Infarcts are often invisible on an initial high resolution diffusion-weighted imaging (DWI) scan, but a follow-up based on the National Institute of Health Stroke Scale (NIHSS) can limit the number of exams and reduce the false-negative rate on follow-up of acute cerebrovascular events, according to a study published online July 11 in Stroke.
DWI is the clinical gold standard to distinguish stroke and transient ischemic attacks, explained Benjamin Hotter, MD, and colleagues from Charité University Hospital in Berlin. However, there have been significant reports of DWI-negative strokes, in which nearly one quarter featured DWI lesions on follow-up.
“No screening tool is currently available to guide scarce resources toward patients who should receive a follow-up scan to detect initially invisible infarction,” wrote the authors.
To determine the rate of false-negative high-resolution DWI, Hotter and colleagues conducted a prospective analysis of 151 patients undergoing 3T MRI of acute cerebrovascular events within 24 hours of onset. Sixty-three of these patients received follow-up scans the next day.
Of the patients who received a follow-up scan, seven had DWI abnormalities the next day resulting in a false-negative rate of 11.1 percent for the initial scan.
The median NIHSS score for those with negative second-day DWI results was 0, while the median NIHSS score for DWI-positive patients was 2, reported the authors. “Our analysis showed that the NIHSS on the second day was significantly associated with diffusion restriction in follow-up exams with a strong [odds ratio] of 17.5.”
The negative predictive value of follow-up based on NIHSS scores was 0.96, according to the results.
“Using NIHSS to triage for follow-up MRI would have led to a false-negative rate of 3.2 percent, with an overall reduction of examinations of 81.0 percent,” wrote Hotter and colleagues.
The authors noted that a limitation of the study was that it could not distinguish whether infarctions seen on follow up high-resolution DWI were the actual cause of initial symptoms. They also recognized the study’s low follow-up rate, noting a higher rate would allow for increased statistical power.