## 1 Introduction  
Percutaneous electrical nerve stimulation (PENS) is a nonpharmacological, minimally invasive peripheral neuromodulation procedure. It is used, alone or as a complementary therapy, for addressing chronic neuropathic [1; 2] and mixed nociceptive with neuropathic pain, especially in musculoskeletal disorders unresponsive to common therapies [3]. Clinical applications concern low back pain (LBP) [4], migraine and difficult-to-treat primary headache disorders [5], post-herpetic neuralgia [2], and scar tissue pain [2]. Moreover, PENS has also been used to address opioid-resistant cancer pain [6] and other unspecified chronic pain conditions [7].  
This technique combines the principles of both electro acupuncture and transcutaneous electrical nerve stimulation (TENS) [1]. In particular, PENS works through a dedicated probe consisting of two or more needle electrodes (10, 32 gauge) inserted at the subcutaneous level and capable of conveying a mild electric current to a predetermined target (peripheral sensory nerve, soft tissue) with a dermatomal approach. The treatment plan includes one or more thirty-minute (15-60 min) sessions. The neurophysiological bases of the PENS functioning probably involve the modulation of the large-diameter afferent A\(\beta\) fibers and the local release of biochemical agents such as neurotransmitters and endorphins [3]. The stimulation of interneurons located in the Rexed laminae 2 and 3 of the dorsal horn was also suggested [8]. Furthermore, PENS can stimulate serotonin, cholinergic, and opioid receptors in the spinal cord, and ameliorate primary hyperalgesia through GABA signaling activation [3; 8].  
The scientific evidence on TENS is widely available [9; 10]. On the contrary, despite the potential applications of PENS in different clinical conditions, there is no solid evidence to sustain this procedure. To date, evidence is mostly sustained by small-sized, controlled trials. In 2013, the National Institute for Health and Care Excellence (NICE) published guidance confirming the short-term safety and efficacy profile of the PENS technique. Furthermore, it encouraged the implementation of clinical research for establishing the selection of patients suitable for treatment and assessing long-term outcomes. The NICE document also stressed the need for clear documentation useful for the indications of treatment [2]. Previously, the American Academy of Neurology, the American Association of Neuromuscular and Electrodiagnostic Medicine, and the American Academy of Physical Medicine and Rehabilitation published guidelines and recommended PENS therapy for the treatment of painful diabetic neuropathy (level B) [11]. Thus, there is a need for an update.  
Meta-analysis and bibliometric studies are both quantitative strategies. Nevertheless, meta-analyses are commonly implemented as theory extension instruments and summarize empirical evidence by analyzing the features and strength of outcomes and links encompassed by variables. On the contrary, bibliometric analysis summarizes the bibliometric architecture of a field by analyzing \"structural relationships\" between different research elements and features. In other words, bibliometrics is a computable approach that is suitable to investigate emerging trends and the insight structure within a given topic or field for gathering estimable, reproducible, and objective data. Therefore, this method can better broadly outline the knowledge of the research area. This approach is also useful for stimulating multidisciplinary collaboration [12].  
On these premises, we conducted a visualized bibliometric network of all the literature available on the subject for highlighting the strengths and weaknesses of the scientific output.