Interfascicular Resection of Benign Peripheral Nerve Sheath Tumors

Abstract

Background:

Interfascicular resection is a surgical technique used to safely treat benign peripheral nerve sheath tumors through careful dissection of functional neural elements off the tumor surface^1,2.

Description:

Proper operative technique is essential to improving symptoms, preserving neurologic function, and minimizing the chance for recurrence. Accurate tumor localization, ideal patient positioning, and placement of a longitudinal incision permit adequate exposure. Prior to tumor resection, normal nerve should be identified proximally and distally and controlled with vessel loops. This allows functional fascicles streaming around the tumor in the pseudocapsule to be visualized during resection. A fascicle-free window is identified on the tumor surface through visual inspection and intraoperative neurophysiology monitoring if desired. The pseudocapsule layers are divided with a sharp instrument until a smooth and shiny true capsule layer is found. This plane should have minimal resistance and is developed circumferentially until the tumor can be enucleated in toto. At the poles of the tumor, a single nonfunctional nerve fascicle that courses into the tumor is typically found. If there is >1 fascicle running into the tumor, further pseudocapsule layers should be undermined to sweep fascicles off the true capsule surface. The entering-exiting fascicle can be tested for function and is cut sharply. The specimen should be sent to pathology for permanent sectioning. The sides of the pseudocapsule are spread in opposite directions to evaluate for residual tumor, and any remaining tumor is removed if it can be done safely. Meticulous hemostasis is achieved, and the surgical site is closed in anatomical layers.

Alternatives:

Pain is the most common presenting symptom, and neuroleptic medications should be used in escalating dosage prior to surgical intervention. Nonoperative medical therapy does not typically result in symptom freedom, and patients often opt for resection. For tumors that are suspected of being malignant, an image-guided percutaneous or open biopsy and staging (positron emission tomography and/or computed tomography scans of the chest, abdomen, and pelvis) are recommended prior to treatment planning. For symptomatic benign extremity lesions, surgical resection is the treatment of choice, and adjuvant therapies like radiation and/or chemotherapy are not recommended. For malignant lesions, more aggressive surgery (wide resection or amputation) and preoperative, intraoperative, or postoperative radiation with or without chemotherapy are often utilized.

Rationale:

The treatment approach depends on a variety of presenting features such as onset, progression, symptom severity, tumor size, location, imaging features, presence of a syndrome, and patient age. There is little benefit from the resection of an incidentally found, small, nongrowing lesion. The most common reasons for removal of extremity lesions are a painful mass and/or radiating “nerve” pain. There is a high likelihood of relieving the symptoms and minimizing the risk of recurrence, and a relatively low risk of causing neurologic injury. The procedure provides a definitive diagnosis. For patients with severe pain, progressive weakness, rapid tumor growth, or concerning imaging characteristics, biopsy should be considered to determine malignant potential.

Introductory Statement

Interfascicular dissection is a technique used for benign peripheral nerve sheath tumors to preserve functional neural elements surrounding the lesion and permit mass enucleation.

Indications & Contraindications

Indications

• Lesion pain.

• Radiating nerve pain.

• Numbness.

• Patient request (anxiety).

• Weakness (rare for benign nerve sheath tumors).

• Large size or rapid growth.

• Concerning imaging features.

• Definitive tissue diagnosis of a benign solitary schwannoma or neurofibroma and distinction from another pathology.

Contraindications

• Multiple prior surgeries with pain the only symptom (a relative contraindication only; medical management is important).

• Active infection.

• Atypical lesion (obtaining a biopsy and staging studies [positron emission tomography and/or computed tomography scans of the chest, abdomen and pelvis] prior to resection, is recommended).

• Patient is medically unfit to undergo regional anesthetic surgical procedure (general preferred).

Step 1: Preoperative Evaluation and Imaging (Fig. 1)

Fig. 1.

Figs. 1-A, 1-B, and 1-C MRI studies of a 57-year-old man with a 12-year history of left foot pain and radiating paresthesias into the plantar aspect of the foot. He had undergone multiple evaluations for plantar fasciitis and deep vein thrombosis without a diagnosis. There was no history of weakness or tumors in his family. A = posterior tibial artery, F = fibula, G = gastrocnemius muscle, S = soleus muscle, T = tibia, and V = posterior tibial vein. Fig. 1-A T2-weighted fat-saturated axial image demonstrating a lesion (asterisk) arising from the tibial nerve in the mid-calf, deep to the soleus muscle. Fig. 1-B T1-weighted fat-saturated axial image after gadolinium injection showing homogeneous enhancement of the mass (asterisk) adjacent to the posterior tibial artery and vein. Fig. 1-C Sagittal T1-weighted image showing the fusiform lesion (asterisk) with smooth borders arising from the tibial nerve (arrows).

