Old technicque, new applications: Can the H-reflex be a possible Real-Time Indicator in endoscopic surgery for decompression of acute disc herniation at the S1 root?

Old technicque, new applications: Can the H-reflex be a possible Real-Time Indicator in endoscopic surgery for decompression of acute disc herniation at the S1 root? | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Case Report Old technicque, new applications: Can the H-reflex be a possible Real-Time Indicator in endoscopic surgery for decompression of acute disc herniation at the S1 root? Marta Rodrigues Carvalho, Vitor Caldas Marques, Baldomero Pinto Soares, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7707715/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 19 Dec, 2025 Read the published version in European Spine Journal → Version 1 posted 8 You are reading this latest preprint version Abstract Background: The Hoffmann (H-) reflex is a monosynaptic electrophysiological response that reflects S1-root integrity. Although widely used in experimental neurophysiology, its intra-operative application during endoscopic lumbar surgery is rarely reported. We present the first real-time documentation of H-reflex normalization immediately after endoscopic decompression of an acute L5–S1 disc herniation. Case Description: A 49-year-old woman presented with acute right-sided S1 radiculopathy (ankle dorsiflexion/plantar-flexion MRC grade III; hallux extension grade I). MRI showed a large, right sub-articular L5–S1 disc extrusion compressing the S1 nerve root. Unilateral biportal endoscopic discectomy was performed under general anesthesia with multimodal intra-operative neuromonitoring (MEPs, free-run EMG, and continuous tibial-nerve H-reflex). Baseline recordings revealed marked side-to-side asymmetry: right-leg MEP amplitudes were depressed, and the right H-reflex was low and unstable. During foraminoplasty and fragment removal, a sudden, sustained 100 % surge in right H-reflex amplitude occurred, coincidence with root decompression, while MEPs remained unchanged. Free-run EMG discharges abated after the H-reflex improved. Post-operative recovery was uneventful. At 2 months the patient demonstrated near-complete motor recovery (MRC IV–V) and full pain relief. Conclusion: Continuous H-reflex monitoring provided an immediate, sensitive marker of S1-root decompression when conventional MEPs failed to reflect functional recovery. This single-case experience supports the feasibility and potential prognostic value of adding H-reflex surveillance to the neuromonitoring toolkit for endoscopic lumbar surgery. Prospective studies in larger cohorts are warranted to validate threshold criteria, refine stimulation parameters, and determine cost-effectiveness. Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction The H-reflex (Hoffmann’s reflex), originally described by Hoffmann in 1910, is an electrically evoked monosynaptic spinal reflex that directly stimulates the Ia fibers of the tibial nerve and is classically recorded from the soleus muscle. Functionally, it represents the electrophysiological analogue of the stretch reflex, allowing quantitative assessment of α-motoneuron excitability and presynaptic inhibitory mechanisms at the spinal level. Parameters such as H-reflex latency, maximal amplitude (Hₘₐₓ), and the Hₘₐₓ/Mₘₐₓ ratio are widely used to characterize the state of the reflex arc and have been methodologically refined to improve reproducibility of results 1 – 3 Endoscopic decompression of an L5–S1 disc herniation presents unique anatomical challenges: a high iliac crest can obstruct the transforaminal trajectory of the working cannula; the ventral inclination of the disc space reduces the angle of approach; and a narrow neuroforamen compromises S1-root exposure. These factors increase technical complexity and steepen the learning curve, often requiring targeted foraminoplasty or transiliac approaches to bypass the ilium and ensure a safe, effective corridor 4 Intraoperative neuromonitoring techniques, including continuous H-reflex recording and somatosensory evoked potentials, have been employed to provide real-time feedback on the functional integrity of nerve roots, enabling immediate surgical adjustments. Classic studies by Mustafa et al. demonstrated the prognostic value of late responses (H-reflex and F-wave) in L5–S1 discectomy, correlating neurophysiological changes with functional recovery, while recent investigations validate the feasibility of monitoring in endoscopic discectomy, showing reductions in dysesthesias and improved prediction of postoperative neurological deficits 5 , 6 Therefor the objective of the present case report is to, Therefor the objective of the present case report is to describe the utility of intraoperative H-reflex monitoring in lumbosacral spine surgery as a parameter reflecting immediate neurophysiological improvement following surgical decompression of a large herniated disc. Case report A 49-year-old woman, previously healthy, presented to the emergency