We read with interest the article by Pohl and colleagues 1 reporting on the beneficial effect of cooling on the writing performance of patients with writer's cramp. We had earlier reported 2 improvement of hand function (including handwriting) after limb cooling in patients with essential tremor (ET). As in the study by Pohl et al., our ET patients showed no improvement with limb warming. In contrast, those with Parkinson's disease (PD) did not exhibit significant change with either limb cooling or warming. Pohl and coworkers reported a significant improvement in writing time in writer's cramp patients after cooling on comparing cold to warm; however, they did not report the difference between room temperature and cold (which is not likely to have been significant). They then concluded that cooling can potentially provide temporary relief of writer's cramp. In real life, cooling can be a useful physical measure if it can potentially improve a movement disorder compared to baseline, ambient temperature. Thus, picking up a statistical difference between cooling and warming may not signify true improvement.
Of the 10 individuals with writer's cramp studied by Pohl and colleagues, 3 felt there was no change in their handwriting and 5 reported only mild improvement; 80% had little or no subjective change. Interestingly, the 2 patients with the greatest improvement in writing (rated moderate) had a wrist tremor. This suggests that limb cooling may have primarily influenced the tremor rather than dystonia in the patients they studied, particularly so that those with pure dystonia (80%) only had minor subjective improvement (the magnitude of which can easily be a placebo effect) or no improvement at all. In contrast, patients with ET or physiologic tremor may have dramatic or marked improvement of their tremor with limb cooling. 2,3 A study comparing two sets of writer's cramp patients-with or without tremor-may help determine the differential influence of cooling on dystonia and tremor. Given the small number of patients studied by Pohl and coworkers, a subanalysis of these two sets of patients may not be feasible.
Based on Pohl's study and ours, it appears that normal controls, PD patients, and individuals with writer's cramp but no tremor are not significantly benefited by limb cooling. This may be due to the increased muscle stiffness and prolonged relaxation time from lowered muscle temperature. 4,5 Although patients with PD present with a resting tremor, this tremor often disappears with action (e.g., handwriting). Thus, the lack of improvement or worsening of writing in PD from limb cooling may stem from increased rigidity or bradykinesia.
Lakie and colleagues reported that cooling can improve physiologic tremor for as much as 150 minutes. 3 Similarly, we have observed that ET patients may benefit from limb cooling for 2 hours or longer, despite the rather quick return of surface skin temperature to baseline. In Pohl's study, the crossover from cooling to warming or vice versa was only 60 minutes. We question whether this "wash-out" between periods of limb immersion is sufficient to preclude a carry-over effect from one temperature to the other.