Acoustic Neuroma

Acoustic neuroma, also known as vestibular schwannoma, is a benign Schwann cell tumor. These tumors develop from the Schwann cell sheath and might be intracranial or extracranial in nature. They frequently develop near the cochlear and vestibular nerves, with the latter's inferior division being the most common source. Acoustic neuroma tends to occupy the cerebellopontine angle anatomically. Meningiomas make up about 5-10% of cerebellopontine angle tumors and can arise anywhere in the brain. Individuals with type 2 neurofibromatosis are more likely to have bilateral acoustic neuromas. Acoustic neuroma is among the most frequent intracranial tumors, accounting for about 6 to 10 percent of all tumors in most studies. Acoustic neuroma affects about 12 out of every million people in the United States, and about 20 out of every million people in a modern European cohort. However, acoustic neuroma is becoming more common. An examination of 26 years of prospectively collected data in Denmark, from 1976 to 2002, revealed that the incidence increased from 7.8 for every million to 19 million. Smaller tumors are being detected in greater numbers as a result of these measures. Between 1976 and 2008, Danish statistics revealed a size reduction from an average of 30 mm to an average of 10 mm. These changes in the epidemiology of acoustic neuroma could indicate that the presentation of patients with this disease is changing as well. As a result, healthcare professionals in primary health care, hospitals, and surgical subspecialties would benefit from being aware of any emerging patterns in the manifestations of acoustic neuroma patients. The clinical signs and manifestations of acoustic neuroma described in the literature are mostly from older research which may or may not be pertinent to current practice. In addition, a proper statistical study of these variables using multifactorial regression has yet to be completed.

 

Epidemiology

Schwannomas are responsible for about 8 percent of all cerebral tumors that present medically. Acoustic neuromas are mostly unilateral and sporadic. Bilateral acoustic neuromas are hereditary in nature and account for less than 5 percent of all schwannomas. Acoustic neuromas are more common in people in their fourth to sixth decades of life. Acoustic neuromas that form in people with neurofibromatosis type 2 are more likely to appear earlier, peaking in the third decade of life. Acoustic schwannomas can appear in childhood, albeit they are uncommon. There is a slight female predilection, with issues worsening during pregnancy. Inherited acoustic neuroma is substantially more frequent in neurofibromatosis type 2 than in neurofibromatosis type 1, despite the latter being much more prevalent. Only 24 percent of patients of neurofibromatosis type 2 have been documented with unilateral acoustic neuroma, whereas bilateral acoustic schwannoma is a feature of neurofibromatosis type 2.

 

Acoustic Neuroma Causes

The majority of people who are diagnosed with an acoustic neuroma have no obvious potential risks. The only confirmed etiological potential risk linked to an increased risk of acquiring an acoustic neuroma is exposure to high-dose ionizing radiation. Numerous studies have found that using a phone does not raise the likelihood of getting an acoustic neuroma, while more research on the consequences of long-term cell phone use is needed.

People with neurofibromatosis type 2 have a tumor suppressor gene on chromosome 22q12.2 that is missing. Merlin or schwannomin is the name of the abnormal protein made by the gene. Neurofibromatosis type 2 is characterized by bilateral acoustic tumors, while additional symptoms such as peripheral neurofibroma, meningioma, glioma, and juvenile posterior subcapsular lenticular opacities are also common. Peripheral neurofibroma and cafe au lait patches, on the other hand, are substantially less common than in neurofibromatosis type I.

 

Pathophysiology

The Schwann cell involvement of the vestibular segment of the vestibulocochlear cranial nerve causes the large bulk of acoustic neuromas. The cochlear nerve accounts for less than 5 percent of all nerves. At about equal rates, the superior and inferior vestibular nerves seem to be the nerves of development. Within acoustic tumors, three distinct growth patterns can be identified:

1. No expansion or very slow progression, 
2. Slow growth (0.2 cm/year on diagnostic imaging studies)
3. Rapid growth (1.0 cm/year on neuroimaging)

Although most acoustic neuromas grow at a slow rate, others can double in size in as little as six months to one year. Even though some tumors follow either of these development patterns, others seem to cycle between times of no or moderate growth and times of fast growth. Because of the growth of the cystic element, tumors that have developed cystic degeneration (probably because they have exceeded their blood supply) are occasionally capable of rather aggressive growth. Acoustic tumors form from the Schwann cell that implants and tumor growth compresses vestibular nerves on the surface. Because vestibular fiber degeneration is delayed, most patients have little or no imbalance or dizziness. Once the tumor has gotten big enough to occupy the internal auditory canal, it can either expand bone or spread into the cerebellopontine angle to keep growing. The cerebellopontine angle is characterized by circular growth.

