Complications of Peripheral Nerve Evaluation (PNE)
Siegel et al.  noted 18.2% of adverse effects in 914 test stimulation procedures. The most common adverse events are lead migration from 11.8%  to 18.6% , technical problems (2.6%) and pain (2.1%). One surgical intervention (0.1%) was required to remove a test lead electrode that became dislodged during lead removal. Local infection and subcutaneous hematoma are rare .
For chronic sacral neuromodulation, complication rates range from 22 to 43% [19, 53, 138, 156] and re-operation rates from 6 to 50% [56, 99, 138, 156]. However, many studies do not discuss the complications arising from stimulation implants [21, 71, 156].
A prospective study was performed by the manufacturer Medtronic (Minneapolis, MN, USA) including 14 North American and 9 European centers . Of the 633 patients enrolled in this study, 250 had been implanted with the sacral nerve stimulator system by the end of the reporting period, representing 6506 months of device experience. Of the 250 implanted patients, 157 (62.8%) experienced a total of 368 adverse events associated with the device for use of stimulation therapy. Of the reported 368 events, 56 (15.2%) required no intervention, 151 (41%) required non-surgical intervention and 161 (43.8%) required surgical intervention. Overall, 89.4% (329) events were fully resolved. In the 250 implanted patients, post-implant adverse events associated with the devices or use of stimulation were pain at the internal pulse generator site (14.2%), new pain (10.8%), suspected lead migration (9.1%), infection (7%), pain at lead site (5.5%), transient electric shock (5.6%), suspected device problem (2.2%), adverse change in bowel function (3%), technical problems (3.9%), persistent skin irritation (0.8%), change in menstrual cycle (0.9%), suspected nerve injury (0.4%), device rejection (0.4%) and others (14.1%).
Pain is a frequent adverse event occurring in 4%  to 29%  of patients. Little is known about the severity and treatment of pain related to device implantation. Frequently, no distinction is made between postoperative pain, pain associated with the device, referred pain, pain related to stimulation, neuropathic pain and psychological pain. In one study, placement in the upper buttock reduced the rate of revision surgery but not pain . The symptoms of pain should always be thoroughly analyzed in order to treat it.
Any infection should always be detected and treated early. Removing the device either temporarily or definitively may prove necessary. Despite being a common complication of all implantable devices, few studies refer to this adverse event. No information is available in the literature concerning the etiology, severity or timing of infection. Mention has been made of skin irritation requiring device explantation . Compared to the Brindley technique (sacral anterior root stimulation with posterior rhizotomy) which has a maximum 2.4 infection % rate , the mean of 6.1% related to sacral neuromodulation  appears to be too high. Progress in prevention and device modification is required.
To date, there have been no reports of permanent injury or nerve damage . Sometimes nerve injury is suspected  and there is a potential risk.
The configuration of the electrode itself (incorrect fit to the nerve), surgical trauma, pressure caused by post-surgical edema, excessive scar formation and tension on the electrode cables are all potential contributors to neural damage . The peripheral nerve may be affected adversely by chronic constriction and compression . However, these risks are less important in the case of epineural electrodes than in intraneural ones . In animal studies, excessive or prolonged stimulation may cause early axonal degeneration . The risk of injury is also affected by the duration of continuous stimulation . It is well known that needle insertion into the sacral foramen can result in damage to nerve root and vessels . Because these structures are more likely to be found on the medial aspect of the foramen, injury can be minimized by using a more lateral foramen entry. Increasing the angle of needle entry in the vertical plan can increase the risk of injury to the vessels (venous plexus), and therefore that of hematoma and fibrosis . The S2 foramen is nearly half filled by its nerve root and ganglion, which increases the likelihood of penetration during needle placement. On the other hand, the S3 and S4 foramina are filled mostly with fat and their nerve occupies a relatively smaller portion of the foramen . It has been observed that the therapeutic efficacy of the implant sometimes becomes limited over time, and the potential formation of fibrosis between the electrode and target nerve has been suggested .
Bosch et al.  described difficulties in maintaining proper electrode positioning, breakage of the lead, fracture of the extension cable, electrode dislocation or malpositioning, early failure of pulse generator, contact lead point dysfunction and seroma around the generator site. However, device-related complications appear to be the most frequent. The following complications have been reported in patients undergoing sacral nerve stimulation for urinary urge incontinence:
- Device complications such as pain at the implant site , device rejection , early pulse generator failure , stimulation-dependent pain in leg or buttock  and current-related problems.
- Lead complications such as disturbed toe flexion, lead migration [22, 147], adverse changes in elimination function e.g. bowel (diarrhea) and urinary system , suspected nerve injury , lead site pain , transient electric shock  and fracture of the extension cable  or lead .
- Wound complications such as partial wound dehiscence of the sacral incision , hematoma , infection  or skin irritation .
More than one third of patients go to surgical revision , mostly for repositioning of the lead or the extension. Tempory removal with subsequent reimplantation is normally the result of infection or chronic pelvic pain. Repositioning of the internal pulse generator is performed to relieve pain at the site, or because the battery is dead. Permanent removal is to the result of infection, chronic intractable pain, or because the device has not proved satisfactory. Surgical revision does not appear to affect the overall degree of patient satisfaction , and it seems to decline with time .
Although relatively frequent, complications have until now received insufficient attention. Many patients require re-intervention to reposition or remove the device due to displacement, breakage or migration. However, the procedure is safe in experienced hands.
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