A 50 yo M w/ hx of HIV and IVDU presents to the ED w/ progressively worsening L UE pain and swelling x 2 weeks. He is febrile w/ a temp of 101.1, HR 110 and BP 96/62. On exam, his L upper arm is diffusely swollen, erythematous, tender and warm to to touch.  A CT of his L UE was obtained and shown below.  What's the diagnosis?  (scroll down for answer)

Answer:  L deltoid muscle abscess

• Official CT read "rim enhancing 5.1 x 2.8 x 7.8 cm fluid collection in the left deltoid muscle"
• Risk factors:  HIV, IVDU, DM, other immunocompromised states
• Differential diagnosis:  intramuscular abscess vs pyomyositis (primary abscess of skeletal muscle) vs cutaneous abscess
• Workup:
• labs including CBC, ESR, CRP, blood cultures (consider sending HIV if status unknown)
• CT with contrast
• XR little utility, but can consider if delay to CT to look for free air/underlying signs of osteomyelitis
• MRI may be necessary depending on location 
• Treatment: 
• surgical incision and drainage + antibiotics
• Vancomycin
• Consider Dalbavancin in certain patient populations (noncompliance, lack of follow up)
• Depending on location, may need IR draiange

References:

• Rayzeh M. Pectoralis major muscle abscess in an immunocompromised adult: Case report and literature review. Int J Surg Case Rep. 2020; 75:166-68. 

• Cronin et al. Treatment of deep intramuscular and musculoskeletal abscess: Experience with 99 CT-guided percutaneous catheter drainage procedures. Am J Roent, 2011. 196:5,1182-1188. 

• Tintinalli, JE. Tintinalli's Emergency Medicine: A Comprehensive Study Guide. 8th ed. New York, NY: McGraw-Hill Education LLC, 2016.