There are 3 major nerves that supply the muscles of the hand and forearm as well as sensation to the hand: the median, ulnar and radial nerves. The radial nerve arises from a large plexus of nerves in the neck (the brachial plexus), winds around the back of the upper arm (humerus) to lie in front of the elbow. Just past the elbow, the radial nerve then divides in to a large sensory branch (the superficial radial nerve) that supplies sensation to the back of the hand over the thumb web, and a motor branch called the posterior interosseous nerve (PIN). The PIN then travels dorsally between a number of large muscles to reach the back of the upper forearm, and innervates all of the muscles that raise (extend) the wrist, fingers and thumb. Entrapment or compression of a sensory nerve results in numbness and/or tingling of the fingers. Entrapment of a motor nerve can result in muscle weakness or paralysis, but it some instances it can lead to muscular pain.
Entrapment of the motor branch of the radial nerve at the elbow can result in a constellation of symptoms which has been termed Radial Tunnel Syndrome. This syndrome is much less common and lesser known than median nerve compression at the wrist (carpal tunnel syndrome) and ulnar nerve compression at the elbow (cubital tunnel syndrome). Carpal tunnel syndrome initially results in numbness and tingling that affects the thumb, index and middle fingers. Cubital tunnel syndrome leads to similar symptoms in the small and ring fingers. Radial tunnel syndrome on the other hand, does not result in any numbness nor tingling. In fact, the main symptom is that of proximal elbow pain. It is especially of interest in that it can mimic tennis elbow, or lateral epicondylitis. It may seem paradoxical that compression of a motor nerve can cause pain, since it was originally thought that only the sensory nerves contained pain fibers, but this is now well accepted.
The nerve involved is the posterior interosseous nerve (PIN), which is the motor branch of the radial nerve at the elbow. This nerve branch supplies all of the forearm muscles which are responsible for extension of the fingers, wrist and thumb. Posterior Interosseous Nerve Entrapment also involves the PIN. In this case, the presenting symptoms are due to weakness and/or paralysis of the extensor muscles, which results in a wrist or finger drop. Radial tunnel syndrome conversely has no sensory nor motor loss. Although the same nerve is compressed, the clinical presentation may be different, and merely represent two ends of the spectrum of PIN compression.
Anatomy
The radial nerve divides into it’s two terminal branches, a motor branch (PIN) and a superficial sensory branch just before the elbow. The PIN then travels from the anterior aspect of the elbow, through the supinator muscle at the radial head level to emerge on the back part of the upper forearm. The nerve sequentially innervates the extensor muscles of the wrist, fingers and thumb. The superficial sensory branch remains on the lateral border of the forearm, where it ultimately supplies sensation to the back of the thumb and the 1st web space. This sensory branch is not compressed since it takes off prior to the site of compression, hence sensation remains normal.
The PIN branch may be entrapped by up to 5 separate structures. It is most commonly entrapped by the fibrous upper edge of the supinator muscle as it travels through the muscle. In many instances there is a dynamic entrapment of the nerve. That is, the compression is intermittent and dependent upon muscular action. With repetitive forearm supination and pronation, the fibrous edge of the radial wrist extensor (the extensor carpi radialis brevis) and the fibrous upper border of the supinator muscle have a scissoring action on the nerve which leads to intermittent PIN compression.
Clinical Presentation
Typically the patient will complain of poorly localized pain over the lateral aspect of the elbow, which may radiate up or down the arm. Because it is due to compression of a motor nerve, patients may compare the pain to a muscle cramp, which is deep and aching in nature. The syndrome typically occurs in workers performing hard manual labor that requires forceful elbow extension and repetitive pronation and supination. Because it is a dynamic compression, the pain is worsened by heavy manual activity and relieved by rest. It is important to note that an accompanying tennis elbow is present in many cases. Most patients have already been treated for tennis elbow for a prolonged period with splinting, injections and even surgery before the co-existence of radial tunnel syndrome is recognized.
The two conditions are differentiated mainly by the physical exam. In radial tunnel syndrome, the point of maximal tenderness is over the lower edge of the supinator muscle which is in the upper third of the midforearm, whereas in tennis elbow the point of maximal tenderness is over the lateral epicondyle. Pain with resisted extension of the middle finger and resisted supination which indirectly stress the wrist extensors and the supinator muscle are confirmatory signs, but they may also be present with tennis elbow. One useful diagnostic test is the complete relief of pain following a temporary block of the PIN with a 3-5 cc of local anesthetic. With a successful block, the patient will have temporary paralysis of wrist and finger extension, but will have no further pain! Any coexisting tenderness over the lateral epicondyle will still be present, which helps to differentiate the two disorders.
Nerve Conduction Studies/EMG
Unlike carpal tunnel and cubital tunnel syndrome, the standard nerve conduction study and EMG are almost always normal, and of limited use. A newer technique which measures the comparative nerve latencies with the forearm held in neutral, pronation and supination increases the diagnostic yield. Since this condition is a dynamic entrapment, placing the forearm in the position which entraps the nerve will lead to slower nerve conduction as compared to the other positions.
Treatment
The initial treatment is always directed towards activity modification and rest. A long arm splint and NSAID’s are useful, but cortisone injections have minimal effect. Any coexisting tennis elbow should be simultaneously treated, in which case cortisone injections are a mainstay. If all of these measures fail, operative decompression of the nerve is indicated.
Surgery
It is my preference to decompress the PIN through a combined anterior and dorsal approach (link to radial tunnel 1). The nerve is identified at the elbow and followed down the arm to where it passes underneath the supinator muscle. A number of sites must be decompressed including the deep fascia, the radial recurrent leash of vessels which consist of a number of large arteries that cross over the nerve (link to radial tunnel 2 and 3), and the upper border of the supinator muscle itself (link to radial tunnel 4). I also frequently release the lower border of the supinator as well since in my experience this has led to better outcomes (link to radial tunnel 5,6). Despite the extensive dissection the final scar is still cosmetic and becomes less noticeable over time (link to radial tunnel 7).
Results
Most large studies agree that although surgical treatment is beneficial, many patients have residual symptoms. In one retrospective study of 19 patients the results were found to be consistent with those previously reported: i.e. 75% favorable outcomes. Despite this finding only 8 patients (40%) stated they were satisfied. The main reason was residual pain. Shorter delay between the onset of symptoms and surgical treatment as well as simultaneous release of the lateral epicondylar muscles was found to positively influence patient satisfaction. In another study of 28 patients with an average 28-month follow-up, only 11 of the 28 patients (39%) had excellent or good results. However, 64% subjectively assessed their results as excellent or good. Results were worse in litigated cases and in patients receiving workers’ compensation benefits. Complete relief of pain with unrestricted forearm use is not the norm, hence permanent job modification may be necessary.
Work Considerations
Most people can perform one-handed work activity by the 2nd week followed by light duty including clerical work at 6-8 weeks. Heavy manual labor can often be resumed by 12 -16 weeks.
