When surgery is required to peripheral nerves it may involve other specialities such as plastic or orthopaedic surgeons as well as neurosurgeons dependent on the location of the problem, the particular pathology and the functional challenges faced.
Nerve Compression Syndromes
Peripheral nerves will pass through anatomical restrictions on their journey through the body. Those nerves coursing along the limbs may be at particular risk of constriction or external injury as they cross joints. The tissues surrounding the nerves at the points also plays a role if it becomes inflamed or thickened.
Below are described the most common of these syndromes that present to neurosurgeons.
Carpal Tunnel Syndrome
Compression of the median nerve as it crosses the wrist is one of the commonest nerve compression syndromes. Characteristic symptoms and signs include:
- Pain or numbness in the hand that may be worse at night or most noticeable first thing in the morning.
- Difficulty gripping objects between the fingers.
- Pain or numbness in the hand that may be worse at night or most noticeable first thing in the morning.
- Burning sensations or pins and needles affecting the thumb, index and middle fingers specifically.
- Exacerbation of symptoms
- Reproduction of pins and needles in the fingers on tapping the ventral wrist (Tinel's sign).
- Provocation of symptoms with sustained wrist flexion (Phalen's sign).
The compression develops because of inflammation and thickening of tissue called the flexor retinaculum that lies across the median nerve ventrally as it crosses the wrist into the hand. The condition is associated with diabetes, repetitive strain injury, hypothyroidism, pregnancy, arthritis and the use of vibrating tools. Rarely it may feature alongside other systemic diseases.
A piece of tissue called the flexor retinaculum covers the median nerve protecting it ventrally as it crosses the wrist into he hand. When this tissue becomes inflamed the nerve can be compressed.
Investigation consists of elctrophysiolgical testing to confirm the diagnosis and sometimes ultrasound or MRI in rare cases with structural pathology within the canal.
Initial treatment consists of splinting at night or physiotherapy (nerve glide exercises) as well as analgesic and anti-inflammatory medication where appropriate. If symptoms persist then a steroid injection in the wrist provides relief in most cases. If symptoms return then surgical decompression of the nerve may be indicated. This is usually possible under a local anaesthetic although revisional surgery may be done under general anaesthesia. Repeated steroid injections are not helpful.
Cubital Tunnel Syndrome
On this occasion the ulnar nerve is compressed at the elbow. The symptoms are similar in character to those described above but differ in distribution. Pain may radiate down the elbow to the outside of the hand reminiscent of striking one's "funny bone" at the elbow. Weakness is demonstrable when spreading the fingers apart or when flexing the fourth and fifth finger.
Investigation again is by electrophysiological measurement of the transmission velocity along the nerve as it crosses the elbow. MRI or ultrasound may again be helpful. Initial treatment is again with splinting. Simple decompression of the nerve is usually the first surgical procedure of choice. In cases requiring visional surgery or where the nerve is prolapsing around the bone of the elbow with movement it may be necessary to fix the nerve in a new course at the front of the elbow - transposition.
There are many other nerves that may be entrapped in a similar fashion and in principle at least their management is similar to that outlined above.
Nerve Tumours
Peripheral nerve tumours develop on nerves that have left the cranial or spinal bones and are travelling through the body to provide sensation, movement and the unconscious maintenance of blood flow to organs, temperature and autonomic functions.
There are regions where these nerves intersect rather like a railway network . At these points elements transfer from one group of nerves to another before continuing their journey. Such a junction is called a plexus. In the root of the neck and axilla is found the brachial plexus where the major nerves that innervate the upper extremity are formed before travelling down towards the hands. There is a lumbosacral plexus serving the same function in the lower extremity and transmitting nerves to the pelvic organs. There are many other examples throughout the body although these latter two examples are not infrequently vulnerable to injury in the setting of major trauma. Similarly tumours in these areas will typically require the attention of multiple specialists given the complexity of access and mix of functions affected.
Most tumours that develop on peripheral nerves are benign. In adults the most common arises from the Schwann cells that insulate the nerve - hence Schwannoma. one type of Schwann cell produces a substance called myelin to help them in their role. Schwann cells are support cells that help nourish the neurones and facilitates the transmission of electrical signals up and down the nerve trunk.
A subtype of Schwann cell does not produce myelin and these are the origin of neurofibroma which are more associated with the genetic condition neurofibromatosis type 1. Neurofibromas develop more centrally and are potentially more disruptive to nerve function. In the context of NF1 there is also a risk of malignant transformation over time so a newly painful neurofibroma should be removed for examination if possible.
Perineurioma is a rare tumour that can from within or without a peripheral nerve and is seen most often in children and young adults. Intraneural varieties again are more disruptive to nerve function and they may be more common in predominantly motor nres.
Nerve and Muscle Biopsy
Occasionally neurologists and rheumatologists who are investigating the possibility of certain nerve and muscle diseases will require samples from specific nerves and/or muscles to assist them.
Nerve biopsy may be required when inflammation affects the tiny arteries delivering oxygen to peripheral nerves- vasculitis. As a result then nerve may lose function- peripheral neuropathy. This is usually indicative a systemic problem and treatment is typically with medication. Confirmation of the diagnosis prevents unnecessary exposure to potentially toxic medications. Related non-infectious inflammation in muscle is termed myositis.
Biopsy is usually under local anaesthesia and the sample requires prompt examination by a specialist neuropatholgist without chemical fixation.
