Neurofibromatosis 2

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Chapter 57 Neurofibromatosis 2

Neurofibromatosis 2 (NF2) is a rare syndrome characterized by bilateral vestibular schwannomas, multiple meningiomas, cranial nerve tumors, spinal tumors, and eye abnormalities. NF2 presents unique challenges to the otologist because hearing loss may be the presenting complaint leading to the diagnosis of the disorder. NF2 is quite invasive, requiring a multispecialist team approach for the evaluation and treatment of the disorder. The primary impairment is hearing loss resulting from bilateral vestibular schwannomas. NF2 must be characterized from neurofibromatosis 1 (NF1); although the names are linked, the disease entities are distinctly different. This chapter reviews the clinical characteristics of NF2, and current recommendations for evaluation and treatment.

CLINICAL CHARACTERISTICS OF NEUROFIBROMATOSIS 2

Definition

The NIH Consensus Development Conference also developed guidelines for the diagnosis of NF2. NF2 is distinguished by bilateral vestibular schwannomas with multiple meningiomas, cranial tumors, optic gliomas, and spinal tumors. A definite diagnosis is made on the basis of the presence of bilateral vestibular schwannomas or developing a unilateral vestibular schwannoma by age 30 and a first-degree blood relative with NF2, or developing at least two of the following conditions known to be associated with NF2: meningioma, glioma, schwannoma, or juvenile posterior subcapsular lenticular opacity/juvenile cortical cataract (Table 57-1).1

TABLE 57-1 Neurofibromatosis 2 (NF2) Diagnostic Criteria

INDIVIDUALS WITH THE FOLLOWING CLINICAL FEATURES HAVE CONFIRMED (DEFINITE) NF2
Bilateral VS or family history of NF2 (first-degree family relative) plus
Unilateral VS <30 years or
Any two of the following: meningioma, glioma, schwannoma, juvenile posterior subcapsular lenticular opacities/juvenile cortical cataract
INDIVIDUALS WITH THE FOLLOWING CLINICAL FEATURES SHOULD BE EVALUATED FOR NF2 (PRESUMPTIVE OR PROBABLE NF2)
Unilateral VS <30 years plus at least one of the following: meningioma, glioma, schwannoma, juvenile posterior subcapsular lenticular opacities/juvenile cortical cataract
Multiple meningiomas (≥2) plus unilateral VS <30 years or one of the following: glioma, schwannoma, juvenile posterior subcapsular lenticular opacities/juvenile cortical cataract

VS, vestibular schwannomas.

There may be significant heterogeneity in the presentation of the disease from one individual to the next. Some individuals may have a very mild form of the disease with small vestibular schwannomas manifesting in an older individual. Meanwhile, some children present with multiple intracranial tumors at a very young age. Despite the heterogeneity of the disease within a family, the expression of NF2 tends to be very similar.2 There is a significant genetic component to the disease with much variability within the parameters of the observed phenotype. Studies have shown that a truncating mutation (nonsense and frame shift) may be linked with a more severe form of NF2.35 The more severe form of NF2 is termed Wishart form. Individuals with this severe form present with early onset of the disease with multiple intracranial schwannomas and meningiomas that result in blindness, deafness, paralysis, and death by age 40. Despite the strong genotype-phenotype correlation, individual differences in tumor growth occur within subjects, making it difficult to predict how an individual will change over time even when the genotype is known.

