Influence of L-dopa on subtle motor signs in heterozygous Parkin- and PINK1 mutation carriers

Introduction: A latent nigrostriatal deficit and its possible clinical consequences in asymptomatic heterozygous Parkin and PINK1 mutation carriers (AMC) have been a matter of investigation in recent years. Notably, mild Parkinsonian signs in heterozygous mutation carriers can be so subtle that they may be missed if not specifically investigated.

Methods: We studied 15 heterozygous Parkin and PINK1 AMC and 18 age- and sex-matched mutation-negative controls using a standardized video, instructing the probands to perform relevant parts of the UPDRS III to investigate fine motor movements at baseline and after first-time L-Dopa administration. Additionally, available UPDRS III scores of mutation carriers from the past ten years were reviewed.

Results: AMC showed a reduced number of fine motor movements per second compared to controls at baseline (p = 0.04). L-Dopa improved motor performance numerically but non-significantly in AMC (p = 0.2301), but significantly in healthy controls (p = 6.1·10-5). Although none of the AMC reported symptoms, nine showed rigidity, bradykinesia, tremor, and postural instability when the UPDRS III was applied. Mean UPDRSIII scores significantly decreased after L-Dopa administration (p = 0.005), but did not increase over the past ten years.

Conclusions: (i) Heterozygous AMC show subtle motor abnormalities when a detailed, specialized motor examination is applied and compared to mutation-negative matched control subjects. (ii) The mild motor deficit present in a subgroup of heterozygous Parkin and PINK1 AMC appears to be non-progressive and responsive to L-dopa administration. (iii) Evaluating motor changes, their progression, and treatment response in AMC can provide valuable insights into possible early disease stages and compensatory mechanisms.