RESUMO
A horseshoe abscess is caused by infection that spreads between the flexor tendon sheath of the thumb or little finger through the radial and ulnar bursae through communication between the two and/or the space of Parona. We present a case of an 80-year-old woman with rheumatoid arthritis who presented with 6 months of right hand and wrist soft tissue swelling, initially treated as a rheumatoid arthritis flare. MRI demonstrates the horseshoe abscess and after surgical irrigation and debridement with synovectomy, cultures demonstrated infection with mycobacterium avium intracellulare (MAI). This case demonstrates the importance of MRI in diagnosing and evaluating the extent of hand infections and for considering mycobacterial organisms for appropriate treatment and antibiotic regimen.