Olfactory impairment and traumatic brain injury in blast-injured combat troops: a cohort study

Abstract

Objective: To determine whether a structured and quantitative assessment of differential olfactory performance-recognized between a blast-injured traumatic brain injury (TBI) group and a demographically comparable blast-injured control group-can serve as a reliable antecedent marker for preclinical detection of intracranial neurotrauma.

Methods: We prospectively and consecutively enrolled 231 polytrauma inpatients, acutely injured from explosions during combat operations in either Afghanistan or Iraq and requiring immediate stateside evacuation and sequential admission to our tertiary care medical center over a 2½-year period. This study correlates olfactometric scores with both contemporaneous neuroimaging findings as well as the clinical diagnosis of TBI, tabulates population-specific incidence data, and investigates return of olfactory function.

Results: Olfactometric score predicted abnormal neuroimaging significantly better than chance alone (area under the curve = 0.78, 95% confidence interval [CI] 0.70-0.87). Normosmia was present in all troops with mild TBI (i.e., concussion) and all control subjects. Troops with radiographic evidence of frontal lobe injuries were 3 times more likely to have olfactory impairment than troops with injuries to other brain regions (relative risk 3.0, 95% CI 0.98-9.14). Normalization of scores occurred in all anosmic troops available for follow-up testing.

Conclusion: Quantitative identification olfactometry has limited sensitivity but high specificity as a marker for detecting acute structural neuropathology from trauma. 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.

Classification of evidence: This study provides Class III evidence that central olfactory dysfunction identifies patients with TBI who have intracranial radiographic abnormalities with a sensitivity of 35% (95% CI 20.6%-51.7%) and specificity of 100% (95% CI 97.7%-100.0%).

Figure 1. Combat Casualty Care Pathway: Interhospital transfer of inpatients from Afghanistan and Iraq to Walter Reed Army Medical Center

US Air Force Transportation Regulating Command and Control Evacuation System aircrew mission manifests provided longitudinal tracking of all patient movements from the combat zones to our hospital. Twenty Army/Air Force casualties were diverted to NNMC for treatment of penetrating and complex blunt head injuries; 13 penetrating are included in the totals. However, 7 Army/Air Force closed head injuries at NNMC (5 blast mechanisms and 2 blunt mechanisms) are excluded from our dataset. When penetrating intracranial and blast trauma were in conflict, penetrating was always taken in preference to all other mechanisms. Neuroimaging in Afghanistan or Iraq was often performed as a routine component of whole-body trauma scans: head, chest, abdomen, and pelvis. NNMC = National Naval Medical Center; TBI = traumatic brain injury.

Figure 2. Associations among olfactory performance, neuroimaging status, and TBI status (n = 231)

No attempt was made to further differentiate gradations of hyposmia because the delimiters that define such alterations have not been uniformly standardized. Olfactometry scores differed significantly among groups (p < 0.001, Kruskal–Wallis test) with significant differences noted between the group with abnormal neuroimaging (i.e., moderate/severe TBI) and each of the 4 groups, based on post hoc comparisons using a stepwise step-down procedure. No significant differences were found between any of the other groups. TBI = traumatic brain injury.

Figure 3. Troops with abnormal neuroimaging: Location of injury and its association with olfactory performance and multifocality (n = 40)

For specific brain regions, data do not sum to 100% because more than one region could be affected. A clinically relevant neuropathologic finding was defined as any acute intracranial finding due to trauma. Findings were substratified based only on location and multifocality and not by size or nature of the lesion (e.g., edema vs hemorrhage vs contusion). Multifocal was defined as 2 or more regions of the brain affected. Neuroimaging results involved a review of the most proximal, in-theater scans and follow-up stateside radiographic examinations. All studies were read by neuroradiology. Fisher exact test. The silhouette image of the brain was modified from an image generated by dreamstime.com, with permission.