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One in 14 older surgical patients had covert stroke that was detected by MRI soon after surgery, and these strokes were associated with an increased risk of cognitive decline a year later, the prospective study found.

In people ages 65 and older who had elective, non-cardiac surgery, 7% experienced a perioperative covert ("silent") stroke, reported P.J. Devereaux, MD, PhD, of Hamilton Health Sciences and McMaster University in Hamilton, Ontario, and colleagues.

And 1 year later, cognitive decline -- defined as loss of 2 or more points on the Montreal Cognitive Assessment () -- was more likely to occur in surgical patients who had a perioperative stroke compared with those who didn't (adjusted OR 1.98, 95% CI 1.22-3.20, absolute risk increase 13%; P=0.0055), they wrote in .

"We've found that 'silent' covert strokes are actually more common than overt strokes in people aged 65 or older who have surgery," Devereaux said in a statement.

"Over the last century, surgery has greatly improved the health and the quality of life of patients around the world," added co-author Marko Mrkobrada, MD, of the University of Western Ontario in London, Ontario. "Surgeons are now able to operate on older and sicker patients thanks to improvements in surgical and anesthetic techniques. Despite the benefits of surgery, we also need to understand the risks."

Covert strokes are brain infarcts that lack clinically overt stroke-like symptoms, but are visible on MRI. In non-surgical settings, covert stroke is more common than overt stroke and is with dementia, cognitive decline, psychomotor speed decline, and an increased risk of subsequent overt stroke.

A previous of 100 older adults found "a surprising 10.0% (95% CI 5.5-17.4) incidence of perioperative covert stroke," noted George Mashour, MD, PhD, of the University of Michigan Medical School in Ann Arbor, and colleagues in an accompanying .

"In reproducing this finding in a large, multicenter, international study, NeuroVISION's foundational contribution is to confirm, with precision by neuroimaging criteria, that there is indeed a very high incidence (7%) of perioperative covert stroke in patients aged 65 years and older presenting for non-cardiac surgery," they wrote. "In fact, this incidence is an order of magnitude higher than the incidence of overt stroke in patients at moderate risk undergoing non-cardiac surgery."

NeuroVISION involved 1,114 patients (ages ≥65) from 12 centers in North and South America, Asia, New Zealand, and Europe, who had elective, non-cardiac surgery from 2014 to 2017. Patients received diffusion-weighted MRI within 9 days of surgery to look for evidence of covert stroke and were followed for 1 year to assess cognitive capabilities.

The researchers excluded patients who underwent carotid artery or intracranial surgery, or who were diagnosed by a physician with an acute overt stroke after their index surgery but before the study MRI. The primary outcome was cognitive decline, defined as a decrease of 2 points or more on MoCA.

In total, 78 people (7%; 95% CI 6%-9%) had a perioperative covert stroke. Cognitive decline 1 year after surgery occurred in 29 of 69 people (42%) who had a perioperative covert stroke and in 274 of 932 people (29%) who did not have covert stroke (adjusted OR 1.98).

Covert stroke also was linked to an increased risk of perioperative delirium (HR 2.24, 95% CI 1.06-4.73, absolute risk increase 6%, P=0.030) and overt stroke or transient ischemic attack at 1-year follow-up (HR 4.13, 1.14-14.99, absolute risk increase 3%, P=0.019).

While MoCA is a widely used screening tool for cognitive impairment, "it was not designed for diagnostic use after stroke, and regarding reliability and validity in this setting," Mashour and colleagues pointed out.

"On the basis of the original strategy described by , a MoCA score decrease of 2 or more is associated with a positive predictive value of 20% with respect to the clinically relevant decline based on neuropsychological testing," they wrote. "Thus, false positives were likely to be present, which weakens the conclusion that perioperative covert stroke carries a cognitive cost."

The lack of a non-surgical control group "also precludes more definitive conclusions regarding cognitive trajectory," the editorialists continued. "This aspect of the study design is particularly relevant to the primary outcome because the low preoperative MoCA scores in many patients suggest that they were probably already declining cognitively before their surgery."

Because MRI can show acute stroke for up to 10 days, some strokes seen in this study may have occurred before, not after, surgery, the researchers added. And while the primary analysis was adjusted for known confounders of cognitive decline, residual confounding also may be a factor.

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