Acute Nerve Injury

What is Acute Nerve Injury?

Our understanding of peripheral nerves and the injuries they can sustain, known as peripheral nerve injuries (PNIs), has primarily come from medical experiences during war times. Sir Herbert Seddon created a system to classify these injuries while helping those injured during World War II. However, it’s not rare today to see these injuries in non-military-related accidents. They can significantly affect a person’s life and can lead to serious disabilities. This is especially impactful since these injuries tend to occur in young adults, meaning the implications can affect them for their entire life.

The structure of peripheral nerve trunks consists of three separate layers. The innermost layer, made of collagen and known as the endoneurium, wraps around nerve fibers to provide structural and nutritional support. These fibers and the endoneurium make up the nerve bundles, also known as fascicles. Each of these fascicles is surrounded by a thin layer of cells called the perineurium. The epineurium, another collagenous layer, covers these fascicles. Understanding this structure is crucial for grasping the different types of PNIs, the symptoms they present, their possible outcomes, and the most effective ways to manage them in patients.

What Causes Acute Nerve Injury?

Car accidents are the most common cause of Peripheral Nerve Injuries (PNIs), making up 46% of these cases, with crashes involving motorcycles accounting for 9.9%. In war zones, injuries from shrapnel and explosions are the most likely causes. Other sources of PNIs include accidents involving pedestrians, gunshot wounds, fall accidents, industrial mishaps, stab wounds, recreational vehicles like snowmobiles, and assault. Surprisingly, almost one fifth (17.4%) of surgically-treated PNIs are unintentionally caused during medical procedures.

PNIs can happen in different ways. The most common type involves stretching injuries, where the nerve gets stretched beyond its ability to handle. These injuries often occur alongside broken bones, but usually don’t result in a complete loss of nerve continuity. Injuries caused by sharp objects, like knives or blades, are the second most common and may cause partial or even complete breakage of the nerve.

Another frequent type of injury is a compression injury. Despite the nerve staying intact, it might lead to a complete loss of motor and sensory nerve functions. Believed to be caused by a lack of blood supply and direct compression, these injuries can result in long-lasting nerve damage, especially in severe cases. Other less common ways PNIs can happen include thermal injury, or reduced blood flow due to blood vessel injury. Sometimes, a combination of these factors can occur.

Certain nerves are more prone to injury based on their location. For instance, the radial nerve, ulnar nerve, and common peroneal nerve are open to such risks. Improper sitting habits can cause what’s often referred to as “Saturday night palsy,” which weakens wrist and finger movements. Ulnar nerve can be damaged in surgery due to improper patient positioning. Similarly, during long surgeries, the common peroneal nerve can be compressed, particularly in thin patients.

Bone fractures can also lead to acute nerve injuries. These injuries can occur initially at the time of the fracture or secondarily due to issues like improper medical intervention or scar formation. In fact, nerve damage related to bone fractures is most commonly seen with the radial nerve. Dislocated joints can also cause PNIs. Nerve injuries can also inadvertently happen during certain surgeries due to compression, stretching, or lack of blood supply. Examples of this include injuries to the nerves in the arm during chest surgery, or to the femoral nerve during abdominal surgery.

Risk Factors and Frequency for Acute Nerve Injury

Peripheral nerve injuries (PNIs) happen in roughly 3% of trauma patients. The percentage goes up to 5% if we include injuries to nerve roots, nerve networks (plexuses), finger nerves, or minor nerve injuries. These injuries are most common in patients aged 18 to 35 years old, with the average age being 34.6 years. They are more common in males, who have a ratio of 5 to 1 compared to females. Generally, PNIs affect the upper limbs in 60.5% of the cases. A small percentage around 6.2% show nerve injuries in both the upper and lower limbs. The most frequently injured nerves are the radial nerve in the upper limbs and the peroneal nerve in the lower limbs.

• Around 3% of trauma patients experience peripheral nerve injuries (PNIs).
• With minor nerve injuries included, this increases to 5%.
• Most patients with these injuries are between 18 and 35 years old.
• The average patient age is 34.6 years.
• Males are more likely to have PNIs, with a male-to-female incidence ratio of 5 to 1.
• The upper limbs are affected in 60.5% of PNIs cases.
• About 6.2% of cases have nerve injuries in both the upper and lower limbs.
• The most common nerve injuries are to the radial nerve in the upper limbs and the peroneal nerve in the lower limbs.

