J Neurosurg Spine  Volume 22 • January 2015

spine caSe reportJ Neurosurg Spine 22:70–74, 2015

Spinal stimulator peri-electrode masses: case reportrobert a. Scranton, MD,1 Ioannis M. Skaribas, MD,2 and richard K. Simpson Jr., MD, phD1

1Department of Neurosurgery, Houston Methodist Neurological Institute; and 2Greater Houston Anesthesia, Greater Houston Pain Consultants, Houston, Texas

The authors describe a case of delayed spastic quadriparesis caused by a peri-electrode mass following the implanta-tion of a minimally invasive percutaneous spinal cord stimulator (SCS). Prior reports with paddle-type electrodes are reviewed, and a detailed histological and pathophysiological comparison with the present case is made.The patient developed tolerance to a cervical percutaneous SCS 4 months after implantation, followed by the onset of spastic quadriparesis 9 months after implantation. The stimulator was removed, and contrast-enhanced MRI revealed an enhancing epidural mass where the system had been placed, with severe spinal cord compression. Decompression was carried out, and the patient experienced neurological improvement. Pathological examination revealed fibrotic tissue with granulomatous and multinucleated giant cell reactions. No evidence of infection or hemorrhage was found. Professionals treating patients with SCSs or contemplating their insertion should be aware of this delayed complication and associated risk factors.http://thejns.org/doi/abs/10.3171/2014.10.SPINE1425Key WorDS spinal stimulation; complication; electrode mass; myelopathy; epidural mass; technique

The first use of a spinal cord stimulator (SCS) in a human occurred in 1967 to treat a patient with intractable right-sided chest and abdominal pain secondary to bronchiogenic carcinoma.34 Since that time, thousands of devices have been implanted for an expand-ing list of indications, including failed back surgery syn-drome, complex regional pain syndrome, ischemic limb pain, neuropathic pain, chronic low-back pain, refractory angina, and many other less common indications.2,27,28,38,42

Complications associated with the procedure may in-clude electrode migration, infection, hardware malfunc-tion, hematoma, and, rarely, neurological deficit.2,29,35,38 Allergic reactions with cutaneous manifestations to the components of SCS systems have been reported.3,7,26,36 Six cases are reported in the literature of a mass forming around paddle electrodes, causing spinal cord compression and myelopathy between 14 and 22 months after implanta-tion (Table 1).4,6,18,30,41 We review this literature and report a case of cervical cord compression with spastic quadripa-resis secondary to a mass around percutaneous electrodes that occurred less than 1 year after implantation.

case reportHistory and Physical Examination

This 41-year-old woman suffered from chronic intrac-

table neck pain. She was managed medically for many years, followed by anterior cervical discectomy and fu-sion at C5–6 and C6–7. The patient continued to have pain refractory to further medical treatment. Next, the patient had a successful SCS trial and underwent perma-nent placement of a dual electrode system. Prior to the procedure, MRI of the cervical and thoracic spine without contrast was performed, confirming the absence of canal stenosis or other contraindication.

There were no postoperative complications, and the patient reported full strength and adequate pain control. Slowly, the effectiveness dwindled over the course of 4 months, requiring multiple reprogramming sessions. Nine months after implantation, the patient developed weak-ness and clumsiness of her upper extremities; she was then referred for neurosurgical evaluation in our clinic.

During the initial consultation, the patient complained of intractable neck and bilateral shoulder pain. This was asso-ciated with complaints of coldness, numbness, and clumsi-ness of her hands, as well as gait instability. Visual inspec-tion revealed marked atrophy of the hand intrinsic muscles. The patient’s gait was spastic. Cranial nerve examination was unremarkable. Motor testing revealed 4/5 strength in the upper-extremity muscles except for 3/5 weakness of hand grip. Strength was diminished (4+/5) throughout the

abbrevIatIoN SCS = spinal cord stimulator.SubMItteD January 7, 2014.  accepteD October 2, 2014.INcluDe WheN cItINg Published online November 7, 2014; DOI: 10.3171/2014.10.SPINE1425.DIScloSure Dr. Skaribas is a consultant for St. Jude Medical.

