A 20-year-old woman, with no significant past medical history, presented with left shoulder pain that exacerbated with exercise over a three weeks period. There was no trauma or shoulder dislocation history. The physical examination showed no abnormalities and an MRI was requested for further investigation.
Left shoulder MRI showed an empty shallow bicipital groove with a normal robust short head of the biceps tendon (SHBT). There were no other significant findings and there were no signs that the absence of the long head of the biceps tendon (LHBT) was due to a tear, namely there was no intraarticular stump, no abnormalities on the usual topography of the biceps anchor and no fluid in the bicipital groove. Agenesis of the left LHBT was diagnosed and the patient was recalled for an ultrasound which confirmed similar findings on the right shoulder with an empty shallow bicipital groove establishing a diagnosis of bilateral agenesis of the LHBT.
The biceps brachii has two muscle bellies with two corresponding proximal tendinous origins, the LHBT originates intraarticularly from the supraglenoid tubercle and posterosuperior labrum while the SHBT originates from the coracoid process. Distally the two bellies unite to form one tendon that inserts on the radial tuberosity [6,7]. The LHBT is known for having many anatomic variants, mostly derived from supernumerary heads, also termed accessory fascicles, that occur in 9.1-22.9% of the population, the most common of which is a supernumerary head that arises from the humeral shaft (10% of European whites). The LHBT can also present with additional origins from the articular capsule, humeral tuberosities and even the supraspinatus tendon, some of which can result in a bifid LHBT appearance promptly identifiable on MRI [5,7]. Although the incidence of LHBT congenital agenesis is unknown, it is believed to be extremely rare. Increasing use of MRI and arthroscopy has increased the incidence of this diagnosis. Kumar et al recently reviewed 35 cases, of which only 8 patients had bilateral agenesis [1,2]. Agenesis of the LHBT is a rare challenging diagnosis for a clinician since there are no specific symptoms, Popeye sign is absent and most clinical tests for evaluation of the LHBT, such as the speed test and the Yergason’s test, lack specificity for agenesis. The presentation of these patients is variable, they can be asymptomatic or may complain of shoulder pain or instability and imaging findings may be normal or show cuff or labral pathology. Although some report that shoulder pain may be related to LHBT agenesis alone, it is still uncertain, due to low levels of evidence available, whether the agenesis is just an incidental finding or if it may be the source of shoulder pain and even represent a risk factor for shoulder instability and labral tears [1,3,4]. On MRI an empty bicipital groove usually raises suspicion of LHBT tear. One should be aware of the rare finding of agenesis mainly when there is a shallow bicipital groove and no ancillary findings of LHBT tear, such as an intraarticular tendon stump, a fluid-filled tendon sheath and muscle oedema. Finally, it is important for us radiologists to be aware that a tear is not the only cause of an empty bicipital groove. Kumar et al reported that on 60% of cases, which were diagnosed during arthroscopy, agenesis of the LHBT was initially missed on MRI and later confirmed retrospectively . Written informed patient consent for publication has been obtained.
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