Immediate and 1-week Efficacy of a Single Session of Sciatic Nerve Electrical Stimulation in the Management of Low-back Related Leg Pain

Disability will be assessed using the Oswestry Disability Index (ODI), an instrument designed to evaluate the degree of disability associated with acute, subacute and chronic LBP. The ODI consists of a total of 10 items, one assessing pain and the remaining nine assessing activities of daily living, each rated on a 6-point ordinal scale ranging from the best to the worst scenario (0-5 points). The scores for each item will be summed to obtain a total between 0 and 50 points, with the following categories: no disability (0-4 points), mild disability (5-14 points), moderate disability (15-24 points), severe disability (25-34 points) and complete disability (35-50 points). The ODI has been shown to be reliable and valid for assessing disability in patients with LBP and has been validated in Spanish. The MCID varies across studies; in this case, the smallest reported value (a change of 5 points) will be used as the reference for significant changes.

The SF-12 questionnaire, a shortened version of the SF-36, will be used for this assessment. The SF-12 consists of 12 items that evaluate eight different parameters: physical functioning, role limitations due to physical problems, bodily pain, general health, vitality, social functioning, role limitations due to emotional problems and mental health perception. The items are combined and transformed to provide both a physical and a mental health score, each ranging from 0 (worst) to 100 (best). The SF-12 has been shown to be valid and reliable in populations with chronic LBP. The MCID related to quality of life in patients with LBP has been established as improvements greater than 3.77 points in the mental component and 3.29 points in the physical component.

This variable will be measured using the S-LANSS pain scale, a self-administered instrument designed to identify patients with pain of neuropathic characteristics. It is a simple scale consisting of seven items with binary responses (yes or no), relating to the presence of symptoms (five items) or clinical signs (two items). The maximum possible score is 24 points, with scores equal to or greater than 12 considered clear indicators of the presence of a neuropathic pain component. The S-LANSS scale has been shown to be reliable for diagnosing pain with neuropathic characteristics and has been validated in Spanish.

To assess PPT a pressure algometer will be used, determining the minimum amount of pressure required for the patient to feel pain. PPT will be measured at three different points on each side: tibial nerve (posterior aspect of the knee), peroneal nerve (at the level of the fibular head), and the sciatic nerve trunk (gluteal region), repeating each measurement three times and averaging the results. According to the classification by Shrout and Fleiss, algometry measurement of PPT at the tibial nerve has shown good intra-rater reliability (ICC = 0.64) and acceptable inter-rater reliability (ICC = 0.56) in asymptomatic individuals. For the peroneal nerve, intra-rater reliability is excellent (ICC = 0.84) and inter-rater reliability is good (ICC = 0.70). For the sciatic nerve, intra-rater reliability is excellent (ICC = 0.90) and inter-rater reliability is good (ICC = 0.75).

The test will be performed with the participant in a supine position, passively raising the lower limb by flexing the hip while keeping the knee extended. A recent meta-analysis has highlighted ongoing concerns regarding its reliability and diagnostic accuracy, suggesting that incorporating structural differentiation movements may improve its performance. Based on previous research of limited quality, the test has been reported to have high sensitivity and low specificity, although results vary between studies. Likelihood ratios also differ widely across investigations, with LR+ ranging from 1.07 to 4.73 and LR- from 0.27 to 0.96. It has been determined that diagnosing radicular pain and radiculopathy in patients with LBRLP, when based solely on history and physical examination, has only slight inter-rater reliability (Kappa 0.35). In terms of interpretation, the SLR test is considered positive when it elicits familiar symptoms that relieve upon returning to the starting position.

Categorical scales will be used to record the extent to which the SLR test reproduces symptomatology recognized by the patient as familiar (not familiar, slightly familiar, considerably familiar or exact reproduction of symptoms) and the severity of any dysesthesias or paresthesias experienced (none, mild-moderate or severe). This questionnaire has demonstrated moderate to excellent inter-rater reliability (ICC = 0.78-0.96).

Data on the hip flexion ROM achieved during the SLR test will be collected. The participant will be in a supine position and the hip flexion movement will be recorded using anatomical reference points and assessed using Kinovea, a motion-analysis software. A previous study using this method reported excellent intra- and inter-rater reliability, with ICCs ranging from 0.985 to 0.999 and a minimal detectable change of 1.3-4° of hip flexion in patients with LBP. In contrast, a previous meta-analysis concluded that inter-rater reliability for ROM measurement during the SLR test is poor, with variations ranging from 13° to 20º.

An experienced ultrasound researcher will perform shear wave elastography (SWE) to assess sciatic nerve stiffness using a Logiq P9 XDclear™ system with convex (1-6 MHz) and linear (6-15 MHz) probes. Exams will be bilateral in the subgluteal fossa, with participants prone, ankle neutral, and knee extended. The nerve will be identified axially for thickness, width and area, then imaged longitudinally for SWE. Stable transducer positioning will allow simultaneous B-mode and SWE display. Stiffness will be expressed in kPa, taking at least three measurements that will be averaged for analysis. All images will be acquired by the same investigator. This method shows good reliability (ICC = 0.869), and higher stiffness (≈11.3%) has been observed in affected legs of patients with LBRLP compared to their unaffected side and to healthy controls.

Global perceived improvement will be measured using a 7-point Likert scale ranging from "much worse" to "much better." Clinical improvement will be considered in participants who report feeling "moderately better" or "much better".

Adverse events will be defined as any problem related to the intervention that causes symptoms lasting at least two days or requiring specific treatment.