Olfactory evoked responses and identification tests in neurological disease

To assess the value of smell testing we used olfactory evoked potentials (OEP) and an identification test in multiple sclerosis, Parkinson's disease, motor neuron disease and Alzheimer's disease.

The OEP to H2S (20 ppm) was obtained using an olfactometer designed to stimulate olfactory nerve endings only. Odor recognition was assessed by the University of Pennsylvania Smell Identification Test (UPSIT). In all instances the disease was 'definite' based on standard diagnostic criteria. Controls were derived from 156 healthy people.

1) Multiple Sclerosis: 11/72 patients (15%) were abnormal on UPSIT. For OEP there was significant increase of latency and decrease in amplitude in 6/26 patients (23%). 2) Parkinson's Disease: 126/155 (81%) patients had an abnormal UPSIT score. 12/37 (32%) had prolonged latency with normal amplitude measurement on OEP, but 27 had absent or unclear readings. 4/10 with normal UPSIT displayed abnormality on OEP. 3) Motor Neuron Disease: 9/58 (16%) were abnormal on UPSIT. There was significant delay in 1/10 (10%) patients on OEP. 4) Alzheimer's Disease: UPSIT scores were abnormal in all 8 patients examined. OEP was normal in 4 of these who could be tested.

Smell dysfunction was found in all 4 conditions but most severely in Parkinson's Disease (over 80%). The UPSIT in general showed abnormality more frequently than OEP. The olfactory defect probably involves peripheral structures in all diseases tested except Alzheimer's. A patient with normal olfaction is unlikely to have idiopathic Parkinson's disease.