10 mm Postoperative Hematoma Threshold Predicts Reoperation but Cut-off Is Arbitrary

Postoperative chronic subdural hematoma (CSDH) thickness of at least 10 mm, a radiological threshold long used as an outcome measure in clinical trials, is associated with worse functional outcome and a dramatically higher risk of reoperation, according to a post hoc analysis of the nationwide, multicenter, randomized FINISH trial published in Acta Neurochirurgica. Yet the study also delivers a cautionary message: because the relationship between hematoma thickness and functional outcome appears to be linear, any single cut-off value is inherently arbitrary, and the 10 mm measure should complement rather than replace patient-centered outcomes.

The analysis included 581 patients who underwent burr-hole drainage for chronic subdural hematoma, a condition whose surgical treatment leaves residual subdural fluid collections that vary widely in size. When researchers stratified patients according to hematoma thickness measured at six weeks after surgery, they found that 283 patients, or 49 percent, had residual hematomas measuring 10 mm or thicker. The team then identified optimal thickness thresholds and evaluated their associations with functional outcome on the modified Rankin Scale (mRS), mortality, and reoperation at both six weeks and six months.

The reoperation findings were the most striking. Overall, 91 patients, or 16 percent of the cohort, required reoperation within six months, and 50 of these, or 55 percent, were reoperated within the first six weeks. Patients whose postoperative hematomas measured 10 mm or more had markedly higher reoperation rates than those with thinner collections: 32 percent versus 0.7 percent.

Functional outcomes told a more nuanced story. Patients with hematomas of at least 10 mm had worse functional outcomes on the modified Rankin Scale at both time points assessed: at six weeks the adjusted odds ratio was 1.6 (95 percent confidence interval 1.2 to 2.2, p = 0.004), and at six months it was 1.4 (95 percent confidence interval 1.0 to 1.8, p = 0.049). However, when the researchers examined dichotomized unfavorable outcome, defined as a six-month mRS of 4 to 6, the difference between the thickness groups disappeared, with an adjusted odds ratio of 1.3 (95 percent confidence interval 0.7 to 2.4, p = 0.48). This pattern suggests that the thickness measure captures gradations of disability and the need for further surgery rather than a clean divide between favorable and catastrophic outcomes.

A key methodological finding concerns discrimination. All evaluated thickness thresholds demonstrated poor discriminatory performance when used alone as predictors of outcome, and the relationship between poor functional outcome and hematoma thickness was linear rather than showing any natural break at 10 mm or at any other value the investigators tested. In other words, thicker residual collections are progressively associated with worse outcomes, but there is no magic number at which risk suddenly escalates. The apparent superiority of the 10 mm cut-off for identifying reoperation risk reflects its practical utility as a flag, not a biological discontinuity.

The authors conclude that postoperative CSDH thickness of at least 10 mm at six weeks is associated with worse functional outcome and higher risk of reoperation, and that a 10 mm radiological threshold may serve as a surrogate endpoint in chronic subdural hematoma trials. Given the linear dose-response relationship, however, they argue that such imaging measures should complement, not replace, patient-centered outcomes such as functional status measured on the modified Rankin Scale. As a post hoc analysis of a randomized trial, the study identifies associations rather than proving that residual hematoma thickness itself causes poor outcomes, and the findings apply to patients treated with burr-hole drainage in the Finnish trial setting. Even so, for trial designers and clinicians alike, the message is clear: the ruler remains useful, but the patient, not the millimeter reading, should remain the primary yardstick of success.

Subject of Research: Postoperative chronic subdural hematoma thickness as a predictor of functional outcome and reoperation after burr-hole drainage

Article Title: Chronic subdural hematoma postoperative thickness and patient outcome – is 10 mm a relevant cut-off measure? A post hoc analysis of the FINISH trial

Article References: Netti, E., Knuutinen, O. A., Ahmed, A., Korhonen, T., Luoto, T., Leinonen, V., Koivisto, T., Tetri, S., Posti, J. P., Lönnrot, K., Tommiska, P., Raj, R., & On behalf of the FINISH study group (2026). Chronic subdural hematoma postoperative thickness and patient outcome – is 10 mm a relevant cut-off measure? A post hoc analysis of the FINISH trial. Acta Neurochirurgica. https://doi.org/10.1007/s00701-026-07061-5

Image Credits: AI Generated

DOI: 10.1007/s00701-026-07061-5

Keywords: chronic subdural hematoma, burr-hole drainage, postoperative hematoma thickness, FINISH trial, reoperation, modified Rankin Scale, surrogate endpoint, neurosurgery, functional outcome, radiological threshold, Chronic, subdural