[Study on the clinical characteristics of isolated congenital anosmia]

Objective: To retrospectively analysely the electrophysiological and imaging features of isolated congenital anosmia (ICA) and to assess the clinical phenotypic characteristics and classification of ICA. Methods: Clinical data of 30 ICA patients in Beijing Anzhen Hospital from 2012 to 2019 was retrospectively reviewed, including 13 males and 17 females, aged (35±19) years. The control group consisted of 30 healthy people from medical examination center, including 13 males and 17 females, aged (39±14) years. The clinical characteristics of ICA were analyzed using Sniffin' Sticks test, olfactory event-related potentials (oERPs), trigeminal event-related potentials (tERP) and olfactory pathway MRI. SPSS 17.0 software was used to compare the difference of olfactory function between the two groups. The correlation between olfactory bulb, olfactory sulcus structure and age was observed, and the clinical phenotype characteristics of ICA patients were analyzed. Results: The subjective olfactory function was completely lost in ICA patients. oERP was absent in all of the ICA patients, but showed normal N₁ and P₂ waves in controls. tERP could be evoked in 63.3% (19/30) of ICA patients, and signals in these patients showed higher amplitude in the N₁ ((-10.33±6.93) μV vs (-5.11±2.71) μV, t=-10.113, P<0.01) and P₂ ((+17.25±8.51) μV vs (+7.31±3.46) μV, t=5.443, P<0.01) waves than that of the controls. Olfactory bulbs were aplastic in 80.0% (24/30) of patients and hypoplastic in 20.0% (6/30) of patients. Fifty-six point seven percent (17/30) of patients had bilateral olfactory sulcus deletion while 43.3% (13/30) had dysplasia, and all of the patients exhibited a depth of olfactory sulcus less than 8 mm. Both the structure of olfactory bulbs and olfactory sulcus were not associated with age for ICA patients (r value was -0.174 and 0.325, respectively, all P>0.05). Conclusions: ICA patients show neurophysiologic deficits and some anatomic differences compared with healthy controls. The absence of oERP combining with a depth of olfactory sulcus less than 8 mm is the important indicator for clinical diagnosis of ICA. The structure of olfactory bulb may be a critical factor for clinical classification of ICA.