Olfactory Testing Informs the Decision Process to Obtain Advanced Neuroimaging in Traumatic Brain Injury

MILITARY MEDICINE, 183, 3/4:321, 2018 Olfactory Testing Informs the Decision Process to Obtain Advanced Neuroimaging in Traumatic Brain Injury LTC Robert Shih, MC, USA*†; Col Michael Xydakis, USAF, MC*† ABSTRACT The purpose of this article is to examine the usefulness of olfactory testing as a tool for the evaluation or stratification of traumatic brain injury (TBI) patients. Olfactory dysfunction is more likely to be overlooked by both the patient and the provider, especially in the acute setting, in contrast to deficits in other senses like vision or hearing. This is a retrospective clinical analysis (case series) of eight active duty service members referred to ear, nose, and throat clinic at Walter Reed National Military Medical Center during a 2-yr period between March 2014 and March 2016 for subspecialist evaluation of suspected olfactory impairment after an exposure to closed head trauma. Advanced neuroimaging revealed evidence of frontal lobe injury in all eight patients, which was subtle and subcentimeter in half of the cases, best demonstrated with high-resolution imaging in the coronal plane. In this article, we discuss the correlation between olfactory dysfunction and brain pathology in both TBI and non-TBI settings. We then provide our recommendation for an orbit magnetic resonance imaging (MRI) to evaluate the inferior frontal lobes and olfactory bulbs in patients with unexplained anosmia. INTRODUCTION Traumatic brain injury (TBI) is a broad and heterogeneous category of neurologic disease, which is defined by a temporary or permanent brain dysfunction as the result of an external force. It can present with a wide array of physical, cognitive, behavioral symptoms and can have a wide range of outcomes from complete recovery to severe disability. Clinical management of TBI patients is dependent on having accurate noninvasive tools that can stratify them into different prognostic or treatment categories. For example, head computed tomography (CT) is a fast and widely available tool, which is effective in the acute setting for detecting neurosurgical emergencies, such as hemorrhage, herniation, and hydrocephalus, as well as bony fractures or metallic shrapnel. In a review of the effectiveness of seven neuroimaging modalities for the detection of TBI, the two most useful tools were brain magnetic resonance imaging (MRI) and transcranial Doppler, within specific clinical settings.1 The purpose of this article is to examine the usefulness of olfactory testing as a tool for the evaluation or stratification of TBI patients. Olfactory dysfunction is more likely to be overlooked by both the patient and the provider, especially in the acute setting, in contrast to deficits in other senses like vision or *Department of Radiology, Walter Reed National Military Medical Center, 8901 Wisconsin Avenue, Bethesda, MD 20889. †Air Force Research Lab, Wright-Patterson Air Force Base, OH 45433. The identification of specific products or scientific instrumentation does not constitute endorsement or implied endorsement on the part of the authors, DoD, or any component agency. The views expressed in this presentation are those of the authors and do not reflect the official policy of the Department of the Army/Navy/Air Force, Department of Defense, or U.S. Government. doi: 10.1093/milmed/usx178 Published by Oxford University Press on behalf of Association of Military Surgeons of the United States 2018. This work is written by (a) US Government employee(s) and is in the public domain in the US. hearing. Nevertheless, it is not an uncommon phenomenon, with estimates of complete loss of smell function (anosmia) in 5–10% and partial loss of smell function (hyposmia) in 30–40% of TBI patients, with the possibility of distorted smells (dysosmia) or phantom smells (phantosmia) in the subacute period.2 Although nasal airway obstruction or shearing of the olfactory nerves at the cribriform plate can theoretically cause a conductive or sensorineural post-traumatic anosmia, most TBI patients show no evidence of sinonasal or anterior skull base fractures on CT; therefore, a more likely mechanism is central injury to the olfactory bulbs, tracts, or cortex of the brain, which can be confirmed with advanced neuroimaging.3 METHODS This is a retrospective clinical analysis of eight active duty service members referred to ear, nose, and throat (ENT) clinic at Walter Reed National Military Medical Center (WRNMMC) during a 2-yr period between March 2014 and March 2016 for evaluation of suspected olfactory impairment after a history of closed head trauma. The service members were evaluated by an otolaryngologist who subspecializes in smell and taste disorders. In addition to a standard history and physical examination, including nasal cavity examination, olfactory testing was performed using a commercially available set of felt-tip pens containing liquid odorants: Sniffin’ Sticks (Burghart Messtechnik, Wedel, Germany). Available neuroimaging was reviewed by the otolaryngologist in conjunction with a neuroradiologist. RESULTS Case 1 is a 45-yr-old active duty sailor who was referred for evaluation of suspected hyposmia 11 mo after a severe closed head injury (motorcycle accident), including facial fractures and subdural hematomas. His sense of taste and smell were previously normal and now significantly impaired with loss of MILITARY MEDICINE, Vol. 183, March/April Supplement 2018 321 Olfactory Testing in Traumatic Brain Injury appetite, for example, “things taste generic to me…a filet mignon tastes the same as a corn dog.” He also reported new phantosmia, that is, phantom smells or olfactory hallucinations. Formal testing revealed normal taste with severe hyposmia: 4/4 taste strips, 6/12 Sniffin’ Sticks, 22/40 University of Pennsylvania Smell Identification Test (UPSIT), and unable to smell natural gas or smoke odorants. His brain/orbit MRI revealed subcentimeter encephalomalacia/gliosis at the inferior surface of the left frontal lobe, consistent with mild sequela from previous contusion (Fig. 1). Case 2 is a 31-yr-old active duty sailor who was referred for evaluation of suspected anosmia, after a severe mountain biking accident 6 yr earlier, during which he was traveling 20 miles/h and was thrown over the handlebars. He presented to an outside hospital with headache, transient confusion, and blood in his left external auditory canal. There is outside documentation of left subdural hematoma, bilateral frontal lobe contusions, and left temporal bone fracture. His taste and appetite are normal; his sense of smell was previously normal. Formal testing revealed anosmia by quantitative olfactometry: 4/4 taste strips, 2/12 Sniffin’ Sticks, and 11/40 UPSIT, but he was able to identify some odorants at suprathreshold concentration (e.g., licorice, coffee, and bubble gum). His brain MRI revealed encephalomalacia or gliosis at the inferior surface of both frontal lobes, chronic sequela of known contusions (Fig. 2). Cas (...truncated)