Before resecting a peripheral nerve schwannoma using the interfascicular dissection technique, perform a thorough history and physical examination, assess for evidence of a syndrome or family history, and use imaging studies to evaluate the tumor.

• Perform a thorough neurologic history and physical examination. While patients often report paresthesias and subjective numbness, these lesions rarely present with true sensory or motor deficits³.

• Evaluate for evidence of a syndrome or family history.

• Consider nerve conduction or electromyography studies if a neurologic deficit is found.

• Use high-resolution ultrasound or magnetic resonance imaging (MRI) with contrast medium (preferred) to evaluate the tumor (signal characteristics) and its location. Both schwannomas and neurofibromas are usually globular or fusiform in shape with regular borders, isointense to muscle on T1-weighted sequences, hyperintense on T2-weighted images, and avidly enhance with gadolinium, and they commonly demonstrate the split-fat sign, fascicular sign, and target sign⁴.

Step 2: Patient Positioning and Incision Planning (Fig. 2)

Fig. 2.

The patient is positioned supine with the hip and knee flexed and externally rotated. Natural fascial plans can be exploited via this medial approach to limit muscular dissection and permit visualization of the deep vasculature within this compartment.

Position the patient so that the target nerve is accessible; prepare and drape the patient; center the incision on the mass, using imaging as necessary; and perform a longitudinal incision following the course of the nerve.

• Ensure that the target nerve is accessible along its course and use circumferential draping, which is preferred.

• Plan a longitudinal skin incision centered on the mass; perioperative imaging (ultrasound or MRI with markers) can be helpful when the mass cannot be localized with physical examination.

• Perform a longitudinal incision, following the course of the nerve if possible; an S or Z-shaped path is used across joint creases.

Step 3: Dissection of Soft Tissue to Expose the Parent Nerve and the Tumor (Fig. 3)

Fig. 3.

External neurolysis, superficial tumor exposure, and vessel loop control of the proximal and distal aspects of the normal nerve (in blue vessel loops) and surrounding artery and vein (in red vessel loop). Gentle tension using vessel loops may buttress tumors located more deeply.

Carry the incision down to the nerve, which is located between muscle bellies, and use vessel loop control of the normal nerve proximal and distal to the lesion before dissecting the tumor.

• Carry the incision down to the nerve using electrocautery or gentle dissection.

• Locate the nerve between muscle bellies and obtain vessel loop control of the normal nerve proximal and distal to the lesion prior to dissection of the tumor itself.

Step 4: Superficial Tumor Dissection and Inspection of the Tumor

Perform external neurolysis and search the tumor surface for a fascicle-free window.

• Working from known normal nerve, perform external neurolysis with blunt spreading maneuvers.

• Search the tumor surface for a fascicle-free window; use a nerve stimulator if unsure.

Step 5: Epineurotomy of Tumor Pseudocapsule

Perform an epineurotomy of the pseudocapsule.

• Use bipolar electrocautery on the epineurial vessels in a fascicle-free window.

• Incise the pseudocapsule with a knife or sharp dissector in a longitudinal direction (parallel to nerve fibers).

Step 6: Interfascicular Dissection (Fig. 4, Videos 1 and 2)

Fig. 4.

Tumor dissection between the pseudocapsule and the true capsule. A stitch placed longitudinally in line with the nerve may be used to provide gentle traction and help roll out tumors.

Perform extracapsular dissection until the true tumor capsule is found, work around the tumor using a blunt instrument to remove the pseudocapsule, and use finger dissection to separate the tumor from the pseudocapsule.

• Attempt extracapsular dissection until the true tumor capsule is found; if it is not, perform additional dissection of the pseudocapsule. The true capsule is a shiny smooth yellow color, and there is a decrease in dissection resistance once it is reached⁵.

• Once the true capsule is found, work circumferentially around the tumor with no cutting technique but using a blunt instrument, cottonoid, or gentle finger dissection to sweep away pseudocapsule and the fascicles running along the surface of the tumor.

• If the tumor is firm, place a stitch longitudinally through the true tumor capsule to aid in gentle traction while dissecting off the pseudocapsule.

• Continue dissection to fully separate the tumor away from the pseudocapsule. A transition point from the normal nerve to the tumor can be palpated and utilized to further mobilize the tumor at the poles.