department with acute low back pain radiating into the right lower limb, accompanied by paresthesias and distal motor weakness. On neurological examination, the left lower extremity demonstrated full strength (MRC grade V) in all myotomes; on the right, hip flexion and knee extension were preserved at grade V, whereas ankle dorsiflexion and plantarflexion were reduced to grade III, and hallux extension was markedly weak at grade I. Lumbosacral MRI revealed a right-sided subarticular disc protrusion with an annular fissure indenting the thecal sac and displacing the descending S1 nerve root (Fig. 1 ). After consultation with the neurosurgery team, the patient underwent endoscopic lumbar spine surgery under intraoperative neurophysiological monitoring (IONM). A single bolus of succinylcholine was administered for endotracheal intubation. General anesthesia was induced and maintained with propofol and fentanyl. Before surgical manipulation, MEPs were obtained from the adductor muscles, vastus lateralis, tibialis anterior, peroneus longus, medial gastrocnemius, abductor hallucis brevis, and abductor pollicis brevis. Asymmetrical MEPs were observed on the right, with reduced amplitudes in the extensor hallucis, peroneus longus, soleus, and medial gastrocnemius compared to the contralateral side (Fig. 2 ). Initial free electromyography showed spontaneous firing in the right L5-S1 roots. An H-reflex was performed bilaterally. Stimulation was applied using needle electrodes in the popliteal fossa, with the cathode placed proximally and the anode 3 cm distally. Stimulus duration was 1.0 ms, with intensity adjusted to maximize H-reflex amplitude (up to 15 mA). A low-frequency filter of 100 Hz and a high-frequency filter of 3 kHz were used. Subdermal needle electrodes were inserted into the soleus muscle for recording. The H-reflex waves showed asymmetry in amplitude, with significantly lower values on the right compared to the left (Fig. 3 ). Decompression was effectively completed at 10:23 p.m., with a marked improvement in the right H-reflex amplitude compared to baseline (approximately 100%), which was maintained throughout the procedure (Fig. 4 ). No changes in MEP amplitudes were observed. The patient was evaluated 1 and 2 months after surgery. At the second follow-up visit, she showed substantial improvement in strength in all segments of the lower limb (IV/V) and complete improvement in pain. Discussion The Hoffmann reflex (H-reflex) is one of the most studied reflexes in human and mammalian neurophysiology. It can be elicited in more than 20 muscles and involves a relatively simple circuit: electrical stimulation directly activates 1a afferent fibers, which synapse in the spinal cord and excite alpha motor neurons, resulting in muscle contraction. Changes in H-reflex amplitude can reflect: - Changes in motor neuron excitability; - Variations in neurotransmitter release at synaptic terminals; - Alterations in intrinsic membrane properties of motor neurons 2 . The present study highlights both the diagnostic and prognostic utility of the H-reflex in acute radiculopathy, particularly when MEPs remain unchanged despite adequate neural release. The use of intra-operative neurophysiological monitoring (IONM) for endoscopic spine surgery has consistently been associated with greater patient safety and fewer neurological sequelae. In a retrospective cohort of 127 unilateral biportal endoscopic (UBE) decompressions, Bai et al. (2025) employed multimodal IONM, somatosensory evoked potentials (SEPs), motor evoked potentials (MEPs) and free-run electromyography (EMG), and compared outcomes with an unmonitored control group. The monitored cohort had significantly lower postoperative leg-pain scores (mean VAS 1.8 ± 0.4 vs. 2.1 ± 0.2; p < 0.001) without any prolongation of operative time or deterioration of other peri-operative indices 7 . Furthermore, in a comprehensive systematic review by Zanin et al. (2025) pooled data from twenty-one clinical investigations and two economic evaluations spanning the entire spectrum of spinal surgery. Their meta-analysis confirmed that MEPs offer the greatest sensitivity (90.2 ), whereas SEPs provide the greatest specificity (97.1 ) for early detection of neurological compromise. Economic modelling further indicated that, when the baseline risk of neurological injury exceeds 0.3, routine multimodal IONM is cost-saving, averting an average of USD 23 189 per case 8 . The literature is still scarce regarding the use of H-relfex as prognostic tool for endoscopic spine surgery. Previous studies demonstrated the feasibility of H-reflex monitoring but did not focus on amplitude enhancement during decompression 9 , 10 . Egli et al. (2007) described postoperative improvements recorded days after surgery 11 , whereas Makovec et al. (2010) suggested that continuous H-reflex surveillance can guide intra-operative manipulation by alerting surgeons to prolonged or excessive root traction 12 . More recently, Tang et al. (2025) prospectively monitored 78 UBE discectomies