Median nerve- Excision of tumour and AxoGuard nerve protector wrapping

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Peripheral nerve sheath tumours are rare but the true incidence is unknown as many are asymptomatic and present as spurious findings on imaging for other pathologies. When they are large or occur at natural compression points they are more likely to be symptomatic. Small subcutaneous nerve sheath tumours may present as small lumps that are painful if touched. Most tumours fall into the benign category and are either Schwannoma or Neurofibroma subtypes. Most are solitary but both can present as multiple tumours in neurofibromatosis. Multiple Schwannomas are also a feature of a separate genetic condition Schwannomatosis in which strings of multiple tumours may arise from a single nerve trunk. Following excision superficial nerves may remain sensitive or tether in scar tissue.
The AxoGuard nerve protector (Axogen) is made from layered porcine extracellular matrix and provides protection from scar formation and adherence to surrounding tissues. Rapid revascularisation allows soft tissue incorporation and restores nerve gliding.

Indications:
Nerve tumours are rare and most are asymptomatic. In cases where there are established motor and sensory abnormalities, superficial tumours susceptible to local repeated trauma and where there is diagnostic uncertainty, consideration should be given to surgical removal.
Each patient must be counselled specifically for their particular tumour by virtue of its location and size. Non-critical sensory nerve tumours may be removed with less morbidity than a proximal tumour in a mixed motor and sensory nerve. The patient must be aware of the risk of neurological deterioration following surgery. The deterioration may be transient from oedema but there is a risk of creating an axonotmesis type injury from intraneural dissection as well as the possible need for resection of an involved fascicle group for tumour clearance.
Symptoms and examination:
This tumour presented in the median nerve at the distal forearm and was symptomatic when the arm was resting on a table in the pronated position using a keyboard, when knocked and when irritated by clothes. The tumour was causing a mild degree of baseline pain and with intermittent paraesthesiae in the median nerve distribution of the hand. There was no motor dysfunction in the median nerve distribution in the hand. A 1.5 x 1cm firm mass was palpable along the course of the median nerve with a strong Tinel’s sign elicited at the site of the swelling. The mass did not transilluminate and could not be moved in a longitudinal plane but was readily mobile in a radio-ulnar direction, suggestive that is was situated in a longitudinal structure.
Investigations:
Imaging of the tumour with MRI +/- ultrasound is mandatory prior to consideration of surgical removal. If there are any unusual features on imaging or worrying clinical presentation (nerve pain, rapidly increasing in size, recurrence at a site of previous resection, major motor or sensory dysfunction) then consideration should be given to a nerve biopsy prior to planning definitive surgical management. In uncertain cases the use of MRI spectroscopy shows promise in identifying abnormal proteins (trimethylamine) that may be associated with atypia or malignancy). Perhaps in the future these types of chemical imaging plus functional imaging (PET scanning) may provide diagnostic information sufficiently accurate to prevent open nerve biopsy. Neurophysiology is necessary when there is clinical evidence of deteriorating nerve function.
Imaging of this swelling with ultrasound suggested a nerve sheath tumour of the median nerve and subsequent MRI examination confirmed this location with typical features consistent with a diagnosis of a benign Schwannoma.
Non-operative management:
Small asymptomatic tumours with no adverse features on clinical examination or imaging may be clinically monitored with repeat imaging only if there is a change in size or symptom profile. Tumours can be observed when there are minimal symptoms, no neuropathic pain and no sensory or motor disturbance as long as imaging is typical for a benign peripheral nerve sheath tumour.
This patient was initially treated non-operatively for 3 years by a hand surgeon who had reassured the patient that the swelling was benign and had counselled them that surgical excision would be likely to result in permanent loss of nerve function in the hand and neuropathic pain. The patient came for a second opinion because the pain was significant and intrusive on daily activities.
Alternative operative management:
For benign tumours simple excision biopsy should suffice but the operation should be performed with great care to prevent further loss of nerve function. If there is diagnostic uncertainty and imaging is inconclusive consideration should be given to a formal open incisional biopsy of the lesion. The biopsy should include the interface with normal nerve tissue and so results in damage and scar that makes it more likely that there will be permanent neurological dysfunction after tumour resection. In simple cases with routine imaging and clinical findings we recommend a marginal excision of the tumour by way of excision biopsy to prevent these problems. It must be noted that despite these precautions there are still cases of inadvertent marginal excision of malignant tumours although these tend not to be neurofibrosarcomas but usually atypical presention of tumours such as synovial sarcoma or intra-neural Ewing’s sarcoma. Therefore it is essential that all potential surgical cases are discussed in a multidisciplinary meeting with review of imaging, clinical presentation and any previous histology or biopsy findings. The appropriate surgical procedure can then be planned. For malignant tumour resection wide margins are required with radical excision of surrounding normal tissue to ensure that the tumour is not breached. Reconstruction of the nerve in these rare cases must be planned and depends on the adjuvant therapy planned and the site of the nerve gap. Options include nerve grafting, nerve allografting, distal nerve transfers and tendon transfers.
In this case simple excision was recommended based on the clinical findings, imaging and chronic history. In such cases the epineurium can be left open after tumour removal, however I use a barrier nerve wrap to reduce the risk scar adhesion and wound sensitivity.
Contraindications:
Surgery should not be undertaken outside of a specialist unit if there is a suspicion on imaging of malignant peripheral nerve sheath tumour. In such cases definitive management should be done by a sarcoma specialist and should involve radical excision margins with compartment excision or amputation.

