Differential Diagnosis: Steroid Striae vs. Superficial Venous Thrombophlebitis
These purplish linear abdominal and flank lesions that transiently improved with azithromycin are most likely superficial venous thrombophlebitis (SVT), not steroid striae, and require immediate duplex ultrasound to confirm the diagnosis and exclude concurrent deep vein thrombosis.
Clinical Reasoning
The key distinguishing feature here is the response to azithromycin followed by recurrence—this pattern is incompatible with striae but consistent with an inflammatory/thrombotic process that temporarily improved (possibly due to anti-inflammatory effects or coincidental timing) then recurred when the underlying pathophysiology persisted 1.
Why This Is Likely SVT, Not Striae:
- Striae distensae (stretch marks) from corticosteroids are permanent structural changes in dermal collagen that do not "clear up" with antibiotics or any medication 2
- SVT characteristically presents with pain, erythema, and tenderness along superficial veins, which can appear as linear purplish lesions on the abdomen or flanks 3, 4
- The temporal relationship with azithromycin is a red herring—macrolide antibiotics have mild anti-inflammatory properties that may have temporarily reduced local inflammation, but this does not address the underlying thrombosis 5
- Prednisone for secondary adrenal insufficiency creates a hypercoagulable state, increasing SVT risk through multiple mechanisms including increased coagulation factors and endothelial dysfunction 4, 6
Immediate Diagnostic Workup
Obtain venous duplex ultrasound of the abdomen and flanks immediately to:
- Confirm SVT diagnosis and measure exact thrombus length
- Assess proximity to deep venous system (inferior vena cava, iliac veins)
- Exclude concurrent deep vein thrombosis, which occurs in approximately 25% of SVT cases 1, 4
Baseline laboratory studies before any anticoagulation:
- Complete blood count with platelet count
- PT/aPTT
- Liver and kidney function tests 1
Management Algorithm Based on Ultrasound Findings
If SVT ≥5 cm in Length:
Initiate fondaparinux 2.5 mg subcutaneously once daily for 45 days (first-line), which reduces progression to DVT from 1.3% to 0.2% and recurrent SVT from 1.6% to 0.3% 1, 4
Alternative if parenteral therapy is not feasible: Rivaroxaban 10 mg orally once daily for 45 days 1, 4
If SVT Is Within 3 cm of Deep Veins:
Escalate immediately to therapeutic-dose anticoagulation for at least 3 months, treating as DVT-equivalent due to high risk of proximal extension 1, 4
If SVT <5 cm:
Repeat ultrasound in 7-10 days; initiate anticoagulation only if progression is documented 1
Adjunctive Symptomatic Management
Regardless of anticoagulation decision:
- Warm compresses to affected areas 1, 5
- NSAIDs for pain control (avoid if platelets <20,000-50,000/mcL) 1
- Graduated compression if tolerated 1, 7
- Early ambulation rather than bed rest to reduce DVT risk 1
Critical Pitfalls to Avoid
- Do not dismiss this as "just striae" without imaging—approximately 10% of SVT patients develop thromboembolic complications within 3 months despite treatment 1, 6
- Do not treat with antibiotics—the azithromycin response was coincidental or due to anti-inflammatory effects, not antimicrobial action 3, 5
- Do not use treatment duration shorter than 45 days if anticoagulation is indicated 1, 7
- Failing to perform ultrasound risks missing concurrent DVT in 25% of cases 1, 4
Why Striae Are Unlikely
Corticosteroid-induced striae:
- Develop gradually over weeks to months of exposure
- Are permanent structural changes that fade from purple to white but never resolve
- Do not respond to any medication
- Do not recur after "clearing" 2
The clinical course described—clearing with antibiotics then recurring—is fundamentally incompatible with the pathophysiology of striae distensae.