DOI:10.2214/AJR.05.1281
AJR 2007; 188:984-991
© American Roentgen Ray Society
Noncardiac Implantable Pacemakers and Stimulators: Current Role and Radiographic Appearance
Galina Levin1,
A. Orlando Ortiz and
Douglas S. Katz
1 All authors: Department of Radiology, Winthrop-University Hospital, 259 First
St., Mineola, NY 11501.
Received July 28, 2005;
accepted after revision August 1, 2006.
Address correspondence to G. Levin
(glevin{at}winthrop.org).
Abstract
OBJECTIVE. The purpose of this pictorial essay is to familiarize
radiologists with the clinical functioning, proper anatomic positioning,
appearance on radiographs and CT scans, potential complications, and MRI
safety issues of several implantable noncardiac pacemaker and stimulator
devices.
CONCLUSIONS. The use of noncardiac pacemakers and stimulators is
rapidly increasing because of the utility of these devices in the management
of surgically and medically refractory conditions. Unlike cardiac pacemakers,
electrical stimulators are MRI compatible under certain circumstances.
Keywords: body imaging • CNS • MRI • safety • spine
Introduction
Since the invention of the cardiac pacemaker, research in
neuromodulation has led to the development of several implantable noncardiac
pacemakers and stimulators that have been approved by the U.S. Food and Drug
Administration (FDA). Some of these electrical devices help patients with
debilitating diseases often not amenable to other therapies. The purpose of
this article is to familiarize radiologists with the clinical functioning,
proper anatomic positioning, appearance on radiographs and CT scans, and
potential complications of several implantable noncardiac pacemaker and
stimulator devices. These devices include gastric, bladder, vagal, and
diaphragmatic pacemakers and brain and spinal stimulators. The clinical and
limited radiology literature to date on this topic is reviewed, and MRI safety
issues with use of these devices are discussed.
Devices
Deep Brain Stimulator
In 1997, the FDA approved implantable deep brain stimulation (DBS) therapy.
This treatment is used as adjunctive therapy in patients with Parkinson's
disease, essential tremor, and several types of dystonia. The implantable
brain stimulator is surgically implanted into the thalamus under CT or MRI
guidance. Unilateral versus bilateral positioning of wires and the exact
location of the wire tip in the thalamus depend on the condition for which the
patient is being treated. For example, Parkinson's disease is managed with
bilateral stimulation of the globus pallidus or the subthalamic nucleus.
Dystonia, however, is managed with either unilateral or bilateral stimulation
of the same nuclei [1].
The DBS device consists of an electrical pulse generator and insulated
wires. Electrodes extend from the thalamus subcutaneously down the patient's
neck and connect to a pulse generator usually located under the clavicle (Fig.
1A,
1B,
1C). Results
[1] suggest that at long term
follow-up, hardware complications such as infection and lead migration occur
in approximately 26% of patients.
Vagal Nerve Stimulator
Repetitive vagal nerve stimulation was approved by the FDA in 1997 for
management of medically refractory seizures and more recently has been used
for management of intractable hiccups. The possible mechanism of action
involves neuromodulation of afferent vagal nerve fibers that communicate with
important adrenergic and noradrenergic centers in the brainstem
[2]. The vagal nerve
stimulation generator is implanted subcutaneously below the left clavicle. A
connecting lead is passed through the soft tissues of the neck and implanted
into the left vagal nerve (inside the carotid sheath) inferior in relation to
the origins of the superior and inferior cervical cardiac branches (Fig.
2A,
2B). The vagal nerve
stimulation kit includes a separate magnet the patient uses to boost generator
activity as needed. Complications after vagal stimulator insertion include
infection, lead fracture, and stimulation-induced symptoms such as hoarseness
and dysphagia [2].
Spinal Cord Stimulators
The spinal cord stimulator was introduced in the 1960s, when chronic back
pain was managed with electrical shocks to the spinal cord
[3]. Electric impulses were
thought to interfere with propagation of painful stimuli to the brain. To be
eligible for this treatment, patients must have had no response to
conventional medical and surgical therapies
[4].
Implantation of a spinal cord stimulator is performed under fluoroscopic
control. There are two types of spinal cord stimulators: internal and
external, depending on the location of the generator. External generators are
usually used for trial stimulation to determine the effectiveness of the
treatment in a specific patient. Both types of devices contain a generator, an
extension wire, and one or two leads ending with multiple electrodes, the
total number of which ranges between four and 16
[4]. The leads are implanted
under fluoroscopic control into the epidural space adjacent to the lower
aspect of the spinal cord between T9 and L1 (Figs.
3A,
3B,
3C and
4). The generator is usually
implanted between the skin and the fascial layers. Common complications are
electrode migration, infection, and hardware malfunction
[4].
