Beyond visible blood loss: Rethinking perioperative bleeding in endoscopic spine surgery

Abstract

Minimally invasive spinal endoscopy is widely perceived as a low-blood-loss procedure. However, this assumption may be misleading. Emerging evidence suggests that hidden blood loss (HBL), rather than visible intraoperative bleeding, constitutes the dominant component of total blood loss in endoscopic spine surgery. The recent study by Ye et al published in the World Journal of Orthopedics on dual-media spinal endoscopy for lumbar spinal stenosis reinforces this concept by demonstrating that HBL accounts for the majority of perioperative blood loss and is associated with operative duration and preoperative hematologic parameters. These findings challenge the reliance on measured intraoperative blood loss as a surrogate for surgical burden. Continuous irrigation and limited visualization may obscure ongoing hemorrhage, creating a false perception of hemostasis. As a result, perioperative blood loss may be systematically underestimated, particularly in elderly or comorbid patients. While current estimations of HBL remain indirect and influenced by fluid dynamics, its consistent presence across endoscopic techniques suggests a need to reconsider how blood loss is defined and monitored. Future studies should clarify its clinical impact and guide perioperative risk stratification.

Key Words: Hidden blood loss; Endoscopic spine surgery; Unilateral biportal endoscopy; Lumbar spinal stenosis; Perioperative blood loss; Minimally invasive spine surgery

Core Tip: Hidden blood loss (HBL) represents a substantial and often underestimated component of perioperative bleeding in endoscopic spine surgery. As highlighted by recent evidence, irrigation-based techniques may obscure ongoing hemorrhage, which creates a disconnect between visible and physiological blood loss. While HBL is consistently observed across endoscopic procedures, its predictors remain context-dependent and its clinical relevance is not yet fully defined. Rather than relying on visible intraoperative blood loss alone, clinicians should interpret perioperative bleeding within a broader framework that accounts for procedural complexity and patient factors.

This editorial refers to “Hidden blood loss in dual media spinal endoscopy surgery: Analysis of perioperative risk factors” by Ye et al, 2026; https://doi.org/10.5312/wjo.v17.i6.119695.

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INTRODUCTION

Endoscopic spine surgery has become increasingly used in the treatment of lumbar degenerative pathology because it offers reduced soft-tissue disruption, smaller incisions, earlier mobilization, and lower estimated intraoperative blood loss compared with conventional open approaches[1,2]. In a systematic review comparing full-endoscopic, microendoscopic, and open discectomy, endoscopic techniques demonstrated substantially lower estimated blood loss, which reinforces the current perception that these procedures are “low-blood-loss”[1]. However, this assumption is being challenged[3,4]. The issue is not simply whether endoscopic techniques reduce surgical exposure, but whether visible intraoperative bleeding remains a reliable surrogate for total perioperative blood loss in an irrigation-dependent operative environment.

This distinction is clinically important because endoscopy may make bleeding less visible without necessarily making it physiologically irrelevant. Continuous irrigation dilutes and removes blood from the operative field, while venous plexus oozing, cancellous bone bleeding, and soft-tissue infiltration may remain poorly captured by conventional estimates. Recent studies in biportal endoscopic spine surgery have emphasized that visible blood loss may underestimate total blood loss, with hidden blood loss (HBL) epresenting a substantial component[5,6]. Thus, a paradox emerges: The very conditions that improve visualization during endoscopic surgery may also obscure the true magnitude of perioperative bleeding.

HBL IN DUAL-MEDIA SPINAL ENDOSCOPY

The recent study by Ye et al[7] published in the World Journal of Orthopedics extends the growing body of literature on HBL into the context of dual-media spinal endoscopy for lumbar spinal stenosis[7]. In their retrospective analysis of 146 patients, HBL accounted for the majority of total blood loss, despite the minimally invasive nature of the procedure. Operative time, total blood loss, and preoperative hematologic parameters, including hemoglobin and hematocrit, were identified as independent risk factors, while coagulation-related variables such as activated partial thromboplastin time and fibrinogen demonstrated additional associations. These findings are consistent with prior studies in endoscopic lumbar surgery, which have repeatedly shown that HBL constitutes a substantial proportion of total perioperative blood loss[3,4,6].

Importantly, the contribution of this study lies not in establishing the presence of HBL, which is well documented, but in reinforcing its relevance within a newer endoscopic platform and a stenosis-focused population. Lumbar spinal stenosis surgery often requires more extensive decompression, including bony work and ligamentous resection, which may increase the potential for occult blood loss compared with simpler disc procedures[8]. In this context, the findings of Ye et al[7] suggest that even as endoscopic techniques evolve, the discrepancy between measured and actual blood loss persists. Rather than representing an isolated observation, this study adds to a consistent signal across the literature that conventional intraoperative blood loss metrics may underestimate the true perioperative burden in endoscopic spinal surgery.

