34th Annual Scientific Meeting proceedings

Systematic reviews (SRs) are an invaluable tool for clinicians and are foundational to evidence-based practice, offering a rigorous, transparent, and reproducible synthesis of available evidence. They help clinicians move beyond anecdote and isolated studies, toward conclusions supported by the full scope of published data. In veterinary medicine, where individual studies are often small and heterogeneous, systematic reviews provide a critical tool for drawing meaningful conclusions from limited or inconsistent evidence.

Unlike narrative reviews, SRs are structured, protocol-driven, and aim to minimize bias. A systematic review is defined as a summary of research that addresses a focused clinical question in a systematic, reproducible manner (Murad and Montori 2013). Although the number of SRs in veterinary medicine is still modest compared to human healthcare, the trajectory is rising rapidly. A search of Medline through PubMed reveals over 4,000 SRs published on veterinary topics as of 2025, answering questions such as what are the best option for treatment of cranial cruciate ligament in dogs, what are the complications related to laparoscopy in dogs and what are the treatment option for pericardial effusion in dogs, among others (Bergh et al, 2014, Maurin et al 2020, Scheuermann et al 2021). However, a search made on June 1^st 2025 on Veterinary Surgery, only shows 14 systematic reviews, 11 of them published from 2020. There are many clinical questions that would benefit of being analyzed via a systematic review, and we will see what the advantages and steps are to perform a systematic review on a veterinary surgical topic. Systematic reviews rest on four pillars: well-defined research question, reproducibility, comprehensive methodology, and critical appraisal. Systematic reviews sit at the top of the hierarchy of evidence, but only if conducted with care and based on sufficiently robust primary data. A systematic review typically unfolds in multiple phases:

Formulate a focused question
Using the PICO framework (Population, Intervention, Comparator, Outcome) helps refine a clinically relevant research question. For example: “In dogs undergoing cranial cruciate ligament repair, does the use of postoperative nonsteroidal anti-inflammatory drugs (NSAIDs), compared to placebo or no treatment, improve functional outcome scores within 8 weeks of surgery?”

Develop and register a protocol
Following this initial phase, a protocol should be developed following PRISMA-P guidelines (Moher et al 2015). This protocol serves multiple crucial functions: it allows systematic reviewers to plan carefully and anticipate potential problems, provides transparent documentation of planned methods before the review begins, prevents arbitrary decision-making regarding inclusion criteria and data extraction, and may reduce duplication of efforts while enhancing collaboration. The protocol must be registered and/or published before conducting the actual review, as many journals now refuse to publish systematic reviews without pre-registered protocols. Options to register protocols include platforms like Open Science Framework (OSF), a platform that allows simple and free registration of study protocols. If the SR has significant impact on human health, PROSPERO, an international systematic review registry produced by the Centre for Reviews and Dissemination (CRD), can be considered for registration.  

Assemble a multidisciplinary team
Typically, the review team should include:

• A clinical expert (e.g., a board-certified veterinary surgeon)
• A methodologist or epidemiologist
• A statistician experienced in meta-analysis
• A librarian or information specialist skilled in veterinary database search (including CAB, AGRICOLA, Scopus, Medline)
• In human medicine, patients and public are often included in the team – including owners of animals affected by certain conditions could be considered also in veterinary research

Define eligibility criteria
Establishing eligibility criteria represents one of the most important decisions in systematic review design. These criteria are divided into study eligibility criteria (based on PICO elements) and review eligibility criteria (such as language, publication status, and years of publication). Ideally, eligibility criteria should be set to identify all relevant primary studies while maintaining sufficient homogeneity to allow meaningful synthesis. However, reviewers must balance comprehensiveness with clinical and methodological similarity, as including too diverse a set of studies can compromise the validity of pooled analyses. In veterinary surgery, it is important to consider various type of studies and not limiting to randomized controlled trials. For example, a review on feline urethral obstruction may need to include both prospective cohort studies and retrospective analyses due to limited data, while carefully excluding studies with insufficient outcome reporting or incompatible endpoints.

