Mid-infrared lasers combat neurologic injuries from IED blasts

Blast-induced traumatic brain injury (BI-TBI) has become one of the leading injuries in soldiers returning from Iraq and Afghanistan. Some 60% of all military injuries seen in these regions may have a TBI component.¹ Reports from attending physicians consistently identify problems with cognitive, cochlear, and vestibular (balance) function as the most prevalent injuries, resulting from close proximity to detonated “improvised explosive devices” (IEDs)—homemade bombs that can vary from something the size of a soda can to an entire moving vehicle.

“We have had TBI in every conflict we’ve ever been involved with, but this is more of an urban-warfare situation, with soldiers in alleyways, on street corners, and in among the local population,” says Capt. Michael Hoffer, M.D., of the Naval Medical Center San Diego (NMCSD; San Diego, CA). “The most damaging thing we see most frequently is IEDs. These particular explosives produce a low-pressure sound wave that can do damage due to the wave property. The physics produce TBI even in someone who is not directly impacted by the device.”²

Accurately assessing and treating these injuries poses a number of challenges, in large part because the injuries are not always obvious and because there is a general lack of understanding about the mechanics behind the neurologic damage caused by IED blasts. Given the relatively large number of troops suffering from IED-related injuries, there is clearly a pressing need for technologies and techniques that can provide better insight into the physiology behind IED-related effects on hearing, balance, and cognition and aid in the development of optimal treatment strategies.

Hoffer and his colleagues at NMCSD—which boasts the most experience with TBIs of any military clinic in the United States--have spent the last several years working to do just that. They are now forming a collaborative effort to evaluate and treat returning troops suffering from BI-TBI that includes Harvard Massachusetts Eye and Ear Infirmary, Northwestern University, Walter Reed, and Vanderbilt University. In late 2007 the team submitted a proposal to the U.S. government’s Congressionally Directed Medical-Research Programs initiative; if the funding goes through, the project should begin to move forward in 2008.

“Because we’ve been through the experience of TBI for six to seven years, the surgeon generals, joint chiefs, and Congress are very interested in being organized on how we approach TBI,” Hoffer says. “The patients look like they have no injuries, but when you start talking to them it is clear that they are not thinking, balancing, or hearing as well as before they went over there. We are on the cutting edge of researching and developing diagnostic tools because we see the highest number of patients returning from the conflict with these injuries.”

Another key member of the BI-TBI collaboration is Aculight (Bothell, WA), which has developed proprietary mid-infrared (1850 nm) diode-laser system designed to deliver optical energy directly to certain nerves in order to stimulate specific reactions. The company has been working with Vanderbilt and Northwestern on related projects that are helping to lay the foundation for the BI-TBI project, with particular emphasis on nerve stimulation and hearing. The two go hand in hand, according to Mark Bendett, director of medical products for Aculight.

“Fundamental work done a few years ago at Vanderbilt demonstrates that, just as an electrical current can stimulate nerves, so can light,” he says. “But light is really good at doing other things too, like ablating tissue. So the trick is to find the wavelengths that are absorbed at the proper depth so that you stimulate the nerve tissue without ablating it.”

Aculight and its collaborators have developed the model for what those wavelengths are, he adds, along with specific lasers to address specific nerves, such as sciatic and cochlear (see figure).

Infrared stimulation in the cochlea can selectively stimulate a population of cochlear neurons (blue line). The results are similar to the selectivity of acoustic stimulation at low sound levels (black line). It is not possible to achieve the same selectivity of stimulation with electrical stimulation.

“The key nugget for optical-nerve stimulation is that it is far more spatially specific than electrical stimulation,” Bendett says. “Where you put the light is where it goes. This is important for things like the cochlear nerve, which is extremely tiny, and also for vestibular system.”

In addition to its work in nerve stimulation—the company has exclusive license to this technology and its applications, originally developed at Vanderbilt—Aculight was awarded a Small Business Innovation Research grant in October 2007 to develop an optical cochlear implant in conjunction with researchers at Northwestern. This work has implications for the BI-TBI project; aside from developing tools to restore hearing and balance the project will also utilize mid-IR nerve stimulation for nerve mapping in animals to provide a better understanding of the physiological effects of IED blasts.

“We’ve used laser technology in medicine and surgery for the last 20 years,” Hoffer says. “But some of the things that Aculight is doing with the mid-infrared will expand the ability to do what we’ve done before.” —KK