RTS is a rare, potentially debilitating condition dominated by lateral elbow pain [3]. In affected individuals, repeated or forceful forearm rotation or elbow extension may exacerbate symptoms. This also applies for resisted forearm supination with an extended elbow and resisted middle finger extension [8]. Furthermore, palpation of the radial tunnel, i.e., the area located about 5 cm distal of the lateral epicondyle of the humerus, usually aggravates pain. Next to the moderate to severe pain there may be muscle weakness and decreased motion ranges. Sensory alterations, such as paresthesias and dysesthesias, are not present.

Of note, it is not uncommon that RTS patients present with what they refer to as tennis elbow. Although workup should yield a precise diagnosis, both conditions have been known to occur as comorbidities [9].

If a patient presents symptoms consistent with RTS, he/she should be queried about their occupation and possibly prolonged elbow extension and forearm rotation. Alternatively, RTS patients may report prior trauma. While a considerable subset of cases is still deemed idiopathic, RTS may also indicate ongoing inflammatory processes or space-occupying lesions like cysts and neoplasms [10]. In this context, a tentative diagnosis based on anamnestic and clinical data should be supported by imaging findings. The following techniques may be employed [11]:

• Plain radiography is often carried out to rule out acute injuries of the skeleton, to visualize possible joint effusions, soft tissue swelling and solid neoplasms.
• Ultrasonography allows for a reliable assessment of the radial nerve's condition [5].
• Magnetic resonance imaging may be reserved for cases of chronic elbow pain and non-responders [4].

Electromyographic studies don't usually yield specific findings. Ferdinand et al. analyzed the electromyographic examination findings of 10 patients diagnosed with RTS and could not find evidence of RN or PIN dysfunction in either one [4].

The radial nerve (RN) originates from the brachial plexus, descends through the radial groove of the humerus, and pierces through the lateral intermuscular septum to emerge about 10 cm proximal of the lateral epicondyle of the humerus. The nerve comes to lie between the brachialis and brachioradialis muscles. Up to this point, branches innervating the triceps brachii, anconeus, brachioradialis, and extensor carpi radialis longus muscles, and cutaneous branches have emanated from the RN. Anterior to the lateral epicondyle of the humerus, the RN divides into its two terminal branches: a superficial sensory branch and a deep motor branch. The latter then travels through an enclosed space extending from the radiocapitellar joint to the proximal aspect of the supinator muscle. This space is known as the radial tunnel, and it is delimited by the joint capsule (proximally), the brachialis muscle and biceps tendon (medially), the brachioradialis, extensor carpi radialis longus and brevis muscles (laterally), and the supinator muscle (distally) [1]. After exiting the radial tunnel, the nerve penetrates the supinator muscle and gives off several minor branches that innervate the dorsally located extensor muscles.

The compressive entrapment of the nerve within the radial tunnel causes symptoms consistent with radial tunnel syndrome (RTS). A literature review reveals a certain inconsistency regarding the use of medical termini concerning RTS [2]. On the one hand, distal segments of the deep motor branch of the RN are also referred to as posterior interosseous nerve (PIN), but the point of transition is poorly defined. Therefore, RTS may be defined as an entrapment of the deep motor branch of the RN or PIN [3] [4]. The deep motor branch of the RN penetrates the supinator muscle after it passes under the arcade of Fröhse, a fibrous arch that constitutes the most common site of entrapment of the nerve [5]. Affected individuals are diagnosed with posterior interosseous nerve syndrome, an entity to be distinguished from RTS. Finally, RTS is different from tennis elbow, although historic publications refer to the former as "resistant tennis elbow with a nerve entrapment" [6] [7].

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