Acoustic Neuroma Wikipedia

[Clinio Lofton]

Theterm is also used to refer to any swelling of a nerve, even in the absence of abnormal cell growth. In particular, traumatic neuroma results from trauma to a nerve, often during a surgical procedure. Morton's neuroma affects the foot.Neuromas can be painful, or sometimes, as in the case of acoustic neuromas, can give rise to other symptoms.

The stem neuro- originates from the Greek word for nerve (νεῦρον), while the suffix -oma (-ωμα) denotes swelling.[7] The stem does not imply that neuromas necessarily arise from neurons; neuromas generally arise from non-neuronal nerve tissues. The word was originally used to refer to any nerve tumor, but its meaning has evolved.[7]

A vestibular schwannoma (VS), also called acoustic neuroma, is a benign tumor that develops on the vestibulocochlear nerve that passes from the inner ear to the brain. The tumor originates when Schwann cells that form the insulating myelin sheath on the nerve malfunction. Normally, Schwann cells function beneficially to protect the nerves which transmit balance and sound information to the brain. However, sometimes a mutation in the tumor suppressor gene, NF2, located on chromosome 22, results in abnormal production of the cell protein named Merlin, and Schwann cells multiply to form a tumor. The tumor originates mostly on the vestibular division of the nerve rather than the cochlear division, but hearing as well as balance will be affected as the tumor enlarges.

The great majority of these VSs (95%) are unilateral, in one ear only. They are called "sporadic" (i.e., by-chance, non-hereditary). Although non-cancerous, they can do harm or even become life-threatening if they grow to press on other cranial nerves and vital structures such as the brainstem. Variations in the mutation determine the nature of the tumor's development. The only environmental exposure that has been definitely associated with the growth of a VS is therapeutic radiation exposure to the head.[9][10][11][12]

Sporadic VSs originate within the confining bony walls of the small (ca. 2 cm long) internal auditory canal. The most common early symptoms of these intracanalicular (IAC) VSs are gradual hearing loss and a feeling of fullness in the affected ear, some imbalance or dizziness, and tinnitus (ringing or other noise in the ear).[13] Gradual single-sided hearing loss in the high frequencies is the first most obvious symptom for the great majority of patients. Headache as a presenting symptom of VS specifically is rare; facial symptoms (facial numbness, weakness) usually occur only as the tumor grows out of the canal and/or after therapeutic treatment. Delayed diagnosis and misdiagnosis are not unusual. Initial hearing loss is usually subtle and may be attributed mistakenly to aging, earwax buildup, or perhaps exposure to some loud environmental noise. A sudden hearing loss, which is uncommon, might be misdiagnosed as Mnire's disease, an abnormality of the inner ear that also has tinnitus as a symptom. The brain's vestibular system usually compensates for early balance problems.

There have been cases of tumors that were actually asymptomatic until very large and at a critical stage. Tumor growth rates are highly variable: some small VSs (perhaps 50%) do not grow at all; some few grow for a time and then shrink; some appear dormant but suddenly grow rapidly. In general, although studies differ, VSs that grow are slow-growing at an average rate of 1.2 to 1.9 mm per year. IAC tumors that grow beyond 1.5 cm in diameter expand into the relatively empty space of the cerebellopontine angle, taking on the characteristic 'ice-cream-cone' appearance seen on MRIs. As 'space-occupying-lesions,' the tumors can reach 3 to 4 cm or more in size and infringe on the facial nerve (facial expression) and trigeminal nerve (facial sensation). Advanced hearing loss and spells of true vertigo may occur. Very large tumors are life-threatening when they press on the cerebellum or cause brainstem compression. Late symptoms of very large VS include headache, nausea, vomiting, sleepiness, mental confusion and eventually coma.[3][4]

Preliminary diagnostic procedures include ear examination, hearing and vestibular testing. Typical symptoms include unilateral tinnitus, progressive hearing loss and vertigo. Usually diagnostic sensitivity is increased with one or more otological symptom. The rate of VS pick up with unilateral tinnitus alone using MRI has been shown to be

Computed tomography (CT scan) of the head will detect moderate to large sized VS but can miss small sized VS. VS appears as isodense to surrounding brain parenchyma on CT. VS does not have calcifications in it. A large VS may expand the size of internal acoustic meatus (IAC) and may compromise hearing function because the nerves within the IAC are compressed, particularly the cochlear nerve. However, the facial nerve is less commonly affected. The main advantage of a CT scan is to assess the extent of bony involvement by VS. VS enhances when iodinated contrast is given. A contrasted CT scan of the temporal bone can done if the patient is unable to undergo MRI scan.[19]