with continuous EMG and H-reflex recording. Abnormal H-wave discharges (spike, burst or tonic patterns) closely mirrored direct root manipulation or excessive traction, prompting immediate surgical adjustments; no permanent motor deficits ensued 13 . Finally, the longer-term prognostic value of H-reflex restoration has been corroborated in both open and endoscopic settings. Sarmast et al. (2018) tracked H-reflex latencies and amplitudes in fifty patients undergoing lumbar-disc surgery and found that normalisation by six months strongly predicted pain resolution and functional recovery (sensitivity 56 %, specifcity 67 %) 14 . The authors hypothesise that the H-reflex, by interrogating the monosynaptic S1 circuit at the precise site of injury, provides greater sensitivity to acute conduction changes than MEPs, which traverse longer, polysynaptic pathways and are more vulnerable to anaesthetic attenuation or movement artefact. Mechanical decompression probably alleviates ischaemia or conduction block in Ia afferents or α-motor fibres, restoring reflex transmission almost instantaneously. In contrast, MEP amplitudes may lag because of anaesthetic effects, longer pathway latency, or the subtler magnitude of functional recovery. To our knowledge, this is among the first descriptions of a real-time intra-operative surge in tibial-nerve H-reflex amplitude immediately after S1 decompression, underscoring its potential as a dynamic biomarker of neurological restoration. The main limitation is that it is a single-case study. The findings, while promising, cannot be generalized to a larger patient population. The observed response may be unique to this patient or this specific situation. Further studies with larger cohorts and prospective validation are warranted to define its clinical value and standardize its use in spine surgery. Conclusion This illustrative case highlights both the diagnostic and prognostic utility of the H-reflex in acute radiculopathy, particularly when MEPs remain unchanged despite adequate neural release, suggesting that continuous intraoperative H-reflex monitoring during endoscopic lumbar decompression at the L5–S1 level is both feasible and informative. While promising, these findings derive from a single case and require validation in larger, prospective cohorts. Future studies should aim to standardize stimulation parameters, define threshold criteria for surgical decision-making, and compare clinical outcomes with and without H-reflex integration. Such research will clarify the role of H-reflex monitoring in optimizing the safety and efficacy of minimally invasive endoscopic spine surgery. Declarations Funding No funding was received for this study. Availability of data and materials No data are available online; however, under reasonable request, the data can be shared by the author. Authors roles MRC (Manuscript writing, Manuscript revision, Image elaboration, Data collection, Manuscript Conceptualization) VCM (Manuscript writing, Manuscript revision, Image elaboration, Data collection, Manuscript Conceptualization) BPS (Manuscript revision, Data collection) GP (Manuscript writing, Manuscript revision, Image elaboration) TP (Manuscript writing, Manuscript Conceptualization) MB (Manuscript revision) RG (Manuscript revision) Clinical Trial Number: Not Applicable Ethical Approval: This is an case-report study. The Research Ethics Committee has confirmed that no ethical approval is required. However, the patient provided explicitly authorization for the use of their de-identified data by filling and free-consent form. Consent to participate: Informed consent was obtained from the individual participant included in the study. References Hoffmann P. Beitrag zur Kenntnis der menschlichen Reflexe mit besonderer Berücksichtigung der elektrischen Erscheinungen. Arch Anat Physiol 1910; 1 : 223–246. Misiaszek JE. The H‐reflex as a tool in neurophysiology: Its limitations and uses in understanding nervous system function. Muscle Nerve 2003; 28 : 144–160. Knikou M. The H-reflex as a probe: Pathways and pitfalls. J Neurosci Methods 2008; 171 : 1–12. Chen K-T, Wei S-T, Tseng C, Ou S-W, Sun L-W, Chen C-M. Transforaminal Endoscopic Lumbar Discectomy for L5–S1 Disc Herniation With High Iliac Crest: Technical Note and Preliminary Series. Neurospine 2020; 17 : S81–S87. Mustafa YH, Aboelhassan S, Abdelsalam AI. Prognostic value of late response (F-wave and H-reflex) in surgical management of lumbar disc herniation (L5-S1). Al-Azhar Assiut Med J 2015; 13 : 67–75. de Carvalho PST, Ramos MRF, da Silva Meireles AC, Peixoto A, de Carvalho P, Ramírez León JF et al. Feasibility of Using Intraoperative Neuromonitoring in the Prophylaxis of Dysesthesia in Transforaminal Endoscopic Discectomies of the Lumbar Spine. Brain Sci 2020; 10 : 522. Bai T-Y, Meng H, Lin J-S, Fan Z-H, Fei Q. Application of intraoperative neurophysiological monitoring in unilateral biportal endoscopic lumbar spine surgery. J Orthop Surg Res 2025; 20 : 334. Zanin L, Broglio L, Panciani PP, Bergomi R, De Rosa G, Ricciardi L et al. Intraoperative Neurophysiological Monitoring in Contemporary Spinal Surgery: A Systematic Review of Clinical Outcomes and Cost-Effectiveness. Brain Sci 2025; 15 : 768. Tang S, Liao Y, Pan J, Chen D, Pan D. The preliminary application of electromyography in unilateral biportal endoscopy with general anesthesia for lumbar disc herniation. Front Surg 2025; 12 . doi:10.3389/fsurg.2025.1498878. Sarmast A, Kirmani A, Bhat A. Evaluation of role of electrophysiological studies in patients with lumbar disc disease. Asian J Neurosurg 2018; 13 : 585–589. