What does a string sign in arterial Doppler indicate?

String Sign in Arterial Doppler: Indication of Severe Arterial Stenosis or Occlusion

A string sign in arterial Doppler indicates a critical stenosis (≥95%) or near-occlusion of an artery with minimal residual flow through a severely narrowed lumen, representing a medical emergency that requires urgent evaluation and intervention to prevent limb loss or tissue damage.

Clinical Significance of String Sign

• A string sign represents a severe narrowing of an arterial lumen, typically to less than 1-2mm in diameter, creating a thin, string-like appearance on imaging with minimal flow 1
• It is most commonly associated with Class IIa or IIb acute limb ischemia (ALI), indicating a salvageable but threatened limb that requires prompt intervention 2
• The presence of a string sign correlates with high-grade stenosis that significantly compromises distal perfusion and threatens tissue viability 1

Doppler Findings in String Sign

• Spectral Doppler typically shows abnormal waveforms with loss of the normal triphasic pattern 3
• Color Doppler demonstrates a narrow, string-like vessel with turbulent or minimal flow 4
• Doppler frequencies in vessels with string sign may be either very low (<6 kHz) or very high (>16 kHz), depending on the hemodynamics of the stenosis 1
• Loss of diastolic flow reversal is an early indicator of significant stenosis that may progress to a string sign 3

Clinical Implications and Management

• When detected in the context of acute limb ischemia, a string sign requires emergency evaluation by a clinician experienced in vascular assessment 2
• The finding indicates a limb that is likely "salvageable/marginally threatened" (Category IIa) or "salvageable/immediately threatened" (Category IIb) according to the Rutherford classification 2
• Revascularization should be performed emergently (within 6 hours) for immediately threatened limbs showing a string sign 2
• Initial management includes systemic anticoagulation with heparin (unless contraindicated) while preparing for definitive treatment 2

Diagnostic Considerations

• While Doppler ultrasound can detect a string sign, it has limitations with sensitivity of approximately 83% for detecting residual lumen in cases of ≥95% stenosis 1
• A "pseudostring sign" can occur when vasa vasorum collaterals reconstitute flow distal to a complete occlusion, potentially leading to misdiagnosis 4
• In carotid arteries, a string sign may be associated with fibromuscular dysplasia presenting as a "string-of-beads" appearance 5
• When a string sign is detected, additional imaging (CTA or MRA) may be warranted, particularly in patients with complex vascular history, though this should not delay urgent treatment in cases of limb-threatening ischemia 2

Treatment Approach

• The revascularization strategy depends on local resources, patient factors, and degree of ischemia 2
• Options include catheter-directed thrombolysis, percutaneous mechanical thrombectomy, or surgical intervention (thromboembolectomy, bypass) 2
• The technique that provides the most rapid restoration of arterial flow with the least risk should be selected 2
• For viable limbs with a string sign, revascularization should be performed urgently (within 6-24 hours); for immediately threatened limbs, emergency intervention (within 6 hours) is required 2

Prognostic Implications

• The presence of a string sign is associated with high risk of progression to complete occlusion if not treated promptly 1
• Even with rapid and effective revascularization, patients with acute limb ischemia and string sign have high 1-year morbidity and mortality rates 2
• Analysis of diastolic waveform patterns distal to the stenosis can provide prognostic information about collateral circulation and distal runoff 3