Being able to sense a change in pressure on the brain after a traumatic brain injury would allow for swifter treatment that could stave off debilitating or fatal complications.
By combining electrical impedance sensing with a conventional pressure sensor, Dartmouth College engineer Ryan Halter is creating an early warning system that may help doctors treat traumatic brain injuries before they permanently damage the brain.
Some 1 million people suffer traumatic brain injuries annually. In the most severe cases, internal bleeding increases pressure on the brain, which chokes off blood circulation and can be debilitating or fatal.
Before turning to brain injuries, Halter, an assistant professor at Dartmouth College's Thayer School of Engineering, had spent years developing electrical impedance sensors to image cancer noninvasively. This changed when a graduate student's father, a neurosurgeon, asked if impedance sensing could measure changes in blood flow. Halter realized that measuring the impedance of blood in the brain might alert clinicians to pressure increases inside the skull earlier than existing sensors.
"I would hope that our technology, or other imaging technologies, can better determine intracranial health after an injury," Halter said.
He has a patent and is developing the technology through his startup company, RyTek Medical. "Our long-term goal is to better monitor mild and moderate traumatic brain injuries," he said. "Initially, however, we are going to target patients with severe traumatic brain injuries."
Under Pressure
In healthy adults, intracranial pressure never rises above 20 millimeters of mercury. If it does, doctors must intervene. Since patients with severe traumatic brain injuries are prone to internal bleeding, clinicians must monitor pressure inside their skull.
Current procedures call for neurosurgeons to drill a small hole into the skull and insert an intercranial pressure sensor directly into the brain tissue. If the brain starts to leak blood, it swells and causes pressure inside the skull to rise. The sensor alerts physicians, who try to reduce the swelling with either medication or a decompressive craniectomy, removing a section of the skull so the brain has room to expand, Halter said.
Pressure inside the skull can spike very quickly. The brain contains a reserve space that resembles an empty balloon. Just as water enters an empty balloon without increasing pressure, blood seeps into the empty space without any resistance. But once the balloon—or brain—has filled that space, the addition of any more fluid boosts pressure immediately.
It can happen at any time. Physicians typically scan patients with severe traumatic brain injuries using computed tomography (CT) to image the brain. Yet injuries often evolve after the scan, worsening or developing new hemorrhages that then go undetected.
When intercranial pressure rises above 20 mmHg, patients have much poorer outcomes. If it reaches 40 mmHg, they usually suffer permanent neurological damage.
Halter's invention seeks to monitor how blood fills the space around the brain before it fills up and pressure spikes. In animal trials, he does this by using scalp electrodes and a conventional intracranial pressure sensor with an electrode attached to it to map changes in impedance linked to the amount of blood in the cranium.
Electrodes
Halter places eight electrodes, similar to those used for electrocardiograms, around the skull at 12 o'clock, 3 o'clock, 6 o'clock, and 9 o'clock and the four points halfway between them. He then injects small, imperceptible currents between two electrodes at a time.
He then measures the voltage between the two electrodes. This is directly related to the impedance of the brain tissue and blood between them, Halter says. This enables him to determine whether blood is leaking into the cranium earlier than using a pressure monitor alone. Impedance sensors also indicate where the buildup is occurring.
Halter's system also takes advantage of the conventional intracranial pressure sensor placed within the brain. He couples the sensor's catheter to a very thin electrode that is similar to FDA-approved electrodes for deep brain stimulation and monitoring.
The combined sensor monitors both pressure and impedance within the brain. This is more difficult than it appears, since, in a healthy brain, small cyclic changes in intercranial pressure and voltage occur during each heartbeat. "Ordinarily, when blood is pumped into the [healthy] brain during each heartbeat, you get a small change in fluid volume in the brain," Halter said. In this case, the correlation between pressure and voltage is low, and there is less chance of an injury.
"When you have a large injury, small changes in volume lead to very large changes in pressure," he says. This increases the correlation between pressure and voltage, indicating a likely problem. Together, the brain and scalp electrodes provide a system for continuous monitoring before bleeding causes additional damage.
In a pilot animal study, Halter's invention detected intracranial changes and collection of blood on the brain. Halter's lab recently received a grant from the National Institutes of Health for further research. The lab plans to run animal trials between the fall of 2017 and March 2018.
The Phase 1 trial seeks to validate the technology and show feasibility in an animal model, in this case, a pig. Since the device is similar to today's standard of care, "we don't think it will be too challenging to get this into a human trial," Halter said.
"We're excited to get the next six months of data and see where things stand at that point," he said.
Theresa Sullivan Barger is an independent technical writer.