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 19 Dec, 2025 Read the published version in European Spine Journal → Version 1 posted Editorial decision: Revision requested 14 Oct, 2025 Reviews received at journal 12 Oct, 2025 Reviewers agreed at journal 09 Oct, 2025 Reviewers agreed at journal 08 Oct, 2025 Reviewers invited by journal 08 Oct, 2025 Editor assigned by journal 05 Oct, 2025 Submission checks completed at journal 05 Oct, 2025 First submitted to journal 24 Sep, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7707715","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Case Report","associatedPublications":[],"authors":[{"id":520837863,"identity":"10575b4e-3f12-429c-8e44-5abfe1a6f13e","order_by":0,"name":"Marta Rodrigues Carvalho","email":"","orcid":"","institution":"Instituto Hospital de Base do Distrito Federal","correspondingAuthor":false,"prefix":"","firstName":"Marta","middleName":"Rodrigues","lastName":"Carvalho","suffix":""},{"id":520837864,"identity":"1103aee8-d0eb-472e-baac-0db96bfaf154","order_by":1,"name":"Vitor Caldas Marques","email":"","orcid":"","institution":"Instituto 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A: anterior; I: inferior; R: right; P: posterior.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7707715/v1/1d75329781345f8ceac82262.png"},{"id":92801948,"identity":"15200eb4-8bac-469d-a773-305c5d95da4f","added_by":"auto","created_at":"2025-10-05 11:39:55","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":509664,"visible":true,"origin":"","legend":"\u003cp\u003eIntra-operative transcranial motor-evoked potentials (MEPs) showing marked side-to-side asymmetry. The right-leg montage (top row) exhibits uniformly low-amplitude, polyphasic compound muscle action potentials across distal muscles—most noticeably in the tibialis anterior, peroneus longus, gastrocnemius and soleus—whereas the left-leg montage (bottom row) retains robust, high-amplitude responses on the same channels. The discrepancy, recorded with identical stimulation parameters (750 µV, 25 ms), highlights unilateral corticospinal conduction compromise affecting the right lower limb despite preserved proximal activity (vastus medialis).\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7707715/v1/726271aa40e7c008d907a494.png"},{"id":92803361,"identity":"6b079718-e8c4-4001-8375-89cec74f7683","added_by":"auto","created_at":"2025-10-05 11:55:55","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":389766,"visible":true,"origin":"","legend":"\u003cp\u003eStimulus intensity at which the H-reflex appeared most stable: 13-15 mA. A and B: Left H reflex showing assymetrical responses (bigger in left side than right side) before and after disc herniation decompression. C) Low amplitude H reflex, sometimes appearing in an unstable manner ; D) During decompression: blue arrow indicates the moment when the hernia is removed and immediately after removal H reflex responses emerged in a more consistent and reproducible manner.\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7707715/v1/2078d78e16444e64e4847c13.jpeg"},{"id":92802923,"identity":"fb542c41-1a4e-4a5a-94e3-1e25a053ba56","added_by":"auto","created_at":"2025-10-05 11:47:55","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":183424,"visible":true,"origin":"","legend":"\u003cp\u003eRight Reflex H at the end of the surgery indicating stability and maintenance of responses.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7707715/v1/425e3c98309d4c3ab1d96311.png"},{"id":98813953,"identity":"853081c9-ea16-4fd6-bfb3-5922e4eb872e","added_by":"auto","created_at":"2025-12-22 16:08:21","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1955848,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7707715/v1/7a8f7c5c-7d4c-4b2c-ae0f-48ef7774d62b.