Study design: Secondary analysis of a clinical trial.
Objectives: To analyze adherence to 1-year transcranial Direct Current Stimulation (tDCS) clinical trial in people with chronic pain due to spinal cord injury (SCI). We also explore the association between dropout and several baseline variables such as age, depression levels, pain severity, number of days with pain in the last 7 days, walking ability, sleep, work, relationship with others, and enjoyment with life.
Setting: Boston, USA.
Methods: Forty-six participants were enrolled in this trial, and 33 participants were randomized to receive either active or sham tDCS.
Results: Using the full intention-to-treat (ITT) criteria, only 8 participants (24%) finished the study. The median time to dropout was seven (IQR:6,19) sessions (i.e., immediately after the first follow-up), regardless of the type of stimulation that participants received (active vs. sham tDCS) (χ2 = 0.025, p = 0.875). An exploratory analysis suggested that only the number of days with pain in the last 7 days was moderately associated with dropout, with people experiencing less pain being more prone to dropout from the study.
Conclusions: Despite all the measures to improve study adherence (such as providing parking, flexibility to schedule sessions, follow-up with participants by phone), it seems that long follow-up periods may increase the likelihood of dropout. Given the need to understand long-term effects of interventions, longitudinal trials need to consider alternative designs or methods of treatment (for instance home treatment or home assessment) to decrease attrition rate.