Vagus nerve stimulation: Indications, efficacy, and methods

Introduction and background Vagus nerve stimulation (VNS) therapy is a relatively new form of neuromodulation. The U.S. Food and Drug Administration (FDA) has allowed the commercial availability of VNS devices for patients with treatment-resistant depression (TRD) or intractable epilepsy. The VNS system is an implantable device, similar to the size and shape of a cardiac pacemaker. It provides brief intermittent amounts of electrical stimulation to the left vagus nerve. Neuroanatomy of the vagus nerve The vagus nerve is the tenth and longest cranial nerve in the human body. In the neck it is located between the carotid arteries and the jugular vein. The vagus nerve is a mixed cranial nerve that consists of both efferent fibers sending impulses to extracranial areas, and afferent fibers bringing impulses to the brain. The majority (80%) are afferent fibers. Mechanisms of action Although the exact mechanisms by which VNS therapy works are currently unknown, the antidepressant and anticonvulsant effects of stimulating the vagus nerve may be related to its neuroanatomical pathways. The cell bodies of the afferent fibers that comprise the vagus nerve are located in the nodose ganglion. They project impulses to the nucleus tractus solitarius (NTS) (Sackeim, 2004). These incoming sensory impulses are then projected by the NTS through three main pathways that include (a) an autonomic feedback loop, (b) direct projections to the reticular formation in the medulla, and (c) ascending projections to the forebrain mainly through the parabrachial nucleus (PB) and the locus ceruleus (LC) (George et al., 2000).