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Spinal stimulator peri-electrode masses

J Neurosurg Spine  Volume 22 • January 2015

lower extremities. Reflex testing showed symmetric hy-perreflexia in the upper and lower extremities with an ab-sence of Hoffman’s sign or Babinski reflex. The device was evaluated using cervical spine and abdominal radiographs as well as a cervical spine CT scan. These studies did not reveal any migration or malposition of the electrodes or disconnection between electrodes, extensions, and genera-tor. Evaluation of soft-tissue details was severely limited by artifact from the electrode; no mass could be seen within the spinal canal. The electrodes entered the spinal canal at the level of T-2 with stimulation contacts positioned in the posterior cervical canal extending from C-2 to C5–6.

ManagementThe percutaneous system was removed in the operat-

ing room through an incision overlying the generator and a small dorsal skin incision over the anchor point at T-3. The leads were removed with gentle traction, and no resistance was met. MRI of the cervical spine with and without con-trast was obtained after removal. There were no postopera-tive complications, and the clinical examination findings remained stable. The MRI showed a posterior cervical epi-dural mass extending from the level of C-2 to the superior aspect of T-1 (Fig. 1 upper). The mass was isointense to the cord with areas of central hypointensity and heteroge-neous contrast enhancement. It caused severe spinal cord compression, most profound at C3–4 and C4–5, with T2 hyperintensity within the cord at these levels.

The patient was readmitted for cervical decompres-sion. Prior to surgery, she reported a subjective increase in her hand weakness and that she was unable to ambulate, partially relying on a wheelchair for the previous week. On examination, her hand intrinsic strength remained stable but her lower extremities had declined further with 4-/5 strength throughout except for the anterior tibialis muscles at 2/5.

After preparing and draping the patient, we performed a midline incision from C-2 to T-1 with dissection carried down to the spinous processes and laterally to the facet joints. Laminectomy was performed from the caudal por-tion of C-2 to C-7 and a dense, firm, tan, fibrous mass was then encountered. Sagittal and axial ultrasonography was performed and showed obliteration of the ventral and dor-sal subarachnoid space. The mass was densely adherent to the dura, and no dissection plane could be exploited. Ultrasonography was repeated after subtotal resection and indicated ventral and dorsal CSF with a pulsatile spi-nal cord. The operation was terminated, and the wound was closed in layers in the usual fashion.

Histopathological FindingsRoutine cultures were sent at the time of surgery and

were negative for bacterial or fungal growth. A specimen was sent for pathological examination. Microscopic exami-nation after H & E staining showed dense fibroconnective tissue with marked chronic inflammation, multinucleated giant cells, and several noncaseating granulomas (Fig. 2). Slides were stained and examined for fungal, yeast, and bacterial organisms. None of these organisms were identi-fied. No malignancy or hemosiderin deposit was identified.ta

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 Year

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J Neurosurg Spine  Volume 22 • January 2015

Postoperative CourseFollowing resection, the patient had improvement in the

legs and hand intrinsic muscle weakness over the course of 4 days. She was ambulatory with a walker at the time of discharge to inpatient rehabilitation. At the 2-week post-operative clinic follow-up, the wound was healthy and the patient was ambulating without assistance. The strength in her hands and legs had improved to 4/5. Repeat cervical MRI was performed 6 weeks after surgery and showed adequate decompression (Fig. 1 lower). The patient was re-ceiving no benefit from the stimulator prior to removal and was being managed medically by her pain physician with oral narcotics, benzodiazepines, tizanidine, and pregaba-lin. The pain continues to be controlled with this regimen, and she declines consideration of further surgical manage-ment.

DiscussionSpinal cord compression after SCS implantation is a

rare occurrence and may be caused by epidural hema-toma, infarction, iatrogenic injury, epidural abscess, and tumor.20,29,33,35 Common to all these entities is the need for accurate imaging of the area of concern to characterize the cause and formulate a course of treatment.

