Overview

A peroneal nerve (PN) injury is when a specific nerve (the peroneal nerve) is damaged in the leg as a result of injury or disease (lesion). The PN is responsible for flexing the foot and toes. Due to its location, this nerve is especially vulnerable to damage from traumatic leg injuries, such as knee dislocations or leg fractures.¹ 

Most PN injury patients develop a characteristic gait known as foot drop, which is when patients find it difficult to lift their foot or toes. Since foot drop is a common symptom of other nerve and muscle injuries, assessing nerve damage is key to a prompt and accurate diagnosis.² Doctors might refer you to an electrodiagnostic lab to carry out electrodiagnostic tests such as a Nerve Conduction Study (NCS) or an Electromyography (EMG) test. Both are essential in determining the nature and location of the lesion.

What are peroneal nerve injuries?

Anatomy of the peroneal nerve

The common peroneal nerve (CPN), or fibular nerve, branches off the sciatic nerve in the back of your thigh and then wraps around your knee to reach the front of the outer calf. Right below the knee, it divides into two main branches: the superficial peroneal nerve (SPN) and the deep peroneal nerve (DPN). From there, the two nerves run down to the upper side of your feet (dorsum).¹

Common causes

PN injury is the most common neuropathy in the legs. The causes of the injury vary, and some might be surprising.²

Leading causes of PN injuries are related to traumatic injuries such as:  

• Knee dislocation (in 13-40% of patients)³
• Fracture of the fibula or tibia⁴ 
• Ankle sprain⁵
• Surgery^6,7

PN injury can also arise because of nerve compression, which can be caused by:

• Ganglion cyst⁸
• A tight cast⁹
• Prolonged leg crossing or squatting 
• Prolonged bed rest¹⁰

Predisposing factors include:

• Diabetes¹¹
• Rapid weight loss^5,7

Symptoms of injury

When dealing with a PN injury, you will likely experience some of the following symptoms:

• Difficulty flexing the foot 
• Weak ankle 
• Tingling, pain or numbness on the foot dorsum or outer side of the calf ¹¹

Leaving the injury untreated might lead to the following: 

• Disability 
• Limb deformity 
• Foot drop ⁹

Foot drop

Primary symptoms, such as numbness, if left untreated, often lead to a change in gait known as foot drop.⁹ A patient with a foot drop lifts their knee higher than normal when walking without flexing their foot. Because of this characteristic gait, patients with foot drop often report tripping or falling.¹²

Several nerves are involved in foot movement, and presenting these symptoms does not necessarily imply PN injury.¹² In case of a suspected neuropathy, a physician might perform a physical examination with electrodiagnostic tests. These can guide and confirm the diagnosis of the condition. 

Electromyography and nerve conduction studies 

What are electromyography and nerve conduction studies?

Electromyography (EMG) and Nerve Conduction Studies (NCS) are two types of electrodiagnostic tools that can assess the well-being of your nerves and how well they communicate with muscles.¹³

Both studies rely on the electrical properties of our nervous system. Nerves, like wires, conduct electricity through your body to send an impulse to move a muscle, perceive a sensation, or perform automatic functions, like a heartbeat. All electrical communication produces a signal, which can be measured. EMG and NCS track how well the electrical signals are moving down a specific nerve by measuring this signal and comparing it to a normal signal. 

What do they reveal?

NCS and EMG are often performed together, as the two minimally invasive studies complement each other well. 

NCS looks directly at the nerves, observing how well and quickly they transfer signals. EMG instead looks at how the muscles respond to the electrical impulses sent through the nerves.  

Together, they can reveal important features of a potential nerve injury:

• Severity⁷
• Location
• Prognosis¹⁴
• Nature of injury (caused by muscle or nerve condition)
• Chronicity  

How nerve conduction studies work

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Source: Wikimedia Commons https://commons.wikimedia.org/wiki/File:Nerve_conduction_velocity.jpg

The testing process

An NCS machine has one or two stimulators that send a mild electrical signal down a specific nerve, while a sensor further down the nerve detects the nerve's response. During the test, the stimulator is moved along the nerve to send impulses from different points. Moving the stimulus and recording sites will allow your physician to determine which nerve is damaged and at what location.⁷

Typical findings in peroneal nerve injury

If you are suffering from a PN injury, some features in your NCS results may appear different from normal. Depending on which nerve is affected (deep, superficial or common), the sensory and motor NCS might show different results. 

