Excision of benign tumour posterior interosseus nerve

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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. A decision regarding excision should consider the clinical symptoms, the current neurological status, the likelihood of deterioration if the tumour grows, the site of the tumour, the potential deficits and their functional impact following surgery.

Indications:
This patient presented with a painful swelling of 2 years duration in the extensor compartment of the right mid-forearm. The lesion was felt to have increased in size with constant ache in the wrist and pain on finger and thumb resisted extension. There was no sensory involvement and there was no Tinel’s sign, however the nerve tumour was painful to palpate. MRI confirmed a swelling in the posterior interosseus nerve at the distal edge of supinator. There was no demonstrable weakness in any of the PIN innervated muscles. The wrist pain is likely to be referred pain from compression of the proprioceptive fibres within the PIN that innervate the wrist joint.
Symtoms and examination:
Many Schwannomas are incidental findings on routine imaging or noticed as painless lumps. Due to their location in the peripheral nerves Schwannomas may present with features of neuropathic pain, sensory or motor dysfunction as a result of neural compression. The symptoms are different depending on the nerve involved and the site of compression. Careful examination should identify even mild weakness and this should be noted and may help in determining whether surgery should be performed. The size of the lump should be noted. Typically peripheral nerve sheath tumours may be moved from side to side but cannot be moved longitudinally due to their intraneural location. A Tinel’s sign may be present due to neural compression, focal demyelination or axonopathy.
Investigation:
Neurophysiology is not mandatory but electromyography may confirm features of denervation in distal muscles.
Imaging is extremely important in planning surgery. Ultrasound may ne helpful in locating superficial tumours but MRI is the gold standard. MR neurography sequences are demonstrating promise in identifying fascicle bundles in relation to the tumour. In the future functional imaging combining neurophysiology and structural imaging may help in planning surgery. Atypical peripheral nerve sheath tumours and malignant nerve sheath tumours may show different spectroscopic profiles to benign tumours and in my clinical practice I use MR Spectroscopy in uncertain cases. Trimethylamine peaks are common in malignant peripheral nerve sheath tumours. If there is concern that the imaging is atypical then a biopsy should be considered before surgical resection. For clinically and radiologically typical Schwannomas I plan an excision biopsy as this is likely to achieve satisfactory tumour removal, minimise neurological deficits from surgery and result in very low rates of recurrence. If a biopsy is needed then the biopsy will need to take tumour plus normal nerve to provide a specimen that is meaningful for the histopathologist. The resultant nerve injury and scar will render the neurological deficit more extensive than for a simple excision biopsy. However if a malignant tumour is suspected biopsy is mandatory and the subsequent surgical resection will be radical excising normal nerve and tissues including the biopsy tract. Reconstruction efforts will depend on the site of the nerve resection, the type of nerve, the anticipated deficits, adjunctive therapies and the prognosis for the patient. Typically reconstruction in such cases may include autologous nerve graft, nerve allograft, distal nerve transfers and tendon transfers. In highly malignant tumours of main nerve trunks in the lower limb, amputation my confer functional advantages for the patient.
Conservative management:
Many Schwannomas can be observed as long as the patient is asymptomatic and the imaging confirms a typical benign morphology. Surveillance MRIs are not mandatory. I practice a clinical surveillance with interval MRI if patient s note increased size, pain or neurological deterioration.
Alternative operative management:
Simple decompression may be undertaken when large tumours are subject to external compression due to their location and it is unlikely that surgical excision can be performed without significant functional loss in an otherwise relatively symptom free patient. If the tumour has already recurred consideration should be given to excision and graft reconstruction as long as the resultant deficit can be defined and the patient agrees to this strategy.
Contraindications:
Large, painful, tumours with atypical imaging findings should not be treated with excision biopsy. They should be discussed with the regional bone and softtissue sarcoma services and a biopsy should be planned prior to surgical removal. In the rare case of a neurofribrosarcoma surgical treatment is radical with compartment excision or amputation and the treatment should be undertaken in specialist units.

The right arm was anaesthetised with an axillary regional local anaesthetic block. 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.

The patient can be discharged home the same day as surgery.
The limb is supported in a Bradford sling with elevation advice for 48 hours to reduce wound swelling.
The dressing should be reduced after one week and the wound inspected.
A new occlusive dressing is place over the wound and the patient is encouraged to keep the wound clean and dry until 2 weeks.
After this period moisturising scar massage can commence to aid wound maturation and reduce scar sensitivity.
The motor function should be assessed and documented at each visit.
Any weakness in the PIN and specifically in the EPB branch should be monitored.
It is likely that post surgical oedema may result in a transient conduction block but the expectation is for a full recovery by 6 weeks.

Schwannomas are benign tumours and well encapsulated. They may arise form a fascicle within the nerve which may need to be sacrificed. Presenting symptoms depend on the nerve involved and the anatomical location. They can cause pain, sensory and motor dysfunction. Careful surgical removal can remove the whole tumour with low recurrence rates and low morbidity.

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
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