Timothy C. Hain, MD
Acoustic neuromas, also known as vestibular schwannomas, are non-malignant tumors of the 8th cranial nerve. Most commonly they arise from the covering cells (Schwann cells) of the inferior vestibular nerve (Komatsuzaki and Tsunoda, 2001). They comprise about 6 percent of all intracranial tumors, about 30% of brainstem tumors, and about 85% of tumors in the region of the cerebellopontine angle -- another 10% are meningiomas. Only about 10 tumors are newly diagnosed each year per million persons in the United States, corresponding to between 2000 and 3000 new cases each year. In patients with hearing asymmetry, it is believed that only about 1 in 1000 has acoustic neuroma (source: NIH).
Acoustic neuroma occurs in two forms: a sporadic form and a form associated with an inherited syndrome called neurofibromatosis type II (NF2). About 95 percent of all cases are sporadic. NF2 is rare; there are only several thousand affected individuals in the entire United States, corresponding to about 1 in 40,000 individuals. Roughly 5% of patients with acoustic neuroma have type II neurofibromatosis. There is presently no evidence that radiation from cellular phones causes acoustic neuroma (Muscat et al, 2002).
Hearing loss is the most frequent symptom, occurring in more than 95 percent of patients. About 90 percent present with a one-sided, slowly progressive hearing impairment. An example is shown below. A high-frequency sensorineural pattern is the most common type, occurring in approximately two-thirds of patients. In the remaining third the next most common observation is hearing loss at low frequency (which would be more typical of Meniere's disease). Even less commonly, some have the "cookie bite" pattern (suggestive of congenital hearing loss). A sudden hearing loss occurs in about 25 percent of patients with acoustic neuroma. However, because acoustic neuroma is a rare condition, sudden hearing loss attributable to an acoustic tumor occurs in only 1-5 percent of patients with sudden hearing loss as there are many more common causes (Daniels et al, 2000). Hearing can be completely normal in about 11% of patients (Morrison and Sterkers, 1996). Tinnitus is very common in acoustic neuroma, is usually unilateral and confined to the affected ear.
In spite of the origin of acoustics in the inferior vestibular nerve (Komatsuzaki and Tsunoda, 2001), vertigo (spinning) is not common, occurring in only about 20 percent of persons with acoustic neuroma. Vertigo is more common with smaller tumors. Unsteadiness is much more prevalent than vertigo, and approximately 70 percent of patients with large tumors have this symptom. Cerebellar symptoms (i.e. poor coordination of the arms) are unusual.
Facial sensory disturbances occurs only in large tumors (about 50 percent of those greater than 2 cm in size). The facial sensory disturbance may respond to carbamazepine medication for neuralgia. Facial weakness is uncommon. Facial twitching, also known as facial synkinesis or hemifacial spasm, occurs in about 10 percent of patients. Headache prior to surgery occurs in roughly 40 percent of those with large tumors.
Conventional audiometry is the most useful diagnostic test for acoustic neuroma. The most common abnormality is an asymmetrical high-frequency sensorineural hearing loss. No more than 1 out of 20 patients with large tumors have symmetry within 15 db at 4000 hz. However, recall that only about 1 in 1000 patients with hearing asymmetry have acoustics. It has been estimated that 5 percent of persons with sensorineural hearing loss have acoustics (Daniels et al, 2000), but this estimate is suspect as it would imply a much higher prevalence of acoustic neuromas than are commonly accepted. Speech reception (SRT) is normal in many patients with small tumors (SRT). Excellent speech discrimination is found in about 50% of patients with small tumors, and one third of patients with large tumors still have near-normal (> 80%) speech discrimination.
When abnormal with a progressively worsening pattern, audiometry usually leads to further testing such as ABR (auditory brainstem response) and gadolinium enhanced MRI (magnetic resonance imaging) which establishes the diagnosis. ABR testing is less sensitive than MRI, but it is considerably less expensive. A new technique called "summated ABR", essentially several ABRs compared over time, may provide better sensitivity. A characteristic finding on ABR would be a wave I with nothing after it (10-20% of cases). A wave I-III interval delay is common, and a wave V delay occurs in 40-60% of cases. ABR's have high false-positive as well as false negative rates. As many as 1/3 of patients with small tumors (on MRI) have normal ABR.
Electronystagmography, (ENG testing) is frequently abnormal and about 50 percent of all tumors are associated with unilateral loss of calorics. Nevertheless, ENG is not a reasonable diagnostic test because it is not specific. Rotatory chair testing is less sensitive than caloric testing. Posturography is insensitive to acoustic neuroma. Reflex decay is also insensitive (about 36%). Otoacoustic emissions are also considered a poor test for acoustic neuroma.
MRI scan of brain showing an acoustic neuroma (the white spot on the left side of the picture).
