“Congenital Muscular Pseudohypertrophy of the Upper Limb: Morphology, Anatomy and Surgical Guidelines of An Unique Entity”

Background: Isolated congenital muscular pseudohypertrophy of the upper limb is a very rare anomaly. With our case series spanning twenty years, we aim to illustrate the pathological anatomy associated with this rare disorder and present a surgical framework in the operative treatment of this elusive condition. Methods: Patients presenting at two institutions were retrospectively reviewed. Exclusion criteria include syndromes (e.g. CLOVES) or isolated macrodactyly. Patients who presented with worsening deformities such as increasing metacarpophalangeal joint ulnar deviation, hyperabduction of thumb with widening of webspace affecting grasp or wrist deformities were recommended surgery. Surgical procedures include removal of aberrant muscles, rebalancing procedures or osteotomies. Results: A total of 25 patients were reviewed, of which 18 underwent surgery and 13 attended postoperative review. The median age was 7.5 years, and mean follow-up period was 42 months. Radial abduction improved from 54° to 36°, and ulnar deviation improved from 39° to 13°. All patients regained postoperative thumb opposition to the little finger, a function that had been previously lost. However, only 8/13 could oppose the thumb to the index finger postoperatively. Surgical findings revealed interesting additional layers of muscles which are unnamed, with some of these accounting for the deformities and others for bulk. The predominant finding was that of extra muscles rather than hypertrophied muscles. Conclusions: This is one of the largest reported series of congenital muscular pseudohypertrophy of the upper limb. We developed a scoring system for severity and an accompanying algorithm to guide when to offer surgery for moderate or severe deformities. These new muscle morphologies may shed light on evolutionary developmental biology pathways, allowing their safe removal during surgery. Level of evidence: IV