RADIAL TUNNEL SYNDROME

Radial tunnel syndrome describes a compression neuropathy of the PIN as it passes through what is known as the radial tunnel.  Reports as early as 1883 describe “resistant tennis elbow” and may have been referring to this entity. Our understanding of this disorder evolved over time, the nomenclature adapted, from “radial pronator syndrome” in 1954 to “resistant tennis elbow with nerve entrapment” in 1972. The term “radial tunnel syndrome” (RTS) was introduced by Eversmann in 1993 to describe the effects of the supinator brevis muscle compressing the radial nerve in the elbow (ie, the tunnel). It is now known that the radial nerve also may be compressed by the bands of fascia radial recurrent vessels, or (rarely) a hemangioma, lipoma, dislocated head of radius, inflamed synovium, or accessory muscles. The diagnosis is one of exclusion that depends on clinical signs and symptoms.  The existence of RTS remains a controversy, with many surgeons believing it to be severe recalcitrant lateral epicondylitis, because there are no significant findings on imaging modalities or electrodiagnostic studies. Additionally, it is important to understand the relationship between RTS and PIN syndrome.^3,4,5

Pathophysiology

• In one study, the most common MRI finding in 84% (21/25) of patients with RTS was muscle denervation along the posterior interosseous nerve distribution within the supinator muscle.
• RTS is caused by an injury to the posterior interosseous nerve (motor portion of the radial nerve).^3,4,5

Related Anatomy

• The radial nerve divides, just proximal to the arcade of Frohse, into superficial (superficial radial sensory nerve) and deep branchs (posterior interosseous nerve [PIN]) at the lateral elbow.
• The radial tunnel extends from the radial head to the inferior border of the supinator muscle. The tunnel boundaries are formed by the deep and superficial heads of the supinator, extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), and brachioradialis muscles.
• Entrapment of the radial nerve can occur at five sites within the radial tunnel. A number of physicians believe the most common site is the arcade of Frohse.
• The five sites include:
  1. Fibrous bands anterior to radiocapitellar joint between brachialis and brachioradialis
  2. Leash of Henry – recurrent radial vessels that fan out across PIN
  3. Proximal edge of ECRB
  4. Arcade of Frohse – proximal edge of supinator
  5. Distal edge of supinator

Incidence and Related Conditions

• RTS is relatively rare, with an annual incidence rate of 0.003%
• Female to male ratio varies among reports, from 1:1 to 6:1
• Conditions that place the patient at risk of RTS include diabetes, underactive thyroid gland, tumors or ganglion cysts, swelling or fluid in the arm, and trauma to the forearm
• It is important to note that RTS and PIN syndrome are both compression of the same nerve but are differentiated clinically.  PIN syndrome will present with motor deficits while RTS is characterized by pain only.

Differential Diagnosis

• Biceps muscle/tendon injury
• Brachial plexus injury
• Impingement of the articular branch of the radial nerve
• Lateral epicondylitis (LE)
• Muscle tear of the ECRB
• Osteoarthritis of the radiocapitellar joint
• PIN syndrome
• Posterior plica impingement
• Parsonage-Turner Syndrome

The typical patient is aged 30–50 years and his/her occupation/hobby usually includes repetitive forearm supination and pronation (eg, carpentry, heavy manual labor, factory work, sports), and s/he will complain of diffuse forearm pain in the dominant arm. History of a previous surgical procedure, including for carpal tunnel syndrome, trigger finger, or de Quervain’s tenosynovitis, is common in patients with RTS. 

• Damage to blood vessels (rare)
• Loss of strength/muscle atrophy
• Loss of sensation
• Partial to complete loss of wrist and hand movement
• Permanent nerve damage
• Residual pain 

• Predictors of a poor response to conservative therapy:
  • Age >50 years
  • Symptom duration >10 months
  • Constant paresthesia
• Radial tunnel release mitigates symptoms, but mild pain may remain.
• Surgical success ranges from 67–92%.
• There is a paucity of literature on the long term effects of conservative management for RTS. It is also important to note that reported outcomes of surgical intervention vary widely in the literature due to the rarity of the condition, the lack of accepted diagnostic criteria and the frequency of concurrent pathologies including lateral epicondylitis and PIN syndrome.^3,4,5

• Pain in lateral aspect of forearm; exacerbation of pain during resistance of supination, wrist hyperextension, and pointing of the middle finger is consistent with RTS
• Pain associated with RTS increases when traction is applied to the nerve by extending the elbow, pronating the forearm, or flexing the wrist
• Rule of Nine is done by applying pressure to nine areas on the anterior,  dorsal and proximal forearm just distal to the elbow crease.  The patient is assessed for pain in the boxes superficial to the radial tunnel.⁵
• Differentiate from radial tunnel syndrome (RTS) from lateral epicondylitis (LE) by determining point of maximal tenderness is important.  In RTS, it is over the anterior radial neck 3-4 cm distal to the lateral epiconyle.  In LE, it is the origin of the extensor carpi radialis brevis muscle
• MRI is rarely needed for initial evaluation of patients with possible RTS except when evaluating a mass that maybe compressing the radial nerve.
• If nerve injury is chronic, axial MRI at the elbow may show an enlarged bicipitoradial bursa, hypertrophy of extensor carpi radialis brevis muscle, or vascular pathology; these are abnormal signals. However, a normal MRI does not rule out nerve injury.
• There is controversy about whether RTS and PIN syndrome are two separate entities or a continuum of the same condition. 
• There are no definitive tests to confirm the diagnosis of RTS.
• Most patients achieve full recovery with conservative treatment. 
• Electromyography and nerve conduction studies are typically normal in patients with RTS.  The EMG/NCV studies the findings that have been reported do not consistently support the diagnosis of RTS.  This limnited support is based on the lack of consensus regarding EMG technique for RTS and the EMG/NCV thresholds associated with the RTS diagnosis.⁵

New Articles

1. Vergara-Amador E, Ramirez A. Anatomic study of the extensor carpi radialis brevis in its relation with the motor branch of the radial nerve. Orthop Traumatol Surg Res 2015;101(8):909-12. PMID: 26547256
2. Meng S, Tinhofer I, Weninger WJ, Grisold W. Ultrasound and anatomical correlation of the radial nerve at the arcade of Frohse. Muscle Nerve 2015;51(6):853-8. PMID: 25297493
3. Dang AC, Rodner, CM.  Unusual compression neuropathies of the forearm, Part I: radial nerve.  J Hand Surg Am 2009; 34(10):1906-1914.
4. Urch EY, Model Z, Wolfe SW, Lee SK.  Anatomical study of the surgical approaches to the radial tunnel.  J Hand Surg Am 2015; 40(7):1416-1420.
5. van den Ende KIM, Steinmann SP.  Radial tunnel syndrome.  J Hand Surg Am 2010; 34:1004-1006.

Reviews

1. Hobson-Webb LD, Juel VC. Common entrapment neuropathies. Continuum (Minneap Minn) 2017;23(2, Selected Topics in Outpatient Neurology):487-511. PMID: 28375915
2. Moradi A, Ebrahimzadeh MH, Jupiter JB. Radial tunnel syndrome, diagnostic and treatment dilemma. Arch Bone Jt Surg 2015;3(3):156-62. PMID: 26213698

Classics

1. Roles NC, Maudsley RH. Radial tunnel syndrome: resistant tennis elbow as a nerve entrapment. J Bone Joint Surg Br 1972;54(3):499-508. PMID: 4340924
2. Lister GD, Belsole RB, Kleinert HE. The radial tunnel syndrome. J Hand Surg Am 1979;4(1):52-9. PMID: 759504