Acoustic neuromas or vestibular schwannomas are benign tumors that usually arise from the balance, or vestibular nerves. These paired nerves leave the brainstem and pass through a space called the cerebellopontine angle before entering the inner ear. These tumors usually grow slowly and do not metastasize. As they grow, they compress the balance and hearing nerves and cause hearing loss, tinnitus and sometimes imbalance or vertigo. Large tumors may cause headache, worsening dizziness and facial numbness. They occur in 1/100,000 people and make up about 10 percent of all brain tumors. Ninety percent of patients with vestibular schwannomas cause hearing loss and tinnitus in one ear, which is why we investigate patients with asymmetric ear symptoms. About 5-10 percent of patients who experience a sudden hearing loss may also have a vestibular schwannoma or other tumor.
Early diagnosis of a vestibular schwannoma is key to preventing its serious consequences. Due to the confined anatomy, continued growth can cause involvement of other important cranial nerves, or cause pressure against the brain. They can occasionally grow quickly, or their diagnosis be delayed and require urgent treatment. They are diagnosed and measured with an MRI of the brain. Tests of balance are sometimes performed after diagnosis to assist in the decision regarding treatment.
The three options for managing a vestibular schwannoma are observation, microsurgery or radiation therapy. The treatment is based on patient age, health and hearing, tumor size and patient preference. The risks and complications of treatment are, in general, related to the size of the tumor, but many tumors do not show growth for many years, and so observation is reasonable in many patients until tumor growth is documented. Close follow-up with serial MRIs and audiograms are required for many years if this “watchful waiting approach is taken.
Radiation treatment of vestibular schwannoma is different from other forms of radiation. Stereotactic radiation involves a fine, focused beam of radiation to minimize the side effects of radiation on the brain and other nerves. The tumor is not removed but the treatment usually stops the tumor from growing and in some cases causes it to shrink. It was previously recommended primarily for sick or elderly patients with growing tumors who need treatment. Many patients in midlife now choose this treatment modality. Researches are still evaluating the long-term effectiveness of this therapy in young, healthy patients.
As in surgery, the complications of hearing and balance function may occur, but often occur more slowly than with surgery. Radiation is not an option for the treatment of very large tumors with pressure effect on the brain.
Microsurgical removal is usually recommended in healthy people with growing tumors or when the tumor is very large and there are no other options. The surgery is a fairly extensive operation performed over several hours and requiring observation after surgery in the intensive care unit. A team approach is usually taken which includes a neurotologist, a neurosurgeon, an anesthesiologist, a specialist who monitors the cranial nerves during surgery and intensive care unit physicians and staff. This approach facilitates the highest standard of care and best outcome for the patient.
The exact type of operation done depends on many factors. The most significant are the size of the tumor and the level of hearing in the affected ear. If the tumor is very small, hearing may be saved and accompanying symptoms may improve. As the tumor grows larger, surgical removal is more complicated and hearing and balance may be lost. The facial nerve accompanies the nerves of hearing and balance along their course from the brain through the ear and this is a very important nerve to protect during microsurgery. The preservation of the facial nerve is of utmost importance, and is more of a priority than the patient’s hearing. The three approaches to remove an acoustic neuroma are now briefly reviewed.
In this approach, the surgeon works through the inner ear, or labyrinth, to remove the tumor. The patient is totally and permanently deaf after this operation, however there are advantages to this procedure that warrant considering this approach even if the patient had residual hearing in the ear before surgery. The facial nerve is most visible, and therefore protected during the surgery. There is usually less stretch and injury to the facial nerve and the facial nerve function results are usually excellent. As most of the exposure of the tumor with this procedure occurs outside of the envelope for the brain, the dura mater, there is less post-operative headache and so this is often a good choice of approach for patients who suffer from migraine or other headaches. If balance problems are present prior to surgery, this is often a good choice of surgery as it allows the abnormal balance function to be removed and the patient can then usually compensate and regain balance. Another complication, spinal fluid leakage, is on average slightly lower with this approach. A fat graft is taken from the abdomen and placed into the ear to seal the hole.
In this approach, the tumor is exposed via entering the skull and opening the dura mater (a craniotomy) behind the ear. The back of the brain, or cerebellum, is retracted to gain access to the location of most tumors. Removal of some bone is usually required while working within the open dura mater, and this can be a source of headache after surgery. The advantage of this procedure is the possibility to preserve the patients hearing. The surgeon also gains ample access to the area where the tumor is in contact with the brain, and is a good choice in patients with large tumors or when the tumor is not a vestibular schwannoma. The skull bone is repaired with bone graft or bone cement to minimize headaches. Spinal fluid leakage may be slightly higher for this approach.
In this approach, the tumor is exposed through the skull above the ear and the temporal lobe of the brain is lifted and gently retracted to access the location of the tumor. The advantage of this approach is the potential ability to preserve hearing and is often chosen in the setting of healthy patients with small tumors and good hearing. Tumors over 1.5 cm are difficult to remove via this approach. Hearing preservation generally depends on the size of the tumor, but is still not guaranteed even with very small tumors. The skull bone is reconstructed with tiny bars of titanium and special screws.
