Olfactory function may inform Parkinson’s prognosis

Olfactory function is preserved in about a quarter of patients with Parkinson’s disease (PD), and may be predictive of disease course, say researchers.

Olfactory testing has been used as a screening tool to identify patients at high risk of PD for study enrolment. However, Young Sohn (Yonsei University College of Medicine, Seoul, Korea) and colleagues report that 25.5% of a cohort of 208 patients with clinically diagnosed PD had normal olfactory function on the Cross-Cultural Smell Identification Test (CCSIT; ≥9 points).

A further 28.4% of the patients were borderline hyposmic (7–8 points) and 46.1% were hyposmic (≤6 points).

Patients with normosmia were significantly younger than those with hyposmia, at 62.4 versus 68.7 years, and had lower motor scores on the Unified Parkinson’s Disease Rating Scale (UPDRS), at 18.7 versus 24.9. And the difference in UPDRS scores persisted after accounting for age, gender, symptom duration and dopamine transporter (DAT) activity in the posterior putamen.

Patients with normal olfaction also had higher ratios of DAT activity on positron emission tomography, relative to hyposmic patients, between the anterior caudate and the posterior putamen, and between the ventral and posterior putamen.

This implies “relative sparing of DAT reduction in the striatal subregions other than the posterior putamen”, write the researchers in Neurology.

In line with this, the levodopa equivalent dose after an average follow-up of more than 2 years tended to be lower in normosmic than hyposmic patients, at 425 versus 495 mg, although this was not statistically significant (p=0.055).

Patients with borderline hyposmia had DAT activity and follow-up medication needs that tended to be midway between those of patients with hyposmia and normomsia.

In a linked editorial, Simone Rossi and Monica Ulivelli, both from the University of Siena in Italy, say that the study “represents an important step toward better understanding” of the role of olfactory function in PD.

But they note that patients were not systematically screened for alternative causes of hyposmia, and that they were not assessed in an off-medication state, raising the possibility that normosmia may identify patients with a better treatment response, rather than a more benign disease course.

They conclude that “the real clinical importance of olfactory dysfunction in PD remains open, especially concerning the predictive value of normosmia or hyposmia toward disease evolution in a single patient.”

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