Acoustic tumors, like other space-occupying tumors, cause symptoms through one of four distinct processes: pressure or distortion of spinal fluid compartments, dislocation of the brain stem, pressing of arteries causing ischemia or necrosis or crushing and/or attenuation of nerves.

Tumors can progress until they approach 3-4 cm in size before they impact key tissues because the cerebellopontine angle is usually vacant. The facial nerve can typically conform to the stretching caused by tumor progression without medically obvious impairment of function because tumor growth is often gradual enough. By squeezing the cochlear nerve, vestibular nerve, or labyrinthine artery against the bone wall of the internal auditory canal, tumors in the internal auditory canal can cause hearing loss or vestibular instability at an early stage.

The tumor begins to press the lateral side of the brain stem as it reaches 2.0 cm in diameter. Only by squeezing or shifting the brain stem to the opposite side can it grow farther. Tumors that are larger than 4 cm in diameter frequently spread far enough anteriorly to press the trigeminal nerve, causing face hypesthesia (high sensitivity to the pain). Increasing effacement of the cerebral aqueduct and fourth ventricle happens as the tumor grows beyond 4 cm, leading to the occurrence of hydrocephalus.

 

Acoustic Neuroma Symptoms

Unilateral hearing loss is by far the most classic symptom at the initial diagnosis, and it is almost always the sign that contributes to a diagnosis. Until proven differently, consider that any unilateral sensorineural hearing loss is induced by an acoustic neuroma. Hearing loss can be caused by the tumor in one of two ways: direct harm to the cochlear nerve or disruption of the cochlear blood flow. Slow progressive sensorineural hearing loss seen in a large number of individuals with an acoustic neuroma is most likely due to gradual damage to cochlear nerves. Hearing loss that occurs suddenly and fluctuates is more easily explained as a result of a disturbance in the cochlear blood flow. In 5-15 percent of people with acoustic neuroma, hearing loss might be abrupt or variable. A hearing loss like this can improve on its own or in response to steroid medication. As a result, even if hearing returns to normal, a gadolinium-enhanced MRI should be conducted on anyone who has a sudden or fluctuating loss.

As gadolinium-enhanced MRI becomes more widespread, the finding of acoustic neuromas in people with normal hearing has been on the rise. Furthermore, tumor size has a weak relationship with hearing status, since patients with massive tumors can have normal hearing while those with small tumors can be profoundly deaf in the affected ear. The existence of unilateral tinnitus is enough to rule out the possibility of an acoustic tumor. Although tinnitus is most often linked with hearing loss, a small percentage of people with acoustic tumors (approximately 10%) seek therapy for unilateral tinnitus that is not accompanied by subjective hearing loss.

Patients with acoustic tumors rarely experience dizziness or imbalance as clinical presentation. Rotational vertigo (the sensation of moving or dropping) is a rare symptom that can occur in people with smaller tumors. Larger tumors, on the other side, seem to have more imbalance (a sense of fluctuation or instability). In general, roughly 40-50 percent of patients with an acoustic neuroma express some form of balance problem when asked. Balance disturbance, on the other hand, is the presenting symptom in less than 10% of patients. Vestibular fiber loss appears to be gradual enough to allow for correction.

Headaches are present in 50-60 percent of patients at the time of identification; however, headache is only the clinical manifestation in less than 10 percent of cases. Headache appears to become more common as tumor size grows larger, and it is a common symptom in patients who develop obstructive hydrocephalus as a result of a large tumor.

Facial numbness affects around a quarter of patients and is more frequent than facial paralysis at the time of diagnosis. Larger tumors are related to objective hypoesthesia (decreased sensation) involving the teeth, buccal mucosa, or skin of the cheeks, although medium-sized and smaller tumors usually experience a subjective decrease in the sensation that cannot be proven on objective assessment. A reduction in the corneal reflex occurs more frequently and earlier than objective facial hypoesthesia. Although 50-70 percent of people with large tumors have objectively detectable facial hypoesthesia, they are largely ignorant of it, and it is rarely the first symptom they experience.

