When the shock wave hits

Study evaluates how eyes respond

Imagine you are a soldier,  patrolling the streets of a war-ravaged city. Without warning, a suicide bomber sets off an improvised explosive device 50 yards from where you stand. The powerful blast flips you backward into a wall. In seconds you open your eyes. How are you seeing? Will this affect your vision permanently?

Researchers from the joint biomedical engineering program of the Health Science Center and The University of Texas at San Antonio have evidence that eyes subjected to that kind of pressure, even if not punctured, are seriously injured and could suffer partial or complete blindness.

In 2014, the team published a study of blast effects in a postmortem pig eye model. For two years, researchers tested blasts contained inside a tube system at the U.S. Army Institute for Surgical Research, located on Fort Sam Houston in the San Antonio Military Medical Center complex. The U.S. Department of Defense funded the study.

Postmortem pig eyes were exposed to various levels of blast energy and were evaluated with photography, ultrasound and microscopic examinations. Researchers found that an initial shock wave of over-pressurized air from a blast is enough to damage the eyes, even in the absence of shrapnel or other particles. The optic nerve, which serves as the conduit of messages between the eyes and the brain, can also be damaged, leading to numerous visual deficits.

The study is crucial given the dangers soldiers face today. During the Civil War, 1 percent of battlefield injuries involved the eye; this increased to 3 percent through World War II. During the last several combats in the Middle East, 13 percent of injuries have involved the eye. Ocular injuries are now the fourth most common military-deployment injury.

“We think it’s largely due to the use of explosive devices,” said study co-author Randolph Glickman, Ph.D., professor of ophthalmology and radiological sciences in the School of Medicine at the Health Science Center. “They are everywhere, they are messy devices, and they produce lots of eye injuries.”

The problem is not restricted to the battlefield. Each year, 2.4 million eye injuries occur in the U.S. A fifth of Americans will experience one in their lifetime, ranging from a scratched cornea to loss of vision. Ocular trauma is the leading cause of blindness in the U.S.

“Obviously it’s a serious issue in both the military and civilian communities,” said Dr. Glickman, who is cross-appointed at UTSA in physics and biomedical engineering.

Blast injuries can be categorized in four levels: primary, resulting from the pressure wave produced by the blast itself; secondary, due to shrapnel or debris thrown up by the explosion; tertiary, concussive injuries resulting from the whole body being thrown forcefully against a solid object; or quaternary, long-term effects such as post-traumatic stress disorder.

The blast wave hits people so fast that they don’t have time to blink for protection, and even low blast levels can cause eye damage.

The work came out of a pilot study of paintball injuries by geologist Walter Gray, Ph.D., and ophthalmologist Rick Sponsel, M.D., both faculty at UTSA. The researchers found that computer models used by engineers to evaluate how materials break under mechanical stress could also be used to predict eye injuries sustained from different levels of trauma. The machine is so correct, they found, that the type and severity of injury predicted by the computer model matched the actual injuries sustained in lab trials using animal eyes.

Dr. Glickman uses the information to investigate the biological responses to blast trauma.

Before the research, this damage to the eyes was frequently attributed to traumatic brain injuries.

“Internal effects in the eye are something no one has really studied,” said co-author Matthew Reilly, Ph.D., assistant professor of biomedical engineering at UTSA. “Previous studies looked at whether the eye ruptured during the secondary stage, but those research teams didn’t have the sophisticated equipment to look at the inside. In this study, we’ve been able to distinguish between primary and secondary blast effects.”

With the primary blast, the pressure is more widely distributed, Dr. Reilly said.

“The pressure’s not hitting at one point on the eye but across the whole surface,” he said. “The eye isn’t punctured at this step, but the soft parts inside the eye get scrambled. Bad things happen.”

The research team observed retinal detachments, separation of tissue layers, optic nerve damage and injuries to a group of structures that help maintain fluid and pressure balance in the eye.

Those things don’t have to happen, the researchers said. Existing eyewear shields against objects such as shrapnel but doesn’t address blast waves. In the future, they would like to collaborate with Department of Defense specialists to design new types of protective eyewear. Blast modeling would lead to development of prototypes that could be tested to determine which designs best prevent primary blast injuries.

Over time, the project could also help physicians better screen patients for ocular damage and create appropriate treatment plans for the increasing number of soldiers who suffer eye blast injuries while in harm’s way.

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