Radial tunnel syndrome refers to a rare compressive neuropathy of the posterior interosseous nerve (PIN) as it passes through the radial tunnel resulting in pain without motor or sensory dysfunction. This pain-only phenomenon is contrasted with PIN compression syndrome, which describes a more severe PIN compressive neuropathy, ultimately causing injury to the large myelin fibers of the PIN resulting in wrist extensor weakness as well as pain. The PIN is a branch of the radial nerve at the elbow and supplies motor innervation to the dorsal extensor compartment of the forearm and sensory fibers to the dorsal wrist capsule. The PIN courses between the two-headed origin of the supinator and around the radial neck posteriorly. It crosses above the abductor pollicis longus (APL) and passes just posterior to the interosseous membrane down the forearm. Its sensory supply to the dorsal wrist capsule is located in the fourth dorsal wrist compartment. The PIN does not provide cutaneous innervation. Of note, radial tunnel syndrome is an entity with much debate with some experts in the field, even questioning its existence.
The radial nerve originates from the posterior cord of the brachial plexus (C5-T1) immediately posterior to the axillary artery. It passes along the posterior aspect of the axilla. It gives off three distinct branches, the posterior cutaneous nerve of the arm and two branches that provide innervation to the long and short head of the triceps. It then passes through the rotator interval along with the profunda brachii artery. The nerve then courses between the humerus and the long head of the triceps in the posterior compartment of the arm. Next, it crosses the posterior border of the humerus in the spiral groove in a medial to the lateral direction. There, it gives off the inferior lateral cutaneous nerve of the arm, posterior cutaneous nerve of the forearm, and branches to the lateral head of the triceps, medial head of the triceps, and anconeus muscles. Next, the radial nerve pierces the lateral intermuscular septum and courses anterior to the lateral condyle of the elbow between the brachioradialis and brachialis muscles. There, it innervates extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), brachioradialis, and brachialis. At the level of the elbow, it divides into its terminal branches, the superficial sensory branch, PIN, branch to ECRB.
The radial tunnel is defined as a five centimeter (cm) potential space in the dorsal aspect of the forearm extending from the level of the radiocapitellar joint to the proximal edge of the supinator. It is bordered laterally by the brachioradialis, ECRL, and ECRB muscles. The medial border consists of the biceps tendon and brachialis muscle. The floor of the radial tunnel is the capsule of the radiocapitellar joint.
Although the radial artery proper is not found within the radial tunnel, its first branch (radial recurrent artery) is. The radial recurrent artery courses proximally on the supinator and forms an anastomosis with the anterior branch (radial collateral artery) of the profunda brachii (deep brachial) artery. The radial recurrent artery, along with its venae comitantes, is a site of PIN compression known as the leash of Henry. The radial artery continues distally in the forearm between the brachioradialis and flexor carpi radialis (FCR). It passes between FCR and the first dorsal extensor compartment at the level of the carpus and gives off the superficial palmar branch. Finally, it courses between the two heads of the first dorsal interosseous muscle and forms the deep palmar arch as it joins the deep branch of the ulnar artery.
Radial tunnel syndrome has an estimated annual incidence of 0.003%. It is believed to be implicated in 2.97 per 100,000 cases of radial neuropathy in males and 1.42 per 100,000 cases in females. Secondary to its rarity and purely clinical nature, radial tunnel syndrome is often misdiagnosed as PIN compression syndrome, which is defined as PIN compression distal to the supinator with true muscle weakness. Some consider radial tunnel syndrome to be an early form of PIN compression syndrome, which occurs before the structural integrity of the PIN is affected, resulting in weakness.
Possible areas of compression of the PIN in the radial tunnel include (from proximal to distal) fibrous bands anterior to the radiocapitellar joint, the leash of Henry (radial recurrent artery anastomosis), medial edge of ECRB, the arcade of Frohse (proximal edge of the supinator), and distal edge the supinator. The most common site of compression is the arcade of Frohse. A risk factor for the development of radial tunnel syndrome is repetitive prono-supination with the elbow at 0 to 45 degrees of flexion. This type of repetitive motion increases pressure in the radial tunnel, causing irritation of the nerve.
History and Physical
Radial tunnel syndrome manifests as dorsal and radial proximal forearm aching pain. Discomfort is exacerbated with movements requiring prono-supination of the forearm or resisted elbow flexion (lifting heavyweight). Although the PIN does not provide cutaneous innervation, paresthesias over the dorsal thumb-index webspace have been reported. Patients are tender to direct palpation over the mobile wad of the forearm with the point of maximum tenderness 2 to 5 cm distal to the lateral epicondyle of the humerus. This area of point tenderness is the primary finding in radial tunnel syndrome on physical examination. Radial tunnel syndrome is often difficult to distinguish from lateral epicondylitis or tennis elbow and represents concomitant pathology in 5% of cases. Classically, patients with lateral epicondylitis are point tender over the lateral epicondyle and not the dorsal forearm. Provocative physical examination maneuvers include resisted supination and long finger extension. Both should reproduce pain without weakness, although it is possible that any perceived weakness may be due to pain. Additionally, wrist flexion and passive pronation can reproduce the dorsoradial proximal forearm pain by increasing the pressure in the radial tunnel to 250 mm Hg. Notably, patients with radial tunnel syndrome should not have a weakness. This is contrasted with PIN compression syndrome, where the weakness of the wrist extensors is a significant examination finding secondary to damage of the myelin of the PIN.
