Smell Test Helps Identify TBI in Blast-Injured Soldiers

Megan Brooks

April 02, 2015

Testing olfactory function in blast-injured soldiers may help diagnose traumatic brain injury (TBI) on the battlefield, a new study suggests. Researchers found that olfactory impairment was highly predictive of abnormal neuroimaging findings in blast-injured troops with TBI.

This study shows that quantitative identification olfactometry has "limited sensitivity but high specificity" as a marker for detecting acute structural neuropathology from trauma, the researchers say.

"When considering whether to order advanced neuroimaging, a functional disturbance with central olfactory impairment should be regarded as an important tool to inform the decision process," they write.

The study was published online March 18 in Neurology.

The study included 231 soldiers acutely injured from explosions during combat operations in Afghanistan or Iraq and immediately evacuated to Walter Reed National Military Medical Center in Bethesda, Maryland, where they were evaluated for TBI and olfactory function using the University of Pennsylvania Smell Identification Test (UPSIT).

The 136 troops with mild TBI (concussion) and the 55 troops without TBI (blast-injured controls) had normal sense of smell. "More important," say the researchers, all troops with normal neuroimaging findings had normal sense of smell.

Central olfactory impairment was observed only in troops with concurrent acute traumatic radiographic abnormalities, they report. Olfactometric score predicted abnormal neuroimaging significantly better than chance alone (area under the curve, 0.78; 95% confidence interval [CI], 0.70 - 0.87; P < .001).

For troops with olfactory impairment, the most common radiographic abnormalities involved injury to the frontal or temporal lobes.

Frontline Test

"Our data are entirely consistent with the historical literature whereby damage to the brain, whether from trauma, stroke, or through neurodegenerative processes (eg, Alzheimer's disease, multiple sclerosis, et cetera) have been demonstrated to significantly impair memory and thereby the ability of the brain to correctly match-up and link common inhalational odorant molecules to past learning and experience," lead author Michael Xydakis, MD, US Air Force Colonel, Uniformed Services University of the Health Sciences, told Medscape Medical News.

"This study is the first to investigate olfactory impairment in combat casualties during the acute and subacute phase of injury," added Dr Xydakis.

He said research is being conducted throughout the military health system on detection, diagnosis, and surveillance of mild TBI and on which blast-exposed troops require immediate neuroimaging. "Despite advances in radiology and the arduous search for confirmatory neuro-diagnostics, the diagnosis of a mild TBI (concussion) remains clinical and is entirely dependent on each individual caregiver's depth of knowledge and breadth of experience in neurotraumatology," Dr Xydakis said.

Olfactory function may serve as a "frontline test" to alert providers to the need for neuroimaging in blast-injured troops, he said. "Currently the indications for neuroimaging remain ill-defined and highly subjective. Since there is no way to know who needs to be scanned, essentially everyone is scanned. We conclude that when considering whether to order advanced neuroimaging, a functional disturbance with central olfactory impairment should be regarded as an important tool to inform the decision process."

Reached for comment, Douglas H. Smith, MD, director, Penn Center for Brain Injury and Repair and professor of neurosurgery, University of Pennsylvania, Philadelphia, told Medscape Medical News that very little is known about blast TBI. One problem, he said, is that in studies of blast injury and TBI, almost all participants have had head contact, so it's hard to differentiate the "blast" TBI from civilian TBI.

"There is a high need for a rapid screening test to diagnosis TBI and there are a lot of emerging olfaction tests. Since olfactory difficulties are so commonly associated with damage to the brain it is kind of a surrogate marker. It could help determine who should be removed from battle or training," Dr Smith said.

"Current olfactory testing is not widely used, but it's certainly well known that this could be a useful tool to identify individuals with subtle brain injury like concussion all the way up to more severe TBI. It can also be used for Parkinson's and other neurodegenerative diseases," Dr Smith added.

The study was funded by the US Department of Defense Combat Casualty Care Medical Research and Development Program. The authors have disclosed no relevant financial relationships.

Neurology. Published online March 18, 2015. Abstract

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