Although it is relatively costly compared to audiometry or ABR, the optimal test for excluding an acoustic neuroma is a gadolinium enhanced T1 MRI (see picture above). On MRI, acoustic neuromas are frequently uniformly enhanced, dense, and expand the internal auditory meatus. A fast spin-echo T2 variant of MRI is very sensitive to acoustics, and in some clinical settings, can be done fairly inexpensively. If an MRI cannot be done, as in persons with a pacemaker or metallic clips, an air-CT scan should be obtained in high-risk individuals, particularly if the ABR is suggestive of an acoustic neuroma.
Acoustic neuromas range in size up to 4 cm. The smallest, the intracanalicular acoustic, is measured in mm. A "small" acoustic is less than 1.5 cm. A "moderate" acoustic is 1.5-3 cm, and a "Large" acoustic is 3cm or greater. Tumors are staged by a combination of their location and size. An "Intracanalicular" is small and in the IAC. A "cisternal" tumor has extended outside the IAC. A "compressive" tumor is touching the cerebellum or brainstem, and a "hydrocephalus" tumor is obstructing CSF drainage pathways in the IV'th ventricle.
Rarely, acoustic neuromas are inherited. Acoustic neuroma caused by type-II neurofibromatosis should be suspected in young patients and those with a family history of neural tumors. There are several other tumors that can occur in the same region of the brain, the cerebello-pontine angle or CPA, as acoustic neuromas. Of all lesions in the CPA, acoustic neuromas account for 70-90 percent. Meningiomas are second most common (10 percent), followed by epidermoids, and then lipomas. Occasionally tumors in other locations, such as the lung, can metastasize to the CPA.
Nikolopoulos and O'Donoghue recently reviewed 111 articles on acoustic neuroma treatment and stated that "Well-designed comparisons between treatment methods do not exist, and therefore claims by clinicians favoring a particular treatment are unfounded" (2002). We do not think that clinical wisdom can be discounted to such a great degree, but certainly the present situation seems to be that acoustic neuroma treatment is an art.
There are three distinct options:
Medical Management: About 25% of all acoustic neuromas are treated with medical management. Medical management consists of periodic monitoring of the patient's neurological status, use of hearing aids when appropriate, and periodic imaging studies. It is felt to be an appropriate method of management in some patients (Hoistad et al, 2001). There is no medication known to have a substantial effect on the growth of acoustic neuroma tumors. The tumors may grow very slowly, about 1 1/2 mm/year, and one may elect to follow a tumor with serial audiometry and/or MRI scans (Shin et al, 2000). In individuals of advanced age, a serious threat to life or bodily function from tumor growth may be judged unlikely in the remainder of a patient's expected lifespan, and for this reason, medical management may be elected (Perry et al, 2001). Once a tumor is diagnosed, a repeat scan is obtained at 6 months and then at yearly intervals (Perry et al, 2001).
This procedure has its own risks. Even when the tumor is not growing on MRI, there is a risk of losing useful hearing in this situation, making the individual no longer a candidate for hearing preservation type surgery. Somewhere between 10 and 43% of patients followed for about 2 years lose "useful" hearing (Warrick et al, 1999; Shin et al, 2000). A reasonable estimate is that over a year, about 75% of tumors will have visible enlargement, averaging 1.5 mm, and about 25% will not. Some variants grow much faster than others.
Gamma Knife: When the risk of surgery is high because of other medical problems, or where the patient simply refuses surgery, the "gamma knife" procedure may be used. This is a method of irradiating the tumor, invented by Lars Leksell in 1971. Although it avoids surgery with its attendant risks, this option is usually recommended only for high-risk surgical cases because of the possibilities of late radiation complications, hydrocephalus in about 10 percent of patients, and the need for ongoing MRI monitoring of the results of the procedure. However, as the operators of gamma knife become more proficient, these statistics are likely to improve substantially. As of 1996, approximately 25 percent of all acoustic neuromas were treated in this way. In most instances the reason was that patients refused surgery. Gamma knife does not generally make tumors go away -- the figure above is actually that of a patient who had gamma knife surgery several years prior. Patients are best followed with periodic MRI scans for the remainder of their lives. The recurrence rate of the tumor is about 3% after surgery, and 14% after gamma knife, but of course, this figure will vary with the surgeon and the gamma knife protocol.
If surgery is eventually required, surgical complications in this situation such as severe facial nerve weakness are nearly 100%. This occurs because the facial nerve often becomes "fused" to the tumor, after Gamma knife. Like surgery, hearing loss is common after gamma knife. Delayed facial weakness, and facial numbness also occur in roughly 1 third of patients after gamma knife. Hydrocephalus has been reported to occur in between 3 and 12.8 percent (Noren et al, Pollock et al). Dysequilibrium is reported in 8-31%, a figure which is analogous to surgical management. Occasionally tumors enlarge after treatment (Ho and Kveton, 2002)
Surgical Treatment: About half of all acoustic neuromas are presently treated with surgery. In most instances surgical removal of the tumor is the preferred option because it prevents potentially fatal complications of tumor growth. Surgery may enable preservation of hearing. Usually the surgery is done at an academic center by a team of surgeons including a neurotologist (a specialized otolaryngologist) and a neurosurgeon. There are several operations as noted below.
