Restoration of olfaction: is there light at the end of the tunnel?

Abstract

Anosmia is said to affect an estimated 5% of the population, but with the exception of chronic rhinosinusitis (CRS) therapeutic options are limited. As with the symptoms of CRS, when anosmia develops slowly and progressively, it appears to create significantly less patient distress than with sudden onset. However, it still creates a substantial quality-of-life (QOL) issue, with loss of the primary component of food enjoyment, reduced environmental interaction, and potentially even significant life-threatening dangers, such as the loss of ability to smell gas, spoiled food, or smoke. Although otolaryngologists have been able to quantify olfactory loss and diagnose causes, there are very limited therapeutic options when it is not associated with sinonasal inflammation or skull-base pathology. Although olfactory training has shown significant benefit in hyposmia, the effects are more marked in terms of identification and discrimination of odors than they are in olfactory threshold.1 In terms of future therapeutic options, 2 potential approaches have emerged. Goldstein et al.2 have been studying the potential for olfactory regeneration with embryonic olfactory basal cells in mice, and the signaling mechanisms involved in neuronal differentiation. As a potential alternative approach, Holbrook et al.3 publish the first report of smell induced through transethmoid electrical stimulation of the olfactory bulb. In this proof-ofconcept pilot study, the authors show that it was possible to induce a sense of smell in 3 of 5 subjects. Although for this pilot study the authors utilized volunteers with an intact sense of smell, it opens the door for future exciting research and for a potential future therapeutic option. The benefits of topical and systemic steroids in olfactory loss secondary to sinonasal inflammation are well-established. Yan et al.4 report on an evidence-based literature review evaluating the role of either topical or systemic steroids in olfactory dysfunction unrelated to sinonasal disease. Their review shows no benefit from topical nasal steroid sprays, level 3B evidence for the use of oral steroids, and level 1B evidence for the use of topical steroid nasal irrigations, the latter based upon 1 recent randomized controlled study.