Signs and Symptoms of Acute Nerve Injury

Understanding when an injury happened can give us clues about what type of injury took place and how severe it is. For instance, if neurological symptoms like nerve pain or loss of function didn’t start right away, the injury could be due to something pressing on the nerve. But if symptoms started immediately after the injury, the nerve might have been directly damaged. The severity of the injury also often relates to how strong the impact was. The pattern of neurological symptoms and pain can also point to where the nerve damage is located.

Looking for signs of injury like cuts, stab wounds, bullet wounds, scrapes, or bruises is crucial. We also need a detailed neurological exam. This involves checking how strong the muscles served by the relevant nerves are, and making sure the person can feel things correctly. We need to be precise in our examinations to avoid misinterpreting where the injury is. Other types of injuries to blood vessels or muscles can also signal that nearby nerves might be damaged. An important detail is that injuries to the muscles can complicate our assessment of how strong the muscles are, so we need to keep this in mind.

In cases of injury to the brachial plexus, a group of nerves in the shoulder area, certain findings can help us understand the injury better. For example, drooping of an eyelid, a constricted pupil, and not being able to sweat (a condition known as Horner’s syndrome) could suggest injury to the lower part of the brachial plexus or damage to the nerve roots at the base of the neck. On the other hand, if the person has paralysis of one side of the diaphragm, sagging of the shoulder blade, and weakness of the rhomboid muscles in the back, these could signal injury to the upper part of the brachial plexus.

We can assess muscle strength using a scale provided by the Medical Research Council of Great Britain:

• Grade 0: No muscle contraction
• Grade 1: Muscle twitching, but no movement
• Grade 2: Muscle contraction resulting in motion, but not against gravity
• Grade 3: Muscle strength overcomes gravity, but without resistance
• Grade 4: Can contract muscle against resistance, but not at full power
• Grade 5: Full strength (normal)

The testing of sensory function has to be accurate to avoid confusion from overlapping areas of skin served by different nerves. We need to examine specific areas of skin served by only one nerve (fine touch sensations, pinprick, and temperature sensation) to minimize misinterpretation. For example, we might test the back of the hand for a radial nerve injury and the inside surface of the little finger for an ulnar nerve injury.

Testing for Acute Nerve Injury

Electrodiagnostic tests are crucial when assessing nerve injuries, particularly those that aren’t open wounds. Two types of these tests, nerve conduction studies (NCS) or electromyography (EMG), can provide different types of useful information, depending on when they are performed after the injury.

In the first week after injury, NCS can help pinpoint where the injury is located because it measures how well the nerves are conducting or transmitting signals. EMG, on the other hand, can show if the injury is complete or partial. If you’re still able to control your muscles voluntarily, this means the injury is not total.

After the first week, these tests can help determine the severity of the injury by checking if the nerves are still transmitting signals. Traditionally, further tests are done between 3-4 weeks and 3-4 months after the injury, which can provide evidence of early recovery and help guide the next steps in treatment.

These electrodiagnostic tests can also help identify the exact location of the injury. For example, if certain tests show normal responses to sensory stimuli but no responses to touch, then the injury might be located before the nerve ganglion, a cluster of nerve cells.

Magnetic resonance imaging (MRI) is another useful tool, especially for injuries to the brachial plexus, a network of nerves that control the muscles of the shoulder, arm, and hand. MRI can map out the structure of the nerves without invasive procedures or radiation exposure. It can’t, however, determine the severity of nerve injuries on its own.

Interestingly, MRI can show evidence of muscle damage as early as four days after the injury, which would be slightly earlier than what an EMG can pick up. Promising research in animal models shows that other MRI techniques might be helpful in diagnosing and tracking recovery of nerve injuries, but these findings haven’t yet been confirmed in humans.

Treatment Options for Acute Nerve Injury

The approach to treating nerve injuries depends on the type and severity of the injury. Nerve injuries can be of two types: open or closed. For open injuries, surgery is usually required to inspect the damage. If the nerve has been sharply cut, surgery should be performed within 72 hours to repair the ends of the nerve before they pull apart. On the other hand, injuries resulting from a blunt force should be treated with surgery 2 to 3 weeks later. This delay allows any damaged nerve ends to form scar tissue, which can then be removed during surgery. Injuries that have not caused the nerve to be cut can usually be treated without surgery, monitoring them over time with clinical, electrodiagnostic, and radiological evaluations.

