Working/Final Diagnosis: Complicated Skin and Soft-Tissue Infection with Possible Deep Extension
The most likely diagnosis is a complicated skin and soft-tissue infection arising from a ruptured epidermal inclusion cyst that has progressed to a deep abscess, potentially with necrotizing fasciitis or osteomyelitis given the systemic symptoms, diabetes, and radiating pain pattern. 1, 2
Clinical Reasoning
Why This is NOT a Simple Infected Cyst
The 2-year history of a cyst that suddenly progressed to an abscess with systemic malaise and radiating pain indicates rupture of the cyst wall with deep extension beyond superficial tissues. 1, 3
Inflammation in epidermal inclusion cysts occurs as a reaction to cyst wall rupture and extrusion of keratinous contents into the dermis, not simple bacterial infection, but when this progresses to systemic symptoms, it represents complicated infection requiring aggressive management. 1, 4
The presence of body malaise (systemic toxicity) in a diabetic patient with a longstanding cyst that has transformed into an abscess mandates immediate evaluation for deep soft-tissue infection, as diabetes significantly increases risk of necrotizing infections. 5
Critical Red Flags Present in This Case
Pain radiating from anterior trunk to posterior shoulder suggests deep fascial or muscle involvement rather than superficial abscess—this is pain disproportionate to superficial findings, which is a hallmark of necrotizing soft-tissue infection. 5
Systemic malaise indicates the patient meets criteria for systemic inflammatory response and requires hospitalization consideration, blood cultures, complete blood count, creatinine, bicarbonate, creatine phosphokinase, and C-reactive protein levels. 5
Diabetes is a major risk factor for progression to severe soft-tissue infections including necrotizing fasciitis, and diabetic patients may not mount typical inflammatory responses, making clinical assessment more challenging. 5
Immediate Diagnostic Workup Required
Draw blood immediately for culture, complete blood count with differential, creatinine, bicarbonate, creatine phosphokinase (looking for elevation 2-3 times upper limit of normal), and C-reactive protein (concerning if >13 mg/L). 5
Check vital signs urgently: fever or hypothermia, tachycardia (>100 beats/min), and hypotension (<90 mm Hg systolic) all indicate need for hospitalization. 5
Obtain imaging with CT or MRI to detect deep soft-tissue abscesses, sinus tracts, fascial involvement, or bone involvement—plain radiographs alone are insufficient for soft tissue but can detect gas in tissue. 5
Emergent surgical consultation is mandatory if any of the following are present: pain disproportionate to findings, violaceous bullae, cutaneous hemorrhage, skin sloughing, skin anesthesia, rapid progression, or gas in tissue. 5
Management Algorithm
Immediate incision and drainage with surgical exploration is the cornerstone treatment—antibiotics alone will fail without source control. 1, 2, 5
During surgical exploration, obtain tissue specimens (not swab specimens) for Gram stain, aerobic and anaerobic culture before initiating antibiotics, as tissue samples provide more accurate results than superficial swabs. 5, 1
If deep fascial involvement is found during exploration, aggressive debridement of all necrotic tissue is required, potentially requiring multiple operations—this is necrotizing fasciitis until proven otherwise. 5
Empiric broad-spectrum antibiotics covering MRSA, streptococci, and gram-negative organisms should be initiated immediately after cultures are obtained: vancomycin 15-20 mg/kg IV every 8-12 hours PLUS piperacillin-tazobactam 3.375 g IV every 6 hours (or a carbapenem). 5, 2
For diabetic patients with soft-tissue infections, consider hospitalization if hypotension, elevated creatinine, low bicarbonate, elevated creatine phosphokinase, marked left shift on CBC, or C-reactive protein >13 mg/L are present. 5
Common Pitfalls to Avoid
Never assume this is a simple abscess requiring only outpatient drainage—the combination of diabetes, systemic symptoms, and radiating pain demands inpatient evaluation for deep infection. 5, 1
Do not delay surgical consultation while waiting for imaging results if clinical suspicion for necrotizing infection is high—surgical exploration is both diagnostic and therapeutic. 5
Avoid prescribing oral antibiotics and discharging the patient—this clinical presentation requires IV antibiotics and close monitoring for progression. 5, 2
Do not rely on absence of fever to rule out serious infection, as diabetic patients may not mount typical febrile responses. 5
Recognize that ruptured epidermal inclusion cysts can mimic malignancy or other sinister pathologies on imaging and clinical examination, requiring tissue diagnosis. 3, 4