Membrane Surface Area to Volume Ratio in Chronic Subdural Hematomas: Critical Size and Potential Postoperative Target

BACKGROUND: It is unknown why some chronic subdural hematomas (CSDHs) grow and require surgery, whereas others spontaneously resolve. Although a relatively small CSDH volume (V) reduction may induce resolution, V percent reduction is often unreliable in predicting resolution. Although CSDHs evolve distinctive inner neomembranes and outer neomembranes (OMs), the OM likely dominates the dynamic growth-resorption equilibrium. If other factors remain constant, one previous hypothesis is that resorption could fail as the surface area (SA) to V ratio decreases when CSDHs exceed a critical size. We aimed to identify a critical size and an ideal target, which implies resolution without recurrence. METHODS: Three-dimensional computed tomography CSDH SA to V ratios were obtained using computer software to compare CSDH SA to V between cases requiring surgery (surgical) and cases managed conservatively with spontaneous resolution (nonsurgical). RESULTS: Data were obtained in 45 patients (surgical: n = 28; nonsurgical: n = 17). CSDH risk factors did not significantly differ between surgical and nonsurgical cases. Surgical V was 2.5x the nonsurgical V (119.9 +/- 33.1 mL vs. 48.4 +/- 27.4 mL, respectively; P < 0.0001). Surgical total SA was 1.4x nonsurgical SA (256.63 +/- 70.65 cm(2) vs. 187.67 +/- 77.72 cm(2), respectively; P = 0.004). Surgical total SA to V ratio was approximately one half that of nonsurgical SA to V ratio (2.14 +/- 0.90 mL(-1) vs. 3.88 +/- 1.22 mL(-1), respectively; P < 0.0001). Surgical OM SA (SA(OM)) was 120.63 +/- 52 cm(2), and nonsurgical SA(OM) was 94.10 +/- 41 cm(2) (P < 0.0001). Nonsurgical SA(OM) to V ratio was 1.94 mL(-1), whereas surgical SA(OM) to V ratio was 1.005 mL(-1) (i.e., surgical SA(OM) approximate to V). CONCLUSIONS: Because surgical total SA to V ratio was approximate to 2:1, one neomembrane may indeed dominate the dynamic growth-resorption equilibrium. CSDH critical size therefore appears to be when SA(OM) approximate to V, which is intuitive. Practically, subtotal CSDH evacuation which approximately doubles total SA to V ratio or SA(OM) to V ratio implies CSDH resolution without recurrence. This could guide subdural drain removal timing, discharge, or transfer. Prospective validation studies are required.