Distinguishing early-onset PD from dopa-responsive dystonia with transcranial sonography

Dystonia is frequent in early-onset parkinsonism (EOP) and sometimes its presenting sign.^1,2 Conversely, parkinsonian signs occur in dopa-responsive dystonia (DRD), especially in later stages, and may even be the only finding in relatives of patients with clinically typical DRD.³ Both conditions may have a similar age at onset and respond well to treatment with levodopa, and patients may report sleep benefit and diurnal variation of symptoms in both conditions. Due to this phenotypic overlap, categorization of the early-onset dystonia–parkinsonism syndromes may pose a diagnostic challenge.

Dominantly, inherited mutations in the GTP cyclohydrolase I (GCHI) gene can be found in over 80% of the clinical typical cases of DRD but only when conventional screening methods and expensive gene dosage analyses are combined.⁴ The most common known cause of EOP is recessively transmitted mutations in the parkin gene, the detection of which also requires comprehensive screening for small mutations and exon rearrangements. Given these technical considerations and, more importantly, because of the different prognosis of the two conditions, a fast, inexpensive marker or tool would be helpful in establishing a diagnosis of neurodegenerative EOP vs nondegenerative DRD.

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