Acoustic Neuroma Statpearls

[Krysta Cirilo]

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Acoustic neuroma is also known by other names such as vestibular schwannoma (VS), acoustic neurinoma, vestibular neuroma, and acoustic neurofibroma. These tumors originate from the Schwann cells in the sheath surrounding the vestibular and cochlear nerves. Acoustic neuroma can occur either intracranially or extra-axially.

Acoustic neuromas are commonly located in the cerebellopontine angle, near the cochlear and vestibular nerves, particularly the inferior division of the vestibular nerve. About 5% to 10% of tumors in the cerebellopontine angle (CPA) are meningiomas, which may occur elsewhere in the brain. While most cases are sporadic, bilateral acoustic neuromas are typically associated with type 2 neurofibromatosis.

Acoustic neuromas are tumors that develop from the sheath of Schwann cells. They tend to occupy the cerebellopontine angle and are usually found adjacent to the cochlear or vestibular nerve, either intracranially or extra-axially. This activity reviews the etiology, evaluation, and management of acoustic neuromas and highlights the collaboration among the interprofessional team in caring for patients with this condition.

Objectives:Identify the signs and symptoms of acoustic neuromas, such as gradual hearing loss, tinnitus, and balance problems.Assess the size, location, and growth rate of acoustic neuromas to determine the appropriate treatment approach.Utilize various treatment modalities, including surgical resection, stereotactic radiosurgery, or medical management, to develop individualized treatment plans for patients with acoustic neuromas.Collaborate with a multidisciplinary team, including neurosurgeons, radiation oncologists, audiologists, and rehabilitation specialists, to provide comprehensive care and optimize patient outcomes.Access free multiple choice questions on this topic.

Acoustic neuromas are commonly located in the cerebellopontine angle, near the cochlear and vestibular nerves, particularly the inferior division of the vestibular nerve. About 5% to 10% of tumors in the cerebellopontine angle (CPA) are meningiomas, which may occur elsewhere in the brain. While most cases are sporadic, bilateral acoustic neuromas are typically associated with type 2 neurofibromatosis.[1][2][3]

Bilateral acoustic neuromas can be associated with neurofibromatosis type 2, which is caused by a defect on chromosome 22q12.2 at the location of the neurofibromin 2 gene, responsible for encoding the merlin protein. Studies have indicated a predisposing mutation for the development of acoustic neuroma. Additionally, radiation exposure may increase the likelihood of developing this condition.[4]

While there have been concerns regarding mobile phone radiation, several studies have failed to establish a direct causal relationship between mobile phone radiation and the development of vestibular schwannomas.[5]

Schwannomas account for approximately 8% of all clinically manifested intracranial tumors. Most acoustic neuromas are unilateral and occur sporadically. However, genetic factors contribute to developing bilateral acoustic neuromas, constituting less than 5% of all schwannomas.

Generally, acoustic neuromas are diagnosed between the fourth to sixth decades of life. However, individuals with neurofibromatosis type 2 (NF II) tend to present earlier, with the peak incidence occurring in the third decade of life. Although rare, acoustic schwannomas can occur in children.

There is a slight female preponderance, and symptoms can worsen during pregnancy. The hereditary form of acoustic neuroma is more commonly associated with NF II than neurofibromatosis type 1 (NF I), despite NFI being more prevalent. NF I is exclusively linked to unilateral acoustic neuroma in 24% of cases, while bilateral acoustic schwannoma is a hallmark feature of NF II. Both NF I and NF II are autosomal dominant, with genetic defects localized to chromosome 17 and chromosome 22, respectively.[6][7]

On gross examination, acoustic neuromas typically exhibit a rubbery-firm consistency and appear pale with a gray color. They may display varying degrees of vascularity and possess a well-defined capsule. The presence of the tumor can lead to the displacement and stretching of nerve fibers, which may result in discernible changes to the capsule's appearance.