 

Acoustic Neuroma Diagnosis

Acoustic Neuroma MRI

Gadolinium-enhanced MRI is the gold standard diagnostic technique for patients with acoustic neuroma.

1. Lesions as small as 1-2 mm in diameter can be detected with properly done scanning. Thin-cut CT scanning, on the other side, can overlook lesions as large as 1.5 cm, even with intravenous contrast enhancement.
2. Non-enhanced MRI can overlook tiny lesions; hence gadolinium contrast is essential.
3. Fast-spin echo methods do not require gadolinium augmentation and can be completed in a short amount of time for a low cost. However, such precise procedures run the danger of overlooking other causative factors of unilateral sensorineural hearing loss, such as intra-axial malignancies, demyelinating diseases, and infarctions.
4. MRI is not recommended for people who have magnetic devices.

 

Other Imaging Studies

Fine-cut Computed tomography of the internal auditory canal with contrast can exclude a big or medium-sized tumor, however, it didn't spot lesions smaller than 1-1.5 cm.

Air-contrast cisternography has good sensitivity and can identify fairly small intracanalicular lesions if confidence is high and MRI is not an option.

 

Diagnostic Studies

In the mid-twentieth century, a number of audiological tests were developed in an effort to identify people who were more likely to have an acoustic neuroma. When conclusive radiographic scanning consisted of either pneumoencephalography or formal arteriography, it was a worthy effort. This type of testing is no longer carried out. Even the auditory brainstem evoked response (ABR) is no longer commonly utilized as an acoustic neuroma screening method. 20-35 percent of acoustic lesions smaller than 1 cm are missed by ABR screening approaches. Furthermore, ABR is prone to overlook malignancies in patients with good hearing, which are the circumstances where hearing conservation therapies are most beneficial.

 

Histologic Findings

Acoustic tumors have been found to have two kinds of histopathological tissue. Elongated spindle cells with a palisading pattern make up Antoni A tissue. Antoni B tissue, on the other side, has a spongy consistency and is significantly less cellular. There may be patches of Antoni A and Antoni B tissue in a single auditory neuroma. Vero-cay bodies, which are rows of palisading nuclei, are another histologic hallmark of acoustic neuroma. Although acoustic lesions have a fairly basic histological appearance, they can occasionally be difficult to identify from meningiomas. In challenging scenarios, immunohistochemical staining can help identify schwannomas from meningiomas. S-100 antibody reacts with schwannomas, whereas epithelial membrane antibody reacts with meningiomas.

 

Acoustic Neuroma Treatment

Observation

In the following patients, continuous observation with no specific treatment has been used:

• Patients who are old
• Patients with smaller tumors, particularly if their hearing is intact, may benefit from this treatment.
• Patients with medical issues that put them at a higher risk of undergoing surgery.
• Refusing to receive therapy.
• Patients who have a tumor on the side of their only hearing or seeing ear or eye.
• Patients being studied in a variety of studies published to date need therapeutic strategies between 15 and 40 percent of the time.
• The majority of patients who are qualified for hearing conservation operation (70 percent or more) lost their qualification during an observation period.

 

Stereotactic Radiotherapy

For some individuals with acoustic neuroma, stereotactic radiation has emerged as an effective option for microsurgery. Stereotactic radiotherapy uses one of several distinct radiation sources and is delivered by a variety of equipment with various names.

To optimize the quantity of radiation supplied to target tissues while reducing the exposure of nearby normal tissues, stereotactic treatment employs radiation given to a precise point or set of points. It can be given as a single injection or as a series of fractionated dosages.

Radiation at the current smaller dosages causes obliterative endarteritis of the vessels supporting the tumor, which likely stops continued tumor progression. Radiosurgery can disrupt mitosis in tumor cells by producing double-strand DNA breaks. Hansen et al. found that acoustic neuroma cells are radioresistant to the minimal radiation currently utilized in radiosurgery.

The following are some of the benefits of radiotherapy:

1. Duration of hospital stay has been reduced.
2. Cost savings
3. Return to full-time work quickly
4. Lower morbidity and mortality the period after treatment

The following are some of the disadvantages of stereotactic radiation:

1. Constant monitoring and periodic rescanning are required
2. Does not remove the tumor and may fail to inhibit tumor progression, necessitating salvage surgery in some cases.
3. Injury to the trigeminal nerve is more common.
4. Long-term occurrence of secondary cancers is unclear. Over thirty years, the best recent predictions of getting a subsequent cancer from radiosurgery are 1 in 1000 people.
5. Does not alleviate imbalance and could result in long-term balance issues.

