What is Posterior Interosseous Nerve Syndrome?
There are many conditions where the nerves in the upper arm get compressed or pinched. Some, like carpal tunnel syndrome, are quite common, while others, like posterior interosseous nerve (PIN) syndrome, are not as often seen. Understanding the layout and function of each nerve is important for diagnosing the right nerve and pinpointing where the compression occurred.
To understand PIN syndrome, it helps to look at the radial nerve, which is an offshoot of the brachial plexus, the network of nerves in your shoulder. This nerve travels down your arm and splits into two parts, superficial (closer to the skin) and deep (closer to the bone), at the top part of the forearm. Usually, the deep part of the radial nerve goes into the back of the forearm and comes out from the supinator muscle (the muscle that allows you to twist your palm upward) as the posterior interosseous nerve. Sometimes, the deep radial nerve goes through a tunnel, known as the Frohse arcade, before becoming the posterior interosseous nerve, which can make it more likely to become pinched. The posterior interosseous nerve is responsible for controlling certain muscles at the back of your forearm. Its end portion reaches into a part of the wrist, where it helps in moving the wrist joints.
There are different conditions where the radial nerve can get compressed or pinched, causing pain and other symptoms. These include radial tunnel syndrome, posterior interosseous nerve syndrome, and Wartenberg syndrome. Each of these conditions has its own set of symptoms, which can help in making the correct diagnosis. Posterior interosseous nerve syndrome is a condition where the posterior interosseous nerve, which controls the muscles that straighten out the forearm, gets compressed. This condition typically starts slowly and often results in weakness in extending the fingers and thumb. However, the ability to extend the wrist should remain intact because the extensor carpi radialis longus, a muscle involved in wrist extension, is controlled by the radial nerve. In most cases, this condition improves on its own with time and conservative treatments like rest and physical therapy. However, for symptoms that don’t improve with these measures, surgery to relieve the pressure may be necessary.
What Causes Posterior Interosseous Nerve Syndrome?
The Posterior Interosseous Nerve Syndrome is a condition that involves a certain nerve in your arm being damaged or put under pressure. This can be caused by several factors such as injuries, diseases that take up space (like rheumatoid arthritis), brachial neuritis (an inflammation of nerve bundles in the upper arm), or it may just happen on its own due to compression.
The most common place where this nerve gets compressed is a part of your arm called the arcade of Frohse, which is located near the beginning of the muscle that helps you rotate your forearm.
Lastly, consistent and repeated actions involving the rotation of your forearm can also lead to this nerve syndrome.
Risk Factors and Frequency for Posterior Interosseous Nerve Syndrome
Posterior interosseous nerve syndrome is a condition that is more common in males, with males being affected twice as often as females. Additionally, it is also twice as likely to occur in the right arm as compared to the left arm.
Signs and Symptoms of Posterior Interosseous Nerve Syndrome
Patients with a history of trauma or fractures to a limb might suffer from a condition called Posterior Interosseous Nerve syndrome. This condition is often seen in cases of Monteggia fractures or dislocation of the radial head in the elbow. Some symptoms to look out for include struggling to extend the fingers and an abnormal bending of the wrist towards the thumb side – this is due to weakness in the extensor carpi ulnaris muscle, which is responsible for straightening and adducting the wrist. A significant indicator of nerve damage is a positive Tinel sign, where tapping over the site of injury generates a tingling sensation. However, this sign’s presence may depend on the extent of the injury.
Testing for Posterior Interosseous Nerve Syndrome
As part of the assessment, the doctor might conduct a test known as electromyography (EMG) and a nerve conduction study (NCS). These tests help to measure the electricity and signals in your nerves and muscles. In this case, these tests can show changes in the muscles controlled by the specific nerve affected in our case, called the posterior interosseous nerve.
Now, there are several muscles which wouldn’t show these changes because they are controlled by a different nerve, the radial nerve. These muscles include the triceps (the muscle at the back of your upper arm), anconeus (a small muscle near your elbow), brachioradialis (a muscle of the forearm), and extensor carpi radialis longus (a muscle in your forearm that helps in moving your wrist and elbow).