BEYOND DUAL-MEDIA ENDOSCOPY: PATTERNS ACROSS TECHNIQUES

Continuous irrigation improves visualization but dilutes blood, making direct quantification unreliable and creating a mismatch between visible intraoperative and total blood loss[9,10]. This is particularly relevant in decompressive procedures, where low-pressure epidural venous bleeding, cancellous bone oozing after laminotomy or drilling, and soft-tissue microvascular bleeding may persist under irrigation despite an apparently clear operative field[11]. In this sense, endoscopy may not eliminate blood loss as much as it redistributes it into less visible compartments. In a prospective study by Smorgick et al[12], they reported that HBL attributed to approximately 40% of total perioperative blood loss, which supports the fact that this concept is not unique to endoscopic procedures only.

This discrepancy is consistently observed across techniques. In unilateral biportal endoscopy (UBE), Wang et al[13] reported mean HBL of 469.5 ± 195.3 mL (57.6% of total blood loss), while Guo et al[14] reported 361 ± 217 mL (77.9%)[13,14]. Similar patterns are seen beyond UBE: In percutaneous endoscopic lumbar discectomy, supplementary decompression, foraminoplasty, and operative time increase HBL, suggesting that more extensive access and bony work amplify occult bleeding[15,16]. Cervical endoscopy shows comparable trends, with surgical segments and operative time independently associated with HBL, indicating that this phenomenon is not confined to lumbar procedures[4,16].

HBL is not uniform across endoscopic procedures. Its magnitude depends on operative corridor, decompression extent, number of levels, fusion requirement, and operative duration. In endoscopic lumbar fusion, Ge et al[17] identified fusion levels, hypertension, prolonged prothrombin time, and preoperative hemoglobin as associated factors, supporting the view that fusion procedures have a distinct bleeding profile compared with decompression-only surgery. Comparative studies reinforce this technique-specific interpretation: Feng et al[18] demonstrated differing total and hidden blood-loss profiles between UBE-LIF and uniportal full-endoscopic fusion, while Tuerxunyiming et al[19] showed that endoscopic fusion approaches vary in surgical invasiveness rather than sharing a single “minimal blood loss” identity. Accordingly, “endoscopic” should be regarded as a technical category, not a reliable surrogate for physiological blood-loss burden.

This creates the central tension in the field. While comparative studies generally support lower total or visible blood loss with endoscopic approaches, the hidden component remains substantial enough to challenge the clinical meaning of “low blood loss”. Peng et al[20] compared open TLIF with unilateral biportal endoscopic TLIF, providing a more procedure-matched framework than studies contrasting endoscopy with open decompression alone. Similarly, BESS studies suggest that total blood loss may be reduced relative to open surgery, yet HBL remains clinically relevant and should not be overlooked[5,21]. The strongest synthesis-level evidence, a 2025 meta-analysis of six UBE studies including 601 patients, identified BMI, operative time, hypertension, and preoperative hematocrit as significant risk factors for HBL[22]. However, these findings should be interpreted cautiously, as most studies are retrospective, geographically concentrated, and dependent on hemoglobin- or hematocrit-based estimation models rather than direct measurement.

The controversy, therefore, is not whether HBL exists, but how confidently it can be interpreted. Most studies have been based on Nadler and Gross formulas[23,24] to estimate the HBL by using the predicted blood volume and the perioperative hemoglobin or hematocrit changes according to patients sex, height, and body weight. Specifically: For men: Predicted blood volume = (0.3669 × height³) + (0.03219 × weight) + 0.6041. For women: Predicted blood volume = (0.3561 × height³) + (0.03308 × weight) + 0.1833. In these formulas, height is expressed in meters and weight in kilograms[23]. The total blood loss is then estimated as follows: Total blood loss = predicted blood volume × (preoperative hematocrit - postoperative hematocrit)/mean hematocrit.

Mean hematocrit is the average of the preoperative and postoperative hematocrit values[24]. HBL is then calculated by subtracting visible blood loss from total blood loss, with correction for transfused blood if it is used. However, when postoperative drainage is included as visible blood loss, the true blood content may vary over time. This may also affect the accuracy of estimation of HBL[25]. These methods are inherently sensitive to fluid administration, hemodilution, drain management, and timing of postoperative laboratory testing. Apparent consistency across studies may therefore reflect shared methodology rather than true biological uniformity. HBL should be viewed neither as a definitive standalone endpoint nor as a negligible artifact, but as a signal that current blood-loss reporting in endoscopic spine surgery remains incomplete. Patterns of HBL should be interpreted in relation to surgical approach and extent (Table 1).

Table 1 Hidden blood loss patterns across endoscopic spine procedures.

Procedure type	Typical HBL pattern	Main drivers	Practical implication
UBE/BESS decompression[12,13,21]	Often > 50% of total blood loss; commonly 250-500 mL	Operative time, BMI, hypertension, preoperative Hct	Visible blood loss markedly underestimates true perioperative burden
PELD/endoscopic discectomy[14,15]	Increases with access complexity and additional decompression	Foraminoplasty, number of punctures, operative time	“Minimal access” does not guarantee minimal blood loss
Cervical endoscopic procedures[4]	Present despite smaller operative field	Number of levels, operative time	HBL is not confined to lumbar procedures
Endoscopic lumbar fusion (endo-TLIF/UBE-LIF)[16-18]	More variable; often higher than decompression-only cases	Fusion levels, operative duration, comorbidity burden	Fusion should be considered a distinct bleeding profile
Endoscopic vs open surgery[5,19,20]	Lower total blood loss, but hidden component remains substantial	Technique, surgical extent, measurement method	“Low blood loss” depends on how it is measured

HBL: Hidden blood loss; UBE: Unilateral biportal endoscopy; BESS: Biportal endoscopic spine surgery; BMI: Body mass index; PELD: Percutaneous endoscopic lumbar discectomy; TLIF: Transforaminal lumbar interbody fusion; LIF: Lumbar interbody fusion.

