1. A method comprising: detecting a posture transition of a patient; monitoring autonomic nervous system activity of the patient for a defined period in response to detecting the posture transition; determining a baseline indicator of the autonomic nervous system activity during the monitoring period; following the monitoring period, continuing monitoring of the autonomic nervous system activity of the patient during a subsequent period; determining, based upon monitoring during the subsequent period, an indicator of an autonomic nervous system activity of the patient; comparing the indicator based upon monitoring during the subsequent period to the baseline indicator; and estimating a probability that the patient will experience vasovagal syncope as a function of the comparing step.