johns Hopkins experience of nearly 100 years of surgery of the brain. Experience with this type of surgery appears to be essential in minimizing the risk of complications as acoustic neuromas. With a high level of experience has come an understanding of the technical aspects of exposing the tumor, removing the tumor, and closing the surgical site in a way that minimizes neurological deficits and promotes a return to usual activities.
Thousands of patients have received radiation treatment for acoustic neuromas, several hundred at Johns Hopkins. Radiosurgery is a non-invasive treatment that uses precisely focused, narrow beams of radiation to both treat the acoustic neuroma and to reduce the dose of radiation delivered to the surrounding tissues including the hearing, balance and facial nerves.
Controversy exists regarding the optimal form of treatment for the acoustic neuromas. Small tumors that do not pose a risk to brain function and do not produce symptoms, may be watched with follow-up MRI scans to insure "control". This is most often an attractive option in older individuals with small tumors. When treatment is needed, however, the patient and their family should strive to understand the options for treatment, including the success rates, potential complications, and implications for follow-up. In some cases a patient's age, or the size of the acoustic neuroma will hold sway in determining our recommendations for treatment. In other cases, the final decision will be left to a patient and their family. In making a final decision, knowledge is key.
For both surgery and radiotherapy approaches to acoustic neuroma treatment, the important issues in the treatment of the acoustic neuromas are preservation of the facial nerve, preservation of hearing and control of the tumor. "Control of the tumor" is a phrase that should be considered carefully.
Currently, long-term follow-up after treatment that documents a high cure rate is only available for surgical removal, and only when the vast majority of the tumor is completely resected. (There is a suggestion that small "flecks" of tumor may be left without risk of regrowth, but significant portions of the tumor left behind present a significant risk of regrowth). In some cases, however, radiation treatment of the tumor may be an option. With radiation treatment, however, tumor control can only be insured with repeated scans, each year, for life. Few studies to date have documented the effects of radiation beyond 5 years. Moreover, there now exist several reports of malignancies (cancers) developing within the field of radiation treatment for acoustic neuroma. These observations underscore the need for ongoing follow-up of approaches to acoustic neuroma that involve radiation therapy.
Microsurgical resection of acoustic neuromas can be accomplished through three different operative approaches to the tumor. Traditionally the suboccipital approach has offered preservation of hearing and preservation of seventh (facial) nerve function. The success rates with hearing preservation are inversely related to tumor size (large acoustic neuromas have worse results). Hearing preservation after surgery for acoustic neuroma varies between reported groups of patients. However, a general rule is that roughly half of patients with small tumors who have useful hearing prior to surgery will maintain useful hearing following surgery.
The skull base team may favor the translabyrinthine approach for removal of the acoustic neuroma as it offers early visualization of the seventh nerve. This approach results, however, in complete loss of ipsilateral hearing in virtually all patients.
The middle fossa approach has been used for small intracanalicular acoustic neuromas in patients with intact hearing. This approach, however, may have a higher incidence of seventh nerve palsy than for the suboccipital approach, particularly if the tumor lies in a low, dependent position within the internal auditory canal.
Intraoperative monitoring of brain and nerve function is now routinely performed with all surgical procedures for the resection of acoustic neuromas at Johns Hopkins. A team of neurologists and electrophysiologists is dedicated to detecting any changes in the ability of nerves in the area of surgery, as well as the brain, to normally conduct electrical impulses. Such changes can signal the need to change the strategy of dissection, thereby enabling the surgery to proceed while avoiding neurologic complications.
Image-guided surgery brings together the skills of experienced surgeons with 2- and 3-dimensional images of the skull base obtained using CT or MRI scans. Graphic displays in the operating room link those images to the sterile instruments used by the surgeons, so that the instrument tips in real space also appear in the virtual space of the CT or MRI images. The virtual surgical field allows the surgeon to predict what lies ahead, to avoid damaging vital structures, and to assure complete tumor removal.
Most modern surgical series report complete tumor removal with both anatomic and functional preservation of the facial nerve in over 90% of patients having surgery for the acoustic neuromas (Buchman CA, Chen DA, Flannagan P, Wilberger JE, Maroon JC. The learning curve for acoustic tumor surgery. Laryngoscope 1996;106:1406-1411; Sampath P. Facial nerve injury in acoustic neuroma (vestibular schwannoma) surgery: etiology and prevention. J Neurosurg 1997;87:60-66.)
Surgery: Control of Growth
To review complications that occur during the course of acoustic neuroma surgery a retrospective case review was published (Otol Neurotol 2001 Nov;22(6):895-902 Perioperative morbidity of acoustic neuroma surgery. Slattery WH 3rd, Francis S, House KC). A series of 1,687 patients undergoing acoustic neuroma surgery between 1987 and 1997 included 822 male and 865 female patients ranging in age from 10 to 87 years (mean age at time of surgery, 50 yr; standard deviation, 14 yr). The most common surgical approach was translabyrinthine (72.5%), followed by middle fossa (25.7%). The tumors ranged in size from 3 to 7 cm in diameter (mean, 2.0 cm). The main outcome was the frequency of occurrence of all surgical and medical complications. The most common complications were cerebrospinal fluid leaks (9.4% though only 2.1% required a surgical repair of the leak as the remainder healed spontaneously) and meningitis (1.5%). The authors conclude that the findings of this study provide a basis for comparison with other treatment approaches and also are useful for preoperative patient counseling.
The surgical removal of Acoustic Neuromas that grow after radiotherapy presents a unique set of challenges. Acoustic neuromas that grow despite radiotherapy present either as active growth demonstrated on MRI scans (after radiation) or neurologic deficits, particularly facial muscle weakness and spasm that slowly worsen. In such cases, repeated radiation is not thought to be safe and surgical removal may be required.
A recent study based at Johns Hopkins documented the experience of removing acoustic neuromas that grow after radiation therapy (Lee, Westra, Staecker, Long, Niparko: Clinical and histopathological features of recurrent acoustic neuroma following stereotactic radiosurgery. Otology & Neurotology, in press, 2003). As stereotactic radiosurgery for acoustic neuroma entails uncertain long term risk with regard to growth potential and function of adjacent cranial nerves, these authors evaluated the clinical course and characteristics of the tumors in four cases of tumor growth after radiotherapy treated surgically in 2001. All four patients underwent microsurgical resection of VS following primary stereotactic radiation therapy. The report notes that highly inconsistent radiation changes in areas surrounding the regrown tumor were found. Fibrosis outside and within the tumor bed varied markedly, complicating microsurgical dissection of the tumor from adjacent nerves.
Once resected the tumors were evaluated for their cellular architecture. Viable tumor was present in all cases. Despite the scarring of the areas surrounding the tumor, there was no significant scarring or other changes within the tumor. That is, for unknown reasons, these tumors appeared unaffected by the prior radiation treatment. It may be that "pockets" of the tumor evaded the effects of the radiation and gave rise to recurrent growth. The report noted variable scarring surrounding the tumor and a lack of radiation change within the tumor--suggesting that a uniform treatment effect was not achieved in these cases. Although all four patients had preoperative facial nerve weakness that was worsening and/or had expanding tumors on MR scan, excellent preservation of facial nerve anatomy and function was possible. The authors concluded that additional analyses are needed to clarify the effects of radiotherapy on acoustic neuromas.