Each of these approaches has advantages and disadvantages that must be considered in selecting an optimal approach. Surgical treatment where the brain is exposed is nearly always performed by a team of surgeons, usually including a neuro-otologist and a neurosurgeon. Most patients are admitted to the hospital a day before the operation. After surgery, they spend a night in a monitored unit. Most are discharged from the hospital within 4-6 days after surgery, and return to work is usually possible in 6 weeks. MRI's are usually obtained at 1 and 5 years to detect residual or recurrent tumor. Total or near total (95%) removal of the tumor is advised (Sanna et al, 2002).
Vestibular rehabilitation. In most instances, acoustic neuroma surgery results in complete loss of vestibular function on the operated side. Depending on the amount of vestibular function there is present prior to surgery, patients may experience vertigo post-surgery. Vestibular rehabilitation may speed recovery from this deficit. We think it is best that the patient who is planning to have acoustic neuroma surgery visit a vestibular physical therapist to make sure that there is a "good fit" and to learn the basic procedures, and for the individual to begin a weekly program of PT for 1-2 months following discharge. It is important that the otologic surgeon be involved with the therapy as in some situations (i.e. patients with CSF leak), therapy shouldbe delayed.
Surgical treatment, per se, has a substantial risk. Overall, the risk of death from acoustic neuroma surgery is about 2 percent. Unexpected post-operative complications occur in roughly 20 percent with more complications occurring in elderly and infirm individuals and those with large tumors (Kaylie et al, 2001). Complications, ordered from rare to frequent, are listed.
In a recent review of results of 258 patients operated via the translabyrinthine approach, stroke or cerebellar injury occurred in 1.1%. Cerebellar injury can occur due to traction as well as due to injury to branches of the anterior inferior cerebellar artery (Hegarty et al, 2002).
Other complications include CSF leak in 7.8%, meningitis in 1.6%. Facial weakness of various degrees appeared in most, but severe weakness with House-Brackman scores of V-VI at 1 year occurred in 6% (Mass et al, 1998). Wiet and others have recently reported results in 500 cases (Wiet et al, 2001). Overall success at retaining useful hearing was 27%, with considerably better results obtained when operating via the middle-fossa approach.
Significant headache can occur following acoustic neuroma surgery (reviewed by Driscoll and Beatty, 1997). The incidence is very variable among surgeons and also depends on the choice of approach, but an overview of the literature suggests an incidence of about 20%. Schessel et al (1996) observed and documented adherence of neck muscles to the dura after craniectomy and reported a dramatic decrease in headache in patients who had craniotomy with replacement of the bone flap. Similarly, Harner et al also noted a drop in headache when cranioplasty with methyl methacrylate was used instead of craniectomy alone (Harner et al, 1995). The mechanism here is thought to be traction on the dura by movement of neck muscles. Many patients with this syndrome note aggravation by coughing or straining.
Schessel et al (1996) suggested that patients having surgery via the retrosigmoid approach had significantly higher frequency of headache than those who had the translabyrinthine approach. Several other groups have found a similar pattern. Currently there is little information about incidence of headache using the middle fossa approach, but the few series available suggest a rather low incidence (Driscoll et al, 1997).
Management of post-operative headache utilizes analgesics, muscle relaxants, antidepressants and anticonvulsants, in a way similar to migraine management. Migraine abortive agents, however, and specific prophylactic drugs for migraine are not recommended in most instances. Nevertheless, a recent report found that sumatriptan (a migraine drug) improved headache in 9/10 patients with post-surgical headaches
Persistent incisional pain may occur from entrapment of the occipital nerve or from formation of an occipital neuroma. Massage, local heat, and analgesics may help. Occipital nerve blocks may also be beneficial.
Another mechanism that have been suggested is that bone dust trapped within the intracranial cavity may cause a protracted inflammatory response resulting in chronic headache (Driscoll, 1997). MRI images sometimes show dural enhancement and CT images may show calcification along the brainstem. In these patients logically treatment might include anti-inflammatory agents and possibly corticosteroids. Narcotic analgesics are occasionally indicated.
Very rarely, a person with acoustic neuroma might desire a cochlear implant. This might occur if an acoustic tumor is present in the only hearing ear or after surgery to remove bilateral acoustic neuromas. Belal (2001) reported that cochlear implantation is possible only if there is an intact cochlear nerve (as shown by a positive response to promontory stimulation), and if the implantation is done at the time of acoustic tumor removal, before the cochlea ossifies.