When an acoustic neuroma is sectioned, the cut surface reveals a pale gray and firm appearance with a finely trabeculated pattern. Evidence of cystic degeneration, hemorrhage, xanthomatous changes, and calcification points may be present in larger tumors. These changes contribute to a variegated appearance in terms of consistency and color of the giant tumors. The blood supply to the tumor primarily originates from the internal auditory artery, which branches into several tiny vessels on the tumor's surface. In the case of larger tumors, there may be blood supply from small branches of neighboring cerebellar and pontine arteries.

Antoni A tissue is characterized by a compact and organized structure consisting of an interwoven arrangement of elongated bipolar cells. The nuclei and fibers in Antoni A tissue may demonstrate a distinctive pattern, occasionally forming a spiral framework that resembles the appearance observed in meningiomas.

Antoni B, commonly observed in large acoustic neuromas, is characterized by a less organized arrangement of cells. In Antoni B tissue, cells are randomly dispersed and often clustered around areas of cystic changes, necrosis, old hemorrhage, and blood vessels. In addition, this tissue type has varying amounts of lymphocytic infiltration. The Antoni B tissue type, generally seen in larger tumors, is thought to be the outcome of ischemia.

The relative proportions of Antoni A and Antoni B tissue types determine the consistency of the acoustic neuroma. Although these changes are benign and malignant transformation is rare, nuclear pleomorphism is a typical feature of schwannomas. Mitotic figures, which are indicative of cell division, are relatively rare. Necrosis, if present, is attributed to poor blood supply rather than rapid tumor growth.

In electron microscopy (EM), the characteristic basement membrane of Schwann cells can be observed. Additionally, wide-spaced collagen is often present, indicating abnormalities in collagen fiber arrangement.[8]

The signs and symptoms of acoustic neuroma are primarily caused by the compression of the surrounding structures, including cranial nerve VIII (the vestibulocochlear nerve), adjacent cranial nerves, cerebellum, brainstem, and the resulting increase in intracranial pressure (ICP). Most individuals with acoustic neuromas present unilateral hearing loss due to cochlear nerve interruption or impaired blood supply to the nerve. Other clinical features include tinnitus, decreased word understanding, vertigo, headaches, and numbness. As the acoustic neuroma grows, the mass within the CPA will eventually compress the brainstem. This compression of the brainstem can result in gait abnormalities. The following is a summary of the clinical features of acoustic neuroma.

An acoustic neuroma is diagnosed using contrast-enhanced magnetic resonance imaging (MRI) or computed tomography scans. Contrast agents, such as gadolinium, enhance the visualization of tumors and obtain information about the tumor's characteristics and relationship to surrounding structures. Using contrast in these studies is also essential to view smaller tumors. If a patient has a hearing impairment, audiometric tests are often necessary to assess their hearing abilities. While auditory brainstem evoked response is a useful test, it is not commonly used as a primary screening tool for acoustic tumors, as it cannot detect small malignancies reliably.[9][10][11]

An intracanalicular component in the porus acusticus can cause a characteristic finding known as the "trumpeted internal acoustic meatus sign," observed as a widening or enlargement of the opening. Alternatively, when the tumor extends into the extrameatal space, it can produce a distinct appearance known as the "ice cream cone" sign.

Most acoustic neuromas appear hypo to isointense on T1-weighted MRI images and demonstrate heterogeneously hyperintensity on T2-weighted images. Furthermore, these tumors often exhibit significant contrast enhancement when administering a contrast agent, such as gadolinium.

1. Retrosigmoid approach: This involves removing the tumor through an opening at the skull base. This approach provides good access to this region's tumor and cranial nerves. A tumor of any size can be approached while potentially preserving hearing.

2. Middle cranial fossa approach: In this approach, the surgeon gains access to the tumor through the middle cranial fossa of the skull base. This approach is most suitable for tumors with dominant intracanalicular and small cisternal components. Hearing is preserved in most cases. The main disadvantage is the need for temporal lobe retraction, which may produce postoperative seizures and venous infarction if the vein of Labbe is damaged.

3. Translabyrinthine: This technique involves removing the tumor through the inner ear and is most commonly used in patients with large tumors and no serviceable hearing. The main advantage of this approach is that facial nerve can be exposed early and be protected. Also, the cerebellum does not need to be retracted. But the access to the contents of the jugular foramen and lower parts of the foramen magnum is limited.

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