The use of electricity in medical treatment has always been technology-driven, rather than aetiology-driven; as new techniques have appeared, clinicians have quickly looked to try them in the treatment of all sorts of conditions where existing treatment options are limited. Functional disorders--as identified anachronistically in our analysis--have been key contenders for emerging electrical treatments: with Leyden jars, with galvanic and electromagnetic machines, and more recently with TMS and TENS. Parallels can be drawn with the history of electrical treatments for migraine and headache (Koehler and Boes, 2010). Regardless of the mode of delivery of electricity, stimulating a limb to produce movement has repeatedly been found to aid and assist recovery in functional motor disorders. This may also be true of non-electrical methods: we have found benefits using both therapeutic sedation and explanatory demonstration of a positive Hoover's sign as therapeutic methods of demonstrating normal movement in functionally weak limbs (Stone et al., 2014). Each surge in enthusiasm for new electrical treatments has been followed by questions about the nature of the disorder and validity of the treatment response. Physicians have tended to attribute therapeutic success initially to powerful biological or even metaphysical effects, but with time and experience these explanations have been replaced by views that the treatment works through suggestion and placebo. Discomfort with these conclusions has in the past discouraged ongoing development of electrical treatments, even if the end result for patients has been encouraging. In Edwards's Bayesian model, functional motor and sensory symptoms are hypothesized to arise when 'pathologically precise prior beliefs' mediated by attentional processes cause experience of symptoms via a hierarchy of false inferences (Edwards, 2012). It can be argued that use of TMS or peripheral stimulation to produce movement of a functionally weak limb has the specific potential to modulate pathological expectations. To reject these treatments as no more than placebo may mean missing an unusual opportunity to manipulate key elements in the mechanism of the disorder. However, changes to these 'priors' may also be dependent upon patient expectations, and as we see through history, this may only happen if the patient believes there is an actual neuromodulatory effect. This may give rise to significant ethical issues in that the treatment may well directly benefit patients but only if they are (mis)informed that there is an underlying biological rationale. We conclude that modern trials of TMS in functional disorders are part of a repeating cycle of experimentation recurring since the mid-18th century. We suspect that emerging technology, including transcranial direct current stimulation, will follow a similar pattern of experimentation, speculation and marginalization. We suggest that considering our modern efforts in a historical context could aid our ability to further expand and maintain our use of electrical therapies that have proven helpful in the past for patients with functional disorders.
Keywords: conversion disorder; electric stimulation therapy; functional; history of medicine; hysteria; psychogenic; transcranial magnetic stimulation; transcutaneous electric nerve stimulation.