For every hour an acute ischemic stroke is untreated, the human forebrain loses 120 million neurons, 830 billion synapses, and 447 miles of myelinated fiber–the equivalent of 3.6 years of normal aging.
The numbers lend a new urgency to the phrase “time is brain,” said Dr. Jeffrey Saver, who created the quantitative analysis of neural death in stroke (Stroke 2006;37:263–6).
“The figures stagger and motivate,” wrote Dr. Saver of the University of California, Los Angeles Stroke Center. “For patients experiencing acute ischemic stroke, and for the physicians [and others] treating them, every second counts.”
Dr. Saver used an extensive literature review to estimate total counts for neurons, synapses, and myelinated fiber length in the forebrain.
He then used published data to estimate the impact of the duration of infarct growth, infarct volume, and time to treatment on these structures.
He concluded that the forebrain contains an average of 22 billion neurons, 157 trillion synapses, and 84,500 miles of myelinated fibers. The accepted estimate for the rate of neuronal loss in the neocortex alone (which is 86% of the forebrain) is about 31 million per year during normal aging, he said.
The impact of an untreated large vessel ischemic stroke is sudden and severe. Every untreated minute results in the loss of 2 million neurons, 14 billion synapses, and 7.5 miles of myelinated fibers, Dr. Saver said. On a per-second basis, the damage is 32,000 neurons, 230 million synapses, and 218 yards of myelinated fiber. Thus, such a stroke, at an average duration of 10 hours, can age the brain 36 years, destroying 1.2 billion neurons, 8.3 trillion synapses, and 4,470 miles of myelinated fibers.
In an accompanying editorial, Dr. Steven R. Levine suggested that transient ischemic attacks might actually be microstrokes that inflict permanent–although clinically undetectable–neural damage.
“Even after a few seconds of the focal cerebral ischemic process, tens of thousands of neurons and hundreds of millions of synapses are lost–if the time function is linear,” said Dr. Levine of Mount Sinai School of Medicine, New York.
Dr. Levine suggested that the unanswered clinical question then becomes: “Are these very brief episodes then really 'microstrokes' invisible to the neurological examination or current imaging modalities, with very subtle, if any, detectable parenchymal or functional change?” (Stroke 2006;37:10).