Derivation of a Bleed to Brain Ratio to Predict the Need for Surgery in Head Injury.

Background: The Monro-Kellie hypothesis states that the cranial compartment is incompressible, and cranial volume cannot expand. Human cranial volumes vary widely. However, the present guidelines for surgical evacuation are based on the volume of a single bleed. We hypothesized that the ratio of the total volume of blood clots in the brain to the volume of normal brain tissue must reflect the chances of developing increased intracranial pressure more accurately. Objective: Compare the ratio of bleed volume to intracranial brain volume (bleed--brain ratio, BBR) in head trauma patients who had undergone surgery with those who did not require surgery. Estimate the cutoff value for this ratio that could predict the need for surgery. Materials and Methods: An exact planimetric method using a Hounsfield unit bracketing strategy was used to derive total bleed volume and the intracranial volume in 212 head trauma patients. Receiver operating characteristics (ROC) curve was used to assess the predictive power of BBR for surgery. A cutoff value for the BBR with clinically significant sensitivity and specificity was estimated. Results: The intracranial volume varied from 955 ml to a maximum of 1,525 ml. The total bleed volume and BBR was significantly different in the two groups. Using the ROC curve, we found that a BBR of 0.0535 had a sensitivity of 90.6% and a specificity of 52.6% to predict the need for surgery. Conclusions: We demonstrated that the BBR was superior to bleed volume alone in predicting the need for surgery. Although the ratio estimated by the tedious planimetric method was more accurate, an easier less precise method had reasonable accuracy and can be considered for everyday use. BBR could be particularly useful in those head trauma patients who have multiple irregular sized bleeds where the surgeon is undecided on the need for surgery.