Kramer and colleagues studied the effect of intra-dermal injection of 5–20
mouse units of BoNT/A or placebo in a RCT in 15 healthy volunteers, followed up for
14 days [
Kramer et
al. 2003]. Transcutaneous electrical stimulation was used to
produce pain, hyperalgesia and neurogenic flare. BoNT/A caused an anhidrotic skin
area in all cases, and had a significantly smaller axon reflex flare, lower pain
rating, but similar amount of hyperalgesia to pinprick and allodynia after
electrical stimulation, as compared to placebo. This study suggested that BoNT/A
reduces peripheral neuropeptide release, but the evidence for a direct analgesic
effect was limited.
Yuan and colleagues performed a double-blind crossover trial of intradermal BoNT/A
for diabetic neuropathic pain in 18 patients [
Yuan et al. 2009]. There
was a significant reduction in visual analog scale (VAS) pain by 0.83 at 1 week,
2.22 at 4 weeks, 2.33 at 8 weeks, and 2.53 at 12 weeks after injection in the BoNT/A
group, as compared to the respective changes of 0.39, −0.11, 0.42, and 0.53
for the placebo group at the same time points (
p < 0.05).
Within the BoNT/A group, 44.4% of the participants experienced a reduction in
VAS of 3 or more within 3 months of injection, whereas there was no similar response
in the placebo group.
Patti and colleagues studied 30 patients with thrombosed external hemorrhoids who did
not desire surgery [
Patti
et al. 2008]. Patients were randomized to a single
intrasphincteric injection of 30 units of BoNT or saline. After 5 days of treatment,
the maximum resting pressure on anal manometry fell more significantly in the BoNT
group (
p = 0.004). The reduction in pain intensity with BoNT
was noted within 1 day of injection (
p < 0.001).
Ranoux and colleagues performed a RCT of single intradermal injection of
20–190 units of BoNT/A into the painful area in 29 patients with focal
painful neuropathies and mechanical allodynia [
Ranoux et al. 2008].
Patients were followed for 24 weeks. BoNT/A injections were associated with
persistent sustained effects on spontaneous pain intensity, neuropathic symptoms,
and general activity from 2 weeks after the injection to 14 weeks, compared to
placebo.
In a randomized, double-blind study, Sycha
et al. compared a single
intracutaneous injection of BoNT/A 100 units to placebo in six healthy volunteers
using a human acute inflammatory skin model with ultraviolet B radiation induced
sunburn [
Sycha et
al. 2006]. No significant differences were noted between the
groups in heat or cold pain perception threshold, skin blood flow, mechanical pain
threshold or mechanical allodynia. BoNT/A produced changes in sudomotor function.
The sample size in this study was very small, leading to a possibility of type II
error, ie failure to notice a difference when one truly exists. Blersch and
colleagues compared the heat and cold thresholds in 50 healthy volunteers who were
each injected with BoNT/A 100 units or placebo in the two forearms in a double-blind
fashion [
Blersch et
al. 2002]. Using quantitative sensory testing, no
differences were found between the groups with regards to heat and cold pain
thresholds at 4 and 8 weeks. In addition, no differences were found in electric
stimulation pain thresholds between the groups. This study suggested that there was
no evidence of direct antinociceptive effect of BoNT/A in this skin model of
nociception, and that antinociceptive action may be secondary to other effects of
BoNT/A such as chemodenervation or anti-inflammatory effects.
Voller and colleagues studied 16 healthy volunteers after injection of either 30
units of BoNT/A or placebo intracutaneously and responses were studied up to 28 days
after the injection and after capsaicin application at 28 days [
Voller et al.
2003]. The groups did not differ significantly in any of the following
outcomes at day 28: heat pain perception and tolerance thresholds; electric pain
perception and tolerance thresholds; area of secondary hyperalgesia after capsaicin
application; and for the capsaicin-induced flare.