 

Acoustic Neuroma Surgery

Surgical removal is still the most effective way to get rid of a tumor. Acoustic tumors can be removed using a variety of surgical techniques:

1. Retro-sigmoid approach: in skull-base surgery, the retro-sigmoid technique is the workhorse. It also allows for an easy approach to the cranial nerves in this area. This method can be used to treat any type of tumor with the prospect of hearing preservation.
2. Middle cranial fossa approach: this is another hearing-preserving procedure that works best for tumors that have a strong intracanalicular element and a small cisternal element. The most significant downside is the requirement for temporal lobe retraction, which might result in postoperative convulsions and venous infarction if the Labbe’s vein is injured.
3. Trans-labyrinthine approach: the advantage is that the facial nerve can be accessible early and preserved, and the cerebellum does not need to be withdrawn, which is ideal for patients with massive tumors and no functional hearing. However, entrance to the contents of the jugular foramen and the lower foramen magnum is restricted.

Patients under the age of 65 with medium-, moderately large–, or large-grade tumors, expanding tumors, substantial hearing loss, and greater headache severity scores are more inclined to attempt surgical resection rather than close monitoring.

 

Differential Diagnosis 

• Meningioma
• Ectodermal inclusion tumors
• Epidermoid
• Dermoid
• Metastases
• Neuroma from cranial nerves other than 8th cranial nerve 
• Trigeminal neuroma.
• Facial nerve neuroma
• Neuroma of the lowest four cranial nerves (9th, 10th, 11th, 12th)
• Arachnoid Cyst
• Neuro-enteric Cyst
• Cholesterol granuloma
• Aneurysm
• Dolicho-Basilar ectasia
• Extensions of nearby lesions in the cerebellopontine angle

 

Complications of Acoustic Neuroma Surgery

The majority of problems are surgical in nature and include the following:

1. Loss of hearing

When a tumor is microscopic, it is occasionally acceptable to save hearing by eradicating it. However, most tumors are larger, and the surgical treatment causes hearing loss in the affected ear. As a result, the patient can only hear with the healthy ear after the operation. However, innovative treatments such as the CROS hearing aid or a Bone Anchored Hearing Aid are now available to help transfer sounds from the deaf ear to the good ear.

2. Tinnitus

Tinnitus (ear ringing) in the ear of people with Acoustic Neuroma can be quite unpleasant. Tinnitus is often the same following therapy for Acoustic Neuroma as it was before the operation. After the tumor is removed, few patients may report that their tinnitus improves. Tinnitus may be more apparent in ten percent of people.

3. Dryness of the mouth and taste disturbances

It's not unusual for a person to have taste disturbances and tongue dryness as a result of their surgery.

4. Disturbance of balance and dizziness

Acoustic neuroma surgery necessitates the removal of a portion or all of the equilibrium nerve, as well as the inner ear balance system in some situations. Because the tumor usually damages the equilibrium nerve, removing it leads to a reduction in preoperative unsteadiness. Dizziness is a common side effect of surgery, and it lasts for days or weeks.

5. Paralysis of the facial nerve

Acoustic tumors are in close proximity to the facial nerve, which regulates the motion of the muscles that close the eyelids as well as facial expression musculature. Following the removal of an Acoustic Neuroma, transient paralysis of the face and muscles that seal the eyelids is frequent. Weakness can last for six to twelve months, and in rare cases, residual weakness can be lifelong.

 

Conclusion

In medical practice, acoustic neuroma is a rare lesion. They do, however, frequently appear with unilateral hearing loss, so healthcare providers should keep the lesion in mind while making a diagnosis. An interdisciplinary team comprised of a neurosurgeon, neurologist, ENT surgeon, and hearing expert is most suited to treat the tumor once it has been identified. Tinnitus is a common symptom in at least 10 to 20 percent of individuals after surgery. After removal, the risk of recurrence is less than 5 percent. Facial nerve palsy tends to affect 15-30 percent of patients; however, the majority recover completely with time. Hearing loss affects more than half of patients, and it may not improve with time. Hearing loss that persists has a substantial negative influence on one's quality of life.