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Old technicque, new applications: Can the H-reflex be a possible Real-Time Indicator in endoscopic surgery for decompression of acute disc herniation at the S1 root?","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe H-reflex (Hoffmann\u0026rsquo;s reflex), originally described by Hoffmann in 1910, is an electrically evoked monosynaptic spinal reflex that directly stimulates the Ia fibers of the tibial nerve and is classically recorded from the soleus muscle. Functionally, it represents the electrophysiological analogue of the stretch reflex, allowing quantitative assessment of α-motoneuron excitability and presynaptic inhibitory mechanisms at the spinal level. Parameters such as H-reflex latency, maximal amplitude (Hₘₐₓ), and the Hₘₐₓ/Mₘₐₓ ratio are widely used to characterize the state of the reflex arc and have been methodologically refined to improve reproducibility of results \u003csup\u003e\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eEndoscopic decompression of an L5\u0026ndash;S1 disc herniation presents unique anatomical challenges: a high iliac crest can obstruct the transforaminal trajectory of the working cannula; the ventral inclination of the disc space reduces the angle of approach; and a narrow neuroforamen compromises S1-root exposure. These factors increase technical complexity and steepen the learning curve, often requiring targeted foraminoplasty or transiliac approaches to bypass the ilium and ensure a safe, effective corridor \u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eIntraoperative neuromonitoring techniques, including continuous H-reflex recording and somatosensory evoked potentials, have been employed to provide real-time feedback on the functional integrity of nerve roots, enabling immediate surgical adjustments. Classic studies by Mustafa et al. demonstrated the prognostic value of late responses (H-reflex and F-wave) in L5\u0026ndash;S1 discectomy, correlating neurophysiological changes with functional recovery, while recent investigations validate the feasibility of monitoring in endoscopic discectomy, showing reductions in dysesthesias and improved prediction of postoperative neurological deficits \u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eTherefor the objective of the present case report is to,\u003c/p\u003e\u003cp\u003eTherefor the objective of the present case report is to describe the utility of intraoperative H-reflex monitoring in lumbosacral spine surgery as a parameter reflecting immediate neurophysiological improvement following surgical decompression of a large herniated disc.\u003c/p\u003e"},{"header":"Case report","content":"\u003cp\u003eA 49-year-old woman, previously healthy, presented to the emergency department with acute low back pain radiating into the right lower limb, accompanied by paresthesias and distal motor weakness. On neurological examination, the left lower extremity demonstrated full strength (MRC grade V) in all myotomes; on the right, hip flexion and knee extension were preserved at grade V, whereas ankle dorsiflexion and plantarflexion were reduced to grade III, and hallux extension was markedly weak at grade I. Lumbosacral MRI revealed a right-sided subarticular disc protrusion with an annular fissure indenting the thecal sac and displacing the descending S1 nerve root (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAfter consultation with the neurosurgery team, the patient underwent endoscopic lumbar spine surgery under intraoperative neurophysiological monitoring (IONM). A single bolus of succinylcholine was administered for endotracheal intubation. General anesthesia was induced and maintained with propofol and fentanyl. Before surgical manipulation, MEPs were obtained from the adductor muscles, vastus lateralis, tibialis anterior, peroneus longus, medial gastrocnemius, abductor hallucis brevis, and abductor pollicis brevis. Asymmetrical MEPs were observed on the right, with reduced amplitudes in the extensor hallucis, peroneus longus, soleus, and medial gastrocnemius compared to the contralateral side (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Initial free electromyography showed spontaneous firing in the right L5-S1 roots.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAn H-reflex was performed bilaterally. Stimulation was applied using needle electrodes in the popliteal fossa, with the cathode placed proximally and the anode 3 cm distally. Stimulus duration was 1.0 ms, with intensity adjusted to maximize H-reflex amplitude (up to 15 mA). A low-frequency filter of 100 Hz and a high-frequency filter of 3 kHz were used. Subdermal needle electrodes were inserted into the soleus muscle for recording.\u003c/p\u003e\u003cp\u003eThe H-reflex waves showed asymmetry in amplitude, with significantly lower values on the right compared to the left (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eDecompression was effectively completed at 10:23 p.m., with a marked improvement in the right H-reflex amplitude compared to baseline (approximately 100%), which was maintained throughout the procedure (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). No changes in MEP amplitudes were observed.\u003c/p\u003e\u003cp\u003eThe patient was evaluated 1 and 2 months after surgery. At the second follow-up visit, she showed substantial improvement in strength in all segments of the lower limb (IV/V) and complete improvement in pain.