In times of peace, peripheral nerve injuries, the most common cause of a peripheral nerve injury is a motor vehicle accident and penetrating trauma, industrial accidents, and falls.  Of all patients admitted to a Level I trauma center, about 2-3 percent have peripheral injuries.

The most commonly injured nerve in the upper limb is the radial nerve, where it is frequently injured by fractures of the humerus.  The ulnar nerve is the second most commonly injured nerve, where it is injured by a problem in the elbow area.  The median nerve is least likely to be injured and occurs when either the radius or ulna are fractured in the forearm.  Lower limb peripheral nerve injuries are less common, but occasionally an injury happens with the sciatic nerve with pelvic or hip fractures.  Rarely the tibial and femoral nerves are injured. 

In wartime, peripheral nerve injuries are much more common, and we owe much of our knowledge about the injury, repair, and recovery from incidents happening in the wars since WWII.  In peacetime and in wartime, injuries to the peripheral nerve occurring in young males between the ages of 20 and 25. 

The most frequent causes of peripheral nerve injury are motor vehicle accidents (46 percent), motorcycle crashes (10 percent), and, to a lesser extent, pedestrian accidents and gunshot wounds.  Brachial plexus injuries are most likely related to motor vehicle injuries and motorcycle crashes.  The injury to the peripheral nerve is often made worse by other injuries to the central nervous system.  About 60 percent who have peripheral nerve injury also have a traumatic brain injury.

Peripheral nerve injuries are significantly important as they interfere with recovery of function and return to work abilities and carry the risk of secondary complications of falls, fractures, and other injuries. 

There are several classifications of nerve injuries.  These include:

• First-degree injuries—there is an injury to the myelin sheath or lack of blood flow to the nerve, which has an excellent recovery in a few weeks or months.
• Second-degree injuries—there is disruption of the nerve fibers themselves with the surrounding coverings intact.  There is good recovery, depending on the distance to the muscle.
• Third-degree injuries—axons are disrupted, but some of the nerve coverings are disrupted.  The recovery is poor because the nerves grow in the wrong direction without surgery.
• Fourth-degree injuries—there is disruption of nerve axons and some more of the coverings of the nerve.  The prognosis is poor unless there is surgery.
• Fifth-degree injuries—All of the nerve and its coverings are disrupted.  There is no chance of recovery without surgery, and even that may not work. 

This can be caused by a lack of blood flow to the nerve or an injury to the nerve’s myelin coating.  If the lack of blood flow occurs for a period of time less than 6 hours, there are usually no structural changes in the nerve. Because there is a loss of some of the myelin sheath, the conduction of electricity from one mode to another is slowed because the electrical current leaks out of the nerve.  If a lot of myelin coating is missing, the conduction of electricity will be completely blocked.  The muscles can suffer from atrophy due to disuse, especially when the lack of blood flow to the nerve is prolonged. 

When the nerve is injured, there is a process of degeneration that takes place.  The body of the nerve and its long axon are changed by this type of injury.  There is leakage of fluid from the nerve at the site of injury, and there is swelling of the part of the nerve that has been cut off.  Nerve fibers may also degenerate proximal to the site of the injury and may extend for several centimeters. 

The optimum time to do electrodiagnostic testing of the injured nerve depends on the clinical circumstances.  For circumstances in which it is important to identify a problem early, these studies can be done as early as 7-10 days after the injury.  When clinical circumstances permit waiting, testing can be done at 3-4 weeks after the injury and provide much more information.  If the nerve has been repaired in surgery, an electromyogram can be done a few months after the injury to document the return of nerve function. 

The localization of the injury can be either straightforward or complicated.  Localization can be done by looking for a slowing of the electrical impulses or a block of impulses on the nerve.  It can also be done to assess the pattern of nerve loss on an electromyogram. 

The doctor must stimulate the nerve both above and below the level of the injury.  Nerve injuries caused by lack of blood flow and loss of myelin (called neuropraxia) and injuries to the actual axon of the nerve can be identified using motor nerve conduction studies. 

In some cases, with more severe nerve damage, an electromyogram will not help because there will be a diffuse slowing of electrical activity across the entire nerve due to loss of the fastest nerve fibers.  There may be no response at all.  There can be indirect inferences of the localization of the injury by checking the nerve function of all the nerves of the affected area. 

Another major method of determining the size of the nerve injury is by needle electromyogram, which measures the nerve impulses to the various muscles.  Unfortunately, the way a muscle is innervated by the various nerves is different from person to person.  Thus the typical branching scheme of the nerves may not apply to the person being studied, and consequently, the injury site can be misconstrued.

There can be problems identifying the nerve site of injury with a needle electromyogram because there is an injury to the muscle itself. This can result in the doctor diagnosing more than one area of injury or misidentify the site of the lesion altogether.  Partial injuries to the nerves can also make for misdiagnosis of the site of injury. 

There are several ways a nerve can recover after an injury.  In motor nerves, healing of the conduction block at the injury site may lead to recovery of strength of the muscles.  This type of resolution of the conduction block is probably the first mechanism to promote recovery of muscle strength after a nerve injury.  This recovery occurs rather quickly.  It may, however, take several months, depending on how much of the myelin coating the muscles is lost.

