Because spasticity arises from an imbalance of excitatory and inhibitory factors leading to a relative deficiency in GABA in the spinal cord, surgical procedures for treating spasticity either (1) interrupt the stretch reflex to decrease afferent excitatory input, or (2) increase the inhibitory GABA influence on the alpha motor neuron pool in the spinal cord. These procedures can be classified into central or peripheral, and ablative or nonablative (Tables 2 and 3). Because upper motor neuron lesions frequently result in negative symptoms such as weakness and loss of motor control, as well as positive symptoms such as spasticity, careful clinical evaluation and patient selection are required to maximize improvement in symptoms and to avoid creating new deficits or exacerbating pre-existing deficits. It is important to consider that spasticity itself may provide a positive functional component by supporting weak voluntary muscle contraction with involuntary spastic contractions. A reduction in spasticity may actually decrease functional ability.
Anterior (motor) rhizotomies have been performed on adults for the treatment of both upper and lower extremity spasticity, but they result in unacceptable degrees of motor weakness and atrophy and are only performed on children in rare cases of hyperkinetic movement disorders such as hemiballismus. DREZotomy is a modification of selective posterior rhizotomy in which the afferent fibers are divided as they enter the spinal cord in the dorsal root entry zone (DREZ) in the posterolateral sulcus of the spinal cord. A 3-mm deep microsurgical incision is made at a 45-de-gree angle in the posterolateral sulcus at the spinal levels thought to be involved.
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