Neurilemmoma (Schwannoma)

The cause of these neoplasms is unknown.[2] Neurilemmoma can be associated with von Recklinghausen disease; when this is the case, multiple tumors often are present.

Neurilemmoma is the most common neurogenic tumor, but precise prevalence figures are not available. These tumors affect persons aged 20-50 years. No racial or sex predilection is recognized. Common locations for neurilemmomas are, in order of decreasing frequency, the head, the flexor surfaces of the upper and lower extremities, and the trunk.

Recurrence is unlikely after complete resection. Patients usually have rapid and complete relief of pain, with excellent long-term results.[3]

Rare descriptions exist of malignant change in long-standing neurilemmomas, usually in patients with an underlying diagnosis of neurofibromatosis. Malignant change is extremely rare in isolated lesions.

Kano et al evaluated tumor control and hearing preservation relating to tumor volume, imaging characteristics, and nerve and cochlear radiation dose after stereotactic radiosurgery with a Gamma Knife (Elekta, Stockholm, Sweden) in patients with acoustic neuroma.[4]

At a median of 20 months after surgery, none of the patients required further treatment.[4]  Serviceable hearing was preserved in 71% of all patients and in 89% of patients with Gardner-Robertson (GR) class I hearing. Patients who received a radiation dose lower than 4.2 Gy to the central cochlea had significantly better hearing preservation of the same GR class, and all 12 patients younger than 60 years who received a cochlear radiation dose lower than 4.2 Gy retained serviceable hearing at 2 years after surgery.

Because neurilemmomas (neurilemomas; also referred to as schwannomas) can present in many locations, the clinical presentation can be varied.[5]  Some may involve the spinal nerve roots and present with symptoms that mimic those of herniated disk disease of the spine.[6, 7]

Tenderness to palpation is often present; secondary neurologic symptoms may occur if the tumor is large. When involving the C7 nerve root, neurilemmoma has been described as a cause of thoracic outlet syndrome. Lesions in the sciatic nerve can mimic diskogenic low-back pain.

In the extremities, neurilemmomas can present either as an asymptomatic mass (usually mobile in the transverse plane and tethered along the axis of the nerve from which it arises) or as mild, localized pain and paresthesia resulting from pressure on the nerve of origin. Masses are slow-growing and can exist for months to years without producing symptoms. The average time from onset of symptoms to diagnosis is 5.5 years.

Lesions in proximal nerves may cause distal symptoms. If these masses occur in well-defined compartments (eg, wrist or ankle), they can present as either carpal tunnel or tarsal tunnel syndrome.

Findings on plain radiography generally are not specific for neurilemmoma (neurilemoma; also referred to as schwannoma). The rare intraosseous lesion presents as a benign-appearing, well-circumscribed lesion. Differential diagnoses for these lesions include giant cell tumors, chordomas (when involving the spine), and chondroblastomas. Massive bony destruction may be present, especially when the lesion involves the sacrum.

Special studies to consider include computed tomography (CT) and magnetic resonance imaging (MRI).[8] MRI is particularly useful (see the image below); it shows a usually round or oval mass with a moderately bright signal on T1-weighted images and a bright, heterogeneous signal on T2-weighted images.[9] The mass is usually less than 2.5 cm in size. The lesion enhances uniformly with gadolinium contrast.

Neurilemmomas have very distinctive appearances on magnetic resonance images. Many investigators believe that the presence of a target sign on a peripheral nerve is diagnostic for a neurilemmoma.

Wu et al conducted a study to determine the value of ultrasonography (US) in the diagnosis of neurilemmoma and to determine whether the presence of a round or roundlike shape on the ultrasound image is diagnostically useful.[10] They concluded that US is capable of diagnosing neurilemmomas with a high degree of accuracy but that round or roundlike shape and the ratio of longitudinal maximal diameter to short maximal diameter (L/S) are of little value in identifying neurilemmomas of the limbs.