The milder form, or Gardner form, of NF2 is less debilitating. The schwannomas may remain stable for many years, few meningiomas develop, and patients may not develop symptoms until later in life and often have fewer disabilities. The genetic basis of the mild form has not been well characterized. Many of these may be mosaic forms of the disease, however.2-4,6-12

Prevalence and Incidence

The average age of diagnosis of NF2 is 25 years; however, many patients present with symptoms before the diagnosis. There is an average delay of diagnosis of approximately 7 years (Fig. 57-1). There is no difference in the proportion of men versus women who develop NF2, and no prevalence has been described based on ethnicity. Epidemiologic studies place the incidence of NF2 between 1 in 40,000 live births13 and 1 in 87,410 live births.14

Imaging Studies

All patients suspected to have NF2 should have a high-quality magnetic resonance imaging (MRI) scan performed with thin cuts through the internal auditory canal (IAC). All patients diagnosed with a unilateral vestibular schwannoma should have a dedicated IAC series to ensure there is not another tumor on the opposite side. Patients diagnosed with NF2 should have a complete spine series to evaluate the spine and stage the disease. Small spinal tumors commonly may be found in the cauda equina area, and occasionally a large asymptomatic schwannoma or meningioma may be found in the spine that could require treatment. Early treatment of spine tumors can significantly reduce the mortality associated with these tumors. Older patients who present with bilateral IAC tumors must be worked up for other carcinomas. It is unusual for patients older than 40 years to present with NF2, although with more sensitive MRI scanning techniques, more of these individuals are being diagnosed at an older age. Metastasis may rarely manifest with bilateral IAC lesions, and it is important that carcinoma be ruled out in any patient older than 40 years who presents with bilateral IAC lesions.

A patient identified with NF2 should have a complete cranial MRI scan with cervical thoracic and lumbar spinal imaging. This scan serves as a baseline. A 6-month follow-up MRI scan is recommended for the intracranial tumors. If the tumors exhibit stability, a yearly MRI scan is performed on the intracranial structures. Large tumors in the spine may be monitored with a similar frequency. If no spinal tumors are present, spinal imaging should be performed if the patient becomes symptomatic. Monitoring spinal tumors every 3 years is recommended when these are present. Intracranial imaging is performed on a yearly basis unless studies over several years indicate stability of the tumors.

Molecular Genetics

The NF2 gene was mapped to chromosome 22 q 12-2in 1993.1517 The NF2 gene located at chromosome 22 codes for a tumor suppressive protein termed Merlin or Schwannomin. This protein negatively regulates Schwann cell production. The loss of this protein allows overproduction of Schwann cells. The mutation in the NF2 chain predisposes individuals to developing a schwannoma when the second hit occurs to the gene; control of Schwann cells is lost or mutated within the cell. Various types of mutations have been identified, including single base substitutions, insertions, and deletions.4,1820 The mild, or Gardner, type of NF2 may be associated with missense mutations, whereas associations between the other mutations and phenotypes are not as clear.21 The occurrence of NF2 is not restricted to families known to carry the mutation. Frequently, genetic mosaicism occurs, which may not be detected by common mutation analysis techniques.22 Unilateral vestibular schwannomas may exhibit the same type of genetic markers as NF2.23 The mutations in unilateral vestibular schwannomas are confined to the affected tumor tissue. In patients with NF2, the mutation is present in all cell types.22

Screening

MRI screening of potentially affected individuals uses a postcontrast T1-weighted sequence of the full head with thin cuts through the IAC. A dedicated IAC MRI scan identifies most NF2 patients by showing any vestibular schwannomas. Screening of the spine and ophthalmologic examination should be considered if the cranial MRI scan is positive. An audiogram (pure tone thresholds) or current clinical standard auditory brainstem response (ABR) testing is likely to miss small vestibular schwannomas. MRI can diagnose presymptomatically.

MRI is recommended for at-risk children when this test can be performed without sedation; this usually can be done when the child is 7 to 9 years old. A recommended first step for children younger than 7 years is an audiogram. Any child with an NF2-associated symptom, such as hearing loss or facial weakness, should be screened without regard to the need for sedation or age; MRI should be performed as soon as possible after the symptoms become apparent.

