Introduction Definition variations of extensor tendons of the hand usually discovered incidentally during surgery for other reasons (e.g. ganglion excision) Epidemiology Incidence common 1-3% in cadaveric studies Etiology Mechanism symptoms arise because of increased muscle volume within small muscle compartment pain from synovitis or ischemia 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 discovered incidentally during surgery (e.g. ganglion removal) 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 mass does not transilluminate moves with movement of local muscles (flexion and extension of hand and wrist( becomes firmer with grasp provocative tests resisted extension triggers pain Imaging MRI indications exclude other more common conditions e.g. ganglion findings mass is isointense with muscle tissue anomalous extensor indicis proprius (aEIP) extensor digitorum brevis manus (EDBM) extensor medii proprius (EMP) extensor indicis et medii communis (EIMC) Differential Ganglion cysts Synovitis both produce dorsal wrist pain Treatment Nonoperative observation indications first line treatment Operative surgical decompression of 4th dorsal compartment (aEIP) or reduction of muscle belly (EDBM) indications failed conservative treatment, and symptoms, signs and imaging point to anomalous muscle, with no associated conditions (e.g. ganglion)