19.2.1 Patient Selection

The most common condition to require selective dorsal rhizotomy (SDR) is spastic diplegia associated with cerebral palsy), a consequence of non-progressive injury to the fetal or infantile brain. This injury leads to insufficient inhibitory interneuron input, excessive alpha motor neuron excitatory activity, and thus spasticity. ^{5 , 7} The resultant hypertonicity makes ambulation difficult. It may also cause orthopedic deformities of the lower extremities. ⁸ Patients that benefit from SDR are those with disabling hypertonicity secondary to spasticity. The presence of significant dystonia is, however, a contraindication to SDR because the dystonia can be unmasked or exacerbated by rhizotomy. ⁷ Before considering SDR, patients should have undergone a trial of physical and occupational therapy, oral medication, intramuscular botulinum toxin injections, and tendon release. Candidates should be between the 3–10 years of age. Most procedures are done before the age of 15 years. ^{9 , 10}

19.2.2 Preoperative Preparation

For both procedures, the degree of spasticity and extent of motor function must be quantified before surgery. Hypertonicity in upper and lower extremity muscle groups is measured using the Modified Ashworth Scale (MAS). Motor function is quantified using the Gross Motor Function Classification System Score (GMGMS). Gait is assessed using the Visual Gait Assessment Scale (VGAS). Patients should have some motor strength and control so as to prevent deterioration in crouching, lateral trunk sway, and knee hyperextension post operatively. ¹¹ A magnetic resonance imaging scan should be done before surgery to identify the level of the conus medullaris and any possible structural abnormality.

19.2.3 Operative Procedure

Before 1991, SDR was done with L2-S2 laminectomies but the technique has been modified to a selective, single level (L1) or two level (L1–2) osteoplastic laminotomy to avoid delayed spinal deformity. ^{8 , 12 , 13} A short acting, non-depolarizing neuromuscular blocking agent is used during intubation for general anesthesia so as to not interfere with neurophysiologic monitoring. Direct stimulation and electromyography (EMG) are used to guide the surgery along with sensory and motor evoked potentials. After the monitoring leads are placed and initial levels obtained, the patient is positioned prone on the operating room table over gel rolls. The L1 spinous process is identified for the skin incision and a high-speed drill is used to create a laminotomy, which is plated to the spine after completion of the rhizotomy. Ultrasound can be used to identify the conus location, but this can be confirmed grossly when the dura is opened.

A midline durotomy is made and, once the dura is opened, saline irrigation is no longer used so as to not interfere with the EMG responses. Under the operating microscope, the spinal roots between L2 and S2 bilaterally are dissected so as to separate the dorsal roots and ventral roots both anatomically and electrophysiologically. After the rootlets are anatomically identified, a single 0.1 msec square-wave pulse is applied to each dorsal rootlet at a rate of 0.5 Hz. The intensity of the stimulus is increased until a reflex response is obtained from the ipsilateral corresponding muscle. Once the reflex is established, a 50-Hz train of tetanic stimulation is applied for 1 second and the response is graded on a scale of 1 + to 4 + (▶ Table 19.1). Rootlets that produce 1 + and 2 + responses are spared and those that produce 3 + and 4 + responses are sectioned. The decision to transect a given rootlet depends on the intensity of the response and on the number of rootlets that produce that response at a given level. Abnormal roots are identified in this manner, coagulated using bipolar electrocautery and cut sharply with micro-scissors. To achieve maximal clinical benefit, 50–70% of motor roots are transected. ^{5 , 11} Stimulation of the penis or clitoris and perianal area while monitoring the activity in the S2 rootlets is done to identify those rootlets to preserve in order to avoid bowel and bladder dysfunction. ^{5 , 9 , 11} The portion of the dorsal S2 root that is sectioned is limited to less than 35% to limit such morbidity. ⁵

The subdural space is copiously irrigated, and the dura is closed with a running suture in a water-tight fashion. The bone is secured in placed either using sutures absorbable or titanium miniplates. The wound is then closed in multiple layers including muscle, fascia, and skin.