To investigate and compare the morphologic effect at nerve terminals and muscle (extrafusal and intrafusal) following botulinum toxin type-A (BNTA) versus botulinum toxin-B (BNTB) among experimental rats.

Thirty experimental rats were injected with either botulinum toxin type A (Dysport®) (n=10), Botulinum toxin B (Neurobloc®) (n=10) or placebo (n=10). The following parameters were obtained at two endpoints (Day 14 and Day 28): (a) histopathological studies that determined mean diameter and percentage of muscle fibers (type 1, 2A and 2B); (b) muscle atrophy; (c) intrafusal muscle fiber diameter (Bag 1, Bag 2, chains): (d) endplate lengths and (e) terminal innervation ratios (TIA).

Degeneration, necrosis, inflammatory cell infiltration, and nuclear abnormalities were not found on haematoxylin-eosin stain. On D28, a slight increase in fibrous connective tissues (using modified Gomori-trichrome stains) as well as few focal reductions in oxidative enzyme activities (seen in NADH-TA stains) were noted in both BNTA and BNTB groups. Extrafusal muscle fiber diameter measurements (using NADH-TA and ATPase stains) indicated the following changes: (a) whichever toxin is used, there was atrophy of muscle fibers on D14 and D28, compared to controls; (b) whichever toxin is used, there was prominent atrophy of muscle fibers on the injected side compared to the minimal atrophy on the non-injected side; (c) whichever toxin is used, there was more atrophy noted on D14 compared to D28; (d) whichever time point is taken, muscle fiber type 1, followed by muscle fiber type 2A showed more vulnerability to atrophy, and (e) BNTB tended to induce more muscle atrophy than BNTA at the two time points. With regard to intrafusal muscles, the following changes were found: (a) whichever is the toxin used, atrophy of intrafusal fibers on the injected side were noted at the two time points, compared to placebo; (b) whichever is the toxin used, no significant atrophy of intrafusal fibers was noted on the non-injected side at the two time points; (c) Bag1 fibers, followed by chain fibers are more vulnerable to atrophy, and (d) BNTA tended to induce more atrophy of intrafusal fibers than did BNTB at the two time points. With regard to terminal innervation patterns using silver-cholinesterase stains (SCE), BNTA and BNTB groups had higher TIA on D14 and D28, compared to placebo.

BNTA and BNTB produce neuromuscular changes comparable with one another. Both decrease the muscle strength and induce extrafusal and intrafusal muscle changes to varying degrees when compared to placebo. Spread to the opposite corresponding limb muscles occurred to a mild degree with both BTNA and BNTB, also to varying degrees.