Understanding Vestibular Schwannoma (Acoustic Neuroma)

Vestibular schwannomas, otherwise known as acoustic neuromas, are generally benign tumors that are slow-growing. These benign tumors form from the balance nerve which is the vestibular portion of cranial nerve VIII. Vestibular schwannomas most frequently develop in or near the internal auditory canal and may extend to the cerebellopontine angle (CPA). Once extended, there is potential for impact to adjacent nerves and brain structures.

It is important to note that while these tumors are not cancerous, vestibular schwannomas could lead to the development of progressive symptoms. With consideration to potential progression, depending on the size and location, individualized treatment plans are made.

Common Symptoms

Vestibular schwannomas can lead to a range of symptoms. These include:

Progressive hearing loss, typically one-sided
Tinnitus (ringing in the ear)
Vertigo, or imbalance
Numbness or tingling in the face
Facial weakness, headaches, or symptoms from brainstem compression may present in larger tumors

Why Tumor Size and Location Matter

The size and anatomical location of a vestibular schwannoma directly affects treatment type and the urgency of it. Surgical risk also depends on both size and location of the tumor. Additionally, the potential for nerve preservation can change based on these factors.

Tumor size is characterized by three categories:

Small (less than 1.5 cm):

Small tumors may be restricted to the internal auditory canal and can typically be managed through observation or radiosurgery. This size category features the best candidates for hearing preservation.
Medium (1.5-2.5 cm):

Medium tumors extend to the CPA but do not cause brainstem compression. In these cases, surgery or radiosurgery, depending on symptoms or tumor growth, are often discussed as potential treatment plans. Facial nerve preservation remains feasible but is more challenging.
Large (greater than 2.5 cm):

Large tumors often lead to brainstem compression or hydrocephalus and almost always require microsurgical removal to treat. They also involve higher risks for facial weakness, hearing loss, or cerebrospinal fluid (CSF) leaks.

Treatment Options

Vestibular Schwannoma treatment plans are individualized based on patient needs and the conditions of the tumor. Treatment options include:

Observation:

Observation, or “watchful waiting,” involves monitoring the vestibular schwannoma site. This treatment plan is ideal for small or asymptomatic tumors as well as those discovered incidentally during imaging for unrelated conditions or as part of annual check-ups. Serial MRI scans are completed every 6-12 months to monitor the tumor. This treatment may be recommended for older patients or for patients experiencing other medical conditions.
Stereotactic Radiosurgery (CyberKnife):

Stereotactic radiosurgery (SRS), delivered using CyberKnife technology, is performed over the course of 1-5 outpatient visits. This procedure involves using focused radiation to target the specific site of the vestibular schwannoma. CyberKnife radiosurgery candidates present small to medium sized tumors and are typically not candidates for surgical intervention. In most cases, this treatment plan can arrest tumor growth. Despite its name, CyberKnife radiosurgery is not surgery in a traditional sense and does not involve any incision or resulting hospital stay. It is a non-invasive procedure.

This procedure includes a few limitations that may impact final decisions regarding treatment. At the 5 year mark after the procedure, tumor control rates are approximately 85-95%. SRS does not typically shrink tumor sizes; rather, the treatment is utilized to prevent future growth. Hearing preservation rates can be variable at around 50-70% and could further decline with time. Control rates and long-term efficacy of treatment tend to be lower in younger patients (age 40 and below). For these patients, there is a greater likelihood that tumors grow or recur. If tumors are large or compress the brainstem, radiosurgery is not recommended.
Microsurgical Resection:

Surgical intervention is often the preferred treatment for tumors that are large, rapidly growing, and/or compressing the brainstem. With microsurgical resection, the goal is to maximize safety and preserve facial nerve function.and hearing when possible. Three different approaches can be used:
1. Retrosigmoid is used when hearing preservation is possible
2. Translabryrinthine is used when hearing has already been lost
3. Middle fossa is used to access small tumors with good hearing

Risks and Outcomes

Risks may vary with tumor size, location and treatment type. The following are examples of possible complications and the outcomes that may be associated with them:

Facial nerve weakness is approximately 10-20% transient with less than 5-10% being permanent.
Hearing loss is common with large tumors, but there is still a 30-50% risk of it occurring with small tumors when treated surgically or radiosurgically.
Cerebrospinal fluid (CSF) leak occurs approximately in 5-10% patients after surgery.
Balance issues can arise at first but usually improves with therapy.
Infections, such as meningitis, occur in less than 5% of patients.
Brainstem injuries and/or stroke are rare and only occur in less than 1% of cases.

