A neural stimulation system automatically corrects or adjusts the stimulus magnitude in order to maintain a comfortable and effective stimulation therapy. Auto correction of the stimulus magnitude is linked to the measurement of coupling efficiency. Because the events that lead to the necessity of an output amplitude change are all associated with how much electrical energy is coupled to the neural tissue, and because there are several physiologic parameters that reflect in some measure how much energy is actually coupled to the tissue, the measurement of the one or more of such physiologic parameters is used as an indicator of the electrode's effectiveness in providing therapeutic stimulation. The physiologic parameters that may be measured, and used by the invention as a measure of coupling efficiency, include: action potential(s); the optical transmissive and/or reflective properties of the tissue and fluids surrounding or adjacent the target neural tissue; the chemistry of the fluids and tissue near the electrodes, e.g., in the epidural space; or the changes in pressure that occur near the electrodes, e.g., in the epidural space. The relative change in the measured physiologic parameter is used as an indicator of the obstruction to current flow between the electrodes and the neural tissue. Such knowledge with respect to time thus permits the neural stimulation system to effectively auto-correct the output amplitude, thereby minimizing the occurrence of over-stimulation or under-stimulation.
Neural stimulation system providing auto adjustment of stimulus output as a function of sensed coupling efficiency