The rule of nine, as described by Loh et al., can be used to help evaluate patients with a diffuse, ill-defined elbow, or forearm pain. Using this test, nine circular areas are marked out on the patient's volar elbow in a 3x3 grid. The numbering starts on the proximal lateral side with 1 and moves distally so that number 4 is the proximal circle in the middle row, number 7 is the proximal circle in the medial row, and number 9 is the distal circle in the medial row. Each zone is palpated, and patients are given the option of responding with "painful," "uncomfortable," or "nothing." Areas 1 and 2 correspond to the course of the radial nerve and will be painful in cases of radial tunnel syndrome. Areas 5 and 6 correspond to the median nerve, and the medial row serves as controls. Tenderness to palpation in areas 7 to 9 suggests that the correct diagnosis may lie elsewhere.
Advanced imaging, including magnetic resonance imaging (MRI), has a limited role in the diagnosis of radial tunnel syndrome as it is often negative. MRI is useful; however, to evaluate common locations of PIN entrapment and to identify other causes of compression such as tumors, ganglion cysts, or heterotopic ossification. Electrodiagnostic studies are also negative as the large myelinated fibers of the PIN are not affected. Again, these studies can be used to rule out other causes of neuropathy such as foraminal stenosis in the cervical spine. An injection of local anesthetic into the radial tunnel can be a helpful diagnostic tool, but care must be taken to ensure that the anesthetic does not travel to the lateral epicondyle, which would relieve pain from lateral epicondylitis.
Differential diagnoses of radial tunnel syndrome include lateral epicondylitis, PIN compression syndrome, brachial plexopathy, cervical stenosis, and cervical foraminal stenosis. Electrodiagnostic studies can be helpful in cases of PIN compression syndrome and spinal pathology but are normal in radial tunnel syndrome.
Patients with a brachial plexus injury have a variety of presentations depending on the location of the injury. A detailed physical exam and EMG can help make this diagnosis. An MRI of the neck/shoulder may be considered to differentiate stretch injuries from true nerve disruptions.
Cervical Foraminal Stenosis
Compression of cervical nerve roots as they exit the spinal cord and pass through neural foramina. These patients present with radicular type pain, numbness, tingling, and weakness depending on which level is affected. An MRI can be used to assess for foraminal stenosis, and targeted injections can be useful in determining symptomatic levels.
Compression of the cervical cord in the spinal canal. The hallmark of cervical stenosis is myelopathy or hand clumsiness with or without gait disturbances. Positive Hoffman reflexes should raise suspicion for cervical stenosis.
Lateral Epicondylitis ("Tennis Elbow")
This is inflammation or tendinitis of the common extensor tendon on the lateral condyle of the distal humerus. These patients are point tender over the lateral condyle and have severe pain with resisted wrist/finger extension.
A study of 25 patients with radial tunnel syndrome demonstrated that 72% of patients had complete resolution of their forearm pain with an injection of 2 ml 1% lidocaine and 40 mg of triamcinolone at 6 weeks and 60% of cases remained pain-free after 2 years. Relief from operative radial tunnel release is not immediate with delayed recovery taking up to 9 to 18 months with a modest 60% to 70% success rate.
Complications following surgical radial tunnel release include recurrent symptoms of nerve entrapment, bleeding, infection, and iatrogenic nerve injury.
Deterrence and Patient Education
Radial tunnel syndrome can be differentiated from PIN compression syndrome by the chief complaint of isolated dorsoradial proximal forearm pain without weakness. Unfortunately, some patients may seem weak on physical examination secondary to pain, making the diagnosis difficult. Sotereanos et al. performed a retrospective review of 28 surgical radial tunnel releases and reported good results in 11 cases (39%). Poor satisfaction following surgical release was observed in patients with other nerve compression syndromes (carpal tunnel or cubital tunnel syndrome) and those involved in worker’s compensation claims.
Enhancing Healthcare Team Outcomes
Proper communication between the peripheral nerve surgeon (neurosurgeon/orthopedic surgeon/plastic surgeon), physician (neurologist or general physician), radiologist, a neurophysiologist, and neuro/orthopedic nurse is mandatory for the diagnosis and management of this disease, as the differential diagnoses can be tricky at times. It is also essential when to consider surgery, as many cases do not respond if the diagnosis is incorrect. Advanced imaging may be helpful in unclear cases.