Smell Loss and Onset of Phantosmia are Critical Aspects of Head Injury Cognition

Head injury is a common event occurring in 1–2 million people in the U.S. yearly. Head injuries may cause minor inconvenience or it may induce major cognitive changes such as memory loss, concentration difficulties, headache, hearing loss, and visual difficulties, associated with or without loss of consciousness. Two of the major events occurring after head injuries which are not commonly recognized but which significantly alter behavior are loss of smell acuity and subsequent onset of smell distortions or phantosmia. Most patients are unaware that these two events can occur but they are present in most patients who experience head injury of any severity. Initially, patients recognize that they cannot recognize any odors and subsequently recognize that they cannot obtain any flavor from any food or drink. Over time they recognize that they experience a peculiar, usually unpleasant, odor in their nose in absence of any external odor. These odors may be persistent or transient, usually unpleasant, and may be described as fecal‐like, burned, chemical‐like, or metallic. The phantom, if persistent, can be severe enough to initiate loss of appetite with subsequent weight loss and depression. We have evaluated over 200 patients with smell loss and phantosmia after a head injury at The Taste and Smell Clinic in Washington, DC over the past 40 years attempting to understand their cognitive changes in smell function. We have found that the mechanism for these cognitive changes relate to metabolic changes mainly in biochemical changes in both sensory and brain function in which specific growth factors which maintain olfactory function have been diminished and specific brain neurotransmitters, mainly gamma‐aminobutyric acid, have also been diminished. Correction of these abnormalities commonly corrects the smell loss, inhibits the phantosmia and improves cognition.