PENS (Percutaneous Neuromodulation) for Pain

What PENS Is

PENS (Percutaneous Electrical Nerve Stimulation) is a neuromodulation modality in which fine needles are inserted near peripheral nerves or myofascial trigger points and connected to a low-frequency electrical current generator. Unlike conventional TENS, which applies current through the skin surface, PENS delivers the stimulus directly into the subcutaneous or perineural plane, allowing more selective activation of specific nerve fibers.

The difference from TENS is not only technical: stimulation depth enables preferential activation of A-delta fibers (myelinated, fast-conducting) and C fibers (unmyelinated, related to chronic pain), with less interference from high-impedance skin tissues. In clinical practice, this translates into a more consistent analgesic effect per session and longer duration, though this advantage varies by condition treated.

The technique was developed and formalized in the 1990s. The seminal study by Ghoname et al., published in JAMA in 1999, compared PENS with TENS and sham in patients with chronic low back pain, demonstrating PENS superiority in pain, function, and sleep outcomes. In some contexts, when needle insertion follows dermatomal points related to the painful segment, the technique is described as "neurofunctional electroacupuncture" — a designation that reflects methodologic proximity to electroacupuncture practiced by acupuncture physicians.

Mechanism of Action

The analgesic effects of PENS operate at multiple levels of the nervous system. The first is the gate control theory of pain, proposed by Melzack and Wall in 1965: activation of large-caliber myelinated fibers (A-beta) and A-delta fibers promotes presynaptic inhibition of C fibers in the dorsal horn of the spinal cord, reducing transmission of the nociceptive signal toward the brain.

The second mechanism involves release of endogenous opioids. Experimental studies with animal models (reviews by Han and colleagues in the acupuncture neurophysiology literature) suggest that the frequency of the electrical stimulus may differentially modulate the release of neuropeptides: low frequencies (2-4 Hz) would tend to favor the release of enkephalins and beta-endorphin, whereas high frequencies (80-100 Hz) would favor dynorphin. This frequency dependence is one of the proposed bases for parameter selection according to clinical profile — although direct extrapolation to outcomes in humans is limited.

The third mechanism is activation of the descending inhibitory modulation pathway, involving the periaqueductal gray (PAG), the nucleus raphe magnus, and serotonergic and noradrenergic projections to the dorsal horn. This system is particularly relevant in chronic pain, in which there is frequent dysfunction of endogenous inhibitory mechanisms.

Scientific Evidence

Evidence on PENS is heterogeneous and condition-dependent. Most available randomized clinical trials (RCTs) were conducted between the late 1990s and mid-2000s, with small samples (n < 100) and varied protocols — limiting the strength of aggregated conclusions.

The pivotal study is Ghoname et al. (JAMA, 1999): a crossover RCT with 60 patients with chronic low back pain, in which PENS was superior to TENS and sham in pain reduction, improvement in physical function, and sleep quality. Subsequent trials by the same team (Hamza et al., 2000) investigated frequency parameters and session duration, confirming benefit in chronic low back pain and exploring application in cancer pain and painful diabetic neuropathy.

In painful peripheral diabetic neuropathy, small RCTs and case series suggest symptomatic relief, but there is no specific Cochrane meta-analysis on PENS in this condition, and estimates such as NNT vary by study and should not be generalized. The most recent comprehensive systematic review on PENS in chronic pain (published more than a decade ago) pointed to modest benefit and emphasized the need for multicenter studies with larger samples — a gap that has not yet been filled.

For myofascial pain, chronic headache, and postoperative pain, evidence is considered low — predominance of small studies, without adequate sham control, and protocol heterogeneity. The NICE 2021 guideline on chronic primary pain does not specifically recommend PENS owing to insufficient high-quality evidence, although it acknowledges potential in selected subgroups.

Indications

PENS indications reflect available evidence and the pathophysiologic response profile. Adequate patient selection is one of the most important factors for therapeutic success — conditions with a localized neuropathic or myofascial component tend to respond better than diffuse or predominantly nociplastic pain.