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe Hoffmann reflex (H-reflex) is one of the most studied reflexes in human and mammalian neurophysiology. It can be elicited in more than 20 muscles and involves a relatively simple circuit: electrical stimulation directly activates 1a afferent fibers, which synapse in the spinal cord and excite alpha motor neurons, resulting in muscle contraction. Changes in H-reflex amplitude can reflect: - Changes in motor neuron excitability; - Variations in neurotransmitter release at synaptic terminals; - Alterations in intrinsic membrane properties of motor neurons\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. The present study highlights both the diagnostic and prognostic utility of the H-reflex in acute radiculopathy, particularly when MEPs remain unchanged despite adequate neural release.\u003c/p\u003e\u003cp\u003eThe use of intra-operative neurophysiological monitoring (IONM) for endoscopic spine surgery has consistently been associated with greater patient safety and fewer neurological sequelae. In a retrospective cohort of 127 unilateral biportal endoscopic (UBE) decompressions, Bai et al. (2025) employed multimodal IONM, somatosensory evoked potentials (SEPs), motor evoked potentials (MEPs) and free-run electromyography (EMG), and compared outcomes with an unmonitored control group. The monitored cohort had significantly lower postoperative leg-pain scores (mean VAS 1.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4 vs. 2.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2; \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001) without any prolongation of operative time or deterioration of other peri-operative indices\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. Furthermore, in a comprehensive systematic review by Zanin et al. (2025) pooled data from twenty-one clinical investigations and two economic evaluations spanning the entire spectrum of spinal surgery. Their meta-analysis confirmed that MEPs offer the greatest sensitivity (90.2 ), whereas SEPs provide the greatest specificity (97.1 ) for early detection of neurological compromise. Economic modelling further indicated that, when the baseline risk of neurological injury exceeds 0.3, routine multimodal IONM is cost-saving, averting an average of USD 23 189 per case\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eThe literature is still scarce regarding the use of H-relfex as prognostic tool for endoscopic spine surgery. Previous studies demonstrated the feasibility of H-reflex monitoring but did not focus on amplitude enhancement during decompression \u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. Egli et al. (2007) described postoperative improvements recorded days after surgery\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e, whereas Makovec et al. (2010) suggested that continuous H-reflex surveillance can guide intra-operative manipulation by alerting surgeons to prolonged or excessive root traction\u003csup\u003e12\u003c/sup\u003e. More recently, Tang et al. (2025) prospectively monitored 78 UBE discectomies with continuous EMG and H-reflex recording. Abnormal H-wave discharges (spike, burst or tonic patterns) closely mirrored direct root manipulation or excessive traction, prompting immediate surgical adjustments; no permanent motor deficits ensued \u003csup\u003e13\u003c/sup\u003e. Finally, the longer-term prognostic value of H-reflex restoration has been corroborated in both open and endoscopic settings. \u003cb\u003eSarmast et al. (2018)\u003c/b\u003e tracked H-reflex latencies and amplitudes in fifty patients undergoing lumbar-disc surgery and found that normalisation by six months strongly predicted pain resolution and functional recovery (sensitivity 56 %, specifcity 67 %)\u003csup\u003e14\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eThe authors hypothesise that the H-reflex, by interrogating the monosynaptic S1 circuit at the precise site of injury, provides greater sensitivity to acute conduction changes than MEPs, which traverse longer, polysynaptic pathways and are more vulnerable to anaesthetic attenuation or movement artefact. Mechanical decompression probably alleviates ischaemia or conduction block in Ia afferents or α-motor fibres, restoring reflex transmission almost instantaneously. In contrast, MEP amplitudes may lag because of anaesthetic effects, longer pathway latency, or the subtler magnitude of functional recovery. To our knowledge, this is among the first descriptions of a real-time intra-operative surge in tibial-nerve H-reflex amplitude immediately after S1 decompression, underscoring its potential as a dynamic biomarker of neurological restoration.\u003c/p\u003e\u003cp\u003eThe main limitation is that it is a single-case study. The findings, while promising, cannot be generalized to a larger patient population. The observed response may be unique to this patient or this specific situation. Further studies with larger cohorts and prospective validation are warranted to define its clinical value and standardize its use in spine surgery.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThis illustrative case highlights both the diagnostic and prognostic utility of the H-reflex in acute radiculopathy, particularly when MEPs remain unchanged despite adequate neural release, suggesting that continuous intraoperative H-reflex monitoring during endoscopic lumbar decompression at the L5\u0026ndash;S1 level is both feasible and informative.