This study aims to introduce the LOEM system as a compact but descriptive tool to standardize the extent of ESS performed in individual CRS cases, by providing descriptions of both the extent of the surgical approach and the mucosal treatment. With an evaluation of an expert, the LOEM system could be connected to diagnostic cases and surgical interventions allowing for a proof-of-concept in the field of Rhinology. Furthermore, the LOEM classification condenses expert surgical knowledge in CRS into a structured and simplified format, which may pave the way towards the collection of more comprehensive datasets in CRS.

Need for CRS surgical standardization

There is an outstanding need for standardization of surgeries in CRS management [30]. Although various metrics are used to quantify radiologic [31], nasal polyp size [32] and endoscopic outcomes [33], few attempts have been made to comprehensively classify ESS. Some approaches exist in the literature for frontal sinus surgery [22, 23, 34], maxillary ostium enlargement [35] and sphenoid sinus ostium [36]; however, developing a more comprehensive system able to group interventions conducted in all sinuses remains a challenge. In 2013, the Japanese Rhinology Society proposed a classification system for ESS, which comprises five types: removal of the ostiomeatal complex (Type I), single sinus procedures (Type II), multiple sinus procedures (Type III), full-house surgeries (Type IV), and extended procedures beyond the sinus wall (Type V) [37]. Nevertheless, this system only numbers the procedure types without specifying the anatomical structures involved and does not describe relevant surgical details, such as the intervened paranasal sinuses, the extension of the ostium drainage, or the mucosa treatment. Another approach is the ACCESS system [38], which based on the Lund-Mackay score, provides a quantitative measure of the extent of ESS based on postoperative CT images. This classification allows the assessment of functional sinus patency and topical medication but does not define the actions performed on the mucosa. The LOEM system provides a comprehensive framework that encompasses the anatomical extent of surgery, ostium modifications and mucosal treatment. This enhances precision, evaluation and international comparability in endoscopic sinus surgery, while supporting the development of evidence-based guidelines for these procedures.

Although there is a consensus on the need for collecting more comprehensive datasets to analyze the true impact of different ESS, interchangeable definitions of complete, extended, and radical FESS based on non-standardized terminology persist in the literature, which hinder our ability to establish guidelines for patient management and evaluation of surgical outcomes [17, 25]. This ambiguity has led to misunderstandings about the extent of ESS and the management of the sinonasal mucosa. This is especially problematic at a time when new pathways for sinonasal mucosal inflammation are being identified, promoting more extended surgical techniques both on bony structures and the mucosa [21, 24]. In this sense, reboot surgery focuses on the complete removal of the mucosa under severe inflammatory states by promoting regrowth of healthy neomucosa [13]. In contrast with the classical functional mechanistic viewpoint, such a disruptive proposal has promoted the development of new complementary techniques such as regenerative surgery (i.e., mucoplasty), which promotes the regeneration of healthy mucosa from large free grafts from the floor of the nasal fossa [14, 15].

Similarly, CRS guidelines do not resolve the ambiguity of the terms functional and extended surgeries [9, 10, 39]. Some researchers consider that the objective of FESS should be the improvement of nasal breathing functionality [4], while others argue that it should encompass extensive procedures to facilitate effective delivery of intranasal medical therapy [3]. Additionally, there is a school of thought that limits functional surgery to the ethmoid and osteomeatal complex [2]. However, these guidelines fail to provide a clear delineation of the modifications carried out within FESS on nasal structures. The same ambiguity exists for the term extended surgery, also referred to as complete or radical ESS. Some authors, in an attempt to optimize surgical outcomes, use the term expanded FESS [20, 40]. Thus, specific descriptions of extended surgery procedures are provided by certain groups [12, 13, 18]. Conversely, other authors classify all previous definitions of extended surgery under the term conventional FESS, reserving extended or radical surgeries for procedures involving extensive sinus openings (such as frontal sinusotomy type DRAF III, mega-antrostomy or medial maxillectomy, or complete sphenoidotomy) [21]. Further consensus is needed to provide standardization of surgical procedures in CRS management.

