What is a nerve?
A nerve is a cable like structure that carries information back and forth from the brain and other parts of the body. Within the nerve structure, there are fibres that carry sensory and motor components. The sensory fibres detect temperature and pain and the motor fibres carry impulses to muscles allowing them to contract or move. Some nerves carry either sensory or motor fibres but many in the upper limb contain both.
The structure of a nerve is akin to that of an electrical cable. The nerve fibres are like the copper wires. Each nerve fibre is held together by an insulating layer (endoneurium). The nerve fibres are then grouped into bundles and held together by another insulating layer of connective tissue (perineurium). Like the outer casing of an electrical cable, these bundles themselves are in turn grouped and held together by a final insulating layer (the epineurium).
Classification of nerve injuries:
Doctors divide nerve injuries into 3 simple groups, and understanding the difference between these groups can help predict whether or not surgery to the nerve is required, and secondly whether the nerve is likely to work again. The names given to these types of injury are based on the Latin words which best describe the injuries.
- Neuropraxia – This is where the nerve has been squashed or bruised, but the outer layer and the inner nerve fibres remain intact. The nerve stops working for a period of time (usually weeks). This type of injury usually does not need treatment.
- Axonotmesis – This occurs when the nerve is crushed and the inner nerve fibres are damaged, but the outer layers remain intact. This type of injury is often caused by a ‘stretch’ of the nerve and around the wrist and hand usually does not need treatment.
- Neurotmesis – The nerve has sustained significant damage and all layers of the nerve have been injured and are no longer in continuity. This is the most severe nerve injury.
The majority of injuries we deal within the Fife Hand Service are OPEN injuries – the nerve has been cut in two by a knife or glass. These injuries are sometimes isolated, but often just one part of a wound in which there may be damage to tendons, muscles and bone as well as the nerve.
What are the symptoms?
When a nerve is cut, it immediately loses all function. A cut nerve in the finger will produce loss of sensation along the side of the finger. A cut through one of the bigger nerves at the wrist gives numbness in the hand and loss of muscle function in the hand. The pattern of sensory and muscle loss is unique to each nerve, and it is usually possible to tell clinically whether the nerve has been cut or not just by examining the hand.
What is the treatment?
Where a nerve has been cut, it is always worth considering surgery to repair the nerve. Sometimes the decision is easy – if there is a significant wound and a large nerve – but sometimes not so easy. Small nerves in the middle and ring fingers particularly may be better left alone rather than repaired, simply to avoid having a long scar down the finger when the initial wound is small. The decision to repair a nerve is a shared one between you and your surgeon.
Where the injury is closed, that is no wound is present, it is not always easy to establish the degree of a possible nerve injury. Often, we recommend a “wait and see” policy in order to assess whether the injury will recover on its own. Sometimes a nerve conduction test may be performed to ascertain any degree of nerve damage.
How is a nerve repaired?
The outer layers of the nerve are sutured together. The ends of the nerve are lined up so that the fibres are matched from opposite sides. It is only possible to repair the outer layer of the nerve.
How do nerves re-grow or regenerate?
When a nerve is cut, the nerve fibres above the level of injury (ie closer to the brain) continue to work, but the nerves below the injury die. This leaves an empty tube. The nerves can regrow from the undamaged part of the nerve down this tube.
Nerve fibres begin to re-grow across the repair site within 3 to 4 weeks. They generally grow at a rate of 2-3mm per week. Regrowth, however, is not always straightforward. Some nerve fibres re-grow down the wrong tubes resulting in a mismatch of connections. We usually expect around 20-30% of the nerve fibres to regenerate.
Factors that influence nerve healing include:
- Age: Children have the ability to regenerate nerves better than adults, particularly those above the age of 60.
- Scar tissue: If the degree of injury is significant, more scar tissue will be present. This can hamper recovery.
- The level and type of injury: The worse the damage, unfortunately, the worse the outcome. Higher energy injuries and injuries that occur nearer to the brain signify a worse prognosis.
- Smoking: The chemicals in cigarettes reduce the blood supply the hand and arm resulting in less oxygen, and therefore nutrients getting to the damaged tissue to allow repair.
What can I expect during recovery?
The results of nerve repair are very variable. Some patients get a fantastic result with very good recovery of function, where others get very little back and the loss of function can be devastating for people who do not recover well.
As the nerve regenerates, there is generally a staged recovery:
Stage 1 – Anaesthesia: Initially, the area affected by the nerve injury will feel completely numb. Signs of initial recovery include a feeling of pins and needles or electrical shooting type pains.
Stage 2 – Hyperaesthesia: This can be an unpleasant feeling. The pins and needles feeling is more pronounced and you may be sensitive to temperature changes. You must learn to use your hand and get used to the sensations which may last for up to 2 years or more.
Stage 3 – Hypoaesthesia: Eventually the pins and needles settle and sensation should improve. The sensation may not be entirely normal. It can take up to 5 years for sensation to improve but often it will not return to pre-injury levels.
Stage 4 – Muscle Re-innervation: If the nerve that is injured innervates muscles, these muscles will become weak and wasted as the nerve recovers. Once the muscle regains its nerve supply, flickers of muscle contractions may be noticed. Gradually this will continue to improve and can continue for up to 5 years and muscle may regain enough power to move joints. Return of full power may not occur.
What complications may occur?
Neuroma formation: A neuroma is essentially a ball of healing nerve tissue. Electric shock sensations may be felt along the course of the nerve it originates from. At best, a neuroma can be uncomfortable when tapped or knocked. At worst, the sensitivity from a neuroma can impact on sleep and all aspects of function. Neuromas are treated with desensitisation programmes and physiotherapy. If severe or next to a scar, then excision of the neuroma will sometimes relieve the symptoms, but at the cost of sacrificing any nerve function.
Persistent pain: Some patients experience significant problems as a result of nerve injuries. A combined approach with our hand physiotherapists will help to address this.
Cold intolerance: Some patients experience a painful response to cold environments. It is difficult to predict how long this will last, however, be reassured that this is a common problem and does not indicate that there is anything wrong with the recovery of the nerve. Cold intolerance can last many years following a nerve repair, but generally settles with time.
Further surgery: If nerve function does not recover significantly, reconstructive procedures may be necessary in order to try and regain function.
© Fife Hand Service 2020