\u003c/p\u003e\u003cp\u003eWhile promising, these findings derive from a single case and require validation in larger, prospective cohorts. Future studies should aim to standardize stimulation parameters, define threshold criteria for surgical decision-making, and compare clinical outcomes with and without H-reflex integration. Such research will clarify the role of H-reflex monitoring in optimizing the safety and efficacy of minimally invasive endoscopic spine surgery.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNo funding was received for this study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNo data are available online; however, under reasonable request, the data can be shared by the author.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors roles\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eMRC (Manuscript writing, Manuscript revision, Image elaboration, Data collection, Manuscript Conceptualization)\u003c/p\u003e\n\u003cp\u003eVCM (Manuscript writing, Manuscript revision, Image elaboration, Data collection, Manuscript Conceptualization)\u003c/p\u003e\n\u003cp\u003eBPS (Manuscript revision, Data collection)\u003c/p\u003e\n\u003cp\u003eGP (Manuscript writing, Manuscript revision, Image elaboration)\u003c/p\u003e\n\u003cp\u003eTP (Manuscript writing, Manuscript Conceptualization)\u003c/p\u003e\n\u003cp\u003eMB (Manuscript revision)\u003c/p\u003e\n\u003cp\u003eRG (Manuscript revision)\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eClinical Trial Number:\u0026nbsp;\u003c/strong\u003eNot Applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical Approval:\u0026nbsp;\u003c/strong\u003eThis is an case-report study. The Research Ethics Committee has confirmed that no ethical approval is required. However, the patient provided explicitly authorization for the use of their de-identified data by filling and free-consent form.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to participate:\u0026nbsp;\u003c/strong\u003e\u003cem\u003eInformed consent was obtained from the individual participant included in the study.\u003c/em\u003e\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eHoffmann P. Beitrag zur Kenntnis der menschlichen Reflexe mit besonderer Ber\u0026uuml;cksichtigung der elektrischen Erscheinungen. \u003cem\u003eArch Anat Physiol\u003c/em\u003e 1910; \u003cstrong\u003e1\u003c/strong\u003e: 223\u0026ndash;246.\u003c/li\u003e\n\u003cli\u003eMisiaszek JE. The H‐reflex as a tool in neurophysiology: Its limitations and uses in understanding nervous system function. \u003cem\u003eMuscle Nerve\u003c/em\u003e 2003; \u003cstrong\u003e28\u003c/strong\u003e: 144\u0026ndash;160.\u003c/li\u003e\n\u003cli\u003eKnikou M. The H-reflex as a probe: Pathways and pitfalls. \u003cem\u003eJ Neurosci Methods\u003c/em\u003e 2008; \u003cstrong\u003e171\u003c/strong\u003e: 1\u0026ndash;12.\u003c/li\u003e\n\u003cli\u003eChen K-T, Wei S-T, Tseng C, Ou S-W, Sun L-W, Chen C-M. Transforaminal Endoscopic Lumbar Discectomy for L5\u0026ndash;S1 Disc Herniation With High Iliac Crest: Technical Note and Preliminary Series. \u003cem\u003eNeurospine\u003c/em\u003e 2020; \u003cstrong\u003e17\u003c/strong\u003e: S81\u0026ndash;S87.\u003c/li\u003e\n\u003cli\u003eMustafa YH, Aboelhassan S, Abdelsalam AI. Prognostic value of late response (F-wave and H-reflex) in surgical management of lumbar disc herniation (L5-S1). \u003cem\u003eAl-Azhar Assiut Med J\u003c/em\u003e 2015; \u003cstrong\u003e13\u003c/strong\u003e: 67\u0026ndash;75.\u003c/li\u003e\n\u003cli\u003ede Carvalho PST, Ramos MRF, da Silva Meireles AC, Peixoto A, de Carvalho P, Ram\u0026iacute;rez Le\u0026oacute;n JF \u003cem\u003eet al.\u003c/em\u003e Feasibility of Using Intraoperative Neuromonitoring in the Prophylaxis of Dysesthesia in Transforaminal Endoscopic Discectomies of the Lumbar Spine. \u003cem\u003eBrain Sci\u003c/em\u003e 2020; \u003cstrong\u003e10\u003c/strong\u003e: 522.\u003c/li\u003e\n\u003cli\u003eBai T-Y, Meng H, Lin J-S, Fan Z-H, Fei Q. Application of intraoperative neurophysiological monitoring in unilateral biportal endoscopic lumbar spine surgery. \u003cem\u003eJ Orthop Surg Res\u003c/em\u003e 2025; \u003cstrong\u003e20\u003c/strong\u003e: 334.\u003c/li\u003e\n\u003cli\u003eZanin L, Broglio L, Panciani PP, Bergomi R, De Rosa G, Ricciardi L \u003cem\u003eet al.\u003c/em\u003e Intraoperative Neurophysiological Monitoring in Contemporary Spinal Surgery: A Systematic Review of Clinical Outcomes and Cost-Effectiveness. \u003cem\u003eBrain Sci\u003c/em\u003e 2025; \u003cstrong\u003e15\u003c/strong\u003e: 768.\u003c/li\u003e\n\u003cli\u003eTang S, Liao Y, Pan J, Chen D, Pan D. The preliminary application of electromyography in unilateral biportal endoscopy with general anesthesia for lumbar disc herniation. \u003cem\u003eFront Surg\u003c/em\u003e 2025; \u003cstrong\u003e12\u003c/strong\u003e. doi:10.3389/fsurg.2025.1498878.