Closed nerve injuries, where the nerve remains enclosed, are usually treated conservatively as many remain connected. Urgent medical attention is required if the injury results in a compartment syndrome, which could permanently harm the nerve. If there’s no sign of nerve recovery 3 to 4 months after the injury, further surgery will be necessary.

Several surgical techniques are available for treatment. The easiest involves removing scar tissue that has formed around the nerve. In some cases, scar tissue may have formed inside the nerve and requires surgical intervention. However, if no nerve action potential (which indicates nerve functionality) can be recorded, or the nerve is clearly not continuous, nerve repair is necessary. This typically involves joining the ends of the nerve together, or if this is not possible, inserting a graft.

After surgery, it may be necessary to limit movement to avoid disrupting the sutured nerves. Physical therapy is encouraged for joint mobility, maintaining muscle elasticity until re-innervation occurs, and preserving unaffected muscle strength. The patient should be aware that recovery and rehabilitation may take several months or years, and full recovery might not be achieved. Tracking nerve regrowth and assessing motor unit recruitment during follow-up is usually carried out using Tinel’s sign and needle EMG exams.

Early treatment with medication to control nerve pain is important. Commonly prescribed medicines include tricyclic antidepressants, anticonvulsants (such as carbamazepine, gabapentin or pregabalin), or serotonin reuptake inhibitors. Referral to acute pain services could also be beneficial.

What else can Acute Nerve Injury be?

If a doctor suspects that someone has an acute peripheral nerve injury, they also consider other similar conditions, including:

• Damage or disorders of the nerve roots in the spine (spinal radiculopathy)
• Injury to the spinal cord
• Diseases of peripheral nerves (peripheral neuropathy)
• Stroke or brain injury
• Injury to the muscles or blood vessels
• Tumors in the peripheral nerves

By using their knowledge of the nervous system’s structure and function, and carrying out a thorough medical history and physical examination, doctors can rule out these other conditions. They may also perform further investigations, such as imaging scans and electrical tests of the nerve function, to confirm the diagnosis of peripheral nerve injury.

What to expect with Acute Nerve Injury

Predicting the recovery from peripheral nerve injuries (PNIs) can be challenging due to numerous factors. This can depend on things like the severity and nature of the injury, if there was scar formation, the injury’s location relative to the muscle, the patient’s age, and if the nerve ends were brought back together, if necessary. However, one general rule is that the more severe the injury, the less likely a full recovery will be.

In the case of first and second-degree injuries, healing often begins right away, leading to a good or even excellent recovery over a period of weeks or months. This recovery might be due to the reversal of a block in nerve conduction, or through axonal regeneration, which is where the nerve fibers regrow at a rate of about 1mm per day.

However, for more severe injuries, the damaged nerve fibers must first completely break down (a process called Wallerian degeneration) before they can start to regrow. This regrowth can be hindered if the nerve’s structure has been disrupted, which can cause the nerve fibers to grow into incorrect tubes or even into neighboring tissues. This is particularly relevant for the most severe (fifth-degree) injuries, where recovery cannot be expected without surgical intervention to repair and realign the nerve ends.

Possible Complications When Diagnosed with Acute Nerve Injury

Peripheral nerve injuries (PNIs) can lead to significant complications for patients. These complications can be disabling, long-lasting, or even irreversible. Direct complications often include long-term pain, heightened sensitivity to touch, intolerance to cold, and loss of motor or sensory functions in a limb, which can compromise its functionality.

However, the impact of these injuries can extend beyond just physical symptoms. Disabilities from PNIs can lead to job loss and additional financial burdens, like needing to hire caregivers. They can also negatively impact the patient’s mental health and overall quality of life.

Complications of PNI:

• Chronic pain
• Heightened sensitivity to touch
• Cold intolerance
• Loss of motor or sensory function in a limb
• Job loss
• Additional financial burdens (e.g., caregivers)
• Negative impact on mental health
• Decreased quality of life