Botulinum toxin
Botulinum toxin, a muscle-nerve paralyzing bacterial toxin, is being used for controlled partial muscle paralysis in regulated doses to relax spastic muscles to improve function in Cerebral Palsy. However, the development of anti-toxin antibodies by the body and regeneration of destroyed nerves limits its usefulness.
Moreover, its sudden freedom from spasticity takes away the motivation for continued intensive OT/PT, and often results in the child going back to square one after a few months. In addition, the US FDA has cautioned that the higher doses used for CP spasticity in children have caused at least 16 fatal outcomes due to leakage of the toxin from the muscles into systemic circulation.

Botulinum toxin can produce focal, controllable muscle weakness of predictable duration, without sensory adverse effects. Randomised clinical trials involving patients with spasticity resulting from a variety of diseases (mainly stroke and multiple sclerosis) have clearly shown that botulinum toxin type A (Dysport and Botox) can temporarily reduce spastic hypertonia in the elbow, wrist and finger flexors of the upper limbs, and the hip adductors and ankle plantar flexors in the lower limbs.

The clinical benefits from this reduction of neurological impairment are best shown in the upper limb, with less disability of passive function and reduced caregiver burden. In the lower limbs, there is improved perineal hygiene from hip adductor injections. The benefits of reducing ankle plantar flexor tone are less well established. Pain is also reduced, possibly by mechanisms other than muscle weakness. Improved active function has not yet been clearly demonstrated in RCTs, only in open-label trials.

The safety of botulinum toxin-A is impressive, with minimal (mainly local) adverse effects.There are little data on the use of botulinum toxin type B (Myobloc or Neurobloc) in spasticity and the only RCT that has examined this did not show tone reduction; dry mouth appeared to be a very common adverse effect. There are also very little data to allow a benefit-risk comparison of phenol and botulinum toxin injections; each have their clinical and technical advantages and disadvantages, and phenol is much less costly than botulinum toxin.

Source: "Botulinum toxin treatment of adult spasticity : a benefit-risk assessment", Geoffrey Sheean