Curved Brain Artery Segment Tied to Temporary Clipping, but Bias Concerns Remain

A letter published in the Egyptian Journal of Neurosurgery has raised methodological concerns about a study linking the shape of the M1 segment of the middle cerebral artery to the use of temporary clipping during aneurysm surgery. The letter, authored by Abdul Basit Munir, Sohaib Mushtaq, Nashmia Faraz and Dua Jamil, acknowledges the clinical relevance of the original findings while arguing that the reported effect size should be interpreted with additional caution.

The original study, conducted by Fonseca and colleagues, examined whether the morphology of the M1 segment influences surgical management of middle cerebral artery aneurysms. Its central finding was that curved M1 segments were associated with a higher likelihood of temporary clipping during surgery than straight segments, a result the letter writers describe as anatomically plausible and potentially useful for surgical planning.

One strength the letter highlights is the original study’s assessment of interobserver agreement. Two experienced cerebrovascular surgeons independently classified M1 segments as straight, upward-curved, or downward-curved, achieving 95.3 percent raw agreement and a kappa value of 0.92, which supports the reproducibility of that classification system.

However, the letter points out that the original manuscript does not state whether those reviewers were blinded to the surgical outcome, including the use of temporary clipping, when retrospectively classifying M1 morphology. Because morphology assessment and outcome ascertainment were performed within the same institutional dataset, the letter argues that unblinded retrospective classification raises the possibility of ascertainment bias favoring the study hypothesis, a pattern consistent with recognized hindsight and contextual bias in retrospective image interpretation.

A second concern involves the small number of temporary-clipping events. Among 86 patients, only 13 such events occurred, including just 3 among the 47 cases with a straight M1 segment. The letter notes that such sparse cell counts can produce unstable odds ratio estimates depending on the calculation method used.

The letter also raises a mediation concern. Temporary clipping may be applied either electively or reactively during intraoperative rupture. According to the letter, Table 3 of the original study showed that intraoperative rupture was numerically more frequent in curved M1 cases, at 15.4 percent versus 6.4 percent, though the difference was not statistically significant. This means part of the observed association between curved M1 morphology and temporary clipping may reflect reactive clipping following rupture rather than a direct anatomical effect on planned proximal control. The letter writers suggest that distinguishing elective from reactive clipping events would help clarify the mechanism underlying the reported association.

A final concern addresses the study’s timeframe. The cohort spans a 12-year period, from 2006 to 2018, under a single surgeon. While the original authors discussed how this design limits external generalizability, the letter notes they did not address potential temporal confounding within the cohort itself. Surgical experience, technique, and preoperative imaging modalities plausibly evolved over that interval, and the effect of surgeon experience on aneurysm-clipping outcomes has been documented elsewhere. If curved-M1 cases were unevenly distributed across the period, calendar time could confound both morphology classification and the surgeon’s threshold for applying temporary clipping.

The letter emphasizes that these considerations do not diminish the anatomical plausibility or clinical relevance of the reported association between M1 morphology and temporary clipping. Rather, they suggest the effect size warrants more cautious interpretation than the raw odds ratio alone might imply.

To strengthen confidence in the findings, the letter calls for clarification of several points: whether raters were blinded, which statistical method was used for the sparse-event odds ratio, whether clipping events were elective or reactive, and whether temporal trends existed within the cohort. Such clarifications, the authors conclude, would better guide the design of the multicenter validation studies that the original authors themselves rightly call for.

The letter concludes that clarifying rater blinding, the statistical method used for sparse-event odds ratios, the elective-versus-reactive nature of clipping events, and any temporal trends within the cohort would strengthen confidence in these hypothesis-generating findings.

Subject of Research: Methodological critique of a study associating M1 artery segment morphology with temporary clipping in middle cerebral artery aneurysm surgery

Article Title: M1 segment morphology and temporary clipping in middle cerebral artery aneurysm surgery

Article References: Munir, A. B., Mushtaq, S., Faraz, N., & Jamil, D. (2026). M1 segment morphology and temporary clipping in middle cerebral artery aneurysm surgery. Egyptian Journal of Neurosurgery, 41(1), Article 167. https://doi.org/10.1186/s41984-026-00673-0

Image Credits: AI Generated

DOI: 10.1186/s41984-026-00673-0

Keywords: middle cerebral artery aneurysm, M1 segment morphology, temporary clipping, cerebrovascular surgery, ascertainment bias, odds ratio, sparse events, intraoperative rupture, retrospective study, temporal confounding, interobserver agreement, Egyptian Journal of Neurosurgery