Simple Risk Predictions for Arteriovenous Malformation Hemorrhage

To the Editor: In November 1995, Kondziolka et al. (2) published an article summarizing a risk prediction formula for arteriovenous malformation (AVM) hemorrhage. Using the multiplicative law of probability formula, the authors reported that the risk of hemorrhage = 1 − (risk of no hemorrhage)expected years of remaining life. The assumptions inherent in this formula are a constant yearly risk of hemorrhage and independent behavior of all years of observation (2). The figures, as reported by Kondziolka et al., are useful in clinical practice for counseling patients and their families regarding lifetime risk of AVM hemorrhage; however, often it is not convenient to use the formula. We have noted a simple way of approximating the lifetime risk of hemorrhage for patients with intracranial AVMs. These figures are based on a 3% annual risk of hemorrhage (1, 3), which is a close approximation of the annual risk of hemorrhage for all AVMs detected without previous hemorrhage. Selected angiographic criteria and other factors are likely to contribute to a higher or lower risk (4). Nonetheless, the following formula closely approximates that of the multiplicative formula: 105 − patient’s age in years = lifetime risk of hemorrhage. As noted in Table 1, the risks summarized by this simple formula, which is easy to use in clinical practice, closely approximate the risk of hemorrhage for a 3% annual AVM risk using the complicated multiplicative formula. The approximation is close enough that it is certainly within the error of the annual risk of AVM hemorrhage (∼3%) and provides a very close approximation for use in clinical practice. The figures from the simple formula differ slightly from those using the multiplicative formula; however, even with the assumption of a 4% per year hemorrhage rate, the simple formula still provides a nice approximation. Table 1: Lifetime Risk of Arteriovenous Malformation HemorrhageKondziolka et al.’s efforts have provided clarification of lifetime hemorrhage for patients with AVMs facing important management decisions. The simple formula described provides an excellent bedside tool for counseling patients regarding lifetime hemorrhage risk in the setting of an intracranial AVM. Robert D. Brown Jr.