Identification of the NF2 gene and chromosome 22 has made genetic testing possible. It is recommended that patients with NF2 see a genetic counselor to discuss the hereditary consequences of the disease. Blood testing for the mutation is able to identify the defect of the NF2 gene in approximately 70% to 75% of patients with a known diagnosis of NF2. If the defect is identified in the affected individual, potential family members may be screened. If the gene is not identified in the affected individuals, blood screening of family members cannot be performed. The use of blood screening for patients without a diagnosis of NF2 or with a suspected diagnosis of NF2 is not recommended. New mutations in patients with mild presentation are most likely to be missense mutations, and this is difficult to identify with genetic testing of NF2 patients.

Tumor Types

Bilateral vestibular schwannomas (acoustic neuromas) are benign neoplasms of the acoustic or eighth cranial nerve (Fig. 57-2).24 The tumors are thought to arise at the glioma–Schwann’s cell junction within the internal auditory meatus. The tumors most commonly arise from the superior vestibular nerve, although with NF2, tumors may be found on the cochlear and facial nerves within the internal auditory meatus. The consequences of a vestibular schwannoma are numerous, including hearing loss progressing to deafness, dizziness and balance problems, tinnitus, facial nerve paralysis, brainstem compression, and, if left untreated, death.

Despite the strong genetic effect in NF2, there is enormous variability in the number of tumor types, the rate of progression, and the disabilities experienced. This enormous variability is also found in patient presentation. Some patients may be asymptomatic. Patients who have no symptoms when diagnosed have generally been identified on the basis of genetic analysis conducted because a blood relative has NF2 or presymptomatic screening. Although the NIH criteria for NF2 require the presence of bilateral vestibular schwannomas for diagnosis, patients may first develop unilateral schwannomas as a young child with no other tumors, or adult patients may present with multiple meningiomas (cranial and spinal) and no vestibular schwannomas.9,25 Although the NIH criteria for NF2 imply that all NF2 patients develop bilateral vestibular schwannomas, some researchers are not convinced of this.36 Evans and colleagues26 based their conclusion on the observation of a possible variant form of NF2 manifesting with skin and spinal tumors in the absence of vestibular schwannomas. Nonetheless, the phenotype generally is reflective of the underlying disorder.

The natural history study of vestibular schwannomas in NF2 conducted at the House Ear Institute showed that 10 of 80 (12.5%) enrolled subjects had no symptoms at diagnosis, and 23 (28.8%) had cranial meningiomas and spinal meningiomas in addition to bilateral vestibular schwannomas. Nearly half (47.5%) had one vestibular schwannoma removed before enrollment. Generally, the tumor resected before enrollment was removed 1.5 years after discovery, and was an average of 2.1 cm at removal. Few patients in the natural history study had spinal tumors or meningiomas removed before enrollment. The preliminary data would indicate that, for this sample of NF2 subjects, the most salient medical issue is the growth of the vestibular schwannomas.

Intracranial Schwannomas

Vestibular schwannomas are the most common intracranial schwannoma associated with NF2. The most frequently identified nonvestibular schwannomas are schwannomas of CN III and V. Bilateral CN III or V schwannomas are the most common additional schwannomas seen. It is important to identify these lesions on MRI. Lower cranial nerve schwannomas may also be identified, but are much less frequently seen. A vestibular schwannoma rarely turns malignant, and sometimes the unilateral vestibular schwannoma may regress in size altogether. Growth of the tumors does not seem to be related either to loss of heterozygosity (genetic level of analysis) or to auditory functioning (phenotype level of analysis). For this reason, it is recommended that a patient have at least yearly MRI scans to track changes in size.3340 All newly diagnosed patients should have a full head and spine study to stage their disease. After the disease is staged, a 6 month study is performed to determine if the tumor is fast-growing or slow-growing.41

CN V, or trigeminal nerve, schwannomas are the most common type seen after vestibular schwannomas. Oculomotor schwannomas are the third most common schwannomas seen intracranially. Occasionally, it is difficult to distinguish whether these schwannomas have arisen from the oculomotor, trochlear, or obducent nerve, especially when the tumor rises within the cavernous sinus. Trochlear and abducent schwannomas are extremely rare with only a handful of cases reported in the literature.

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