Also, the test would show normal activity in the nerves responsible for feeling in the area controlled by the superficial radial nerve (a nerve that provides sensation to the back of the hand and the thumb).
Treatment Options for Posterior Interosseous Nerve Syndrome
Posterior interosseous nerve syndrome, a condition affecting the arm, is usually first treated with non-surgical methods. These can involve the use of a splint to immobilize the affected arm, anti-inflammatory medication known as NSAIDs, physical therapy to improve strength and flexibility, and changing daily activities to minimize pain. Surgery is considered only if these methods do not improve the condition after at least three months.
If surgery becomes necessary, the goal is to alleviate areas where the nerve is compressed or squeezed, causing pain. This can be achieved by releasing certain areas in the arm that may be compressing the nerve, such as fibrous bands (tough tissue) near the elbow joint, or the edge of a muscle in the forearm (“extensor carpi radialis brevis”). The process might also include ligating (tying off) the “leash of Henry”, which is an artery in the arm, and releasing the “arcade of Frohse”, which is a fibrous muscular structure in the forearm. The surgeon can also work on the distal edge of the supinator, which is a muscle involved in rotating the forearm.
After surgery, it’s important for the patient to start gently moving their arm as soon as possible. This early active range of motion helps promote recovery and regain function. Even after surgery, the patient should keep in mind that improvement in symptoms can keep happening for several months.
What else can Posterior Interosseous Nerve Syndrome be?
When examining a patient for possible Posterior Interosseous Nerve Syndrome, a doctor might also consider two other conditions that could cause similar symptoms: Radial Tunnel Syndrome and Wartenberg Syndrome.
Radial Tunnel Syndrome may affect the same regions as Posterior Interosseous Nerve Syndrome. It differs in that it usually causes pain in the forearm, but doesn’t result in any muscle weakness. The discomfort often starts on the back and side of the forearm and can spread down the arm to the back of the hand. An area known as the ‘mobile wad’ may also feel tender to touch, which can sometimes lead to confusion with another condition known as Lateral Epicondylitis. However, the painful areas in Radial Tunnel Syndrome are typically located further away from the elbow than in Lateral Epicondylitis, generally about 1 to 2 inches below the elbow joint.
Wartenberg Syndrome, also referred to as “cheiralgia paresthetica”, is when the nerve that allows sensation on the hand’s back and side closest to the thumb (superficial radial nerve) becomes compressed between two specific forearm tendons. As this nerve doesn’t control muscle movement, Wartenberg syndrome only affects sensation. Therefore, there’s no muscle weakness. Patients may experience tingling, numbness, or undefinable pain on the hand’s back and the side closest to the thumb. Associations have also been reported between Wartenberg Syndrome and a painful condition called De Quervain’s tenosynovitis.
What to expect with Posterior Interosseous Nerve Syndrome
The overall outlook for patients is usually positive with non-invasive treatments. If the individual goes through surgery, they might continue to see improvements for several months afterward. For athletes, they can get back to their sport or activity when they have regained full range of motion (ROM) and strength.
Possible Complications When Diagnosed with Posterior Interosseous Nerve Syndrome
Complications after surgery may consist of:
- Not completely relieving the problem
- Continued symptoms
- Difficulty returning to work at the previous level
- Inability to perform physically challenging jobs
Recovery from Posterior Interosseous Nerve Syndrome
Rehabilitation should begin shortly after the pressure has been relieved from the injured area, with a focus on early active movement. It’s worth noting that full recovery might take as long as 18 months.
Preventing Posterior Interosseous Nerve Syndrome
Patients should be informed that non-surgical treatments should be tried thoroughly before considering surgical treatments. Treatment for PIN, which refers to an issue with a nerve in the forearm, is not as effective as treatments for carpal tunnel or cubital tunnel syndromes – conditions that affect the nerves in the wrist and elbow, respectively.
