IEEE Spectrum February, 2015 - 39

all seizures, but in about 65 percent of
patients the number of seizures was
reduced by half.
The prior generation of VNS devices
stimulated the vagus nerve in 30-second
bursts about 11 times per hour. Cyberonics
found in its clinical trial that when the
new closed-loop device was set to its highest level of sensitivity, it added only a few
extra stimulations to that total. Even if
some of those additional pulses were the
result of false positives, this seems acceptable. Vagus nerve stimulation doesn't
have the side effects associated with antiepileptic drugs, such as sluggishness and
cognitive fog. In fact, patients using VNS
therapy have reported feeling happier
than usual and mentally sharper.
The AspireSR went on the market
in Europe last year; it is awaiting FDA
approval in the United States. Meanwhile,
in the lab, engineers are continuing to
refine the algorithms to detect more seizures with fewer false positives. They're
also considering adding a notification
function, so that the first sign of a seizure would trigger an alert for a caregiver.

t o B e u S e f u l in a closed-loop system, a

sensor needn't always detect the electrical signals that flicker through the brain
or heart; sensors can also pick up physiological signals such as those generated
by movement and activity. Medtronic,
where another of us (Denison) works,
has developed one such system that
uses a three-axis accelerometer as its
sensor, bootstrapping off the technology in your smartphone.
People who suffer from chronic pain
usually try drug regimens or surgery to
alleviate their symptoms, but in some
patients these approaches don't work.
So in the 1980s Medtronic came out with
its first spinal cord stimulation (SCS)
system, in which a pulse generator is
implanted in the abdomen or above
the buttock and leads are routed to the
spinal canal inside the vertebrae. The
system's electrodes send an electric
pulse up the nerve fibers that carry sensory information from the painful body
part to the brain, generally producing
a mild sensation called paresthesia-
the tingling feeling of an arm or leg fall-

ing asleep. This stimulation to the brain
is thought to interfere with the pain signal that the nerve fibers would otherwise transmit.
Today's conventional SCS systems provide patients with remote controls to
adjust stimulation if necessary and to
maintain the desired levels of pain relief
with limited side effects. For example,
many patients find that changing body
position alters the stimulator's effect
on their spinal cords, which can cause
unwanted sensations. To understand
this phenomenon, visualize a patient
lying on his back; in that position, gravity pulls his spinal cord down closer to
the electrodes, causing the current to
activate more nerve fibers. The patient
may then feel a jolt, or the tingling sensation may spread through more of his
body. Conversely, if the patient lies on
his stomach, the distance between the
electrodes and the spinal cord increases,
and because fewer nerve fibers receive
the masking stimulation signal, the pain
may return.
To address this issue, Medtronic set
out to develop a closed-loop device that
automatically adjusts stimulation intensity according to the patient's position.
While the best option might be a sensor that directly measures the distance
between the electrode and the spinal
cord using ultrasound or optical sensors,
these technologies aren't yet practical
for a small implanted device.
A simple three-axis accelerometer,
however, can provide an indirect gauge
of distance: Its inertial measurements
indicate the body's position in real time,
letting the device adjust stimulation as
needed. For this purpose, Medtronic
engineers adapted a type of microelectromechanical system accelerometer
found in cellphones and other consumer products, which wouldn't draw
too much power and drain the device's
battery. The battery is rechargeable-
the patient just holds an induction paddle over the implanted device-but it
would be inconvenient if the patient had
to recharge often.
In Medtronic's adaptive SCS device,
the implanted pulse generator contains
both the accelerometer and a micro-

processor, which runs an algorithm
that classifies the person's position
and determines the appropriate stimulation. To calibrate the device initially,
the patient assumes a series of postures (lying supine, prone, on the right
and left sides, and standing up). The
doctors can then map the data from
each position to a stimulation level,
with the aim of activating a constant
volume of nerve tissue as the patient
goes about the routines of daily life.
The system also adapts over time by
incorporating manual adjustments
made by the patient into the automatic
control algorithm.
Regulators in the United States and
Europe approved this system, called the
Restore Neurostimulator, a few years
ago. The next generation of spinal cord
stimulators from Medtronic may include
other sensors as well-perhaps some that
detect heart rate or signals from the central nervous system. As these implanted
devices get smarter about what's going
on in a patient's body, they should be
able to deliver therapy that is more customized and precise.

a S w e S e e I t, the goal of all these closedloop systems is to let doctors take their
expert knowledge-their ability to evaluate a patient's condition and adjust
therapy accordingly-and embed it in
an implanted device. These dynamic
systems have a number of potential
benefits: They may react faster than
current devices, provide more tailored
therapy to individuals, and free up clinicians' time.
These smart devices may have something to teach doctors as well. As the
stimulators provide therapeutic effects,
they also provide data about how physiological states relate to clinical outcomes.
From this new information, scientists
and engineers hope to learn more about
how the nervous system works, how it is
affected by disease, and how to design
better treatments. Sometimes, it seems,
the way to move forward in science is to
follow a loop. n
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Table of Contents for the Digital Edition of IEEE Spectrum February, 2015