Video 1.

Download video file ^{(12.3MB, mp4)}

DOI: 10.2106/JBJS.ST.18.00077.vid1

Animation demonstrating resection of a benign peripheral nerve sheath tumor. Highlighted is the interfascicular technique to achieve gross total resection while preserving surrounding functional fascicles. (Used with permission of Mayo Foundation for Medical Education and Research. All rights reserved.)

Video 2.

Download video file ^{(109.6MB, mp4)}

DOI: 10.2106/JBJS.ST.18.00077.vid2

Clinical case including intraoperative video of benign nerve sheath tumor resection from the right middle trunk of the brachial plexus. (Used with permission of Mayo Foundation for Medical Education and Research. All rights reserved.)

Step 7: Tumor Enucleation (Fig. 5)

Fig. 5.

A single entering fascicle is identified at the pole of the tumor (taut red vessel loop). These are typically nonfunctional but can be tested with a nerve stimulator to ensure this is not a critical fascicle.

After the tumor is freed circumferentially, identify and dissect any fascicles after testing to ensure they are nonfunctioning, and then transect them to remove the entire tumor.

• After the tumor is freed circumferentially, identify entering and exiting fascicles.

• If >1 fascicle is entering and/or exiting, attempt to dissect the additional fascicle(s) free from the true capsule.

• Test any fascicles directly entering the true capsule to ensure that they are nonfunctional and then transect them to remove the tumor in its entirety as one solid mass.

Step 8: Inspection, Hemostasis, and Closure (Fig. 6)

Fig. 6.

Photograph of the specimen demonstrating the smooth true capsule of the lesion and single entering fascicle.

Evaluate for and remove any remaining tumor if it can be done safely, ensure that the tumor has smooth borders and no nerve fascicles emanating from it, send the specimen for pathologic analysis, and close the surgical site.

• Gently spread the sides of the pseudocapsule in opposite directions to evaluate for residual tumor and remove any remaining tumor if it can be done safely.

• The pseudocapsule is not specifically resected.

• Evaluate the tumor to ensure that it has smooth borders and no major nerve fascicle is emanating from the tumor. Send the specimen to pathology for permanent section.

• Achieve hemostasis with an absorbable gelatin compressed sponge and gentle direct pressure; use bipolar electrocautery sparingly and only if absolutely necessary.

• Close the surgical site in anatomical layers.

Results

Interfascicular resection has been shown to safely treat peripheral nerve sheath tumors through careful dissection of functional neural elements off the tumor surface¹, with approximately 90% of nonsyndromic patients with a solitary lesion having preservation of neurologic function and alleviation of symptoms when proper technique was used^6-8.

While temporary sensory disturbances are common after tumor resection, permanent weakness is rare if functional fascicles are protected and the tumor is enucleated in toto^9,10. Surgical resection typically produces resolution of nerve-type pain, and patients are usually able to wean off neuroleptic mediations. Tumor dissection and resection may be more difficult in patients with schwannomatosis or neurofibromatosis types I or II, in whom slightly lower rates of preserved function have been reported¹¹. Nonetheless, patients with these syndromes still greatly benefit from tumor resection if they are symptomatic. While recurrence of benign solitary lesions is very low (<1%), plexiform lesions may persist or recur and should only require revision surgery if symptoms cannot be controlled medically¹.

As experience is gained in finding the correct tissue plane, gross total resection should be anticipated in most cases. The primary goal for resection of benign nerve sheath tumors is symptom relief and not every tumor cooperates as described. Certain tumors may be scarred to surrounding tissues or occur in regions that are less accessible. Safety and preservation of nerve fascicles should never be sacrificed in order to achieve gross total resection. If a complete resection is achieved, the recurrence rate is very low for pure isolated solitary lesions. As it is a benign pathology, only patients who are symptomatic should be offered surgery.

Pitfalls & Challenges

• Inadequate exposure of the proximal and distal aspects of the nerve can lead to injury, especially during dissection at the poles of the lesion.

• The pseudocapsule is rarely adherent to the true capsule. In these circumstances, intracapsular piecemeal dissection is recommended to protect the fascicles running along the surface of the tumor and piecemeal resection can be performed if necessary.

• Plexiform lesions are frequent in patients with schwannomatosis or neurofibromatosis types I and II. They have multiple entering and exiting fascicles, which should be tested with a stimulator and need to be dissected carefully to prevent injury.

• Most peripheral nerve sheath tumors are benign, and biopsy should be avoided because it can cause pain or substantial iatrogenic neurologic deficit.