Your Personalized Plan

Dr. Yim will carefully examine your MRI, perform hearing and balance tests, and explain a treatment strategy that would be most effective given your tumor’s size, growth trends, and symptoms. Regardless of the final treatment option selected, our priority will continue to be to preserve function, provide preventative care to reduce long-term complications, and maintain your quality of life.

Retrosigmoid Craniotomy for Acoustic Neuroma Surgery

Acoustic neuromas, or vestibular schwannomas, are benign tumors that grow on the balance and ocular nerves near the brainstem. They can be surgically removed using a retrosigmoid craniotomy which is also referred to as a retrosigmoid or suboccipital approach. This method allows neurosurgeons, such as Dr. Yim, to access the tumor site using a small opening behind the ear. Additionally, this approach can provide direct visualization of the cerebellopontine angle, a common site for vestibular schwannomas to be located.

When Is the Retrosigmoid Approach Used?

Retrosigmoid craniotomies may be appropriate for patients with medium to large tumors and/or tumors that extend toward the brainstem or into the cerebellopontine angle, whose goals are to preserve facial nerve function. Dr. Yim may also recommend this approach for patients desiring to preserve hearing and serviceable hearing is still present.

How the Procedure Works

Before the surgery can take place, the patient is placed under general anesthesia and any hair present behind the impacted ear is shaved as part of surgical preparation. The patient’s head will be typically positioned in a lateral or park-bench position so that the area behind the ear is exposed and ready for surgery.

Following preparation, a small incision is made behind the ear. During a retrosigmoid craniotomy, a small silver dollar-sized portion of the skull is removed behind the sigmoid sinus, one of the major veins in the body. The dura mater, the membrane protecting the brain, then can be carefully opened to expose the cerebellum.

By retracting the cerebellum, the neurosurgeon will be able to access the cerebellopontine angle where the acoustic neuroma can be identified as well as any other critical structures (e.g. the facial nerve CN VII, cochlear nerve for hearing, lower cranial nerves, vertebral arteries, and brainstem).

With the identification of the vestibular schwannoma site, the neurosurgeon can begin to remove the tumor microsurgically. Removal starts with internal debulking where the inside of the tumor is removed to collapse it. Throughout the surgery, the facial nerve will be monitored to preserve function of the nerve. Hearing is preserved when possible, but preservation depends on tumor size, location, and preoperative hearing function. In some cases, residual portions of the tumor may not be completely removed in order to avoid injury to any nerves or arteries the tumor may be adhered to.

After the completion of the removal process, the surrounding surgical area is examined for bleeding. When safe to do so, Dr. Yim will close the surgical site in layers. First, the dura mater is closed to form a seal that helps prevent CSF leakage. Then, the skull opening is reconstructed and closed using bone cement (typically polymethylmethacrylate or PMMA) rather than a metal plate.

Dr. Yim prefers the use of bone cement in reconstruction procedures for several reasons. Bone cement leaves a lower profile than a titanium plate would because no hardware is placed beneath the scalp and can be molded precisely to the patient’s skull shape. The material itself is safe, biocompatible, and is typically tolerated with minimal complication by the body. Metal implants have previously been associated with skin irritation and/or infection; the use of bone cement reduces these risks. Furthermore, bone cement has preferable cosmetic outcomes due to its smooth appearance.

Post-operative Recovery

After surgery, patients are carefully monitored in the neuro intensive care unit (ICU) or step-down unit for the following 1-2 days. Most patients remain in the hospital for 3-5 days. Physical therapy is recommended early in the recovery process to help recover balance that may have been lost during the procedure.

After initial recovery, confirmation of the resection is completed with MRI imaging studies. Dr. Yim, along with the neuro medical team, will continue to monitor patients for facial function, hearing, CSF leaks, or other complications.

Potential Risks

Brain surgery includes several risks, but Dr. Yim prepares extensively to minimize the likelihood of them. The following include potential complications and the approximate risks associated with them:

Facial nerve weakness is transient in about 10-20% cases and permanent in about 5-10% of them.
Hearing loss commonly occurs with large tumors but can vary based on pre-operative hearing strength.
CSF leak complications occur in approximately 5-10% of cases.
Balance issues are common at first, but typically improve with rehabilitation therapy.
Infections such as meningitis occur in less than 5% of patients.
Brainstem injuries or stroke are rare complications. Less than 1% of patients experience either of these complications.

Long-Term Efficacy

Many patients recover as expected and are able to return to their normal lives with physical therapy and follow-up appointments. Serial MRIs are used to monitor for any residual or recurrent tumor growth, but this growth is typically rare. In some cases, additional procedures such as radiosurgery may be needed to treat small residual tumors.