Define search strategy and extract data
Systematic searches should cover multiple databases, gray literature (e.g., theses, conference abstracts), and non-English publications if feasible. Tools like Covidence, Rayyan, or RevMan can aid screening and data extraction. In systematic reviews, it is customary to have two operators independently run the search strategies and extract eligible studies. Once the eligible studies are extracted by each operator, these are compared and any discrepancy is discussed. In case of persisting disagreement a third operator is consulted. A similar approach is used when extracting the data from the studies. Once agreed and pilot tested a data extraction sheet, data are extracted independently by two operators and are then compared, discussed and a third operator is involved as needed. While this may be trivial, having duplicate extraction is a critical part of conducting a systematic review properly and should be accounted at the planning stage of the review.

Assessing study quality and bias
Risk of bias assessment constitutes a fundamental component of systematic reviews, though it's important to distinguish between quality assessment and risk of bias evaluation. Quality assessment examines whether methodological safeguards were implemented in studies, such as whether patients were blinded. Risk of bias assessment goes further by evaluating the implications of these methodological features for study results. As Cochrane defines it, bias represents "a systematic error, or deviation from the truth, in results." Tools such as ROBINS-I for non-randomized studies and RoB2 for randomized trials are widely used for this purpose. The RoB2 tool addresses five key domains of bias in randomized controlled trials, presented in the order of conducting a typical trial: bias arising from the randomization process, bias due to deviations from intended interventions, bias due to missing outcome data, bias in measurement of the outcome, and bias in selection of the reported result. Each domain is evaluated to determine whether the risk of bias is low, some concerns, or high for specific outcomes within individual studies.

Meta-analysis
Meta-analysis represents the statistical synthesis component of systematic reviews. Meta-analysis combines data across studies to produce a pooled effect estimate, improving statistical power. For instance, a meta-analysis evaluating the efficacy of maropitant for canine vomiting might reveal a small but significant benefit compared to metoclopramide – even if individual studies were underpowered for this objective. A common misconception is that meta-analysis can be performed without a systematic review. This is not correct, since in order to ensure that a meta-analysis provides accurate results, all studies published should be evaluated, and this can only occur after a proper SR is conducted. Another common misconception is that SRs always should have a meta-analysis. Meta-analyses should only be performed when the studies are at a limited risk of bias and results of the studies are homogenous enough. While some degree of heterogeneity between studies is always present, substantial differences in study populations, interventions, comparators, outcomes, or settings can make pooling inappropriate. Systematic reviewers must carefully consider whether studies are sufficiently similar to justify meta-analysis or whether narrative synthesis alone is more appropriate. Forest plots are typically used to summarize meta-analytical findings. In veterinary contexts, it’s common to encounter wide confidence intervals due to small sample sizes or event rates, reinforcing the importance of interpreting both the effect estimates and its precision. Lack of uniform outcome measures (e.g., how is gait assessed in dogs varies significantly in between studies) makes synthesis difficult. The development of Core Outcome Sets (COS) also for veterinary specialties will be key to enhancing comparability across trials.

Conclusion and Future Directions
Systematic reviews offer significant value to veterinary surgeons:

• They consolidate fragmented literature into usable guidance.
• They identify gaps in knowledge, helping shape future clinical trials.
• They support clinical decision-making, guideline development, and client communication.

Although systematic reviews are time-intensive, their benefits are clear, especially as veterinary medicine moves toward increasingly evidence-based care.

References

1. Bergh MS, Sullivan C, Ferrell CL, Troy J, Budsberg SC. Systematic review of surgical treatments for cranial cruciate ligament disease in dogs. J Am Anim Hosp Assoc. 2014 Sep-Oct;50(5):315-21. 
2. Maurin MP, Mullins RA, Singh A, Mayhew PD. A systematic review of complications related to laparoscopic and laparoscopic-assisted procedures in dogs. Vet Surg. 2020 Jun;49 Suppl 1:O5-O14. 
3. Moher D, Shamseer L, Clarke M, Ghersi D, Liberati A, Petticrew M, Shekelle P, Stewart LA. Preferred Reporting Items for Systematic Review and Meta-Analysis Protocols (PRISMA-P) 2015 statement. Syst Rev. 2015;4(1):1.
4. Murad M, Montori VM. Synthesizing evidence: shifting the focus from individual studies to the body of evidence. JAMA. 2013;309:2217–2218
5. Scheuermann LM, Gordon-Evans WJ, Nault AJ. Systematic review of the treatment options for pericardial effusions in dogs. Vet Surg. 2021 Jan;50(1):20-28.