Olfactory response is a potential sign of consciousness: electroencephalogram findings

Wanchun Wu; Chengwei Xu; Qimei Liang; Xiaochun Zheng; Qiuyi Xiao; Haili Zhong; Na Chen; Yue Lan; Xiyan Huang; Qiuyou Xie

doi:10.3389/fnins.2023.1187471

Olfactory response is a potential sign of consciousness: electroencephalogram findings

Front Neurosci. 2023 May 18:17:1187471. doi: 10.3389/fnins.2023.1187471. eCollection 2023.

Authors

Wanchun Wu¹, Chengwei Xu¹, Qimei Liang¹, Xiaochun Zheng¹, Qiuyi Xiao¹, Haili Zhong¹, Na Chen¹, Yue Lan¹, Xiyan Huang¹, Qiuyou Xie^{1

2}

Affiliations

¹ Joint Research Centre for Disorders of Consciousness, Department of Rehabilitation Medicine, Zhujiang Hospital, Southern Medical University, Guangzhou, Guangdong, China.
² Department of Hyperbaric Oxygen, Zhujiang Hospital, Southern Medical University, Guangzhou, Guangdong, China.

Abstract

Objective: This study aimed to explore whether olfactory response can be a sign of consciousness and represent higher cognitive processing in patients with disorders of consciousness (DoC) using clinical and electroencephalogram data.

Methods: Twenty-eight patients with DoC [13 vegetative states (VS)/unresponsive wakefulness syndrome (UWS) and 15 minimally conscious states (MCS)] were divided into two groups: the presence of olfactory response (ORES) group and the absence of olfactory response (N-ORES) group according to behavioral signs from different odors, i.e., vanillin, decanoic acid, and blank stimuli. We recorded an olfactory task-related electroencephalogram (EEG) and analyzed the relative power and functional connectivity at the whole-brain level in patients with DoC and healthy controls (HCs). After three months, the outcomes of DoC patients were followed up using the coma recovery scale-revised (CRS-R).

Results: A significant relationship was found between olfactory responses and the level of consciousness (χ²(1) = 6.892, p = 0.020). For olfactory EEG, N-ORES patients showed higher theta functional connectivity than ORES patients after stimulation with vanillin (p = 0.029; p = 0.027). Patients with N-ORES showed lower alpha and beta relative powers than HCs at the group level (p = 0.019; p = 0.033). After three months, 62.5% (10/16) of the ORES patients recovered consciousness compared to 16.7% (2/12) in the N-ORES group. The presence of olfactory response was significantly associated with an improvement in consciousness (χ²(1) = 5.882, p = 0.023).

Conclusion: Olfactory responses should be considered signs of consciousness. The differences in olfactory processing between DoC patients with and without olfactory responses may be a way to explore the neural correlates of olfactory consciousness in these patients. The olfactory response may help in the assessment of consciousness and may contribute to therapeutic orientation.

Keywords: diagnosis; disorders of consciousness; electroencephalogram; olfactory response; prognosis.