What is Anterior Interosseous Syndrome?
The anterior interosseous nerve (AIN) is a nerve that comes off from the median nerve located in your forearm, running deep between the two sections of the pronator teres muscle. This nerve helps control the thumb and forefinger, and it provides movement for three key muscles of your forearm. When it’s not working correctly, this can result in AIN syndrome, which usually results in pain and weakness in the forearm, especially when trying to make a pinching motion with the thumb and forefinger.
AIN syndrome can be the result of temporary inflammation of the nerve, compression of the nerve, or physical injury. Although different theories have been suggested as to what exactly causes it, most experts agree that inflammation plays a major role. This syndrome was first described by medical professionals in 1948 and then identified as a separate issue involving the AIN in 1952. It used to be known as the Kiloh-Nevin syndrome.
Various treatment methods have been tried and have shown reasonable success rates. These include surgical as well as medical treatments, each with different results depending on the timing and the person’s specific circumstances.
What Causes Anterior Interosseous Syndrome?
Anterior interosseous nerve syndrome can happen for many reasons, which can be broadly categorized as spontaneous or caused by injury. The trauma-related causes can be things like fractures in the forearm, injuries caused by piercing or stabbing, cast placement, vein puncturing, or complications from surgery to fix fractures. Among the spontaneous reasons, the most common ones include nerve inflammation in the arm, a condition where pressure increases within a muscle compartment and can damage nerves, and a condition where a nerve gets compressed or squeezed.
The most common place where this nerve can get squeezed or trapped is within certain muscles of your forearm. Other places where the nerve can get compressed are:
– At the upper edge of a certain forearm muscle, the flexor digitorum superficialis;
– Within an accessory muscle attached to one of the main muscles that flex the fingers;
– Within extra muscles that some people have in their forearms;
– Near clotted blood vessels in your forearm (either the radial or ulnar artery);
– Within a certain part of the forearm, known as the lacertus fibrosus.
Sometimes, anterior interosseous nerve syndrome can be confused with other conditions that solely affect the tendons that bend the fingers. In some people, the nerve even gets normally compressed by fibrous bands that often originate from deep within the arm muscles, and this pressure can increase with minor variations.
Other conditions that might look like anterior interosseous nerve neuropathy but actually aren’t, include nerve inflamation that doesn’t involve any compression, and tendon rupture in patients who have rheumatoid arthritis. To rule out these other conditions, a medical professional would flex and extend the patient’s wrist to make sure that the tendons are still intact. Conditions like rheumatoid arthritis and gouty arthritis may make it more likely for a person to get anterior interosseous nerve entrapment.
Risk Factors and Frequency for Anterior Interosseous Syndrome
This is a rare disease, making up just 1% of all disorders that cause weakness in the upper limbs.
Signs and Symptoms of Anterior Interosseous Syndrome
The AIN syndrome is a condition that affects movement but does not cause any changes to sensation. Patients with this condition typically experience vague pain in the forearm and inner elbow. Common complaints include difficulty in bringing the tips of the thumb and index finger together as well as struggling to form a fist or button shirts. A physical exam will often show a positive pinch grip test – instead of making an “OK” sign, patients will pinch a sheet between their outstretched thumb and index finger. Despite these movement difficulties, sensation remains unaffected.
- Pain in the forearm and inner elbow
- Difficulty bringing thumb and index finger together
- Difficulty forming a fist
- Struggle to button shirts
- Positive pinch grip test (can’t make the “OK” sign)
- No sensation changes
Testing for Anterior Interosseous Syndrome
Electrodiagnostic studies, which are tests that measure the electrical activity of muscles and nerves, play an important role in diagnosing anterior interosseous nerve syndrome, which occurs spontaneously. Median nerve sensory tests should be normal since the anterior interosseous nerve, which is being studied, does not involve sensory function.
Another element of the electrodiagnostic studies, electromyography, which measures muscle response to nerve stimulation, will yield important findings. In particular, we’ll look at the results from the flexor pollicus longus, the radial part of the flexor digitorum profundus, and the pronator quadratus. These results will provide useful information to differentiate between neurologic muscle wasting (amyotrophy) and nerve compression disease (compression neuropathy).
Equally, magnetic resonance imaging (MRI), a technique that uses a magnetic field and radio waves to create detailed images of the body, proves to be very valuable in assessing these conditions.
Treatment Options for Anterior Interosseous Syndrome
The general approach to treating AIN syndrome, which affects the nerves in the arm, begins with a rest period and limiting movement with a splint. This should put the elbow near a 90-degree angle or whichever position is most comfortable for the patient. Most patients generally see improvement between the 6th and 12th weeks of this adjustment in their daily activities.
Additional measures to consider include using non-prescriptive anti-inflammatory drugs (NSAIDs) and physical therapy treatments like pain-control methods and massage techniques, if the patient can tolerate them.
If these non-surgical methods aren’t effective after several months, surgery might be necessary. The surgical treatment involves exploring, freeing up, and decompressing the nerves. According to medical literature, 75% or more of patients have experienced positive outcomes after surgery. Particularly, success rates are higher in patients with a clear, identifiable mass causing the issue.
However, surgery is usually only offered as an option in certain cases, and this is usually discussed if at least three months of non-surgical treatments have not been successful.
If a surgical decompression procedure is necessary to relieve pressure on the median nerve, careful dissection is crucial to pinpoint the exact areas of compression. This process is essential in locating and relieving constricting edges or fibrous bands.
What else can Anterior Interosseous Syndrome be?
Diagnosing a nerve compression injury, specifically to the anterior interosseous nerve in the arm, can be tough. The challenge lies in the fact that this nerve primarily controls muscle function. So, often, the symptoms can be just like an injury to the finger ligament making it hard to distinguish.
Other conditions that add to the diagnostic complexity are:
- Stenosing tenosynovitis (a painful condition affecting the tendons on the thumb side of your wrist)
- Adherence or adhesion of the flexor tendon (the cord-like structure which allows your finger to bend)
- A complete tear or break of the flexor tendon, and
- Brachial neuritis (an inflammation of nerves in the shoulder area that cause severe shoulder pain).
Especially, conditions like brachial plexus neuritis can present symptoms very similar to those of an anterior interosseous nerve entrapment, making the diagnosis even more challenging.
What to expect with Anterior Interosseous Syndrome
The outlook is generally positive, and most cases don’t need surgery. If non-surgical treatment doesn’t work after three months, though, surgery might be an option for some patients.
Possible Complications When Diagnosed with Anterior Interosseous Syndrome
Complications can arise from two main sources. First, they can come from the disease that led to the syndrome. Second, they can be a result of the surgery that is performed to treat it.
