We observed attenuation of CA in six patients with PD who suffered from thoracic-level camptocormia, when comparing before treatment and 2 weeks after the botulinum injection into bilateral EO. Although the number of patients is small, this report is the first to show efficacy of BT in bilateral EO with statistically significant improvement in the angle of camptocormia; however, there are several limitations and points for discussion. Due to the study design of an open-label and non-blinded trial, the possibility of a placebo effect cannot be eliminated. However, the latency from the injection of BT to the emergence of efficacy (7–10 days) and duration of efficacy (2–3 months) was considered to be the effect of BT because this treatment effect curve is also generally observed with botulinum therapy [15, 16].
In a previous report using lidocaine, consecutive injections were performed every 4–5 days due to the shortness of drug action (the effect of a single dose lasted for several days), to obtain efficacy in 90 days . Both the previous report of BT into unilateral EO  and our results show that a single injection of BT had therapeutic efficacy for several months; however, a direct comparison between BT and lidocaine is needed for conclusive evidence of a difference in their effectiveness.
We used doses of 75–90 units per single EO, i.e. 150–180 units per patient, of onabotulinum toxin A as the initial dose, for the safety of our light-weight patients. Considering the doses of onabotulinum toxin A used in previous studies of 300–600 units per patient  and 200 units in unilateral EO , our doses may have been insufficient. However, given that the escalation of doses in the maintenance injections did not lead to further amelioration, our doses may have reached supramaximal levels. We selected a relatively low concentration of BT to obtain a broad spread because of the width of EO; however, actual spread of BT is difficult to determine and varies between individuals [15, 16]. In addition, more exploration is needed to determine the optimal site of injection in the EO, and the best method of evaluating the CA.
Considering the individual variability in the therapeutic efficacy of BT, the fact that cases 4 and 6 had severe spinal degeneration and very limited efficacy for BT suggests that modification only in dystonia of the EO muscle is suboptimal to attenuate camptocormia comprehensively. As suggested in a previous report of surgical intervention for camptocormia, poor quality of the spine can limit the therapeutic effect . Moreover, the maximal effect of BT in case 3 could not be maintained due to re-exacerbation of symptoms due to falls. Therefore, to modify exacerbating factors of camptocormia it may be necessary to provide long-term treatment. In terms of subjective relief of symptoms, cases 4 and 5 did not perceive the attenuation of the CA; therefore, CA alone may be an insufficient marker to evaluate amelioration of camptocormia. As case 3 achieved the ability to walk faster and with a larger stride, similar to a recent report that showed a relationship between camptocormia and gait , marginal symptoms/signs associated with camptocormia (i.e. painful contraction of abdominal muscles in cases 1 and 2, and dysbasia in case 3) might need intensive examination for the correct evaluation of therapeutic efficacy. We expected objective stiffness of abdominal muscles, MH and/or CRD to act as biomarkers of camptocormia; however, this was not the case. We speculate that the cause of CRD is secondary muscle injury from excessive contraction due to dystonia, although CRD is a nonspecific sign of injury in motor neurons or muscles . Other limitations of this study are heterogeneity in the duration of PD, in the severity of CA, in the exacerbating factors of camptocormia and other backgrounds of patients, the coexistence of spinal degeneration, dropout of cases 4–6 to examine the long-term effect of BT, and a smaller dose of BT in case 1 than the others.
Limitations are small sample size, open-label and non-blinded trial, heterogeneity of the backgrounds of patients, unexamined validity of the maneuver of botulinum injection (i.e. the injection site and dilution), not thorough evaluation of walking ability, and dropout of three patients in the long term.