The Glass scale: a simple tool to determine severity in essential tremor.

Background: The method for measuring disease severity in essential tremor (ET) is not consistent among neurologists in routine clinical practice.

Methods: We have developed a new scale, called Glass scale, which is easy and quick to administer to ET patients with upper limb involvement. Using the scale involves asking the patient one question: "Over the last week, when you were sitting down at the table, how did you drink water from a glass?" Scores: I - I have no difficulties. II - I can drink with one hand, but I have to fill the glass with less liquid to avoid spills. III - I cannot drink with one hand, I need both hands. IV - I cannot drink with my hands, I need a straw. The score is followed by "A" if tremor involves only the upper limbs, and "B" if not. Construct validity of the Glass scale was tested against the Tremor Clinical Rating Scale (TCRS) and the Bain disability scale. A second neurologist blinded to the Glass scale score assessed inter-rater reliability.

Results: The Glass scale displayed strong construct validity compared to TCRS (w. kappa = 0.907) and to the Bain scale (w. kappa = 0.868). High inter-rater validity was also observed (w. kappa = 0.937).

Conclusion: The Glass scale appears to be a reliable and valid tool to determine tremor severity in ET. The simplicity of the scale makes it appropriate for use in routine clinical practice.