Olfactory dysfunction in Parkinson's disease: Positive effect of cigarette smoking.

BACKGROUND: There is compelling evidence from over 60 epidemiological studies that smoking significantly reduces the risk of Parkinson's disease (PD). In general, those who currently smoke cigarettes, as well as those with a past history of such smoking, have a reduced risk of PD compared to those who have never smoked. Recently it has been suggested that a cardinal nonmotor sensory symptom of PD, olfactory dysfunction, may be less severe in PD patients who smoke than in PD patients who do not, in contrast to the negative effect of smoking on olfaction described in the general population.
METHODS: We evaluated University of Pennsylvania Smell Identification Test (UPSIT) scores from 323 PD patients and 323 controls closely matched individually on age, sex, and smoking history (never, past, or current).
RESULTS: Patients exhibited much lower UPSIT scores than did the controls (P < 0.0001). The relative decline in dysfunction of the current PD smokers was less than that of the never- and past-PD smokers (respective Ps = 0.0005 and 0.0019). Female PD patients outperformed their male counterparts by a larger margin than did the female controls (3.66 vs. 1.07 UPSIT points; respective Ps < 0.0001 and 0.06). Age-related declines in UPSIT scores were generally present (P < 0.0001). No association between the olfactory measure and smoking dose, as indexed by pack-years, was evident.
CONCLUSIONS: PD patients who currently smoke do not exhibit the smoking-related decline in olfaction observed in non-PD control subjects who currently smoke. The physiological basis of this phenomenon is yet to be defined.