Bladder Pacemaker
The urinary bladder pacemaker was approved by the FDA in September 1997. It
has been shown to significantly improve the symptoms of incontinence in
patients with severe neurologic diseases
[5]. Patients undergo a trial
with a temporary stimulator that paces the bladder externally through the
skin. Patients with favorable trial results undergo surgical implantation of a
permanent device consisting of a lead wire containing four platinum
electrodes. The wire is inserted surgically into a sacral foramen, usually S3,
on one side adjacent to the sacral nerves
[5] (Fig.
5A,
5B,
5C). The generator itself is
implanted subcutaneously in the hip area. The patient is given an external
control unit that can switch the bladder pacemaker between on and off modes.
The complications of bladder pacemakers are not well described, to our
knowledge. Potential complications include infection, skin irritation, and
abnormal wire migration
[5].
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Fig. 5A —39-year-old woman with urinary incontinence and bladder stimulator.
Frontal (A) and lateral (B) spot radiographs obtained during
surgery show placement of lead (arrow) for bladder stimulator.
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Fig. 5B —39-year-old woman with urinary incontinence and bladder stimulator.
Frontal (A) and lateral (B) spot radiographs obtained during
surgery show placement of lead (arrow) for bladder stimulator.
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Fig. 5C —39-year-old woman with urinary incontinence and bladder stimulator.
CT scan of pelvis without contrast enhancement obtained to exclude abscess
around stimulator shows course of lead (arrow) through right S3
neural foramen.
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Gastric Stimulators
The gastric pacemaker was approved by the FDA in 2000 as a humanitarian-use
device for gastroparesis refractory to medical therapy. A gastric pacemaker
generates high-frequency stimuli that enhance motility and facilitate emptying
[6]. The gastric pacemaker
consists of a neurostimulator, implantable lead, and external programming
system. Two bipolar leads are implanted laparoscopically into the serosa of
the stomach and then joined with a subcutaneous electric generator (Fig.
6A,
6B,
6C). Potential complications
include perforation of the gastric wall, lead migration, infection, and seroma
formation [6].
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Fig. 6B —61-year-old woman with gastroparesis and intractable abdominal pain.
CT scans (B at slightly higher level) show gastric pacemaker with leads
(white arrow) abutting stomach wall (black arrow).
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Fig. 6C —61-year-old woman with gastroparesis and intractable abdominal pain.
CT scans (B at slightly higher level) show gastric pacemaker with leads
(white arrow) abutting stomach wall (black arrow).
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Diaphragmatic Pacemaker
The diaphragmatic (phrenic nerve) pacemaker was invented in the 1970s by
William W. Glenn, a cardiothoracic surgeon. Problems with malfunction and
nerve damage, however, deterred the popularity of this device for at least a
decade. One of the latest models of phrenic stimulator was approved by the FDA
in 1986. This device has been used to treat or has treated hundreds of
patients with respiratory insufficiency by alleviating the need for lifelong
mechanical ventilation [7].
Indications for phrenic nerve stimulation include respiratory insufficiency
due to phrenic nerve upper motor neuron paralysis or dysfunction of the
respiratory control center [7].
Surgical implantation of a diaphragmatic pacemaker is usually performed
laparoscopically. Four electrodes are embedded in each phrenic nerve. A
receiver implanted in the subcutaneous tissues is connected to the electrodes
by platinum leads. An external portable transmitter generates radio waves,
which the receiver converts into electric stimuli
[7,
8] (Figs.
7A,
7B,
7C and
8A,
8B). The pacemaker
implantation procedure can be complicated by pneumothorax in the immediately
postoperative period. Infection of the wires also has been reported
[8,
9].
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Fig. 7A —17-year-old boy with chronic respiratory insufficiency after
resection of tumor of cervical spine. Frontal radiograph of chest shows
bilateral diaphragmatic receivers (black arrows) with leads
(white arrows).
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Bone Stimulator
Uses of electrical stimulation in the management of poorly healing bone
fractures can be traced to the early 1800s. A century later, scientific
experiments revealed that fractured bone has an inherent electrical field that
promotes a repair mechanism. Numerous studies have shown that application of
an external magnetic or electric field produces an artificial stimulus that
promotes healing [10].
Bone stimulators can be subdivided into invasive and noninvasive types.
