Spinal Cord Machine Interface: The Missing Link in Neuroprosthetics

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Brain and Spinal Cord Machine Interfaces:
A View from the Clinical Edge
Peter Konrad, MD PhD
Assoc Professor Neurosurgery and
Biomedical Engineering
Vanderbilt University
Dr. Konrad has no financial
disclosures relevant to this talk
There is discussion of the use of
FDA-approved devices in off-label
applications
Emergence of Extrinsic Neurally
Controlled Systems
 Goals:
 Parastep FES
 Cochlear prosthesis
 ITB / Bladder prosthesis
Sensory Signal Insertion
 Examples:
Stroke
SCI
ALS
Amputation
Motor Signal Extraction
 Restore lost motor fxn
 Restore lost sensory fxn
 Restore internal control
signals
Neuromodulation in Disability
• Brain:
– Extract unique cortical signals
– Insert lost sensory information
• Spinal cord:
– Cervical region: Hand function / UE limb
sensation
– Lumbar enlargement: Standing, walking,
B/B ctrl
– Spasticity control
• Peripheral nerves:
– Focal deficit: amputation, peripheral injury
Brain Machine Interface: Cortex
• Advantage:
– Unique cortical sensorimotor experience
– Anatomically well defined
regions
– Access relatively easy
– Tolerant of implantation (?)
• Disadvantage:
– Coding of information highly
complex
– Sensory plasticity
– Potential seizures with some
types of chronic stimulation
Physiology of Motor Coding
• Human studies show
potential for motor
signal extraction real
time
• Thalamic micro-wire
implants (Patil et al 2004)
• Cortical Utah array in
quadriplegic (Hochberg et al
2006)
– Able to move screen
cursor
– >5 years implanted
Physiology of Motor Coding
• Visual cues are integral
to closed-loop BMI
control
• Speed of processing
circuitry important in a
functional BMI unit
– Shenoy et al: 4 words/min
• Volitional control will
require significant
number of channels and
processor speeds
BMI for Sensory Control
• Sensory experience
easily dissipates /
plasticity
• Role of BMI Less
understood
Sadato et al. Neurosci Lett 2004
BMI for Sensory Input:
State of Technology
• Auditory prosthesis
represent most
extensive clinical
impact of BMI
• Better response with
more centralized
implant
– Present ABI since
1993
– 21 electrodes OK for
speech recognition
Lenarz et al. Otol Neurotol 2006
Spinal Cord Machine Interface
• Advantage:
– Injury affects a segment of cord
– Regionally specific function
(cervical, thoracic, lumbar)
– Integration sensori-motor
function already in place
– Plasticity not known
• Disadvantage:
– Anatomy very compact
– Surgically more risky
– Not well suited for subtle,
volitional control
SCMI: ITB for adjustment of reflex tone
• Spasticity:
– Hypertonicity of AMN
– Baclofen (GABAb)
replaces lost regulation
• ITB therapy: example of
SCMI
– Simple system at present
– Likely to be a significant
adjunct to other SCMI
From Brain
GABA
Organization of Spinal Cord
AUTONOMIC
Motor:
•Anterior horn
•Motor tracts (5)
Sensory:
•Posterior horn
•Sensory tracts (3)
Autonomic:
•Intermediate gray
•Sympathetic tracts T1-T12
•Parasympathetic S2-5
SENSORY
MOTOR
Physiological Organization of the Spinal Cord
• Central Pattern
Generators (CPGs)
– Standing: Extensor tone
predominates
(reticulospinal system)
– Walking: recipricol Flex /
ext activity
– Balance: Agonistantagonist reflex
(vestibulospinal system)
Motor Control in the Spinal Cord
Spinal Cord AMN Pools during
stepping in normal cat
Yakovenko S et al. J Neurophysiol 2002;87:1542-1553.
Motor Spinal Cord Machine Interface
• Concept is to discretely activate
AMN pools (30-60µA; 200µsec)
• Can accomplish standing and
walking through separate stim
points (est 2X4=8)
• Standing: > 20 min body weight
(vs 4 min with IMS)
• Walking: > 140 steps (>200m)
• Implants: 67% functioning at 6
months
Mushahwar et al. J Neural Eng 2007
Motor Spinal Cord Machine Interface
FES Muscular stim in a
complete paraplegic
ISMS in complete
paraplegic cat
Motor Spinal Cord Machine Interface
• Epidural stimulation can also initiate stepping
patterns
• Based on FRA stimulation studied in 1911 by
Graham Brown
Motor Spinal Cord Machine Interface
• L2-3 cord (T12
vert) stim
• Non-weight
bearing patients
• Stepping patterns
only generated
• Rehab potential is
large for this
therapy (maintain
muscle tone at a
minimum)
25 Hz
Q
1 mV
H
1 mV
TA
1 mV
TS
1 mV
1s
KM
3.5 V
4.0 V
4.5 V
5.0 V
Spinal cord stimulation-strength (V)
Q
H
TA
TS
KM
(7V-10V)
vertical markers: 500 µV (EMG), 45° (goniometer); horizontal marker: 1 s (time)
Minnassian et al. Spinal Cord 2004
Sensory Insertion into the Spinal Cord
• Cordotomies - SC
stim tolerated in
awake humans
• ALST: pain, temp
• DCLS: mechanical,
vibration,
proprioception
Sensory Insertion into the Spinal Cord
USEA
Konrad, Diedrich et al. Vanderbilt Univ (unpublished data)
Autonomic SC Machine Interface
Arch Surg 1972;104:195-202
The Bigger Picture
• Bridging a Lesion:
Sensory Signal Insertion
• Spinal Cord contains
Viable lnterface
Options for
Neuroprosthetics
Stroke
SCI
ALS
Amputation
Motor Signal Extraction
– Brain : Peripheral Nerve
– Brain : Spinal Cord
– Spinal Cord : Peripheral
Nerve
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