Motor subtypes of PD show different prognostic traits

By Lucy Piper

The severity of postural instability and gait difficulty (PIGD) appears to be a useful indicator of Parkinson's disease (PD) progression, researchers report.

This makes it a useful proxy of advancing disease and not just a specific PD subtype, says the team. This is in contrast to tremor severity, which did not have a significant effect on the severity and progression of nondopaminergic manifestations and motor complications of PD.

"Our findings do not support the use of the [tremor-dominant] subtype as a prognostic trait in PD", the researchers therefore write in Neurology.

They assessed data on 396 patients from the Profiling Parkinson's disease (PROPARK) cohort at baseline and over five follow-up assessments. The motor subtype was calculated for 362 patients at baseline, with the PIGD subtype diagnosed in 150 and the tremor-dominant subtype in 137. The remaining patients had a mixed type.

Patients with the PIGD subtype had significantly worse scores for cognition (Scales for Outcomes in Parkinson's disease-Cognition [SCOPA-COG]), depression (Hospital Anxiety and Depression Scale), autonomic dysfunction (SCOPA-AUT) and psychotic symptoms (SCOPA-Psychiatric complications [SCOPA-PC]) than patients with tremor-dominant PD, as well as more motor complications.

And when the patients were divided into those with severe and mild forms of each subtype, only the PIGD score predicted symptom severity. The severe PIGD group scored worse than the mild PIGD group on all measures, and additionally for excessive daytime sleepiness (SCOPA-PC), and showed more rapid progression of cognitive impairment.

The findings were largely replicated in a second independent cohort (Estudio Longitudinal de Pacientes con Enfermedad de Parkinson [ELEP]) of 365 patients.

One main difference, however, was that both high PIGD and high tremor scores were associated with worse cognition, suggesting that high tremor scores do not protect against the development of cognitive problems, the team, led by Jorine van der Heeden (Leiden University Medical Center, the Netherlands), notes.

This idea has previously been suggested in studies using the ratio of tremor and PIGD scores to classify patients and their risk of switching from a predominant tremor-dominant subtype to a PIGD subtype, they explain.

"However, since the transition to [PIGD-dominant] is basically due to an increase in PIGD severity, conclusions regarding prognostic value of tremor severity should not be based on the ratio method", the researchers say.

"Instead, the effect of tremor and PIGD severity should be evaluated individually."

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