Classifying tremor: Language matters

Tremor was the first movement disorder ever described and is mentioned in medical literature, with variable terminology, since Hippocrates’ writings. The current term derives from the Proto-IndoEuropean root *ter-, meaning to shake and to fear, that was retained with its double meaning in Sanskrit and in ancient Greek after undergoing a syllabic metathesis: =lfr (trasat), sqE lx (tr em o, to tremble or to shake), sqόlo1 (tr omos, shaking caused by great fear). The two meanings became for the first time distinct in Latin, as the original *terroot was retained to mean having fear (terreo; see English: terror), whereas the *tremetathesis referred specifically to the involuntary movement of tremor. This language evolution also accompanied medical interest: Galen, who practiced under the Roman empire, dedicated a detailed monograph to tremor that he considered a phenomenological entity distinguished from any other hyperkinetic condition, specifically palpitations and spasms (considered early descriptions of dystonia). Notwithstanding such a long history of detailed observations and distinctions, the classification of tremors remains a challenging task. The IPMDS task force on tremor recently proposed a new consensus where tremors are classified along two axes: clinical characteristics and etiology. At first sight, this new classification recalls the recent classification of dystonia that is also organized in two axes with identical names. However, there are significant differences. First, Axis I in the tremor classification includes laboratory tests, whereas in the classification of dystonia Axis I only includes physical examination and medical history. The difference is understandable, since tremor is the clinical expression of an oscillation, whose characteristics (e.g. frequency, amplitude, etc.) can be measured in a neurophysiology or movement disorders lab. Other laboratory findings, such as structural imaging, are also listed under Axis I. By contrast, in the dystonia classification, nervous system pathology (including imaging) has a dedicated listing under Axis II (etiology) and other lab tests are not mentioned. The dystonia classification had to accommodate the terminological heritage of “primary dystonia”, a term used until recently as a combined phenomenological and etiological descriptor for “genetic or idiopathic cases in which dystonia is isolated and there is no consistent pathologic change”. Therefore, the dystonia task force felt it was important to retain an etiological entry listing pathological changes under Axis II. To this respect, the dystonia classification is backward compatible: former “primary dystonia” is now called “idiopathic isolated” or “inherited isolated”, depending on the genetic status. A second difference between the two classifications is that, for tremor, common syndromic aggregations were outlined as part of the classification exercise. By contrast, the dystonia classification did not list dystonia syndromes, that were the object of an independent publication. A reason for such difference is the historical heritage of “essential tremor”, a syndrome with variable definitions over time that needed systematization. In that respect, the new classification of tremors is backward compatible, accommodates and updates earlier descriptions of essential tremor, and recognizes two distinct syndromes: “essential tremor” and “essential tremor plus”. Experts on dystonia, who succeeded to get rid of the “plus” descriptor used in earlier dystonia classifications, may not be happy to read this. Notwithstanding, there are substantial advances in the new definitions of essential tremor, the most common syndrome of isolated tremor, and the exclusion criteria are also very useful to highlight what is not essential tremor. These innovations pave the way for the common language needed to make advances in knowledge, the main purpose of any classification. The interrelation of tremor and dystonia, observed since the time of Galen, still remains a fascinating clinical and pathophysiological relationship. A subtle red line further connects the dystonia and tremor classifications: what is the minimal requirement for making a diagnosis of dystonia? In other words, are any of the -------------------------------------------------------------There are no relevant financial disclosures related to this letter.