xRead - Olfactory Disorders (September 2023)

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INTERNATIONAL CONSENSUS ON OLFACTION

TABLE VII.16a (Continued) Study

Year LOE Study design Study groups

Clinical end point

Conclusions

Schriever et al 299

2020 4

Retrospective case series

Patients with hyposmia

Chart review of etiology Two thirds of children with OD have ICA, but it becomes progressively less common into adulthood

Shushan et al 939

2015 4

Case-control

Patients with ICA vs controls fMRI with odor stimulus fMRI activity in patients with ICA suggests

odormaybe subclinically perceived

CCCRC = Connecticut Chemosensory Clinical Research Center; CT = computed tomography; ERP = event-related potential; fMRI = functional magnetic res onance imaging; ICA = isolated congenital anosmia; LOE = level of evidence; MRI = magnetic resonance imaging; OD = olfactory dysfunction; ODT = odor detection threshold; OF = olfactory function; OIT = odor identification test; QOL = quality of life; SDOIT = San Diego Odor Identification Test; SS-ID = Sniffin’ Sticks identification only; SS-TDI = Sniffin’ Sticks threshold, discrimination, identification combination; T&T = Toyoda and Takagi; UPSIT R = University of Pennsylvania Smell Identification Test.

(two of which exclusively had structural lesions in the amygdala on neuroimaging), all patients had resolution of olfactory symptoms after mesial temporal lobectomy. 880 The prevailing view is that these changes explain change in smell and olfactory hallucinations, 881 but another possi bility is that changes in the OB play a role. 883 Patients with temporal lobe epilepsy and a unilateral epileptic focus perform worse on standard measures of olfaction. The impairment is typically bilateral and sur gical treatment such as mesial temporal lobectomy may exacerbate the problem. 884,885 Because of the highly overlapping anatomy between the regions involved in smell and the regions involved in seizure activity, discussing olfaction and performing olfac tory testing may be important in this patient population. OD can be present in patients with epilepsy Aggregate grade of evidence : B (Level 2: one study; Level 3: one study; Level 4: one study). Olfaction can also be linked to headache syndromes on several levels: potent smells provoking headache, fear or sensitivity to smells being a component of headache, and smell being altered in patients with headache syndromes. Our emerging understanding of its pathophysiology sug gests multiple reasons for the olfactory changes that have been described in migraine. Functional changes in the lim bic system, 885 cortical spreading depression in the piriform cortex, 886,887 activation of the amygdala, 888 and the release of calcitonin gene–related peptide by olfactory stimuli 889 are among the factors that may explain this relationship. In one MRI study, patients with migraine and osmophobia had lower OBVs than controls. 890 While most patients with migraine have normal olfaction, 891,892 it may be impaired in a minority of patients, especially in deeply affected patients. 893,894

Osmophobia is the fear, dislike, or aversion to odors. Prior literature has cited osmophobia as being present in patients with migrainous headaches with up to 95% preva lence, and yet it is not mentioned in the International Clas sification of Headache Disorders ( ICHD ). 895 Photosensi tivity/photophobia and phonosensitivity/phonophobia are mentioned and noted as part of the diagnostic criteria, yet osmophobia is not, perhaps because of lower prevalence. Whether it is truly present in such a large proportion of patients with migraine is debated, but osmophobia is a common associated symptom of migraine in patients of all ages, 896,897 with a prevalence of 25% to 86% found in various clinical studies 898,899 A prospective study was per formed in migrainous patients with and without aura, as well as in patients with episodic tension-type headache. A total of 67.2% of migraineurs reported osmophobia in at least a quarter of their attacks, whereas zero patients with episodic tension-type headache reported this as a symp tom, suggesting that osmophobia is a highly specific symp tom that can be used to differentiate migraine without aura and episodic tension-type headache. 900 This hyper sensitivity to odors and even tastes may persist between attacks. 901,902 Olfactory stimuli such as smoke or perfume can precipitate migraine attacks 903 and pleasant odors such as lavender may improve it. 905,906 Osmophobia is most common in patients with migraine, but has also been reported in patients with other headache disorders such as cluster headache. 907 There are some data to suggest that while certain smells are particularly offensive to migraineurs, even when in between attacks, this does not change their baseline olfac tory ability. 893 However, there are also data demonstrat ing that baseline olfactory acuity is more abnormal in migraine patients compared with controls, 908 as well as evidence suggesting that OBV is diminished in patients