Frontotemporal dementia (FTD) is a progressive neurodegenerative disease. Diagnosis of FTD, especially the behavioural variant, is challenging because of symptomatic overlap with psychiatric disorders (depression, schizophrenia, bipolar disorder). Olfactory dysfunction is common in both FTD and psychiatric disorders, and often appears years before symptom onset. This systematic review analysed 74 studies on olfactory function in FTD, depression, schizophrenia and bipolar disorder to identify differences in olfactory dysfunction profiles, focusing on the most common smell measures: odour identification and discrimination. Results revealed that FTD patients were severely impaired in odour identification but not discrimination; in contrast, patients diagnosed with schizophrenia showed impairments in both measures, while those diagnosed with depression showed no olfactory impairments. Findings in bipolar disorder were mixed. Therefore, testing odour identification and discrimination differentiates FTD from depression and schizophrenia, but not from bipolar disorder. Given the high prevalence of odour identification impairments in FTD, and that smell dysfunction predicts neurodegeneration in other diseases, olfactory testing seems a promising avenue towards improving diagnosis between FTD and psychiatric disorders.

额颞痴呆（FTD）是一种进行性神经退行性疾病。由于症状与精神疾病（抑郁症，精神分裂症，双相情感障碍）重叠，FTD的诊断尤其是行为变异的诊断具有挑战性。嗅觉功能障碍在FTD和精神疾病中都很常见，并且通常在症状发作前数年出现。这份系统的综述分析了有关FTD，抑郁症，精神分裂症和双相情感障碍的嗅觉功能的74项研究，以识别嗅觉功能障碍特征的差异，重点是最常见的气味测量：气味识别和辨别。结果显示，FTD患者的气味识别受到严重损害，但没有歧视。相比之下，被诊断为精神分裂症的患者在两种措施中均显示出损害，而那些被诊断出患有抑郁症的人则没有嗅觉障碍。躁郁症的发现参差不齐。因此，测试气味识别和辨别力可以将FTD与抑郁症和精神分裂症区分开，但不能与双相情感障碍区分开。鉴于FTD中气味识别障碍的患病率很高，并且气味功能障碍预示了其他疾病的神经退行性变，嗅觉测试似乎是改善FTD与精神疾病之间诊断的有前途的途径。