Imaging modalities usually considered in evaluating SCS include radiography, CT scanning, and postmyelog-raphy CT scanning. However, these studies are subop-timal and do not afford the resolution needed to appro-priately evaluate soft tissues compared with MRI. Our patient underwent high-resolution cervical CT scanning; the artifact created by the SCS electrodes was such that no useful interpretation could be made. In our experience, the dispersion artifact created by SCS electrodes during CT scanning makes this modality of little clinical utility.

The gold standard for evaluation would be MRI; un-fortunately, most SCSs are incompatible with this modal-ity. Uncomplicated MRI has been performed in patients with an SCS in place.6,22,23,33,37 However, there is inherent danger that cannot be overlooked. One early report de-tailed a case of neurological injury to a patient who was believed to have suffered an electrical injury via an SCS after entering the magnetic field of an antitheft device in a store.10 Pulsed radiofrequency diathermy was thought to cause brainstem lesions and vegetative state in a patient with bilateral subthalamic nucleus electrodes.25 Ruggera et al. performed an in vitro experiment using pulsed ra-diofrequency diathermy and found a 2.8°C temperature increase after 1.49 seconds with an SCS electrode.32 This heating is seen similarly with MRI where the system acts as an antenna and energy from the radiofrequency mag-netic field is absorbed and concentrated.14 Other possible complications of MRI with incompatible stimulators in-clude device damage, reset, and spontaneous discharge. Our patient was receiving no benefit from the stimula-tor, which made the choice to remove prior to undergoing MRI simple. One could have advocated for a more in-vasive procedure that included exploratory laminectomy during the initial surgery. In this case, our patient was not experiencing an acute decline, and a more conserva-tive approach was taken. Had the device been MRI com-patible, the mass could have been diagnosed sooner, thus possibly preventing a second operation. It is important to counsel patients on the lack of compatibility prior to SCS implantation.

Six cases of spinal cord compression from a peri-elec-trode mass have been reported, all believed to have in-volved a paddle-type electrode.4,6,18,30,41 Reports described scar tissue forming around electrodes placed via cervical laminectomy, presenting between 14 months and 16 years after implantation. Our patient presented with neurological complications after 9 months, which is much earlier than prior reports, and is the only case in this review associated with percutaneous electrodes. This device has a smaller surface area and is placed in a significantly less invasive manner, creating minimal disruption to the native anatomy.

Prior histological descriptions include reports of scar tissue, fibrosis, and foreign body giant cell reaction (Table 1). The histology in our case was consistent with that in

FIg. 2. Photomicrograph of the epidural mass. H & E, original magnifica-tion ×10. Figure is available in color online only.

FIg. 1. Sagittal MR images (left) of the cervical spine obtained before (upper) and after (lower) decompression, with axial images (right) at the level of C4–5.

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prior reports. In all prior cases except one, a plane could be exploited to completely resect the mass. Wada and Kawai41 reported “severe adhesion between the dura and lamina was caused by scar tissue.” Complete resection of the mass was not achieved in the present case because of the dense adhesion to the dura.

Most reports described the development of tolerance preceding symptomatic compression, defined as a pro-gressive decrease in clinical response despite increasing or maximal stimulation in the absence of malposition or malfunction. The average latency to tolerance is 12.8 months (range 4–24 months); in the present case it was 4 months. Proposed mechanisms for tolerance include neu-ral plasticity at various sites of the pain pathway and local fibrosis around the electrodes.8,15–17,21,24,31

The pathophysiological mechanism of peri-electrode masses is unknown. Dam-Hieu et al. proposed that the def-icit in their patients was a result of both the fibrotic mass and spondylosis, although the former was thought to be the primary factor.6 Cervical laminectomy can be complicated by postlaminectomy kyphotic deformity.19,40 Risk factors include preexisting loss of lordosis, kyphotic deformity, and violation of the facet joints at the time of surgery. Pre-existing instability with the addition of a stimulator could lead to the development of repetitive local trauma and pro-gressive scarring. Spondylosis and loss of cervical lordosis is seen in other reports of cervical peri-electrode mass-es.18,41 The mass in our patient extended the entire length of the electrodes within the spinal canal from C-2 to T-2 and included the fused segments at C5–6 and C6–7. The involvement of fused segments suggests that spondylosis is not the only factor in the development of peri-electrode masses, although a contributory role cannot be excluded.