For PN injury, sensors placed in these areas might detect abnormal signals:

• Foot dorsum
• Ankle
• Shin 

If there is a nerve injury, the signal along the PN will likely be weaker than expected or transfer more slowly.⁷

How electromyography works

The testing process

Unlike NCS, EMG does not require an external stimulator. In an EMG study, small needle electrodes are inserted into your muscles. When you want to contract a muscle, your brain sends an electrical signal through the nerves, down to the desired muscle. The electrical signal from your muscles will vary depending on how strongly you contract them. The more strongly you contract, the greater the electrical signal will be. EMG detects the electrical impulses in your muscles during these contractions.¹⁵

As the EMG specialist asks you to contract a muscle, the electrodes record the local electrical activity. The signal is sometimes transformed into an audio feedback that your physiatrist will listen to to detect abnormalities. In other words, instead of looking for abnormalities, your physician will be listening to them. 

Typical findings in peroneal nerve injury

EMG is not as sensitive as NCS, so if you are experiencing a milder PN injury, your EMG results might come back normal. In more severe cases, EMG results may show unusual electrical activity in affected muscles, such as the calf and shin muscles, both when they are at rest and contracting.¹³

Why these tests matter

Guiding treatment

Once diagnosed with a PN injury, EMG and NCS can help determine the severity of the lesion. Depending on the extent of the damage and its cause, different invasive and non-invasive treatment paths might be indicated. The first line of treatment for less severe cases is observation and lifestyle or activity changes. In case of nerve compression or recurring symptoms, treatment plans may include decompression or repair surgeries.⁹

Predicting recovery

EMG and NCS can help evaluate the potential of nerve regeneration in a patient. In particular, the motor signal recorded by NCS is a good indicator of recovery. In cases where there was electrical activity in the shin or dorsal foot muscles before treatment, over 80-90% of patients have been reported to recover their ability to flex the foot.¹⁴

Differentiating from other conditions

Symptoms of PN injury, such as foot drop, overlap with signs of other lower limb neuropathies like sciatica and sciatic neuropathy. Sometimes, multiple nerve problems can occur at once, which is why thorough testing is important to confirm the exact cause of symptoms.¹⁰ By testing the nerve performance and muscular response in different locations, EMG and NCS can rule out the presence of other nerve neuropathies⁷

What you can expect during a test

Before the test

When referred to a physiatrist or neurologist for an EMG/NCS, inform them of any: 

• Medications (bring a list if necessary)
• Bleeding disorders 
• Pacemakers or  cardiac defibrillators¹³
• Needle phobias¹⁶

On the day of the test 

• Shower to remove oils from the skin
• Refrain from using body lotions or oils
• Wear comfortable clothes that make it easy to access your legs

During the test

Depending on your pain tolerance and physiological state, NCS and EMG might cause some discomfort due to the mild shocks from the NCS stimulator or the insertion of EMG needles. Both will test for a PN injury.

During an NCS test

1. Your provider will stick sensors and stimulators on your foot or leg
2. Small electrical impulses will be sent through the stimulator and detected
3. The stimulator or sensor will be moved to a different area
4. The process repeats until sufficient information is collected

During an EMG test

1. Your provider will insert a small needle in your back, leg or gluteus muscle
2. You will be asked to relax and then increasingly contract your muscles
3. Needles will be moved to different muscles
4. The process repeats until sufficient information is collected

The duration of the tests will depend on the number of nerves or muscles being tested, but they usually last from 20 minutes to 1 hour.

After the test

Following the tests, you might experience some muscle soreness, bruising or minimal bleeding.¹³

Summary

Peroneal nerve injuries are common lower limb injuries and can significantly impact mobility, often causing a foot drop. Electrodiagnostic tests like NCS and EMG are essential tools for accurately diagnosing the injury, determining its severity, and guiding treatment. Together, these tests provide a detailed picture of nerve and muscle function, helping physicians differentiate PN injury from other conditions and predict the likelihood of recovery. Early evaluation and proper diagnosis are key to effective management and better long-term outcomes.