Phenol Blocks

Clinically, this treatment appears effective in decreasing spasticity: there is however no good control studies to document its effectiveness. Recent popularity of Botulinum toxin leads to a recurrence of the interests on the use of nerve block. It has been reported to be safe and useful in treating children with cerebral palsy. It probably could be considered as one of the useful treatment options of spasticity. It is also a very economic treatment.

Use of intrathecal phenol for the treatment of spasticity was first reported in 1950s (Nathan; Kelly & Gauthier-Smith). Perineural phenol block and intramuscular phenol injections have been subsequently performed for relief of spasticity since 1960s.

Perineural block with aqueous phenol of concentrations less than 2% will have only local anesthetic properties. It may cause some microvascular damage to the nerve but no significant nerve destruction. At concentrations more than 3%, phenol denatures protein and causes nerve destruction with axonal degeneration. It is commonly observed after nerve block that voluntary strength is often more preserved in the good relief of spasticity. It was first speculated that phenol may preferentially destroy sensory fibres with resulting reduced spasticity and preserved muscle power. This speculation was not substantiated after the animal histological studies which showed similar degenerations of all types of fibres. Interruption of both sensory and motor pathways may be already sufficient to explain this observation. The synergistic effect of interruption of both the sensory and motor pathways contributes to the reduced reflex contraction while the interruption of the motor pathway alone affects the active muscle strength. In addition, gamma efferents which is part of the motor pathway act functionally as afferents since the interruption of the gamma efferents causes relaxation of the intrafusal fibres. The result is a reduction of reflex contraction and spasticity.

Clinical Applications in Cerebral Palsy: Phenol nerve block can be used for the treatment of spasticity in cerebral palsied children. Concentrations below 3% usually give unsatisfactory results. Frequent repetition of nerve blocks may be required. In clinical practice the most commonly used concentrations are between 3 - 6%. Compared with the Botulinum toxin injection, phenol may have more adverse effects. Technically it is also more difficult. It however have more persistent effect and so may require less injections. The treatment involves all muscles the blocked nerve supplies. It hence has the advantage of relieving the spasticity of the whole muscle group with a single injection. On the other hand, it has the disadvantage of poorer selectivity. As the nerve is recovering by axonal regeneration, the muscle strength and spasticity return. At concentrations above 3%, the effect lasting more than a year is not uncommon. There are some factors that may affect the effects and duration of the treatment. These include concentration and volume of injection, subsequent treatments such as stretching and casting, and presence of selective control. However, there is no control study on these issues. The treatment is useless to contracture. It nevertheless could be used with casting to relieve and tackle the problems of spasticity and contracture at the same time. Phenol has the additional advantage of anesthesia to relieve the pain of stretching. Phenol block and Botulinum toxin may be used in the same child so that the large muscle groups are treated with phenol block while the selected individual small muscles are treated with Botulinum toxin.