Introduction Definition variations of extensor tendons of the hand usually discovered incidentally during surgery for other reasons (e.g. ganglion excision) Epidemiology Incidence common login to view 1 more bullet Etiology Mechanism symptoms arise because of increased muscle volume within small muscle compartment login to view 1 more bullet Anatomy Normal EIP occupies 4th dorsal extensor compartment (8-10mm wide) ratio of 1:1 for muscle:tendon length origin - posterior surface of distal third of ulna and adjacent interosseous membrane insertion - dorsal expansion of index finger on ulnar side of EDC Classification Anomalous Extensor Muscle Forms Anomalous extensor indicis proprius (aEIP) Most common cause of symptoms Extensor digitorum brevis manus (EDBM) Less common cause of symptoms because muscle belly is proximal to extensor retinaculum Extensor medii proprius (EMP) Extensor indicis et medii communis (EIMC) EIP and EIMC unlikely to be symptomatic because of narrow width Presentation Symptoms usually asymptomatic login to view 1 more bullet mass on the dorsum of the hand intermittent dorsal wrist pain if muscle bellies impinge on and occupy the narrow dorsal compartments of the wrist Physical Exam inspection login to view 3 more bullets provocative tests login to view 1 more bullet Imaging MRI indications login to view 1 more bullet findings login to view 5 more bullets Differential Ganglion cysts Synovitis both produce dorsal wrist pain Treatment Nonoperative observation login to view 2 more bullets Operative surgical decompression of 4th dorsal compartment (aEIP) or reduction of muscle belly (EDBM) login to view 2 more bullets