## 3 Percutaneous Electrical Nerve Stimulation (PENS)
PENS might influence the abnormal electrical discharge typical of chronic pain conditions, decreasing pain perception, a typical feature of neuropathic pain, throughout an electrical stimulation of non-nociceptive fibers [34]. PENS consists of a subcutaneous neuromodulation of nerve structures, through 21 gauge 5 to 20 cm length modellable needle-electrode monopolar probes, inserted in the skin; then, the probe is tunneled percutaneously through the use of an ultrasound guide along the major axis of the painful area, within the subcutaneous tissue at a depth ranging from 0.5 to 3 cm [35], as depicted by Figure 1.  
The probe is commonly connected to a neurostimulator device with a program of sensitive stimulation at 100 Hz, with an intensity of 0.2 V until paresthesia along the nerve pathway is obtained. Subsequently, a pulse frequency is commonly applied with an intensity of 0.5 V and a frequency ranging between 2 Hz and 100 Hz every 3 s. The stimulation protocol usually lasts at least 25 min, with continuous supervision by the physician [35]. However, several stimulation programs are reported in the literature,  
Figure 1: The PENS probe tunneled percutaneously at the subcutaneous level through an ultrasound guide. characterized by heterogeneous intensity, frequency, duty cycles and treatment duration, according to the different musculoskeletal disorders. The complete PENS treatment set up is depicted in Figure 2.  
To date, the PENS technique has been defined as a \"rapid emerging field\" by the National Institute for Health and Care Excellence (NICE) guidelines [36] and showed to be safe, both in acute and chronic pain management [7; 37; 38]. [PERSON] et al. showed no complications nor side-effects, such as itching or bleeding from the puncture points after removal of the device, in over 96 patients treated for postoperative pain following cesarean section [39]. To date, the most common adverse event complained by patients in the study of [PERSON] et al. was a mild discomfort during the probe implantation and/or withdrawal [7]. PENS is not devoid of contraindications that are mainly related to the infective risk, although it is a mini-invasive procedure. In addition, an implanted pacemaker/defibrillator or neurostimulated is considered a common contraindication in the studies currently available in the literature [5; 37; 38]. Therefore, in light of these considerations, PENS seems to be a safe and well-tolerated therapy in patients suffering from neuropathic and mixed pain. Moreover, it might represent an additional therapeutical option in refractory pain or in patients with relevant comorbidities that contraindicate a conventional approach or pharmacological therapies. In addition, given the low interaction with other therapies, it could be integrated as a complementary treatment with a multimodal rehabilitative intervention, including pharmacological drugs, physical exercise, and instrumental rehabilitative techniques in order to boost the benefits of conventional therapies [5; 8; 35; 36; 37]. However, further studies are warranted to better investigate its tolerability, safety and effectiveness in different pathological painful conditions.