Calcific tendonitis of supraspinatus simulating acute brachial neuritis (Parsonage-Turner syndrome).

Calcific tendonitis of the shoulder is a common disorder caused by calcium hydroxyapatite deposition;1 the pathogenesis is not well-known. Several authors believe that it is related to degeneration of the tendon with an abnormal reparative response, leading to calcium hydroxyapatite crystal deposition.2 and 3 Usually, calcium deposition causes tendonitis and/or bursitis with acute severe pain and tenderness. It may have a chronic course with acute exacerbations and periods of remission.1, 2 and 3 The shoulder is the most common site of calcium hydroxyapatite precipitation, with an elective localization in the supraspinatus tendon, usually 1 cm from its insertion on the promontory of the humeral greater tuberosity.2 and 3 Other affected sites in the shoulder include the infraspinatus, teres minor, and the subscapularis tendon, in descending order of frequency.1, 2, 3, 4 and 5 If calcium crystal precipitation involves the intramuscular part of the tendon, it may cause massive intramuscular inflammation with an high signal intensity on T2-weighted magnetic resonance imaging (MRI) images. This appearance on MRI can simulate other conditions of high signal intensity on T2-weighted images within muscle, such as denervation injury, focal myopathies, and neoplasms.6, 7, 8 and 9 Parsonage–Turner syndrome (PTs), also known as acute brachial neuritis, is characterized by clinical acute onset with shoulder pain followed by muscular weakness. The typical MRI finding of this condition is a diffuse high signal intensity on oedema-sensitive sequences [fat-saturated T2- and/or proton density-weighted and short-tau inversion recovery (STIR) images], involving one or more muscles innervated by the brachial plexus.10 Muscle hyperintensity is due to denervation and usually appears 2 weeks after denervation but, in some cases, can be seen as early as 4 days after nerve injury.11 The present report describes a case of intramuscular calcific tendonitis of the shoulder mimicking acute brachial neuritis (PTs) clinically and on MRI.