1. Field of the Invention
This invention relates generally to medical devices, and, more particularly, to methods, apparatus, and systems for performing electrical signal therapy by a medical device.
2. Description of the Related Art
There have been many improvements over the last several decades in medical treatments for disorders of the nervous system, such as epilepsy and other motor disorders, and abnormal neural discharge disorders. One of the more recently available treatments involves the application of an electrical signal to reduce various symptoms or effects caused by such neural disorders. For example, electrical signals have been successfully applied at strategic locations in the human body to provide various benefits, including reducing occurrences of seizures and/or improving or ameliorating other conditions. A particular example of such a treatment regimen involves applying an electrical signal to the vagus nerve of the human body to reduce or eliminate epileptic seizures, as described in U.S. Pat. Nos. 4,702,254, 4,867,164, and 5,025,807 to Dr. Jacob Zabara, which are hereby incorporated in this specification in their entirety by reference.
More generally, the endogenous electrical activity (i.e., activity attributable to the natural functioning of the patient's own body) of a neural structure of a patient may be modulated in a variety of ways. In particular, the electrical activity may be modulated by exogenously applied (i.e., from a source other than the patient's own body) electrical, chemical, or mechanical signals applied to the neural structure. The modulation (hereinafter referred to generally as “neurostimulation” or “neuromodulation”) may involve the induction of afferent action potentials, efferent action potentials, or both, in the neural structure, and may also involve blocking or interrupting the transmission of endogenous electrical activity traveling along the nerve.
Electrical signal therapy or electrical modulation of a neural structure (also known as “electrical signal therapy”) refers to the application of an exogenous therapeutic electrical signal (as opposed to a chemical or mechanical signal), to the neural structure. Electrical signal therapy may be provided by implanting an electrical device underneath the skin of a patient and delivering an electrical signal to a nerve such as a cranial nerve. In some cases, the electrical signal therapy may involve performing a detection step, with the electrical signal being delivered in response to a detected body parameter. This type of stimulation is generally referred to as “active,” “feedback,” or “triggered” stimulation. Alternatively, the system may operate without a detection system once the patient has been diagnosed with epilepsy (or another medical condition), and may periodically apply a series of electrical pulses to the nerve (e.g., a cranial nerve such as a vagus nerve) intermittently throughout the day, or over another predetermined time interval. This type of stimulation is generally referred to as “passive,” “non-feedback,” or “prophylactic,” stimulation. It is also possible to combine active and passive stimulation by using a programmed, intermittent signal operating according to a programmed duty cycle, and a detection system that operates when an event, such as an elevated heart rate, is detected. The stimulation may be applied by an implantable medical device that is implanted within the patient's body.
When inducing action potentials in a cranial nerve, action potentials will propagate in both the afferent direction (toward the brain) and efferent direction (toward a body structure innervated by the cranial nerve). If it is desired to provide electrical signal therapy acting on a structure in the brain, the propagation of efferent action potentials may lead to side effects in the innervated body structure. For example, when an implantable medical device induces action potentials in the vagus nerve in the neck, efferent action potentials may travel down the vagus nerve and the recurrent laryngeal nerve (a branch of the vagus nerve), stimulating muscle activity proximate the larynx. As a result, a patient may experience unwelcome modulation of the vocal cords, or other side effects, resulting in impaired speech volume, timbre, or other activity.
Therefore, a need exists for apparatus and methods for performing electrical signal stimulation of the brain via a cranial nerve with reduced side effects in body structures innervated by the cranial nerve.