The diagnosis of adult-onset primary dystonia (AOPD) is mainly clinical, but requires the exclusion of any secondary causes. AOPD is characterised by sustained involuntary muscle contraction resulting in one or more body parts moved away from their neutral position. No other neurological sign is observed, apart from tremor. The 1998 Movement Disorder Society Consensus on Tremor states indeed that tremor and dystonia can be associated, defining the tremor affecting a dystonic body part as ‘dystonic tremor’, and the tremor affecting a non-dystonic body part as ‘tremor associated with dystonia’.1 However, the literature on dystonic tremor is poor and little is known regarding its prevalence and phenomenology in AOPD patients. Recently, an important paper assessing this issue has been published.2 Defazio et al assessed 429 AOPD patients, of whom 72 (16.7%) showed either postural or postural/action tremor. Surprisingly, none of their patients had apparently rest tremor. However, this is not our experience.

We have indeed identified rest tremor in a number of our patients with cervical and other form of adult onset dystonia (see online supplementary video on three such patients). We have also previously reported that a number of patients with unilateral rest tremor and preserved dopaminergic nigrostriatal pathway (ie, patients supposed to be affected with Parkinson's disease, but having scans without evidence of dopaminergic deficit (SWEDDs)), have dystonic tremor or tremor associated with dystonia, as per the consensus classification3 and also by electrophysiology.4

As we have previously suggested,5 on the other hand dystonia can be unrecognised and patients with rest tremor along with postural/action tremor, misdiagnosed as Essential Tremor, Parkinson's disease or SWEDDs. This may depend on the fact that dystonic features can be mild and involve body parts not affected with tremor, or even that have not yet appeared. It is otherwise conceivable that Defazio et al might have missed patients with rest tremor due to lack of study power, rest tremor in AOPD being much less frequent than postural/action tremor.

Whether rest tremor is a non-existent phenomenon in AOPD needs to be debated: further studies specifically assessing rest tremor in a larger population of AOPD are therefore warranted to settle this question.