Archimedes Spiral Looping in Functional and Essential Tremor.

Functional tremor (FT) is the most common functional movement disorder, but diagnosis can be challenging. Archimedes spiral drawings are a useful bedside tool, and looping, also known as the "stretch slinky" sign, has been described as a feature of FT. However, the prevalence of looping in FT and its occurrence in essential tremor (ET) are unclear. This retrospective study examined Archimedes spirals from 22 FT and 28 ET patients. Looping (≥1) was observed in 45.5% of FT spirals and 60.7% of ET spirals, with no significant difference in loop counts between groups (Mann-Whitney U test). A loop-count threshold of ≥7 had the highest positive predictive value for FT (PPV 0.75) and high specificity (0.96), but poor sensitivity (0.14). Receiver operating characteristic analysis yielded an AUC of 0.46. Looping in Archimedes spirals occurs in both FT and ET, and loop counts alone show poor discriminative ability. A loop count ≥7, although insensitive, is highly specific for FT and may provide supportive evidence in the full clinical context.