The RIGHT arm was anaesthetised with an axillary regional local anaesthetic block. This provides a segmental block to conduction in the nerves at the site of local anaesthetic infiltration, without loss of conduction and neuromuscular stimulation in the distal nerve segment. It is important for the anaesthetic team to understand that if the block is incomplete a peripheral top up must not be used and the patient should instead have a general anaesthetic without neuromuscular blockade. The reason for this is that during the procedure intra-operative nerve stimulation is used to define the fascicle anatomy and identify any important branches as the tumour is exposed. When the tumour is removed any fascicles entering the tumour can also be assessed to establish the potential for functional loss and provide a prognosis. Residual function can also be mapped in the preserved nerve.
The limb is prepped and draped to the axilla and a sterile disposable upper arm tourniquet placed around the arm. Exsanguination is performed with elevation rather than an Esmarch bandage to reduce the risk of local trauma to the tumour and the involved nerve. This approach of delayed tourniquet elevation allows 30 minutes of nerve exposure and dissection before the onset of tourniquet induced ischaemic conduction block after which it may be necessary to deflate the tourniquet for 10-15 minutes before conduction is reliably restored.
A lead hand is used to position the forearm and and in a supinated position to provide a stable position and assist with microsurgical excision. Basic hand instruments are required for the superficial exposure, surgical rubber sloops may be used for identification of the nerve, delivery to the wound and gentle retraction. Microsurgical instruments including jeweller’s forceps and curved serrated nerve scissors are ideal for the intraneural dissection.
An AxoGuard nerve protector should be available for ensheathing the nerve at the site of epineurial window. Typically either a 10 x 40mm or a 7 x 40mm size is appropriate for the median nerve in the distal forearm. the AxoGuard nerve protector is sutured in place using 8’0 Prolene sutures on a microsurgical taper cutting needle. This implant useful to protect nerves with extensive epineural dissection in superficial locations.

The limb is elevated in a sling for 48 hours and the bulky dressing kept in place for 5 days.
A wound review and replacement of the occlusive dressing can be performed at that stage.
The patient’s pain and function should be assessed early and any deficits documented and the patient may be reassured.
Further clinical follow up at around 6-8 weeks will demonstrate resolution of any conduction block from oedema and segmental demyelination from the intraneural dissection.
Major deficits and a confirmatory Tinel will confirm axonopathy requiring nerve regeneration and the patient can be advised regarding the prognosis at this stage with further follow up planned as necessary.
Histology should be available at this follow up appointment and should be performed routinely following tumour removal even if the imaging and macroscopic appearance suggest a benighn peripheral nerve sheath tumour.

Generally the results of this type of benign tumour excision are excellent. The prognosis largely depends on the pre-operative neurological status and whether this is a primary or revision surgery. Most patients report resolution of pain and improved sensation and motor function within a few weeks of surgery. This was a Schwannoma and in such cases nerve reconstruction is usually not required. In this case the tumour was resected from the originating fascicle without sacrifice of nerve tissue. In some cases the fascicle of origin must be resected but because there is inbuilt redundancy in the nerve (typified by highly selective fascicle transfer for paralysis) there may be no apparent neurological deficit post-operatively. If the fascicle was large and had a critical function that was not present on stimulation of the remaining nerve consideration should be given to reconstruction of the resected fascicle using a nerve graft. AVANCE processed nerve allograft is useful in sensitised individuals to prevent creation of a secondary nerve injury site from donor nerve harvest. If there is significant trauma to the epineurium and the nerve is superficially located and was sensitised before surgery, I use a layered collagen nerve barrier warp around the nerve to provide some protection from subcutaneous scar formation.
References:
Guha D et al. Management of peripheral nerve sheath tumors: 17 years of experience at Toronto Western Hospital. J Neurosurg 2017 Jul 7:1-9. doi: 10.3171/2017.1.JNS162292 Epub
Tang CY, Fung B, Fok M, Zhu J. Schwannoma in the upper limbs. Biomed Res Int 2013; 167196. doi: 10.1155/2013/167196. Epub 2013 Sep 4
Mansukhani SA, Butala RP, Shetty SH, Khedekar RG. Familial Schwannomatosis: A Diagnostic Challenge. J Clin Diagn Res 2017; Feb;11(2):RD01-RD03. doi: 10.7860/JCDR/2017/20929.9307. Epub 2017 Feb 1