Clinical PD subtypes: tremor-dominant and postural-instability-gait-difficulty (PIGD)

Together with my colleagues at Baylor we first proposed two major subtypes of PD, tremor-dominant and postural-instability-gait-difficulty (PIGD) in 1985 and this observation was confirmed when we analyzed the clinical features of the 800 PD patients enrolled in the DATATOP study (Jankovic et al, 1990). In that study we defined the mean tremor score as the mean of the sum of the baseline tremor (UPDRS part II) and tremor scores (UPDRS part III) for face, right and left hand, right and left foot, and right and left hand action tremor. The mean PIGD score was defined as the sum of an individual's baseline falling, freezing, walking, gait and postural stability UPDRS scores divided by five.

Patients were categorized as having tremor-dominant PD if the ratio of the mean tremor score to the mean PIGD score was 1.5 or higher and as PIGD dominant if the ratio was 1 or lower.  We and others subsequently found that presentation with bradykinesia and the PIGD type of PD seems to be associated with more rapidly progressive course than if PD patients have tremor at the onset of their disease. Using different rating scales Van Rooden et al recently confirmed these initial findings and identified four distinct motor patterns: tremor-dominant, bradykinetic-rigid, and two types of axial patterns: A. rise, gait, postural instability (similar to PIGD) and B. freezing, speech, and swallowing, the latter related to complications of dopaminergic therapy (van Rooden et al., 2009).  The more favorable course of the tremor-dominant form of PD is also supported by various PET scan studies showing slower rate in decline of various dopaminergic indices in patients with the tremor-dominant form of PD (Jankovic, 2005). The tremor-dominant form of PD is associated with a more frequent family history of tremor, raising the possibility of overlap between PD and ET in a subset of cases, a notion that is gaining more and more support (Shahed and Jankovic, 2007). Axial impairment observed in patients with the PIGD subtype, often associated with cognitive impairment, is probably mediated predominantly by nondopaminergic systems. In addition to these subtypes PD population can be separated into young- and late-onset disease.  It is not clear what the subdivision of sporadic PD means and whether the different subtypes have distinct pathogenesis or if they represent a continuum.  With the better understanding of the genetic forms of PD (e.g. LRRK2) and the different clinical and pathological subtypes despite common gene mutations will provide insight into the role of environmental factors in determining the penetrance and the phenotype.