The LOEM classification system: properties

The LOEM coding system standardizes the surgical procedures not only based on the removal of anatomical structures such as the lamella L and ostium O, but also on the mucosa treatment (the letter E indicates whether the procedure was limited or extensive, and M indicates the procedure directed towards the sinonasal mucosa), which may prove useful for categorizing different types of ESS currently available in the clinical arena (Table 2). The LOEM classification lies on well-known anatomical structures and makes use of a simple descriptive lettering and numbering system. This proposed system is not influenced by surgical indication, preoperative or postoperative radiological evaluation or clinical outcomes, making it solely and exclusively limited to the definition of surgical procedures.

Accordingly, based on the extent of bone modifications and mucosal treatment used, LOEM proposes a description of surgical procedures though four compact types (Table 3). The LOEM system aims to overcome current heterogeneity by proposing a simple lettering system that simplifies the descriptions of various approaches reported in the literature and listed in Table 2. This classification attempts to address the lack of a validated tool to assess the extent of ESS and to facilitate research in this field [10, 39]. Moreover, this coding system is also useful for describing primary and revision surgeries on all sinuses and nasal mucosa, avoiding possible ambiguities. In this work, a web-based app is provided to facilitate the description of the surgery using the LOEM system, which may guide future internal and external validation. Hopefully, this approach will allow reliable information to be easily collected and correlated with comorbidities, prognosis factors, biomarkers, and clinical outcomes in specific CRS cases. The proposed classification system has the potential to translate emerging knowledge into personalized surgical treatments to improve patient outcomes [21].

Results of an expert pilot study

Using a systematic Delphi method for information prediction over two rounds to reach consensus among experts, the evaluation of our classification proposal resulted in substantial interrater agreement (overall κ = 0.77) (Supplementary Table S3). This iterative process allows for structured feedback, facilitating the refinement of the LOEM classification system based on expert insights and agreement. However, agreement during the first round for the mucosal M subcategory was only fair (κ = 0.37). After a careful revision of the comments provided by the experts, we concluded that the fair agreement reached in this category was due to heterogeneous interpretations of reboot-type surgery among experts [27, 41]. A report based on surgical details of the reboot surgery discussed in the original publication by Alsharif et al. about the difficulty of achieving a complete excision of the mucosa in distal regions of the paranasal sinuses (e.g. the frontal and sphenoid sinuses, or the alveolar recess of the maxillary sinus), due to limitations of visualization and instrumentation [13] was provided and discussed with the experts. Such clarifications allowed us to ensure alignment of the definition of M with the original description of the reboot surgery, which improved the agreement for this section to a value of κ = 0.79 (Supplementary Table S3).

Conversely, decreases in the degree of agreement for O and E were recorded in the second round of the pilot study (κ = 0.52 and κ = 0.41, respectively). This may be due to a misinterpretation between the enlargement of the ostium (O) and the resection of the paranasal sinus wall (E). It is important to note that, in conventional ESS techniques, modifying the extension of the surgery requires a prior adjustment of the sinus ostium. This fact can be unclear when dealing with R-ESS, where previous modifications of the sinus ostia have already been made [42]. Another possible explanation could be the intrinsic limitation of the LOEM classification in adequately defining how much the ostium must be enlarged to consider that an approach to the sinus drainage ostium changes from O to E. To better explain the extension over the different paranasal sinuses, various specific classifications have been used to better define how much the access pathway to each paranasal sinus should be modified (Table 1) [22, 23, 35, 36].

In addition, the intra-rater reliability after 6 months demonstrated a very high reproducibility of the LOEM classification. Our seven experts achieved an overall agreement of 93% and a Kappa index of 0.82, suggesting highly consistent evaluations over time. While exploring the item-specific findings for LOEM, we still found high agreement in all groups, ranging from 82.55% (E items) to 100% (L items). However, item O had an agreement of 91.27%, but the Kappa value was − 0.05 (p = 0.695), indicating no reliability beyond what would be expected by chance. On one hand, this could be due to the calculation's characteristics, where the expected agreement by chance was very high (93%). This high probability of chance agreement can be clinically explained by the strong tendency to perform antrostomies during ESS, directing approaches not only to the location of the drainage ostia but also to their modification, which makes it a practice carried out in nearly all surgical approaches. Furthermore, the fact that the videos selected for the expert pilot study were mostly from expanded surgeries, with the opening of all ostia, may have also overestimated the expected level of agreement. This lack of agreement for the O item suggests that modifications to the LOEM system may be needed to account for challenges in consistent ostium classification. Future large-scale validations planned for the LOEM system should address this discrepancy through further refinement of the system.

