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.
Symptoms of sporadic VS 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). 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 Ménière'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.
Neurofibromatosis type 2 (NF2) For the most part, unilateral sporadic vestibular schwannomas are readily treated successfully by modern medical techniques. Having bilateral VSs is a more troublesome condition. Bilateralism is considered to be the hallmark and main diagnostic criterion of neurofibromatosis type II (NF2), a genetic disorder that is heritable, progressive, difficult to manage, and has a roughly 50% likelihood of being passed on to each offspring. NF2 patients tend to have a more severe mutation, although there are mild mosaic cases in which only some cells carry the mutation. Genetic testing confirming mutation of the NF2 gene is recommended. About 50% of people with NF2 have a de novo mutation, and about 50% of these new mutations will be mild mosaic cases, which are less likely to be passed on. NF2 patients may develop other cranial and spinal tumors. NF2 develops during the teens or early adulthood, whereas sporadic VSs are diagnosed mostly in patients between the ages of 40 and 60. Both varieties of VS (sporadic and NF2) are very rare, accounting for only about 8% of all primary brain tumors. The incidence of NF2 is approximately 1 per 60,000 people.
Tumor sizes Patient surveys in the U.S. by the national Acoustic Neuroma Association (1998, 2007–08, 2012, 2014) showed that the percentage of diagnosed tumors 1.5 cm or less increased significantly from 23% to 47%. Researchers in Denmark reported (2004): "The size of diagnosed tumors has decreased from a median of 35 mm in 1979 to 10 mm in 2001." In general, tumor size (diameter) is described as small (less than 1.5 cm), medium (1.5 to 2.5 cm), large (2.5 to 4.0 cm) and giant (more than 4.0 cm). (Note: 1 inch = 2.54 cm) Radiologists reporting on MRI scans use the Koos Grading Scale which relates tumor size to its proximity to the brainstem and nearby cranial nerves. Thus Koos grade 1 is a purely intrameatal (IAC) tumor, 1–10 mm in size; Koos grade 2, 10–20 mm, has extended into the cerebellopontine angle (CPA), but with no brainstem contact; Koos grade 3, 20–30 mm, fills the CPA space and touches on the brainstem, but without compression; and Koos grade 4, more than 30 mm in size, compresses the brainstem and nearby nerves, and displaces critical arteries.
Diagnosis
… excerpt ends here. Continue reading the full article.
