Jiří Kozák

Functional magnetic resonance imaging of cerebral activation during spinal cord stimulation in failed back surgery syndrome patients

Spinal cord stimulation (SCS) consisting of electrical stimulation of the dorsal spinal cord using epidural electrodes has been shown to relieve chronic neuropathic pain. To analyze the cerebral activation patterns related to SCS, and to evaluate the effects of SCS on the processing of acute experimental pain, we performed functional magnetic resonance imaging (fMRI) on eight patients suffering from failed back surgery syndrome who were also being treated with SCS for severe pain in their legs and lower back. Three types of stimulation were used, each lasting 36s: (i) SCS, (ii) heat pain (HP) applied to the leg affected by neuropathic pain, and (iii) simultaneous HP and SCS.

Effects of spinal cord stimulation on the cortical somatosensory evoked potentials in failed back surgery syndrome patients

OBJECTIVE To evaluate the functional activation of the somatosensory cortical regions in neuropathic pain patients during therapeutic spinal cord stimulation (SCS). METHODS In nine failed back surgery syndrome patients, the left tibial and the left sural nerves were stimulated in two sessions with intensities at motor and pain thresholds, respectively. The cortical somatosensory evoked potentials were analyzed using source dipole analysis based on 111 EEG signals. RESULTS The short-latency components of the source located in the right primary somatosensory cortex (SI: 43, 54 and 65ms) after tibial nerve stimulation, the mid-latency SI component (87ms) after sural nerve stimulation, and the mid-latency components in the right (approximately 161ms) and left (approximately 168ms) secondary somatosensory cortices (SII) were smaller in the presence of SCS than in absence of SCS. The long-latency source component arising from the mid-cingulate cortex (approximately 313ms) was smaller for tibial and larger for sural nerve stimuli during SCS periods compared to periods without SCS. CONCLUSIONS SCS attenuates the somatosensory processing in the SI and SII. In the mid-cingulate cortex, the effect of SCS depends on the type of stimulation and nerve fibers involved. SIGNIFICANCE Results suggest that the effects of SCS on cortical somatosensory processing may contribute to a reduction of allodynia during SCS.

891 BIO-PSYCHO-SOCIAL ASSESSMENT OF NEUROMODULATION TREATMENT OF PATIENTS WITH FAILED BACK SURGERY SYNDROME

Spinal cord stimulation has been utilized in the treatment of painful conditions and intractable angina. Studies have confirmed an improvement in the quality of life and reduced hospital admissions for these patients. [1,2,3] We report a case of a patient who experienced cross-talk between an implanted percutaneous lead and a surgical plate placed in parallel. Case report: A 64yr gentleman with a pacemaker was referred to the NRAC for management of his intractable angina. He progressed along our treatment algorithm to spinal cord stimulation with no interference to his pacemaker. Lead movement resulted in two lead revisions with short-lived improvements. He was referred to our local neurosurgical unit for a surgical plate. Stimulation remained in the right axilla with the target area central and left chest. He declined revision of the plate and a percutaneous lead was passed parallel to it. Stimulation persisted in the right axilla with the use of the lead only, plate only or lead and plate. This was overcome by passing a continuous ultra-low sub-threshold amplitude of 0.5 v in the surgical plate. Subsequent programming of the percutaneous lead provided stimulation in the target area and symptom control Discussion: Cross-talk between electrodes placed in parallel can be useful to increase the area of stimulation. The unexpected crosstalk between a surgical plate and percutaneous lead can lead to a programming challenge.

267 MODIFICATION OF CORTICAL EVOKED POTENTIALS DURING SURAL NERVE STIMULATION IN FAILED BACK SURGERY SYNDROME PATIENTS TREATED WITH SPINAL CORD STIMULATION

We analyzed whether cortical processing of cutaneous sural nerve stimuli would be altered by spinal cord stimulation (SCS) in patients with failed back surgery syndrome. In nine patients (4 women, 5 men, age 39–56 years) suffering from intractable pain in their left leg and back, the left sural nerve was stimulated with 0.2 ms square pulses (13–34mA) and 5–8 s inter-stimulus intervals. The evoked potentials (111 EEG electrodes), that were recorded during ongoing SCS or in absence of SCS, were modeled using BESATM (MEGIS, Germany). The source model encompassed four source dipoles located in the following regions: the right primary somatosensory cortex (S1, 87ms), the left and right secondary somatosensory cortex (S2, 161 and 167ms, respectively), and the mid-cingulate cortex (314ms). The source amplitudes of the S1 and of both S2 source dipoles were reduced during periods of SCS compared to periods without SCS (P< 0.05). In contrast to the S1 and S2 sources, the amplitude of the mid-cingulate source was increased during SCS (P< 0.05). Subjective intensity of sural nerve stimuli, evaluated using visual analogue scale, was not affected by SCS (P> 0.05). While reduced amplitudes of S1 and S2 sources during SCS suggest inhibition of afferent input possibly occurring at segmental level, the increased source activity in the cingulate cortex points to a positive interaction between SCS and the sural nerve stimulation. These cortical activation changes are not associated with subjective intensity measures of the sural nerve stimuli. Supported by GAUK 66/2005, IGA 8232, RG 0021620816, LC 550 and CNS 1M0517.

250 CEREBRAL ACTIVATION DURING SPINAL CORD STIMULATION AND HEAT PAIN IN FAILED BACK SURGERY SYNDROME PATIENTS

Spinal cord stimulation (SCS) is an established treatment for chronic neuropathic pain. However, in recent studies conflicting results regarding the effect of SCS were noted in a selected group of patients suffering from Complex Regional Pain Syndrome and mechanical allodynia. In the present study we investigated the pain relieving effect of SCS in a rat experimental model of neuropathic pain as related to the severity of mechanical allodynia. Adult male rats (n = 45) were submitted to a unilateral sciatic nerve ligation. The level of allodynia was tested using the withdrawal response to tactile stimuli with the von Frey test. A portion of these rats developed marked tactile hypersensitivity in the nerve-lesioned paw (von Frey test), similar to “tactile allodynia” observed after nerve injury in humans. Then prior to SCS treatment the rats were subdivided into three groups based on the level of allodynia: mild, moderate and severe. All allodynic rats (n = 27) were treated with SCS for 30min (f = 50Hz; pulse with 0.2ms and stimulation at 2/3 of motor threshold) at 16 days post-injury. Our data demonstrate a differential effect of SCS related to the severity of the mechanical allodynia. SCS leads to a faster and better pain relief in mildly allodynic rats as compared to the more severely allodynic rats. Thus, we suggest that the selection and subdivision of patient groups similar to those defined in our experimental setting (mild, moderate and severe allodynic) may provide better pre-treatment prediction of possible therapeutic benefits of SCS.