AASM Membership Sections Newsletter Issue #2
17
the tongue is kept from obstructing the airway via
enhanced tone of muscles such as the genioglossus
(
GG) muscle (Mezzanotte 1992), but this tone is lost
with sleep onset. Tongue muscle stimulation trials
were thus performed using intraoral and/or submen-
tal simulation methods, but stimulation often re-
sulted in arousal and only partial effectiveness (Guil-
leminault 1995, Decker 1993). In a subsequent trial,
arousals were not triggered while flow and the AHI
were markedly improved, but this trial employed
intramuscular electrodes for tongue stimulation, thus
impractical (Schwartz 1996). Given this encourag-
ing result, a more applicable approach was developed
using an implantable hypoglossal neurostimula-
tion device (Inspire I system, Medtronic) where an
implantable pulse generator (IPG) was placed in a
subcutaneous chest pocket and connected to both
a stimulation electrode for one hypoglossal nerve
(
branch to the GG muscle) as well as a intrathoracic
pressure transducer placed in a manubrial drill hole.
(
Schwartz 2001). The pressure transducer was used to
time stimulation to just before inspiration. Reduction
of the AHI with stimulation occurred from about 50
to 20 without induction of cortical arousals and the
patients reported nightly device use without adverse
effects. This trial was promising but various device
malfunctions occurred within 6 months of treatment
and funding was discontinued.
More recently, three medical device companies
developed and began trials of hypoglossal neuro-
stimulation as a therapy for OSA: Apnex Medical
(
Eastwood 2011), ImThera Medical (Mwenge 2012)
and Inspire Medical Systems (Van de Heyning 2012).
Apnex’ and Inspire’s devices use a similar general
stimulation scheme to the original Inspire trial where
an implanted sensing lead is used to time stimula-
tion mostly with inspiration, however the lead is now
a thoracic impedance sensor implanted through an
incision on the lower chest and not via a manubrial
drill hole
(
figure 1
)
.
Apnex’ and Inspire’s stimulation
electrode is applied to the medial hypoglossal nerve
branch which innervates the GG muscle primar-
ily. ImThera’s hypoglossal neurostimulation method
is unique. ImThera’s stimulating electrode is placed
proximally on the hypoglossal nerve
trunk
in the
submandibular triangle and the stimulation design
eliminates the need for implantation of a sensing
lead
(
figure 2
)
.
ImThera’s stimulation is continuous,
but cycled between 6 different contacts on the elec-
trode, allowing stimulation of different portions of
the hypoglossal nerve and hence different arrays of
corresponding extrinsic and intrinsic muscles (Zaidi
2012).
This rotating stimulation scheme reduces the
potential for muscle fatigue.
The site of HGN stimulation and the different
methods of stimulation arise from varied conceptual
and practical considerations. The GG muscle is the
best studied upper airway dilator muscle and tongue
protrusor. Direct GG stimulation resulted in im-
proved flow and reduced upper airway obstructions
(
Schwartz 1996). Hence, this is the likely basis of Ap-
nex Medicals’s and Inspire Medical’s approach which
targets the medial hypoglossal nerve branch that
innervates the GG muscle. To avoid muscle fatigue,
intermittent stimulation, mostly limited to inspira-
tion is employed, and for this reason, implantation of
thoracic sensing leads is needed.
ImThera’s approach is based on the concept of the
lingual hydrostat and HGN neuroanatomy (Zaidi
2012).
The tongue may be considered a muscular
hydrostat in that it is a relatively incompressible
structure which contains multiple interdigitated
muscle arrays at various angles to each other and
with co-activation of different extrinsic and intrinsic
muscles the tongue assumes different shapes and ac-
tivities while maintaining a constant volume (Gilbert
2007).
The concept of multiple tongue muscle activa-
tion for airway function is supported by experiments
where HGN trunk stimulation or surface concur-
rent stimulation of protrusor and retrusor muscles
Continued >>
Neurostimulation for OSA continued