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A B M P m e m b e r s ea r n F R E E C E h o u r s by rea d i n g t h i s i s s u e ! 25 tunnel. Tendinous bands in the supinator muscle that compress the nerve are the most common cause of RTS. RTS symptoms develop either suddenly or gradually, depending on the primary cause of the nerve compression. For example, RTS will often occur due to an acute injury. Acute injuries can cause a change in the positional alignment of the bones in the elbow. In this case, symptoms have a rapid onset in the traumatized area. In other cases, the symptoms are gradual, such as in cases of tumors or tendinous bands in the supinator muscle. Common nerve compression symptoms include paresthesia (pins and needles), sharp pain, and numbness. These symptoms are also common in the carpal tunnel or cubital tunnel syndromes. They result from pressure on many sensory fiber trunks within the nerve. An important distinction is where the pain and sensory symptoms are felt. That is why it is helpful to become familiar with the regions of cutaneous innervation of the distal forearm. Because the PIN is primarily a motor nerve, the usual symptoms of paresthesia and sharp pain are uncommon. Instead, clients with RTS usually describe dull, nonspecific, aching pain in the forearm. That pain results from irritation of the few sensory fibers within the PIN. Pain tends to increase with forearm supination and pronation movements. These movements are usually more painful if performed with elbow extension. The elbow extension further stretches the nerve against adjacent structures. ASSESSMENT Gathering information and the history of the symptom onset and pain patterns is essential. Revisit the patterns and symptoms described above. You might see weakness in the wrist and finger extensors that are innervated by motor fibers in the PIN. Motor or sensory symptoms may exist together or without the presence of the other. the median nerve where compression can occur. Ulnar nerve compression is also common, with cubital tunnel syndrome (elbow) and Guyon's canal syndrome (wrist) being the most prevalent. The radial and other upper extremity nerves are part of the brachial plexus. After diverging from the brachial plexus, the radial nerve courses around the posterior aspect of the upper arm near the spiral groove of the humerus. It then crosses the anterior aspect of the elbow before continuing down the forearm. Just distal to the elbow, the radial nerve divides into the superficial and deep branches. The superficial branch is primarily sensory. The deep branch becomes the posterior interosseous nerve (PIN), mainly carrying motor fibers. Compression of the PIN is the primary problem in RTS. The supinator muscle also plays a critical role in this condition. The supinator has two divisions. One comes off the humerus' lateral epicondyle, and some of its fibers also originate from the radial collateral and annular ligaments. The other division originates from the supinator crest and the fossa of the ulna (Image 1). The radial tunnel is a narrow passage in the elbow region and extends from the radial head to the inferior border of the supinator muscle. In the radial tunnel, the PIN passes between the two divisions of the supinator muscle. RTS is the compression of the PIN within the tunnel. A region along the superior border of the supinator, called the arcade of Frohse, is the most common region for PIN compression in RTS (Image 2). EXPLORING THE PATHOLOGY Several factors can cause radial nerve compression in the tunnel. Trauma to the elbow can displace bones in the elbow region, with the bone ends causing incursion into the radial tunnel and compressing the PIN. Small cysts or tumors can also compress the nerve in the Proximity of the PIN to the edge of the supinator at the arcade of Frohse. Image courtesy of Complete Anatomy. 2 Posterior interosseous nerve (PIN)