The Responsive Neurostimulator (RNS) is being developed by NeuroPace, a company based in Mountain View, California. The device, which is smaller than a playing card, is battery-powered and controlled by microprocessors. It is curved so that, like the cochlear implants on which it is modelled, it can fit into a small cavity carved out of the skull. Two electrodes, connected to the device by thin wires, are implanted into the seizure focus (that is, the part of the brain generating the seizures), which is identified beforehand using electroencephalography (EEG) and neuroimaging.
Unlike earlier devices, such as the vagal and trigeminal nerve stimulators, which stimulate the brain continuously, the RNS is designed to do so only when a seizure is imminent. The electrodes monitor the neuronal activity in the part of the brain into which the electrodes are implanted; when abnormal activity is detected, they emit a burst of electrical stimulation that suppresses the imminent seizure by preventing that abnormal activity from spreading through the brain.
The RNS can store data about the electrical activity in patients’ brains. This information can be transmitted wirelessly to a laptop running specialized analytical software. It can be used by a physician to monitor the brain’s activity in real time, and to detect the “signature” of an imminent seizure.
It is unclear how effective the RNS might be in alleviating the symptoms of epilepsy. The efficacy of the device depends largely on the correct identification of the abnormal electrical activity that is characteristic of an imminent seizure, and the generation of the correct response to that activity. A clinical trial involving of 180 patients is now under way. The trial is expected to last up to 3 years, and is being conducted in about 20 centres across the United States.
The RNS would be especially beneficial to those epileptic patients who either experience severe side effects from, or are completely resistant to, anti-convulsive medication. Such patients constitute up to 50% of all epileptics; in some cases, surgery can be performed to remove the seizure focus; in others, however, the focus is located within or near to areas of the brain that cannot be removed. Surgery would not, for example, be performed on a patient whose seizure focus is close to or within the speech centres in the left hemisphere, as damage to or removal of these parts of the brain would lead to severe speech deficits.