Author(s): Sloop RR
Despite the clinical potential of botulinum toxin type B (BTXB) for treating focal dystonia, hemifacial spasm, and other movement disorders, particularly in those resistant to botulinum toxin type A (BTXA), no objective human data exist to compare the muscle paralysis resulting from these two botulinum toxin subtypes. To objectively compare the human muscle paralysis resulting from intramuscular injections of BTXB with that from BTXA, we measured the extensor digitorum brevis (EDB) M wave amplitude four times before and six times after injection with 17 different doses of BTXB (from 1.25 to 480 units) in 17 healthy volunteers. This established a dose-response curve that we compared with the previously published BTXA dose-response curve. After the establishment of the dose-response curve, we injected 10 new volunteers with five different doses of BTXB and BTXA measuring EDB M wave amplitude 4 times before and 13 times over 57 weeks after injection. The volunteers were randomized by dose and received BTXA and BTXB in opposite EDB muscles. The effect of the toxin in all volunteers was expressed as percent decline in M wave amplitude postinjection (% paralysis). The maximal paralysis 2 weeks postinjection with 320 to 480 mouse units (MU) of BTXB was 50 to 75%, whereas maximal paralysis was 70 to 80% with 7.5 to 10 MU of BTXA. Postexercise M wave facilitation on day 9 postinjection averaged 63% for BTXB and 20% for BTXA. Seven weeks postinjection, BTXB-induced paralysis had improved by 66% with complete improvement by 11 weeks postinjection, whereas BTXA-induced paralysis had improved by only 6% at 7 weeks, and at 57 weeks postinjection 22% of the original muscle paralysis was still present. Thus, human muscle paralysis resulting from BTXB injection is not as complete or long-lasting as that resulting from BTXA.