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Where is the peroneal nerve, and what does it do?
The peroneal nerve is a branch of the sciatic nerve. The sciatic nerve runs down the back of the thigh then divides into two branches. The branch that goes to the outside/front of the leg is called the “peroneal nerve.” This nerve runs just underneath the skin at the outside of the knee where it is vulnerable to injury.
This nerve receives sensation information (touch and feeling) from the anterior (front) and lateral (outer) side of the leg as well as the top of the foot. This nerve also controls the muscles that lift the ankle and toes upward (dorsiflexion).
PICTURE – NERVE AT FIB HEAD, FIB HEAD, SENSATION SHIN AND TOP OF FOOT, DORSIFLEXION AND TOE EXTENSION WITH ARROWS ADDED
What are the causes of common peroneal nerve injury?
The peroneal nerve winds around the top of a bone called the fibula at the outside of the knee joint. At this site, it lies directly underneath the skin and is vulnerable to injury. Any repetitive trauma or prolonged compression at this side can cause nerve injury. Common ways for this nerve to be injured include:
- prolonged side lying
- leg crossing
- squatting “strawberry pickers palsy”
- kneeling (e.g. people who lay tiles)
- Other things that can cause pressure on the outside of the knee like a a leg cast, knee brace, or inside a car.
- In many cases there is no specific cause
What are the symptoms of peroneal nerve injury?
The most common symptom is sudden or gradual onset of weakness in the ankle and toes. Patients notice difficulty to clear the foot from the ground and will often catch their toe and trip. It will often be noticed as their foot tends to “slap” the ground when they walk due to the weakness. There can be some numbness and tingling sensation in the top of foot and front of the leg.
Peroneal neuropathy is most often a problem of pure weakness. There is no pain. Usually there is little numbness or tingling but some people can have mild sensation changes along shin and top of the foot.
How is a peroneal neuropathy diagnosed?
While the story of foot drop without pain or numbness makes doctors think of peroneal neuropathy, foot drop can also come from damage to the sciatic nerve, nerves in the spine, or even a stroke or spinal cord injury. There is a specific pattern of weakness, and sensation changes that are identified on a physical examination. More serious problems like stroke is often considered when it first happens and some people attending an emergency room might have tests to rule out a stroke. Often people will have an EMG and Nerve Conduction Study (hyperlink) in order to electrically confirm the dysfunction in the peroneal nerve and determine how severe the injury is. These testings will also help rule out injury to the sciatic nerve or spinal nerve roots.
Will the nerve heal by itself?
Nerve injuries occur to the myelin (insulation around the nerve) and axons (microscopic wires running through the nerves). In this kind of nerve injury it most often is an injury purely to the myelin (insulation part). The body is able to repair myelin fairly easily but it takes time. Damage to the axons (microscopic wires) is more serious and difficult for the body to repair. The majority of people with peroneal neuropathy have myelin damage and are expected to make a full recovery. The kind of injury (myelin vs. axons) can be determined by the nerve testing (Nerve conduction study and EMG).
How quickly will the nerve recover and when should I go back to the doctor?
The majority of patients will have mostly recovered by three months after they have stopped all activities that might be injuring the nerve. This is the general time frame it takes the body to repair the myelin (though in some cases much faster). This is assuming that there is no further ongoing injury to the nerve. If the nerve is exposed to repeated pressure trauma then it may not heal or healing might be prolonged.
If the nerve testing determined that there was also a significant amount of damage to the axons then full recovery will take longer. Axons regrow but very slowly at the rate of 1mm per day or 1 inch per month. For the nerve to regrow axons from the injury site back to the muscles it may take 6 months or longer. If there is significant axon damage that has resulted in shin and foot numbness then that sensation recovery will take even longer.
As most people will have recovered by 3 months I suggest that anyone with significant ongoing weakness go back to the doctor for re-evaluation if weakness if still ongoing after three months from the time that nerve pressure injury has stopped. Rarely there can be other reasons for damage to the nerve at this spot including a pouch of fluid (ganglion cyst) pushing out from the knee joint into the nerve. Another rare condition is fluid from the nerve backing up pressure through the nerve to cause swelling and damage to the nerve. If weakness has not recovered as expected your doctor will likely order further testing including an MRI to look for these other rare conditions that might benefit from surgery.
What can I do while I’m waiting for the nerve to heal?
The most important thing is to understand the mechanism of this nerve injury and prevent further damage. Try to avoid crossing your legs, squatting, kneeling, or pressure of any kind to the outside of the knee. This even includes sleeping on your side.
Walking ability is the greatest function that is affected by the nerve injury. Your doctor might offer to prescribe you an ankle brace or ankle-foot-orthosis(AFO). In these situations these are usually plastic off the shelf braces that go inside your shoe and help pick up the foot. Wearing a brace will help prevent your toe catching as well as that foot slap walk(gait). Many people figure out a way to walk with the weakness and can’t be bothered with a brace. For those people they will often just wait for the nerve to recover. Other less intrusive devices include different straps and bands.
A good home remedy is wearing high top hiking shoes or boot that will have a bit of built in ankle support.
When parts of our body aren’t moving they tend to get stiff. In order to avoid this I suggest that you stretch the ankle several times a day. To do this, keep the knee fully extended and gently step on something like a rolled up towel and hold for 30 seconds. This doesn’t have to be done frequently. Even 3 times a day would be adequate.
Is there a role for physiotherapy?
In most cases physiotherapy is unnecessary. Most nerve injuries of this kind will recover without any kind of treatments. If there was more severe damage to the nerve and recovery is prolonged then seeing physiotherapy later when there is some recovery of strength is sometimes helpful to go over an exercise program to get the ankle muscles into shape again. If you are planning on returning to high level activities like sports or running then this is definitely recommended.