Thrombocytopenia Absent Radius Syndrome: Presence of Brachiocarpalis Muscle and Its Importance

Purpose: Common clinical manifestations associated with thrombocytopenia absent radius (TAR) syndrome have been reported in the literature since its initial description in 1959. The purpose of this study was to report a clinical observation and describe the anatomic location of this brachiocarpalis muscle. Methods: The radiographs of 20 patients (39 extremities) with TAR syndrome were reviewed and are the focus of this report. The presence or absence of an abnormal muscle crossing both the humero-ulnar and ulno-carpal joint, and its anatomic origin and insertion, were documented by radiographs, surgery, or both. A total of 12 patients underwent surgery on 19 extremities. Furthermore, we compared the radiographs of these 20 patients (39 extremities) with the radiographs of 20 arbitrarily selected patients with non-TAR type IV radial dysplasia, which were part of our database. Results: An abnormal brachiocarpalis muscle was noted radiographically in all 39 of the extremities and was observed surgically in all 19 patients who underwent surgery. It was absent in all 40 extremities of the non-TAR type IV radial dysplasia patients. One TAR patient had only unilateral radial dysplasia. This anomalous muscle has an abnormally high origin on the anterolateral aspect of the proximal humerus, and an insertion into the radial side of the carpus. As such, it has a deforming force at both the wrist and the elbow. Conclusions: We report the consistent finding of a brachiocarpalis muscle in patients with TAR syndrome. Because of its location, the presence of this muscle may influence the surgical treatment of these patients. Type of study/level of evidence: Diagnostic IV.