How It Is Done and What to Expect

The PENS session is performed in the office, with the patient positioned to comfortably expose the region to be treated (often prone for low back pain, lateral decubitus for cervical pain or limbs). After skin antisepsis, the physician inserts fine sterile needles (0.25-0.30 mm gauge) at points mapped to the peripheral nerve location or target trigger point.

The needles are then connected to an electrostimulator via clips. Current is gradually increased until the patient reports a comfortable sensation — typically described as rhythmic tingling, gentle pulsation, or a sensation of "internal massage." Intensity should be perceptible but never painful; reproduction of the original pain during initial adjustment, followed by comfort with pulsation, is often considered a good technical indicator.

Session duration is 20 to 30 minutes. The first analgesic effects are usually perceived after 2 to 4 sessions — patients rarely experience clinically relevant improvement at the first session, and this should not be interpreted as early failure. After each session, mild local pain or hypersensitivity at insertion points is common for up to 24 hours; small hematomas (< 2 cm) are possible in 3-5% of cases and resolve spontaneously.

Adverse Effects, Risks, and Contraindications

PENS has a favorable safety profile when performed by a trained physician, with adverse event rates consistently below 5% in clinical series. Nevertheless, absolute and relative contraindications must be rigorously evaluated before starting treatment.

Limitations and What Is Still Not Known

Despite its clinical potential, PENS faces methodologic and practical limitations that should be considered when discussing the technique with patients. The initial enthusiasm of the 1990s-2000s was not followed by consistent high-quality evidence in subsequent decades, and several clinical questions remain open.

Evidence Gaps

Among the main gaps: (1) absence of recent multicenter RCTs with large samples; (2) lack of standardized stimulation parameters (frequency, intensity, duration, number of sessions) across protocols; (3) heterogeneous patient selection criteria, making result replication difficult; (4) scarcity of data on effect duration beyond 3 months after the initial series; (5) absence of direct comparisons with other neuromodulation modalities in specific conditions.

In addition, limited access is a relevant practical barrier: equipment is not universally available in public health systems or in many private health-insurance services, the cost per session is moderate to high, and the number of physicians trained in the technique is restricted compared with the supply of conventional electroacupuncture. This operational reality influences clinical decision making as much as the scientific evidence.

Relationship with Medical Acupuncture

PENS and medical acupuncture (including electroacupuncture) share common neurophysiologic mechanisms — both activate A-delta and C fibers, modulate the gate-control pain pathway, and stimulate the release of endogenous opioids. The main difference lies in point selection: PENS prioritizes anatomic proximity to peripheral nerves or identifiable trigger points, whereas medical acupuncture uses points defined by clinical tradition and by neuroanatomic correspondence (nerve points, motor points, dermatomal points).

In practice, the two techniques overlap in several situations, especially when acupuncture is combined with electrostimulation. In terms of consolidated evidence by condition, medical acupuncture has a larger volume of available meta-analyses — including a recommendation in the NICE 2021 guideline for chronic low back pain, and moderate-quality evidence in tension headache and chronic migraine. PENS has evidence concentrated in fewer conditions but with similar methodologic design.

On the combination of the two techniques: in general, there is no advantage to applying PENS and medical acupuncture at the same point and in the same session — there is mechanism redundancy and a higher risk of local over-stimulation. The modalities work better as alternatives or in sequential protocols (for example, a PENS series followed by maintenance acupuncture, or vice versa), according to availability, cost, patient preference, and clinical response.

In most Brazilian contexts, medical acupuncture offers broader access and more favorable cost. When electrical stimulation is clinically indicated, electroacupuncture performed by an acupuncture physician can fulfill the same therapeutic objective with comparable evidence. PENS remains a relevant option for patients who have not responded to conventional electroacupuncture or at centers with specific expertise in the technique.

When to Seek Medical Help

Considering PENS as a therapeutic option requires, first, a structured medical evaluation of a chronic pain condition. Neuromodulation should not be the first response to pain — and need not be indefinitely postponed when initial treatment has not produced improvement.