\u003c/li\u003e\n\u003cli\u003eSarmast A, Kirmani A, Bhat A. Evaluation of role of electrophysiological studies in patients with lumbar disc disease. \u003cem\u003eAsian J Neurosurg\u003c/em\u003e 2018; \u003cstrong\u003e13\u003c/strong\u003e: 585\u0026ndash;589.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"european-spine-journal","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"esjo","sideBox":"Learn more about [European Spine Journal](http://link.springer.com/journal/586)","snPcode":"586","submissionUrl":"https://submission.springernature.com/new-submission/586/3","title":"European Spine Journal","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-7707715/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7707715/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003cstrong\u003eBackground:\u003c/strong\u003e The Hoffmann (H-) reflex is a monosynaptic electrophysiological response that reflects S1-root integrity. Although widely used in experimental neurophysiology, its intra-operative application during endoscopic lumbar surgery is rarely reported. We present the first real-time documentation of H-reflex normalization immediately after endoscopic decompression of an acute L5–S1 disc herniation.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCase Description:\u003c/strong\u003e A 49-year-old woman presented with acute right-sided S1 radiculopathy (ankle dorsiflexion/plantar-flexion MRC grade III; hallux extension grade I). MRI showed a large, right sub-articular L5–S1 disc extrusion compressing the S1 nerve root. Unilateral biportal endoscopic discectomy was performed under general anesthesia with multimodal intra-operative neuromonitoring (MEPs, free-run EMG, and continuous tibial-nerve H-reflex). Baseline recordings revealed marked side-to-side asymmetry: right-leg MEP amplitudes were depressed, and the right H-reflex was low and unstable. During foraminoplasty and fragment removal, a sudden, sustained 100 % surge in right H-reflex amplitude occurred, coincidence with root decompression, while MEPs remained unchanged. Free-run EMG discharges abated after the H-reflex improved. Post-operative recovery was uneventful. At 2 months the patient demonstrated near-complete motor recovery (MRC IV–V) and full pain relief.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConclusion:\u003c/strong\u003e Continuous H-reflex monitoring provided an immediate, sensitive marker of S1-root decompression when conventional MEPs failed to reflect functional recovery. This single-case experience supports the feasibility and potential prognostic value of adding H-reflex surveillance to the neuromonitoring toolkit for endoscopic lumbar surgery. Prospective studies in larger cohorts are warranted to validate threshold criteria, refine stimulation parameters, and determine cost-effectiveness.\u003c/p\u003e","manuscriptTitle":"Old technicque, new applications: Can the H-reflex be a possible Real-Time Indicator in endoscopic surgery for decompression of acute disc herniation at the S1 root?","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-05 11:39:50","doi":"10.21203/rs.3.rs-7707715/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-10-14T11:47:11+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-10-12T08:30:37+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"192664442147509788169336134312790128683","date":"2025-10-09T11:15:24+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"131711187219137068680559370902332611165","date":"2025-10-08T13:44:42+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-10-08T13:29:12+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-10-06T02:54:09+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-10-06T02:54:04+00:00","index":"","fulltext":""},{"type":"submitted","content":"European Spine Journal","date":"2025-09-25T01:52:04+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"european-spine-journal","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"esjo","sideBox":"Learn more about [European Spine Journal](http://link.springer.com/journal/586)","snPcode":"586","submissionUrl":"https://submission.springernature.com/new-submission/586/3","title":"European Spine Journal","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"4e022d5a-c234-49a4-b5cd-03fa25db661e","owner":[],"postedDate":"October 5th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-12-22T16:01:27+00:00","versionOfRecord":{"articleIdentity":"rs-7707715","link":"https://doi.org/10.1007/s00586-025-09634-x","journal":{"identity":"european-spine-journal","isVorOnly":false,"title":"European Spine Journal"},"publishedOn":"2025-12-19 15:57:43","publishedOnDateReadable":"December 19th, 2025"},"versionCreatedAt":"2025-10-05 11:39:50","video":"","vorDoi":"10.1007/s00586-025-09634-x","vorDoiUrl":"https://doi.org/10.1007/s00586-025-09634-x","workflowStages":[]},"version":"v1","identity":"rs-7707715","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7707715","identity":"rs-7707715","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}