Noninvasive devices consist of a generator and electrodes attached to the
surface of the skin. Invasive stimulators also consist of a generator and one
or two wires, but both are surgically implanted in the region of the fracture
(Fig. 9A,
9B,
9C). Orthopedists occasionally
use a partially invasive method that involves implantation of one of the leads
into the fracture site and placement of the second lead on the skin, the
generator being incorporated into a cast. In invasive and partially invasive
methods, the generator is removed on completion of therapy, but the
internalized electrodes (which can be single, double, or mesh) are left inside
the patient. The electrodes ideally have maximum contact with viable bone but
no contact with metal implants
[10]. The electrodes may be
surrounded by bone graft material or sutured to underlying bone. It is
acceptable for the electrodes to be folded or coiled or to extend through
drill holes. The battery, which lasts 6-12 months, is usually placed in a
non-weight-bearing region
[10]. Implantable bone
stimulators are used in long bones and the vertebral column. Studies
[11] of electrical bone
stimulation have shown significant improvement in healing of nonunion of
fractures and in the aftermath of spinal fusion procedures. Complications
related to electrical bone stimulation are not well described in the
literature, to our knowledge.
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Fig. 9A —70-year-old man with Charcot foot. Anteroposterior (A) and
lateral (B) radiographs of ankle show bone stimulator with generator
(straight arrow) overlying distal tibia. Single lead (curved
arrow) courses to region of lateral cuneiform bone. Relation of wire to
screws is not entirely clear.
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Fig. 9B —70-year-old man with Charcot foot. Anteroposterior (A) and
lateral (B) radiographs of ankle show bone stimulator with generator
(straight arrow) overlying distal tibia. Single lead (curved
arrow) courses to region of lateral cuneiform bone. Relation of wire to
screws is not entirely clear.
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MRI Safety
The subject of MRI compatibility of pacemaker-like devices is a source of
intense and continuing controversy. Unlike their cardiac counterparts,
noncardiac stimulators and pacemakers are not necessarily considered absolute
contraindications to MRI examinations. However, the manufacturer manuals for
gastric, urinary bladder, diaphragmatic, and spinal cord stimulators list
these devices as absolute contraindications to MRI.
The vagal nerve stimulator manual, for example, states that a 1.5-T magnet
test showed no adverse effects with use of a transmit-and-receive type of head
coil. One study [12] showed
successful use of functional MRI in patients with vagal nerve stimulators.
However, there is concern that most modern high-field-strength MRI machines
have transmit body coils and receive-only head coil combinations
[13], which are strictly
contraindicated by the manufacturer
[14] because of risk of heat
injury.
Like patients with vagal nerve stimulators, patients with DBS devices are
allowed to undergo MRI studies under certain circumstances. It has been shown
[13] that the MRI safety of
DBS devices depends on the MRI technique used, including type of coil,
specific absorption rate, and lead positioning. Temperature changes with use
of transmit-and-receive head coils, 1.5 T, and a local specific absorption
rate of approximately 2.4 W/kg were concluded to be safe in one in vitro study
[15] of bilateral DBS. Because
of lack of sufficient research data regarding these coil combinations, MRI
with full body coils and head transmitonly coils is contraindicated in
patients with DBS devices. In vitro studies have shown significantly higher
maximum temperature ranges with use of body coils versus head coils
(25.3°C vs 7.1°C)
[16]. However, one in vitro
study [17] of transmit body
and receive-only head radiofrequency coils showed acceptable temperature
elevations. Two case reports in the literature describe serious injuries after
MRI of patients with DBS devices. In both patients, either the implantation
technique or imaging technique differed from the manufacturer's
recommendations [18]. The
manufacturer [19] of one DBS
system warns about the possibility of spontaneous voltage induction in the
device that can result in uncomfortable shocks to patients.
MRI compatibility of electrical bone stimulation devices used in the
appendicular skeleton has not been described in the literature, to our
knowledge. The manufacturer of the most widely used model lists MRI as a
contraindication in patients with this device. However, at least one model of
spinal fusion device was tested and cleared by the FDA as MRI compatible if
several important criteria are met. These criteria include use of a 1.5-T
magnet and intact electrodes and avoidance of sequences that deliver high
radiofrequency energy, such as echo-planar sequences, high gradient fields
exceeding 20 T/s, and high whole-body absorption rates greater than 1.0 W/kg
[20].
In conclusion, the use of noncardiac pacemakers and stimulators is rapidly
increasing owing to the unique ability of these devices to assist in the
management of otherwise surgically and medically refractory conditions. Unlike
cardiac pacemakers, some electrical stimulators are MRI compatible under
certain circumstances. It is important for radiologists to recognize these
devices on different imaging studies and to check for proper positioning and
complications. It also is important to check the MRI compatibility of these
devices in patients who need MRI. However, it cannot be overemphasized that
because of the wide spectrum of MRI techniques used at different institutions
and differences in equipment and potential differences in surgical device
implantation, every patient with an implanted stimulator device should be
approached on a case-by-case basis to ensure maximum patient safety.
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Abstract
Introduction
