How to diagnose and treat a vestibular schwannoma

It’s time to explore a type of brain tumor that is less common than metastases, meningiomas, gliomas, and pituitary tumors—vestibular schwannomas. It’s important to learn about this type of tumor because it exemplifies some of the complexities of managing brain tumors, even if they are benign and slow growing in nature.

Vestibular schwannomas (also known as acoustic neuromas) make up about ten percent of primary intracranial tumors. They are benign and tend to grow very slowly off the vestibular nerve as it exits the brainstem and heads into the internal auditory meatus towards the inner ear. This region is known anatomically as the cerebellopontine angle. Many cranial nerves can be found here as well as the all-important brainstem.

Some vestibular schwannomas not only grow slowly but seem capable of lying dormant for many years. So, predicting the behavior of vestibular schwannomas can be quite challenging. Furthermore, slow-growing tumors can reach frightening dimensions with little to no symptoms, despite compressing vital neurological structures. These structures adjust to the compression initially but at some point can experience a relatively rapid functional collapse.

Historically, vestibular schwannomas went undiagnosed until they reached a tremendous size and caused severe brainstem compression with multiple resultant cranial nerve palsies and long tract findings (e.g., hemiparesis). Surgery was the only viable treatment, and morbidity and mortality of the surgery proved frighteningly high.

As intraoperative techniques and technologies improved, tumors were discovered earlier (thanks to better imaging) and mortality for vestibular schwannoma surgery improved. However, morbidity from multiple cranial nerve palsies and brainstem strokes continued to plague surgeons.

With further refinement, morbidity was significantly reduced, but the preservation of facial movement and hearing remained a challenge. The facial nerve (cranial nerve VII) and the vestibulocochlear nerve (cranial nerve VIII) can be intimately tangled up with and stretched to paper thinness by a large tumor. Surgery extended to many hours in duration as surgeons labored to remove tumors from these nerves hoping to not harm their function.

On the affected side, most patients lost their hearing and facial movement after surgery. Since the early 2000s, facial movement preservation rates with surgery have gone up significantly, and there has been moderate success in hearing preservation (particularly with small tumors). This is thanks to more advanced techniques, tumors being discovered earlier, and scaling back surgical aggressiveness by leaving residual small pieces of tumor stuck to these nerves.

Generally, tumors that have been resected receive no adjunctive radiation. They are simply followed and only addressed if they recur; a well-resected tumor seldom recurs.

At the same time, stereotactic radiation treatment has been found to control smaller tumors while allowing for high rates of preservation of facial function and hearing. Stereotactic radiation for small- to medium-sized tumors has been shown to stop the growth of many tumors with less chance of injuring the cranial nerves (at least initially). The tumors can even shrink but do not disappear.

As a result, some authors advocate for the use of stereotactic radiation over surgery for small- to medium-sized tumors. However, not all tumors respond to stereotactic radiation and surgery after radiation is felt to be of higher risk.

More recently, many authors advocate to simply monitor small tumors. A fair number of small tumors seem to sit unchanged for extended periods—even decades.

The net effect has been a shifting (and somewhat confusing) approach to the management of these types of tumors. So, we’ll review the current guidelines for diagnosing and managing vestibular schwannomas.

Patients with a vestibular schwannoma often present with hearing loss and tinnitus in the affected ear, and occasionally disequilibrium. The hearing loss is in the high-frequency range and is often picked up by audiologists; it is considered to be the sensory-neural type.

Audiologists who pick up this hearing loss on testing will usually refer patients to neuro-otologists specialized in operating on disorders affecting the temporal bone and hearing apparatuses.

Patients themselves may or may not notice a gradual hearing loss. Many note that they have had to switch ears to talk to someone on the phone.

Some patients may present with unilateral tic-like facial pain or facial numbness. Interestingly, facial movement is rarely affected preoperatively, despite the tumor’s intimate relationship to the facial nerve (cranial nerve VII). Other signs of cranial nerve dysfunction (such as swallowing difficulties or hoarseness) are uncommon.

Perform a physical and neurological examination

On examination, cognition and speech should be normal unless there is associated hydrocephalus. Hydrocephalus is generally only seen with large tumors.

When examining a patient, pay particular attention to the cranial nerve examination:

• Test eye movement
• Assess facial sensation
• Assess corneal reflex
• Assess facial movement
• Test the ability to hear fingers rubbing
• Assess balance
• Check for nystagmus
• Test gag reflex
• Assess palatal motion (when the patient says “ahhh”)

Look for contralateral hemiparesis, which could result from compression of motor long tracts as they pass through the brainstem. Next, obtain formal audiometric tests and refer the patient for neurosurgery and neuro-otology evaluations.

Obtain diagnostic imaging

Magnetic resonance imaging (MRI) usually demonstrates a brightly enhancing mass on the eighth cranial nerve near the brainstem or in the internal auditory canal. Larger tumors appear like relatively round balls in the cerebellopontine angle with a tongue protruding into and expanding the internal auditory meatus. Larger tumors compress and distort the brainstem and may cause associated hydrocephalus.

Vestibular schwannomas under 15 mm with intact hearing

If the tumor is less than 15 mm in diameter, and the patient’s hearing is intact, monitor the tumor with yearly audiology evaluations. Order follow-up MRI every 6 months for the first couple of years, then gradually space them out after that. It is advisable to check this management plan with a neuro-otologist or neurosurgeon.

Vestibular schwannomas under 15 mm with hearing impaired or lost

If the tumor is less than 15 mm and hearing is impaired (but serviceable), consider conservative management (e.g., following the tumor), surgical resection, or stereotactic radiation. All three management strategies carry similar long-term outcomes. Many favor surgical resection because without surgery the tumor will likely increase in size over time.

Stereotactic radiation boasts better hearing preservation rates in the first couple of years, but hearing tends to decrease over time and long-term hearing loss tends to even out.

Similarly, if the tumor is less than 15 mm, and hearing is lost, consider conservative management (e.g., following the tumor), surgical resection, or stereotactic radiation. All three management strategies have been advocated for. You may consider simply following these tumors and addressing them only if they increase in size.

Vestibular schwannomas ranging from 15 to 25 mm

In tumors 15 to 25 mm in diameter, surgical resection should be considered in healthy candidates (particularly younger ones), no matter the status of their hearing. These tumors will likely continue to grow, risking brainstem and cranial nerve functions. Observation or stereotactic radiation are good options in elderly or more debilitated patients. Hearing preservation rates of surgery versus stereotactic radiation are controversial for this patient group.

Vestibular schwannomas over 25 mm in size

Generally speaking, tumors over 25 mm in diameter should be addressed surgically. Gross total resection is no longer the principal goal of the surgery; decompression of the brainstem and cranial nerves is the goal.

Usually, most of the tumor can be safely removed. But, small amounts of the tumor may be left on the facial nerve rather than risk its structural and functional integrity. The residual tumor is then followed with MRI every 6 to 12 months and is only addressed if there is evidence of a recurrence. A small, recurrent tumor is usually addressed with stereotactic radiation.

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