Heavily calcified carotid plaque reduces stroke risk

The calcium ratio in carotid plaque, determined by multislice CT, may help risk-stratify patients with asymptomatic stenosis to less aggressive therapy, according to a study by researchers at the University of Virginia Health System.

Risk assessments for stroke have traditionally focused on the degree to which arterial plaque has constricted blood flow, said study team member Dr. Christopher Kramer, a professor of radiology and medicine at Virginia.

"You might say we've been worried about the size of the river channel. This study and others like it indicate we also need to pay attention to the river bank and to the kind of silt that's accumulating there," he said.

CT has been used to study this plaque calcification as a marker of plaque stability in ex vivo and animal studies. Both unstable and stable plaques, however, often contain varying amounts of calcium, and an acceptable in vivo method to define plaques based on this component remains to be developed.

The researchers hypothesized that asymptomatic plaques could be differentiated from symptomatic plaques using MSCT volume measurements of the content of calcified and noncalcified plaque.

Led by Dr. Kiran R. Nandalur, investigators retrospectively reviewed CT angiograms and identified 102 patients with 50% stenosis or more. The study group included 35 symptomatic patients (17 stroke, 13 transient ischemic attack, five amaurosis fugax) and 67 asymptomatic patients (Stroke 2007;38:935-940).

Patient with large calcified plaque involving internal carotid artery. A: Multiplanar reconstructed CT image shows large degree of calcified plaque (arrow) with narrowed lumen (line). B: Multiplanar reconstructed CT image used for volume measurements demonstrates cross-section of internal carotid artery with corresponding lumen (asterisk) and calcified plaque (arrow). (Provided by C. Kramer)

In the asymptomatic group, 62 patients were referred for evaluation of stenosis after ultrasound as an alternative to digital subtraction angiography. These referrals followed 45 physical examination findings, 10 precardiac or general surgery clearances, and seven routine screenings. The remaining five were discovered during precoronary artery bypass grafting workup for comorbidities using CT.

Hypertension was significantly greater in the symptomatic than asymptomatic patients, and age was significantly greater in the asymptomatic patients.

The researchers found an inverse relationship between the proportion of calcium in a plaque and occurrence symptoms, even with a model controlling for age, sex, percent stenosis, and cardiovascular risk factors.

Of clinical significance, the study showed a great deal of overlap between stable and unstable plaques between 0% and 45% calcification of the total plaque volume, but in a distinct subset of patients, heavily calcified plaques (>45% calcified) demonstrated a strong predilection toward stability, as defined by a lack of associated symptoms.

Moreover, the researchers found no significant relationship among the absolute volumes of total plaque, calcified plaque, and noncalcified plaque with symptoms, suggesting the ratio of contents may play a more important role than the amount of plaque contents.

In current clinical practice, physicians look at the percent blockage in the carotid artery to identify candidates for plaque clearing surgery to prevent stroke. The new findings suggest that calcification percentages should also be considered, Kramer said.

The study has potential implications for the management of asymptomatic carotid stenosis. Given the serious risks of surgical and endovascular intervention, a noninvasive method of determining which patients are at decreased risk and potentially amenable to medical therapy is critical. The degree of calcification within the plaque causing stenosis may be an important marker for patients who could potentially be treated less aggressively.

The researchers concluded that prospective studies of patients who have not yet developed symptoms are needed to confirm the diagnostic value of calcified plaque measurements.

For more from the Diagnostic Imaging archives:

Desmoteplase clot buster disappoints in trial

Study favors MRI over CT for acute stroke diagnosis

Prototype 256-slice CT proves its might in stroke imaging

Stroke imaging data lag behind technology