It is noteworthy that the internal validation process was strengthened by the knowledge of seven highly experienced rhinologists. They evaluated the surgeries without knowing details about the patients or the operating surgeons, which ensured fair assessments focused solely on surgical outcomes. Additionally, detailed descriptions and sharing of reports after each evaluation round improved transparency and cleared up any uncertainties. Consequently, the LOEM classification system may enable a more precise delineation of different surgical approaches, including more extended mucosal resections. It may also facilitate the inclusion of new regenerative surgery concepts. The implementation and expansion of this system will hopefully enhance consistency in describing surgical approaches for CRS, thereby simplifying comparisons and increasing the reproducibility of the various available techniques.

Pathway to implementation: integrating the LOEM system into clinical practice

To integrate LOEM into clinical practice and research, the following roadmap can be proposed by authors: (i) develop training programs for surgeons and medical professionals on LOEM, including workshops and online resources to ensure widespread understanding and acceptance; (ii) conduct pilot studies to evaluate the effectiveness of LOEM in various clinical settings, gathering data on its utility and outcomes in patient management; (iii) collaborate with professional organizations to create standardized protocols for the implementation of LOEM in clinical practice; (iv) establish a feedback mechanism to collect user experiences and suggestions for improvement from clinicians using LOEM in practice.

During this process, some issues may arise, such as resistance to change from teams that are accustomed to a specific way of working. There may also be a lack of initial knowledge regarding the interpretation of the various subscripts. Furthermore, a gradual outreach process will be necessary to ensure that future users understand how to use it correctly.

Limitations, utility and future perspectives

The major limitation of the LOEM system is that it does not consider conservative modalities, such as balloon sinuplasty or technical variations, especially those performed on the inferior and superior turbinates, or nasal septum. Furthermore, it does not consider the treatment of sequelae such as synechiae, mucoceles or osteitis. These are inherent limitations of any classification system, which cannot encompass all aspects and variations of ESS. Nonetheless, the LOEM system remains an easy-to-use, intuitive and valuable tool for describing numerous types of ESS variants in the treatment of CRS, providing a global view of the surgical approach both regarding bony structures and mucosal treatment.

Other limitations inherent to the pilot study include the small sample size of eleven videos, the pilot study conducted in a limited geographical region (Europe), and the exclusion of specific items (Lxa, Oxa, Exa, O0, E0) due to case selection criteria, which could restrict the generalizability of findings and limit the broader applicability of the LOEM system globally. Furthermore, potential differences in interpretation and a learning curve among less experienced rhinologists were not fully addressed. This calls for the development of new large-scale validation studies that encompass both seasoned and novice rhinologists, a higher volume of cases, subgroup analyses and additional videos of revision surgeries, to achieve greater robustness and reproducibility of the items included in the LOEM system.

The usefulness of the LOEM classification system lies in the possibility of describing, in a unified way, the different types of ESS performed in the treatment of CRS patients, as well as to compare the different surgical approaches described in the literature. In addition, the LOEM system would help solve current limitations in the use of administrative data based on diagnosis-related groups (DRGs) and International Classification of Diseases (ICDs), due to divergent classification terms used across different organizations and countries [43]. The increasing knowledge on the underlying CRS inflammatory mechanisms and biomarkers, and the solid description of phenotypes in a new precision medicine paradigm, require a systematic classification for ESS extension [17, 44]. In addition, new AI and machine learning algorithms have been developed for predictive modeling of CRS outcomes [45, 46]. The labeling of surgical approaches based on classification systems such as the LOEM system here proposed will promote data acquisition and automated training of AI models in Rhinology.