Yuan et al, in a study assessing the diagnostic accuracy of multiparametric US for evaluating signs of peripheral nerve schwannoma, determined that excellent accuracy could be achieved by combining two-dimensional (2D) imaging, color flow imaging, and elastography.[11] ​ specific US signs specific to peripheral schwannoma included polar blood supply sign and target sign on elastography.

Biopsy may be needed to clearly define the tissue type. For bone lesions and for particularly large soft-tissue lesions, biopsy is prudent to ensure correct diagnosis and management. Whenever a biopsy is considered, strict biopsy guideline principles must be followed.

Lesions of the spinal cord often have a dumbbell shape; otherwise, they are fusiform in shape. They have an epineurium encapsulation, frequently with overlying vessels. The cut surface is pink or white. In very large masses, degenerative cysts, hemorrhage, or dystrophic calcification may be present.

Neurilemmomas have a well-defined fibrous capsule. Histologically, there are two distinct regions, as follows:

• Antoni A areas - These are cellular regions with predominantly benign spindle cells in many intersecting bundles; they may palisade around eosinophilic regions that are called Verocay bodies, and they are positive for S100 staining
• Antoni B areas - These are much less cellular and have a background of loose connective tissue that is myxomatous in appearance

Occasionally, a more aggressive histologic appearance may predominate, but such forms usually lack mitotic figures.

Neurilemmomas are commonly classified according to the Enneking system for benign lesions, as follows:

• Grade 1 - Inactive lesions
• Grade 2 - Lesions that deform the surrounding tissues but are not destructive or locally aggressive
• Grade 3 - Lesions that are locally aggressive and may invade local tissues but do not have metastatic potential

Generally, neurilemmomas are grade 2 or 3.

Neurilemmomas (neurilemomas; also referred to as schwannomas) can cause a functional deficit because of local pressure on the nerve of origin. Malignant degeneration, which is extremely rare, was described by Yousem et al in 1985.[12] Primary malignant tumors of this cell type do exist, but they are histologically distinct from neurilemmomas. In an unusual case in which resection would lead to a significant functional deficit, these benign lesions can be merely observed.

In a study assessing trends in management of unilateral vestibular schwannoma at Johns Hopkins University from 1997 through 2007, Tan et al noted an increase in cases observed with follow-up scanning (from 10.5% to 28.0%) and recommended for radiation (from 0% to 4.0%), along with a decrease in surgical cases (from 89.5% to 68.0%).[13] The increased frequency of observation was significant even after age, hearing status, and tumor size were controlled for. The authors concluded that this trend implied changes in both provider philosophy and patient expectations.

Like most benign tumors, neurilemmomas respond well to local resection. On inspection, the nerve is usually splayed out over the lesion. The lesion is excised marginally, and the nerve fibers are spared. Interlesional resection is warranted when complete resection would result in permanent neurologic deficit. Local control is usually excellent.[14, 15, 16, 17, 18]

Effective use of sterotactic radiosurgery (SRS) for these types of lesions has been reported.[19, 20, 21, 22, 23, 24, 25, 26]

Kano et al evaluated tumor control and hearing preservation relating to tumor volume, imaging characteristics, and nerve and cochlear radiation dose after SRS with a Gamma Knife (Elekta, Stockholm, Sweden) in patients with acoustic neuroma.[4] At a median of 20 months, no patients required further treatment. Serviceable hearing was preserved in 71% of all patients and in 89% of those with Gardner-Robertson (GR) class I hearing. Patients who received a cochlear radiation dose lower than 4.2 Gy had significantly better hearing preservation of the same GR class, and all patients younger than 60 years who received a dose lower than 4.2 Gy retained serviceable hearing at 2 years.

Endoscopic approaches have been used for schwannomas of the head and neck.[27, 28]

The most common complication of surgical treatment of neurilemmoma is initial neurapraxia; however, this neurologic deficit can be permanent, depending on the resection of neural tissue. Generally, patients tolerate resection well, with complete and rapid relief of symptoms.