Background: Ultrasound is the most commonly used imaging modality for assessing carotid artery stenosis. A number of studies have demonstrated that surface irregularities, heterogeneous echotexture and hypoechoic plaques are risk factors for acute ischemic stroke. We performed a systematic review and meta-analysis of the literature to better define the risk of stroke based on the sonographic characteristics of carotid plaques. Materials and Methods: We performed a comprehensive search for studies reporting imaging findings of symptomatic and asymptomatic carotid plaques on ultrasound using MEDLINE and EMBASE. We included both case-control and cohort studies examining the relationship between complex plaque and acute ischemic stroke or transient ischemic attack. Complex plaque was defined as plaque that had any of the following characteristics: heterogeneous echogenicity, echolucency, neovascularization, surface irregularity, ulceration, and intraplaque motion. Meta-analyses using the random-effects model were performed for complex plaque and each of the individual complex plaque characteristics. p < 0.05 was considered statistically significant. We explored the impact of publication bias by constructing funnel plots and testing their symmetry. We conducted the meta-analysis using Comprehensive Meta-analysis version 2.2, Englewood, N.J., USA. Results: A total of 1,013 articles were screened and 23 studies with 6,706 carotid plaques were included. Ultrasound plaque characteristics with a higher prevalence in individuals with symptomatic compared to asymptomatic carotid artery stenosis included plaque neovascularity (OR = 19.68, 95% CI = 3.14-123.16), complex plaque (OR = 5.12, 95% CI = 3.42-7.67), plaque ulceration (OR = 3.58, 95% CI = 1.66-7.71), plaque echolucency (OR = 3.99, 95% CI = 3.06-5.19) and intraplaque motion (OR = 1.57, 95% CI = 1.02-2.41). Variables not associated with symptom status included heterogenous echotexture (OR = 2.68, 95% CI = 0.56-12.80) and surface irregularity without ulceration (OR = 2.38, 95% CI = 0.70-8.11). No evidence of publication bias was observed based on Eggers test (p value of 0.05 for complex plaque and 0.53 for plaque echolucency). The remaining plaque features had insufficient data to assess for publication bias. Conclusions: Our meta-analysis and systematic review of the literature demonstrated that plaques with complex features, particularly those with echolucency, neovascularization, ulceration and intraplaque motion are associated with ischemic symptoms. Assessment of carotid plaque on ultrasound may provide stroke risk information beyond measurement of luminal stenosis. Thus, sonographic evaluation of carotid artery stenosis should focus on the detection of these plaque characteristics in addition to quantifying the degree of stenosis.

Keywords:
Carotid ultrasound, Stroke, Carotid stenosis
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