MRI scan is the imaging of choice because it can more accurately differentiate the mass from other tumors such as meningioma, facial nerve schwannoma, epidermoid cyst, arachnoid cyst, aneurysm, and brain metastasis. MRI scan also helps in surgical planning and follow-up of the tumor after surgery.[19] VS is usually isointense on T1 weighted images, hyperintense on T2 weighted images, and enhances after given gadolinium contrast.[20]

The Guidelines on the Treatment of Adults with Vestibular Schwannoma issued in 2018 by the Congress of Neurological Surgeons in the U.S. looked at the long-term evolution of treatments for VS. The Introduction to the Guidelines stated: "The evolution in treatment over the last century has ultimately led to an environment where functional outcome has taken precedence over disease eradication. With multiple noninvasive management options available, the tolerance of cranial neuropathy in patients with small and medium-sized tumors is low. Today, hearing preservation, facial nerve function, and tumor control remain the primary benchmarks used to evaluate treatment effectiveness and compare outcomes." In other words, tumor management was able to give greater attention to preserving quality of life.

The three main surgical approaches to the tumor are the translabyrinthine (incision behind the ear to reach the bony labyrinth), the retrosigmoid (incision behind the ear to reach cerebellopontine angle) and the middle cranial fossa (incision in front of the ear to access the IAC from above).[21] Tumor size is a major factor in determining approach selection. Adjunctive use of the endoscope for enhanced visualization during surgery for IAC tumors has gained attention as an emerging technique with advancing technology. For large tumors, a 'facial nerve sparing surgery' offers partial removals, to be followed (as needed) by stereotactic radiosurgery or radiotherapy for 'residuals'. The rate of 'tumor control' appeared to be similar to that for gross total removal surgeries. For small to medium size tumors, the appropriateness of so-called 'hearing preservation surgery' via either the Middle Fossa or Retrosigmoid approach remained controversial. Data from Denmark indicated that primary observation offered the best chance to preserve good hearing the longest. But preserving good hearing in the affected ear remained an elusive goal. Even during observation, although tumors showed no significant growth, hearing deterioration occurred. Stangerup et al. reported (2010) that most patients with 100% speech discrimination at diagnosis had the best chance of maintaining good hearing after ten years of observation.

The CyberKnife radiation system introduced in 1994 recommends a protocol of three sessions known as hypofractionation. Radiation dosages overall were reduced over the years as experience showed that excellent tumor control rates could be maintained even as dosages were lowered to benefit hearing preservation and facial nerve function. Generally, single-session Gamma Knife radiosurgery is limited in use to VSs less than 3 cm in diameter to avoid possible complications with facial nerves, brainstem and the cochlea apparatus.[26] The risk of radiation-induced secondary tumors is very small, in the range of 0.01-0.02%. The risk for NF2 patients appears to be slightly higher.[27][28][29]

To date, there is no fully efficacious medical therapy for VS. The complexity of the molecular biology research involved is truly challenging. Clinical trials are in progress for other drugs such as everolimus, lapatinib and mifepristone[30][31][32]. Common aspirin has been studied as a low-risk therapeutic option, but emerging evidence suggests that aspirin and other NSAID use may not prevent VS tumor growth.[33]

The 1991 NIH Consensus Statement observed: "There is evidence that some patients with unilateral vestibular schwannoma and a subgroup of patients with NF2 may have tumors that fail to progress rapidly, resulting in stable neurologic function for a long time. The use of MRI with contrast enhancement has resulted in the identification of patients with very small, relatively asymptomatic vestibular schwannomas for whom the natural history is unknown. Conservative management may be appropriate for these patients."[34] At the time, conservative management (i.e., observation, 'wait-and-watch'/'wait-and-scan') was reserved mainly for elderly or infirm patients.

In 1777, Eduard Sandifort of Leiden, the Netherlands, wrote a postmortem first description of a vestibular schwannoma. He observed "a certain hard body adherent to the auditory nerve," and concluded this cause of deafness was beyond the reach of medication or surgery and was therefore incurable. The Schwann cells that multiply to form a VS on the vestibulocochlear nerve were identified 60 years later in 1838 by the German physiologist Theodor Schwann. In 1895, Thomas Annandale, a general surgeon at the Royal Infirmary in Edinburgh, Scotland, was the first to successfully localize and surgically remove a VS.[43] Finger dissection of VS to 'shell out' the tumor was typical. The main goal when dealing with large tumors was preservation of life.

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