Chronic musculoskeletal pain of non-specific origin affects approximately 30% of the global population. Beyond its prevalence, it represents a serious health and socioeconomic problem. It is considered the leading cause of years lived with disability and is characterized by persistent functional limitation, deterioration of quality of life, and poorer mental health, with a high comorbidity of depression, anxiety, and sleep disorders, as well as a greater risk of suicidal ideation and behavior.

From a socioeconomic perspective, chronic pain entails an impact of up to USD 635 billion annually just in United States (USA). In fact, spinal pain alone represents the condition with the highest direct costs (around USD 134.5 billion in USA) and additional indirect costs due to absenteeism and presenteeism, which impair productivity and work performance. These figures are expected to be substantially higher when considered at a global scale.

One of the main aggravating factors of this condition is that in up to 90% of patients experiencing pain there is no identifiable anatomopathological substrate that reliably explains the symptoms (non-specific pain). Radiological findings are extremely common in asymptomatic populations, and making diagnostic or therapeutic decisions based on such findings, promotes overdiagnosis and low-value clinical cascades. It is estimated that up to 50% of imaging requests and 60% of spinal surgeries6 are unnecessary or unjustified. For this reason, Clinical Practice Guidelines recommend prioritizing interventional diagnostic techniques based on functional criteria over the interpretation of radiological findings, as they allow for more sensitive and specific identification of nociceptive sources (with a recommendation grade I-II).

In this context, the motivation of the project is to study PENS as a non-pharmacological, safe, and transferable alternative to nerve blocks, reducing the risk of complications associated with the use of local anesthetics/corticosteroids and large-gauge needles. According to recent systematic reviews and meta-analyses confirming immediate analgesic responses, PENS could be a feasible alternative that maintains functional diagnostic and therapeutic value with a generally mild adverse-event profile and lower cost.

Within the broad spectrum of chronic pain conditions, low back pain (LBP) is the leading cause of years lived with disability worldwide and one of the largest contributors to the global disease burden. According to the latest Global Burden of Disease analysis, approximately 620 million people suffered from LBP in 2020. Projections for the coming decades are not encouraging since, by 2050, the number of affected individuals is expected to rise to 843 million, a 36.4% increase compared to 2020. This growth will be mainly driven by population increase and aging, with the sharpest rises expected in Asia and Africa.

Beyond its direct health consequences, LBP imposes a considerable economic burden on both individuals and societies, as its highest incidence occurs during the most productive working years (approximately 45-55 years old). In the USA alone, direct costs related to spinal disorders reached USD 315 billion between 2012 and 2014, in addition to productivity losses estimated in hundreds of millions of workdays each year. Considering its growing prevalence and socioeconomic implications, LBP is regarded as a global health priority.

LBP may originate from a wide variety of musculoskeletal, neurological, or visceral conditions, and its clinical expression may combine nociceptive mechanisms (related to tissue injury or stimulation of lumbar structures), neuropathic mechanisms (involving lesions or disease of the somatosensory system), and nociplastic mechanisms (involving altered nociceptive processing without structural lesions, often with central sensitization and descending inhibitory dysfunction).

Current clinical frameworks establish that in up to 90% of patients, it is not possible to identify a clear anatomopathological pain source (thus defined as non-specific LBP). The main challenge lies in identifying which structures (i.e., intervertebral discs, facet /zygapophyseal joints, sacroiliac joints, ligaments, or myofascial tissues) are the nociceptive sources. The absence of a defined structural correlate (as most structural abnormalities are also frequent in asymptomatic individuals) largely explains the high recurrence and chronicity rates, as well as the frequency of ineffective treatments, unjustified surgeries, and the economic impact of healthcare utilization without a clearly established etiology. Consequently, diagnosis in these patients is essentially clinical and based on exclusion criteria, with a therapeutic focus on symptom modulation and functional recovery rather than on identifying and correcting a single structural lesion.

Although scientific evidence consistently supports the high percentage of non-specific LBP cases and discourage routine imaging in such patients, this practice remains common in hospital care. It is estimated that 30% to 55% of imaging studies requested for LBP are unnecessary or not clinically indicated. The indiscriminate use of imaging not only generates unnecessary costs and delays, but also exposes patients to unjustified radiation and overdiagnosis risk. For example, the prevalence of disc protrusions in asymptomatic individuals increases from 30% at age 20 to 84% at age 80, which may lead to misinterpreting benign degenerative changes as pathological, fueling overtreatment and patient anxiety.

Consequently, this practice directly impacts the high rate of unnecessary spinal surgeries. It is estimated that up to 60% of spinal surgeries are performed unnecessarily, without clear indications, neurological deficits, or relevant radiographic findings. These procedures not only expose patients to avoidable risks with limited effectiveness, but also represent an annual cost of USD 4 billion in the USA.

In the absence of clinical or radiological criteria with sufficient diagnostic precision, international clinical practice guidelines, such as those of the American Society of Interventional Pain Physicians, recommend diagnostic anesthetic blocks as the reference procedure to confirm the nociceptive source (with evidence level I-II and moderate to strong recommendation strength). These diagnostic blocks consist of controlled injections of local anesthetics to determine the involvement of specific structures depending on the patient's pain relief response. For example, the facet joint can be functionally identified as the pain source through controlled infiltration of local anesthetic into the medial branches of the dorsal rami that innervate the lumbar facets. If a transient and reproducible analgesic response occurs after the block (particularly when comparative blocks with anesthetics of different duration are used) the diagnosis of facet joint syndrome is confirmed, supporting the appropriateness of subsequent interventions.

