Differences between orthonasal and retronasal olfactory functions in patients with loss of the sense of smell

Objective: To investigate differences between orthonasal and retronasal olfaction in patients with loss of the sense of smell without taste complaints. Design: Electrophysiological and psychophysical testing of orthonasal and retronasal olfactory functions. Setting: Outpatient clinics. Patients: A series of 18 patients who had olfactory loss due to various reasons but no "taste" complaints. Main Outcome Measures: Orthonasal and retronasal olfactory functions assessed by olfactory event-related potentials and psychophysical smell tests. Results: Psychophysical testing revealed retronasal olfaction to be normal or slightly altered, whereas orthonasal olfaction was either absent or severely compromised. Findings from nasal endoscopic examinations and computed tomographic scans were within the reference range in all subjects. In response to orthonasal stimulation there were neither detectable olfactory event-related potentials nor any with small amplitudes, whereas olfactory event-related potentials in response to retronasal stimulation were clearly present in some patients. Conclusion: These clinical observations, together with the psychophysical and electrophysiological findings, suggest that orthonasal and retronasal olfaction might be processed differently.