RISK FACTORS: PREDICTION OR POST HOC EXPLANATION?

Although many studies report “risk factors” for HBL, it remains unclear whether these variables are truly predictive or simply explanatory after the fact. Operative time is consistently associated with HBL, but is likely due to the procedural burden rather than a single biological mechanism.

The same limitation applies to patient-level predictors. Variables such as body mass index, tissue thickness, hypertension, diabetes, American society of Aneshesiologists classification, and coagulation indices are inconsistently significant across studies. This variability is not merely a weakness of individual analyses but suggests that HBL risk is highly context-dependent. Accordingly, these variables are better interpreted as markers of specific procedural environments rather than universal predictors.

Hematologic variables require the most cautious interpretation as they could not be considered independent predictors for HBL. Based on the study of Ye et al[7], preoperative hemoglobin and hematocrit not only are frequently associated with HBL but are also embedded in the formulas used to calculate blood loss. Nevertheless, the present literature is more realistic in explaining HBL retrospectively than predicting clinically relevant occult blood loss preoperatively.

CLINICAL RELEVANCE: DOES HBL MATTER?

Most studies focus on quantifying HBL and identifying associated variables, but far fewer examine whether these estimates translate into meaningful patient outcomes. While postoperative decreases in hemoglobin are consistently reported, clear associations with transfusion requirements, symptomatic anemia, delayed mobilization, or increased complication rates are less consistently demonstrated[5,11,22]. As a result, the literature establishes the presence of occult blood loss more convincingly than its clinical consequences.

This creates a central paradox. If HBL frequently represents the majority of total blood loss, one would expect a measurable impact on perioperative outcomes. However, this relationship is not consistently observed. This may be due to the fact that endoscopic procedures still impose a lower overall physiological burden compared with open surgery, or that the magnitude of occult loss rarely reaches clinically actionable thresholds. In addition, many studies are not designed to evaluate outcome-based endpoints, limiting the ability to determine when HBL becomes clinically relevant[11,22].

Before surgery, anemia, coagulation abnormalities, anticoagulant or antiplatelet use, and poorly controlled hypertension should be identified. This is important because there is an association between HBL and hypertension as well as preoperative hematologic status[7,22]. During surgery, careful hemostasis of the epidural venous plexus, paraspinal soft tissues, and cancellous bone surfaces after drilling or decompression is very of primary importance[9-11]. Controlled irrigation pressure, bipolar or radiofrequency coagulation, gentle compression, and selective use of absorbable topical hemostatic materials may also help to reduce persistent occult bleeding[9,10]. After surgery, repeat hemoglobin or hematocrit testing should be considered in high risk patients even when obvious clinical symptoms are absent[5,22].

The implication is not that HBL should be dismissed, but that it should be interpreted within clinical context. HBL may serve as an indicator of surgical magnitude or perioperative physiology, but it should not be treated as a surrogate endpoint in isolation. Instead, its value lies in informing perioperative awareness and risk stratification rather than defining clinical decision-making on its own.

FUTURE DIRECTIONS

Future research should move beyond reliance on indirect estimation models and focus on improving the estimation and interpretation of perioperative blood loss in endoscopic spine surgery. Standardization of HBL calculation, particularly the timing of laboratory measurements and control of perioperative fluid balance, is essential to improve comparability and reliability across studies.

Equally important is a shift toward patient-centered outcomes. Future studies should determine whether HBL translates into clinically meaningful endpoints such as transfusion requirements, symptomatic anemia, delayed recovery, or complications. Without this, HBL risks remaining a descriptive rather than actionable metric.

Further prospective and technique-specific studies with large samples and adequate power are also needed for extracting accurate information as most of the current available evidence is retrospective with limited number of patients, which increases susceptibility to confounding, selection bias, and measurement variability. Rather than treating endoscopic spine surgery as a uniform category, future research should account for differences in surgical approach, decompression extent, and fusion strategies. Integrating blood-loss assessment into broader perioperative risk models may ultimately clarify when HBL is clinically relevant and how it should inform management.

CONCLUSION

Endoscopic spine surgery may appear “low-blood-loss,” but accumulating evidence shows that HBL represents a substantial and often overlooked component of total perioperative bleeding. The study by Ye et al[7] reinforces the gap between visible and physiological blood loss in modern endoscopic techniques. However, current predictors are inconsistent and the clinical relevance of HBL remains uncertain. HBL should therefore be interpreted within context, not as a standalone metric, but as a signal that challenges how perioperative blood loss is defined and understood.