After several weeks of recovery, the muscle fibers become enlarged; this causes a further increase in muscle strength.  In patients with partial injuries to the nerve, it is not clear how much the nerve changes alone contribute to increased strength in the presence of some nerve loss.  Likely, working the existing muscle fibers to the point of fatigue in the setting of partial nerve injuries does produce enlargement of the muscle fibers and an increase in muscle strength.

If there is a partial loss of the nerve fibers within a motor nerve, there can be sprouting of new motor nerves from intact nerve fibers.  This happens within 4 days following the injury.   Partial recovery of muscle function (twitching) can be seen as early as 7-10 days after the injury.  Sometimes the muscles become innervated by these new sprouts of nerves.

Regeneration of the nerve axon (nerve fiber) contributes to the recovery of nerve function.  This is often the only mechanism for muscle recovery.  Sprouting from the nerve stump has been found to occur 24-36 hours after injury, and these start to penetrate the area of injury. 

If the coverings are intact in relatively minor nerve axon injuries, the nerve can regenerate at a rate of 1-5 centimeters per day.  In more severe nerve axon injuries, the potential for spontaneous regrowth of the muscle is far less.  Extensive scarring occurs that reduces the speed at which the nerves can regenerate.  It reduces the likelihood that the nerve will ever reach the organs which they innervate.  When regrowth occurs, it can be misdirected to the wrong muscle.  Sometimes it forms a ball of nerve fibers called a neuroma, which will have to be removed surgically. 

If the nerve and all its coverings are disrupted, regrowth will not likely occur unless the nerve ends are freed up from scar tissue and the nerves reconnected.  A graft can be placed to connect the nerves and, while it will die off, it provides a pathway for new nerve growth to travel. 

Most muscles will remain viable for 18-24 months without being innervated.  Within that period of time, nerves have to reach the muscle to regrow muscle.  If the lesion is high up, say, in the axilla, recovery of hand function is not likely to occur because a lot of nerve has to regrow to reach the hands. 

Surgical repair of the nerve can occur shortly after the injury or later as part of a delayed reconstruction.  Immediate surgery is used for sharp nerve lacerations, such as a knife or glass cutting injury.  If nerve grafting is necessary to approximate the nerve endings, this type of surgery is delayed for several weeks. 

Early reconstruction occurs at about 3-4 weeks after the trauma.  The trauma is usually due to blunt trauma or traction on the nerve but can be done in the direct cutting of the nerve if it hasn’t already been done.  The nerve ends have already formed small nerve bundles at the end of the nerves, which have to be cut out and a graft to connect the nerve endings.  

There are some trade-offs to consider when deciding when to do the surgery.  Ideally, one would like to wait until the nerve grows back on its own.  On the other hand, if the surgeon waits too long (past the 18-24 month level), the muscle may not allow itself to accept the new nerve.  Most surgeons will wait about six months to see if there is any evidence of re-innervation of the muscles before performing an exploratory surgery and doing grafting.  Delayed nerve reconstruction is generally reserved for repairing the most proximal lesions as there is little point in attempting grafting to try and save the distal muscles. 

Tendon transfers are another surgical choice.  This requires some retraining of the muscles after the tendons are grafted onto the muscles so that the new tendons can learn how to make the muscles move. 

Outcomes after surgery vary according to the nerve-injured and the technique used to repair the nerve.  Some patients can recover function from returning to work with intact muscle strength, while others will not. 

Rehabilitation of these types of injuries should focus on avoiding complications and further disability, improving the muscle’s function, and enhancing recovery after surgical repair.  Contractions can occur after injury to a nerve supplying a muscle.  Patients and caregivers need to be taught a passive range of motion exercises daily to prevent contractures.  The limb must be positioned in ways that enhance a full range of motion. 

Use of the limb should be encouraged for daily activities whenever possible.  There are several benefits of using the limb early.  It will prevent atrophy of the muscles from disuse and will help prevent contractures.  The use of the limb may also participate in the improvement of nerve function. 

Some rehabilitation specialists use electrical stimulation of the muscles to prevent atrophy.  The use of this practice is considered controversial.  It is difficult to say that no form of electrical stimulation will help regain muscle function. 

Ultrasound is another physical modality that may enhance recovery after peripheral nerve injury.  It has been used in carpal tunnel syndrome with encouraging results.  It may help a nerve regenerate faster, although not enough studies have been done to prove that this is true.

Splinting is an important part of rehabilitation care after a peripheral nerve injury. Splinting can prevent contractures and can substitute for the loss of motor function.  It can also enhance recovery after surgery, encouraging early use of the limb during daily activities.  The design of the splint depends on the location of the nerve injury and the various muscles involved. 

Both tendon repair surgery and nerve recovery surgery have been found to have better outcomes if there is less tension on the nerves and muscles during recovery.  There are static splints that allow for no motion and dynamic splints that allow for some function.  The therapist must understand the mechanics of splinting forces so the tissue can be protected with fewer contractures and better protection of the nerve.

Editor’s Note: This page has been updated for accuracy and relevancy [cha 4.13.21]