Allergic cutaneous reactions to stimulator components have been described, resulting in local dermatitis and, rare-ly, systemic dermatitis.7,26,36 Manufacturers may provide samples for patch testing to aid in identifying the offending agent. A search of the literature relevant to spinal stimula-tors, deep brain stimulators, and pacemakers did not reveal any reports of allergic reactions manifesting as a peri-elec-trode mass. Our patient had no cutaneous manifestations suggesting allergic response. The platinum and iridium al-loy composition of the contacts is common for stimulation electrodes because it has a high Warburg capacitance (thus low electrode-tissue capacitance), low allergenicity, and reportedly minor tissue capsule formation compared with many other materials such as copper and steel.9,13 Reactions to platinum and iridium have been reported, largely limited to industrial workers with chronic exposure in recycling centers processing catalytic converters.1,5,12 Testing should include evaluation for immediate and delayed hypersensi-tivity through skin prick and patch testing, respectively.39 In our patient the reaction involved the entire length of the electrode within the spinal canal and was not isolated to the area around the contacts. A reaction to the insulation is possible. However, one would expect it to extend beyond the spinal canal.

Epidural hematoma can be considered in the differen-tial diagnosis, although it is usually an early complication and is unlikely to develop later. A small subclinical hem-orrhage could have occurred during implantation, and the

mass formed as a result of continued organization. Histo-logical sections in our patient did not reveal any hematoi-din or hemosiderin deposition, arguing against hemorrhage in our patient, although sampling error cannot be excluded.

Contamination during implantation of an SCS system may produce an inflammatory reaction. Some centers im-plant SCSs in interventional suites outside the rigorous sterile techniques and other procedures mandated in an operating room.29 Additionally, a common technique when accessing the epidural space is to use approximately 5 ml of saline to dissect the plane. This is believed to reduce epidural blood vessel damage or cannulation, but it can also introduce contamination.11 Our patient had no subjec-tive or objective evidence compatible with infection. In our experience, when an SCS system component becomes in-fected, the infection tracks along the device as is seen with intrathecal drug infusion systems. No reaction or mass was found outside the spinal canal, and cultures of the mass along with specialized staining of the histological sections revealed no evidence of infection. Again, sampling error cannot be excluded.

The development of a peri-electrode mass is a rare complication of SCS therapy. In this review the average latency to tolerance was 12.8 months (range 4–24 months); the average latency to clinical presentation for neurological deficit was 54.4 months (range 9–192 months). The small number of cases precludes a complete understanding of the pathophysiology but appears to be an exaggerated inflam-matory reaction. Inciting events of inflammatory reactions could include foreign body reaction, subclinical allergic response, infection, hemorrhage, and dynamic instability with local repetitive trauma. We recommend assessing pa-tients preoperatively for spinal deformity, canal diameter, cord compression, and dynamic instability. It is our belief that the paucity of cases does not support a mandate for long-term follow-up or routine surveillance imaging in all patients. Surgeons and pain physicians should be vigilant for the development of tolerance, especially in the first 2 years after implantation. Neurological deterioration should be thoroughly investigated. These cases further highlight the need for MRI-compatible devices.

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author contributionsConception and design: Simpson, Scranton. Acquisition of data: Scranton, Skaribas. Analysis and interpretation of data: Scranton, Skaribas. Drafting the article: all authors. Critically revising the article: all authors. Reviewed submitted version of manuscript: all authors. Approved the final version of the manuscript on behalf of all authors: Simpson.

correspondenceRichard K. Simpson Jr., Methodist Neurological Institute, Department of Neurosurgery, 6560 Fannin St., Scurlock Tower #944, Houston, TX 77030. email: rsimpson@houstonmethodist.org.

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