Anterior interosseous nerve syndrome is not a very common condition. As an orthopedic physical therapist, I have seen and treated it on occasion. It can be confused with pronator teres syndrome or carpal tunnel syndrome. However, it does have its own distinguishing symptoms. Read on to learn about this uncommon condition. Let’s start with anatomy!
The pronator teres
The pronator teres is a muscle on the palmer side of the forearm. It has two head which means that it has two upper attachment sites near the elbow. One head attaches to the medial epicondyle of the humerus (the upper arm bone). The medial epicondyle is the bony bump on the inside of the elbow when the palm faces up. The other head attaches to the coronoid process of the ulna (one of the forearm bones). The coronoid process is a bony notch near the top of the ulna at the elbow. The muscle starts at the two heads, then combines and travels down to attach to the middle of the outside of the radius (the other forearm bone).
“RCSI – Drawing Flexor muscles and tendons of forearm – English labels” by Royal College of Surgeons of Ireland, license: CC BY-NC-SA
The median nerve
The median nerve is one of the major nerves of the upper extremity. It begins near the armpit and travels all the way down to the hand. As it travels down the arm, the median nerve will pass between the two heads of the pronator teres.
The median nerve innervates many of the muscles of the forearm and hand, including the pronator teres. It provides sensation to half of the palm of the hand on the thumb side. The fingertips and palmer side of the thumb, index finger, middle finger, and half of the ring finger also get their sensation from the median nerve.
The anterior interosseous nerve
The anterior interosseous nerve (AIN) is a branch off of the median nerve. It branches from the median nerve between the two heads of the pronator teres. It is a motor nerve only. This means that it innervates muscle but does not provide sensation. It innervates the flexor pollicis longus, part of the flexor digitorum profundus, and the pronator quadratus. These muscles aid in movements of the thumb, index finger, long finger, and wrist.
What is AIN syndrome?
AIN syndrome is a motor neuropathy that affects the three muscles that it innervates. It is caused by some type of damage or injury to the AIN. There are many different mechanisms of injury to the AIN. They can be categorized as either spontaneous or traumatic.
- Traumatic causes include:
- Fractures at the elbow
- Penetrating injuries and stab wounds
- Tight cast
- Complications from elbow surgery
- Spontaneous causes include:
- Inflammation of the brachial plexus
- This is a network of nerves that originate from the neck to innervate the arm and hand.
- Compartment syndrome
- This is dangerous pressure build up in the muscles.
- Compression
- This is the most common cause.
- The AIN is most likely to be compressed between the 2 heads of the pronator teres.
- Inflammation of the brachial plexus
What are the symptoms of AIN syndrome?
There are several symptoms associated with AIN syndrome. These include:
- No sensory changes
- Pain in the elbow and upper forearm
- Thumb, index, and long finger weakness
- Inability to make the “OK” sign
The okay sign - Difficulty making a fist
- Shows hand of benediction instead because of thumb and finger weakness
Hand of benediction
- Shows hand of benediction instead because of thumb and finger weakness
- Difficulty performing fine motor tasks such as buttoning a shirt
Who gets AIN syndrome?
AIN syndrome is rare and makes up only 1% of all upper extremity nerve injuries. Rheumatoid arthritis and gout may be predisposing factors for the condition.
Who should I see?
- Physical therapy – Physical therapy can help decrease pain and inflammation with techniques including manual therapy and dry needling. PT can evaluate and address abnormal joint mobility, muscle weakness, muscle tone, and flexibility. With AIN syndrome it is important to decrease the amount of work the pronator teres is performing. Your PT can help you activate other muscles to decrease the load on the pronator teres. They can assess your posture during work or sports to prevent the problem from happening again.
- Orthopedic physician – An orthopedist might be necessary if the condition is not responding to physical therapy. Your physician can prescribe medication, give a steroid injection, or order imaging if needed. Surgery is an option if all conservative measures fail.
What can I do?
- Relative rest – Resting the area to decrease further pain and damage from occurring is one of the best ways to treat AIN syndrome. Basically, stay away from the activities that are causing pain. This gives the nerve time to heal. If you keep doing what is causing the injury, it is not going to get better. This might include not participating in your sport or changing how you perform a task or job. If you are not able to participate in your activity or sport, then you can supplement with non-irritating activities such as biking, walking, or jogging while your condition is being treated.
- Ice – Ice can help decrease pain and symptoms at the elbow. Ice the painful area for 15 to 20 minutes at a time. I would recommend using an ice pack several times per day if the pain is constant or is preventing you from participating in an activity. Ice immediately after activity.
- Posture – Having forward, rounded shoulders can cause muscular imbalances in the shoulder and elbow. It decreases the ability of the scapular stabilizers to stabilize the shoulder joint. This causes the muscles at the elbow to work more. It also shortens the pectoralis major, pectoralis minor, and biceps and causes them to get tight. Stretching this area will make better posture more attainable. Below is a video showing how to stretch the pecs and biceps, and also how to use a wall to check your posture.
- Soft tissue mobilization – Soft tissue mobilization helps release the muscles around the elbow and decreases the pressure on the nerve. Use a massage ball, lacrosse ball, tennis ball, or your own fingers to massagepainful areas all around the elbow. Spend a good 3 to 5 minutes rolling. Do not roll so hard that you bruise the area, but it should be a pretty painful sensation. Roll out the area no more than a few times a week. See the video below.
- Pronator teres stretch – Stretching out the pronator teres after rolling it out is a great way to lengthen the muscle. It is a good idea to perform soft tissue mobilization first because you will get a better stretch afterwards. Below is a video on how to stretch the pronator teres.
- Median nerve flossing – You can mobilize the median nerve to ensure that it is not entrapped in an area. Flossing will free up the nerve if it is stuck and allow it to move along its pathway. Below is a video showing how to perform median nerve flossing.
- Thumb and finger strengthening – Strengthening the muscles in the thumb and fingers is important since AIN syndrome can affect these areas. Performing opposition and gripping exercises is important for recovering hand strength. You can use a towel, theraputty, or a stress ball. See the video below.
- Strengthen the rotator cuff and scapular stabilizers– Elbow injuries tend to occur because of weakness of the rotator cuff muscles and scapular stabilizers. These are the stabilizing muscles of the shoulder that allow the arm to reach, lift, and hold without injury or irritation. Using loop resistance bands and resistance bands can increase strength and difficulty.
In review
- The pronator teres is a muscle on the palmer side of the forearm.
- It has two head or two upper attachment sites near the elbow.
- The pronator teres pronates the forearm and helps to flex the elbow.
- The median nerve is one of the major nerves of the upper extremity.
- The anterior interosseous nerve (AIN) is a branch off of the median nerve.
- It branches from the median nerve between the two heads of the pronator teres.
- It is a motor nerve only meaning that it innervates muscle but does not provide sensation.
- AIN syndrome is a motor neuropathy that affects the three muscles that it innervates.
- Symptoms include hand weakness and pain in the forearm near the elbow.
- Will have difficulty making “OK” sign and making fist.
- There are no sensory changes.
- Physical therapy is a key component in treatment.
- Orthopedist is necessary if the condition is not responding to physical therapy.
- There are things that you can do.
- Relative rest
- Ice
- Posture
- Soft tissue mobilization
- Pronator teres stretch
- Median nerve flossing
- Thumb and finger strengthening
- Scapular stabilization strengthening
As always, If you have questions about the content of this post or any other musculoskeletal questions, please e-mail me.
Additional resources
https://www.physio-pedia.com/Anterior_Interosseous_Nerve_Syndrome