Despite their strong recommendation level, these procedures are not free of complications and risks, associated with large-gauge needles and local anesthetic administration. Common complications include peripheral nerve injury from direct mechanical trauma, intrafascicular injection, ischemia, or anesthetic neurotoxicity, with reported incidences of transient neuropathic symptoms ranging from 0% to 41%, and permanent injury below 1%. Other potential complications include systemic toxicity of local anesthetics (potentially severe if intravascular injection occurs), local infection or abscess formation, and, less frequently, nerve compression due to fibrosis or hematoma. Risk factors such as middle age (40-65 years), smoking, pre-existing neuropathy, or metabolic diseases (e.g., diabetes, chronic obstructive pulmonary disease) increase susceptibility to neural damage. Although ultrasound-guided techniques have substantially reduced the risk of major events, they do not eliminate the possibility of post-block neuropathies or adverse anesthetic reactions.

PENS constitutes a minimally invasive alternative that avoids corticosteroids and anesthetics, reduces exposure to large-volume injectables, and produces reproducible analgesic responses, useful as a functional test supporting the diagnostic hypothesis, while also serving as a therapeutic option within a multimodal pain management approach. This technique applies low- or high-frequency electrical currents (2-100 Hz) via fine needles, aiming to activate large-diameter myelinated afferent fibers and modulate nociceptive transmission at the spinal level. It has been shown to induce endogenous analgesic mechanisms through opioid neurotransmitter (µ, δ, κ) release and activation of descending inhibitory pathways, reducing central sensitization and improving conditioned pain modulation. These neurophysiological effects explain its capacity to reduce primary and secondary hyperalgesia observed in chronic musculoskeletal syndromes, particularly in the short term. Indeed, systematic reviews and meta-analyses report that PENS reduces pain and disability compared to placebo and other interventions.

These properties suggest that PENS may represent a feasible and safe alternative to anesthetic blocks for modulating facet-origin lumbar pain, offering an option that eliminates the risks associated with drugs used in blocks (e.g., systemic toxicity of local anesthetics, allergic reactions, systemic corticosteroid effects, hyperglycemia, adrenal suppression) by not requiring substance injection. From a mechanical perspective, PENS uses solid fine-gauge needles (≈0.25-0.30 mm; 32-34G), significantly thinner than those typically used for medial branch blocks (≈22-25G; 0.5-0.7 mm), thereby reducing tissue trauma, hematoma risk, and nerve injury probability. The absence of injectables also decreases the risk of local anesthetic systemic toxicity and prevents intra-articular or perineural pressure increases.

In terms of practical safety, adverse events with PENS are usually mild and self-limited (transient localized pain, small bruising, occasional vasovagal reactions), with a particularly favorable profile in patients with comorbidities where blocks carry higher risk (e.g., poorly controlled diabetes, prior adverse reactions to anesthetics, or contraindications to corticosteroids). With aseptic technique and ultrasound guidance when appropriate, PENS offers a minimally invasive alternative that preserves the diagnostic/therapeutic modulation capability over the facet target, while substantially reducing pharmacological and mechanical risks associated with conventional blocks.

Therefore, the general objective is to evaluate PENS as a diagnostic alternative not inferior to nerve blocks and as a therapeutic modality in chronic LBP of suspected facet origin, to establish its optimal dosage, and to assess its safety, resource utilization, and cost-utility in multicenter outpatient settings.

Inclusion Criteria:

• Participants with chronic LBP lasting at least three months and clinical suspicion of facet joint pain.
• A balanced sex distribution and a wide range of body mass index (BMI) will be promoted to allow subgroup analyses and to explore potential associations between obesity and procedural complications, thus strengthening the external validity of the study. This demographic selection is justified by prevalence patterns reported in the literature and by the need to capture clinically relevant effect modifiers in real clinical environments.
• The suspicion of facet joint pain will be based on a compatible axial clinical pattern and the absence of objective neurological deficit, considering that neither physical examination nor imaging tests provide sufficient specificity. Therefore, diagnostic confirmation will be performed functionally through ultrasound-guided medial branch diagnostic blocks, which are the most reliable tool for attributing pain to the facet joint. The dual innervation of each lumbar facet joint will be considered; thus, two adjacent levels will be blocked per joint (the medial branch at the affected level and the one immediately above).
• Eligible patients will be those with a positive functional screening in two independent medial branch diagnostic blocks performed with anesthetics of different duration, in separate sessions, to reduce false positives.
• The diagnosis will be confirmed in cases reporting clinically relevant pain relief within the expected pharmacological window.

Exclusion Criteria:

• Exclusion criteria will include red flags or specific etiologies such as infection, neoplasm, fracture, or cauda equina syndrome, as well as patients with progressive motor radiculopathy or another demonstrated neuropathic pain source other than facet origin.
• Individuals who have undergone prior procedures that could compromise response interpretation such as recent radiofrequency denervation, cryoneurolysis or intra-articular corticosteroid injections within a period that could extend analgesic effects.

The assessor would be a different investigator blinded to the interventions