Original contribution
Evaluation of vertebral artery hypoplasia and asymmetry by color-coded duplex ultrasonography

https://doi.org/10.1016/j.ultrasmedbio.2004.03.004Get rights and content

Abstract

Distinguishing stenosis and hypoplasia of the extracranial vertebral artery (VA) is clinically important, but there is no agreement in defining VA hypoplasia. We endeavored to attain reference values for VA flow volume by color Doppler ultrasonography (CDU), analyze age and gender effects on VA flow volume and develop a definition of VA hypoplasia. CDU was performed in 447 subjects free of cerebrovascular diseases or carotid stenosis. The VA diameter, peak systolic velocity, end-diastolic velocity, time-averaged mean velocity, resistance index (RI) and flow volume were recorded bilaterally. We found significant asymmetries in diameter, flow velocities and flow volume with left-sided dominance. Diameters were different on left (0.297 ± 0.052 cm) and right (0.323 ± 0.057) sides (p < 0.001). Flow volume was different on right (83.0 ± 36.9 mL/min) and left (96.6 ± 42.4) sides (p < 0.001). Women had significantly smaller diameters, higher flow velocities and lower RIs than men. VA flow volume did not change with aging. We defined hypoplasia as a significant decrease in flow velocities and increase in RI for VA diameters < 0.22 cm. This definition is supported by findings of an increase in ipsilateral flow resistance (RI ≥ 0.75), contralateral diameter (side-to-side diameter difference ≥ 0.12 cm), and flow volume (side-to-side flow volume ratio ≥ 5). This definition and these reference values may lead to better differentiation between congenital variation and steno-occlusion clinically. (E-mail: [email protected])

Introduction

Variations in normal anatomy in the extracranial vertebral artery (VA) are relatively frequent, ranging from asymmetry of both VAs to significant hypoplasia of one VA. It has been recognized that the left VA diameter is often larger than the right VA diameter. In a post mortem study, the left VA was larger in 42%, the right VA larger in 32% and the VAs approximately equal in 26% (Krayenbuhl and Yasargil 1957). In an angiographic study, the left VA was wider in 61%, the right VA wider in 33% and both VAs equal in only 6% (Kazui et al. 1989). In duplex sonographic studies, the average VA diameter was more often larger on the left side than on the right side (Bartels et al., 1992, Yuan et al., 1994, Schöning et al., 1994, Seidel et al., 1999). Thus, the blood flow volume assessed by duplex sonography demonstrated higher blood flow volume in the left VA than in the right VA (Schöning et al., 1994, Seidel et al., 1999).

Up to now, there has been no agreement concerning the definition of VA hypoplasia, although several have been proposed by some authors (Bartels, 1999, Steinke, 2001). In a pathoanatomical study, VA hypoplasia was defined as a decrease in lumen diameter below 2 mm (Fisher et al. 1965). This definition of VA hypoplasia was confirmed by an ultrasonographic study that revealed a decrease in blood flow velocity in VAs with diameters less than 2 mm, based on a hypoplasia rate of 1.9% (Delcke and Diener 1992). A definition of VA hypoplasia as a lumen diameter below 3 mm was proposed after another ultrasonographic study of 50 healthy subjects, with the hypoplasia rate being 6% (Touboul et al. 1986). Bartels suggested that a VA is hypoplastic if the diameter is smaller than 3 mm and the Doppler spectrum shows a high resistive pattern (Bartels 1999).

To differentiate between hypoplasia and stenosis of the VA is important clinically. There is no requirement to do more expensive or aggressive studies, such as magnetic resonance imaging (MRI) or conventional angiography in the differential diagnosis of VA stenosis if hypoplasia of the VA is documented ultrasonographically. In addition, the differentiation between hypoplasia and stenosis of the VA can guide decision-making regarding choice of intervention in symptomatic VA stenosis.

The aim of this study was threefold: 1. To characterize the distribution of normal VA hemodynamics by duplex color Doppler ultrasonography (CDU); 2. to analyze the influence of age, gender, and side on hemodynamics; and 3. to propose a definition of VA hypoplasia.

Section snippets

Materials and methods

A total of 447 subjects (231 men, 216 women), ages 54.4 ± 12.0 years (ranging from 16 to 87 years) enrolled in our study during the year 2002. Subjects were either patients with nonspecific neurologic complaints, such as headache, neck pain, dizziness, syncope or memory impairment, or they were subjects receiving physical check-ups. Patients who had a history or signs of cerebrovascular disease or carotid stenosis greater than 15%, or VA total occlusion, were excluded. CDU was performed with a

Results

In all 447 subjects, the V2 segment of the VA was well-visualized on both sides sonographically. The results of mean age and sonographic parameters of the VA for both sides in men and women are shown in Table 1. Compared with men, women had a significantly smaller diameter of the VA, higher flow velocities and lower RI. However, the right or left or net VA flow volume did not differ significantly between men and women.

The correlations of age with these sonographic parameters of the VA are

Discussion

Congenital variations in the size and symmetry of the posterior circulation are well recognized. Recently, hypoplasia of the VA and the basilar artery has been investigated for possible increased risk of posterior circulation ischemia (Chaturvedi et al. 1999) and migraine with aura (Lovrencic-Huzjan et al. 1998). In addition, it is important in clinical practice to differentiate between hypoplasia and steno-occlusion of the VA. Ultrasonography of the extracranial VA is useful, owing to its

Acknowledgements

This study was supported in part by the research grant from the National Science Council, Taiwan (NSC-92-2314-B-002-243). The authors thank Bao-Show Hwang and Win-Hwen Lin for their technical assistance in ultrasonography.

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