Orthonasal, but not Retronasal Olfaction Is Specifically Impaired in Parkinson’s Disease

Abstract

Olfactory dysfunction (OD) in Parkinson’s disease (PD) appears several years before the presence of motor disturbance. Olfactory testing has the potential to serve as a tool for early detection of PD, but OD is not specific to PD as it affects up to 20% of the general population. Olfaction includes an orthonasal and a retronasal components; in some forms of OD, retronasal olfactory function is preserved. We aimed to evaluate whether combined testing components allows for discriminating between PD-related OD and non-Parkinsonian OD (NPOD). The objective of this study is to orthonasal and retronasal olfactory function in PD patients and compare them to a NPOD group and to healthy controls. We hypothesized that this combined testing allows to distinguish PD patients from both other groups. We included 32 PD patients, 25 NPOD patients, and 15 healthy controls. Both olfactory components were impaired in PD and NPOD patients, compared with controls; however, NPOD patients had significantly better orthonasal scores than PD patients. Furthermore, the ratio of retronasal/orthonasal score was higher in PD than in both other groups. In the NPOD group, orthonasal and retronasal scores were significantly correlated; no such correlation could be observed in PD patients. In summary, PD patients seem to rely on compensatory mechanisms for flavor perception. Combined orthonasal and retronasal olfactory testing may contribute to differentiate PD patients from patients with NPOD.

Olfactory bulb surroundings can help to distinguish Parkinson’s disease from non-parkinsonian olfactory dysfunction

“…Our results are in line with the notion of olfactory bulb volume to be a neuroanatomical correlate of olfactory function ( Buschhuter et al, 2008 , Hummel et al, 2015 ), independently of any underlying condition that may affect its volume. Interestingly, a recent study however suggests that overall chemosensory function such as flavor perception may be one method to behaviorally distinguish between PD and NPOD patients ( Aubry-Lafontaine et al, 2020 ).…”

Clinical Implications of Psychophysical Olfactory Testing: Assessment, Diagnosis, and Treatment Outcome

“…It has been suggested that an ideally olfactory battery to detect subclinical smell loss in PD should include olfactory identification and odor threshold. Interestingly, orthonasal and retronasal scores were not significantly correlated in PD patients and it was orthonasal not retronasal olfaction that was specifically impaired in PD patients (Ciofalo et al, 2006;Aubry-Lafontaine et al, 2020). This evidence indicates that the retronasal olfactory system was relatively unimpaired and the decrease in flavor perception is less pronounced in PD patients than what one would expect from the degree of orthonasal olfactory impairment.…”

Opportunities and Pitfalls of REM Sleep Behavior Disorder and Olfactory Dysfunction as Early Markers in Parkinson’s Disease

“…One possibility may be retro-nasal testing, i.e., the perception of olfactory stimuli in the oral cavity perceived via the nasopharynx. This main contributor to flavor perception appears to be distinctly affected in PD [73]. Another method is to combine olfactory testing with other chemosensory tests.…”

“…In fact, olfactory testing enables the identification of iRBD patients at short-term risk of developing DLB or PD [66]. However, one cannot yet distinguish the phenoconversion trajectory with smell tests alone, even if, after phenoconversion, DLB patients and PD patients seem to be distinguishable using olfactory and trigeminal threshold test [73,74]. The combined study of the olfactory and trigeminal systems appears to be a promising avenue, albeit this remains speculative.…”