Carotid Near-Occlusion ('String Sign'): Differential Diagnosis with 99% Stenosis and Occlusion in Duplex Scanning
Definition and Terminology
Subtotal (pre-occlusive) stenosis is referred to as the 'string sign' or 'string symptom'. According to the guidelines by B.V. Blagodir (2026), this condition is characterized by minimal residual flow — a thin, slow stream with very low velocity. International terminology uses the term carotid near-occlusion (Johansson, Fox, 2016; Johansson, Gu, Fox, 2022 — cited in ESVS 2023).
Three Conditions to Differentiate
| Sign | Severe Stenosis (99%) | Near-Occlusion ('String') | Occlusion |
|---|---|---|---|
| Signal in the Lumen | Present, locally increased velocities | Thin slow stream with very low velocity | Absent with adequate settings |
| PSV | Locally sharply increased proportional to narrowing | Very low ('low velocity') | Not determined |
| Additional Findings | Post-stenotic turbulence, tardus-parvus distal | Requires power/microvascular Doppler, low scale | Stump/thump flow proximally, collaterals |
Why Near-Occlusion is Mistaken for Occlusion
In pronounced near-occlusion, the residual flow is so scant that with standard settings, the color signal is not registered, creating a false picture of occlusion. Rule 3 (Blagodir, 2026): 'low velocity' with visible severe plaque is not a reason to write 'normal' or 'occlusion'; it may be near-occlusion. Confirm residual flow with sensitive modes and rely on a combination of signs.
Practical Techniques for Differentiation
To detect minimal flow, use power/microvascular Doppler and a low scale. In true occlusion, there is no color or spectral signal in the lumen with adequate settings; proximally, a stump/thump flow — a short high-resistance signal at the entrance to the closed segment — may be registered; collaterals are often involved. The degree of narrowing cannot be determined by grayscale imaging, and anechoic plaques may not be visualized.
Limitations of Velocity Criteria
SRU/IAC thresholds are derived for short focal stenoses and fail with different geometry. Extensive and tandem stenoses are a special case for velocity criteria; this is one of the recognized pitfalls in carotid duplex interpretation (Gunabushanam et al., 2023). In near-occlusion, velocity criteria are particularly unreliable because the collapse of the lumen distal to the narrowing reduces velocity despite the degree of stenosis.
Significance of ECA and Collaterals
The external carotid artery is always described (IAC/SRU standards). In ICA or CCA occlusion, the ECA becomes the key collateral to the brain through the ophthalmic anastomosis. For reliable identification of the ICA and ECA, the tapping test on the superficial temporal artery is used: rhythmic oscillations ('saw teeth') appear in the ECA, while the ICA does not respond to tapping.
Frequently asked questions
How to distinguish near-occlusion from complete occlusion?
In occlusion, there is no color or spectral signal in the lumen with adequate settings. In near-occlusion, a thin slow stream with very low velocity is preserved, detectable by power/microvascular Doppler and a low scale.
Can PSV be relied upon in near-occlusion?
No. In near-occlusion, the velocity is 'low', which contradicts what is expected in severe stenosis. Rely on a combination of signs rather than a single PSV; suspect near-occlusion with visible severe plaque and low velocity.
What sign confirms true occlusion proximal to the closed segment?
Stump/thump flow — a short high-resistance signal at the entrance to the closed segment; collaterals are often also involved.
Why are standard velocity thresholds unreliable in near-occlusion?
SRU/IAC thresholds are derived for short focal stenoses. In extensive, tandem stenoses and collapse of the distal lumen, they fail — a recognized pitfall in interpretation (Gunabushanam et al., 2023).
What modes are needed to detect residual flow?
Power/microvascular Doppler and a low scale — they allow registration of the minimal stream that standard color mapping settings miss.