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Neuroanatomical Substrates of
Functional Recovery After
Experimental Spinal Cord Injury:
Implications of Basic Science Research
for Human Spinal Cord Injury
Human spinal cord injury (SCI) is a devastating condition that results
in persistent motor deficits. Considerable basic and clinical research is
directed at attenuating these deficits. Many basic scientists use animal
models of SCI to: (1) characterize lesion development, (2) determine
the role of spared axons in recovery, and (3) develop therapeutic
interventions based on these findings. In this article, current research
is reviewed that indicates: (1) most individuals with SCI will have some
sparing of white matter at the lesion epicenter even when the lesion
appears clinically complete, (2) even minimal tissue sparing has a
profound impact on segmental systems and recovery of function, and
(3) facilitatory intervention such as weight bearing and locomotor
training after SCI may be more effective than compensatory strategies
at inducing neuroplasticity and motor recovery. Body weight sup-
ported treadmill step training is discussed as an example of new
facilitatory interventions based on basic science research using animal
models. [Basso DM. Neuroanatomical substrates of functional recovery
after experimental spinal cord injury: implications of basic science
research for human spinal cord injury.
Phys Ther.
2000;80:808 – 817.]
Key Words:
Experimental models, Flexor withdrawal reflex, Locomotion, Neuroplasticity, Treadmill
training.
D Michele Basso
808
Physical Therapy . Volume 80 . Number 8 . August 2000
cord injury (SCI) each year, with the total
SCI population in the United States ranging
from 175,000 and 275,000 individuals.
1
The
majority of these individuals experience moderate to
severe motor impairment, and 60% require some assis-
tance to function on a daily basis.
2
Few individuals with
SCI recover functional ambulation, with estimates for
recovery ranging from 7% to 38%.
3– 6
Considerable
effort by biomedical scientists has been directed toward
understanding lesion development, mechanisms of nat-
ural recovery, and neuroanatomical substrates of motor
recovery. In this article, I examine some of these studies.
Thus, a clinically complete lesion does not necessarily
indicate an anatomically complete SCI. A functionally or
clinically complete lesion is typically described as the
absence of sensory and motor function below the level of
the lesion (American Spinal Injury Association [ASIA]
Impairment Scale classification A).
10
In contrast, an
individual with an incomplete SCI has sensory and/or
motor function below the lesion (ASIA B, C, and D).
11
Thus, tissue sparing after complete SCI suggests that the
remaining axons are available to possibly mediate some
recovery of function but do not appear to be recruited
using traditional rehabilitation approaches such as com-
pensation. The emergence of the full capacity of these
spared axons may depend on the type of therapeutic
interventions that are used.
Lesion Development After SCI
Based on the histopathology of 50 spinal cords exam-
ined post-mortem in The Miami Project, Bunge and
colleagues
7–9
broadly categorized lesions into: (1) con-
tusion injuries in which the glial limitans and spinal cord
surface remain intact or (2) maceration or laceration
injuries that disrupt the glial and pial interface and may
directly tear the spinal cord tissue. Contusion and some
maceration injuries may be present with loss of central
gray and white matter, which creates a cavity that is
surrounded by a rim of intact white matter at the
periphery of the spinal cord.
8,9
Of particular interest to
clinicians is the fact that intact, continuous central
nervous system (CNS) axons traverse the lesion center in
many individuals with functionally complete SCI.
8,9
Several experimental models of SCI have been devel-
oped in order to understand factors that may facilitate
recovery of function after SCI in humans. The advan-
tages of using an animal model rather than studying
humans with SCI are: (1) the severity of the lesion can be
controlled and reproduced across animals and experi-
ments, (2) direct neuroanatomical evidence of lesion
development over time can be attained using invasive
procedures, and (3) effective parameters of therapeutic
interventions can be established and refined before
translating them for human use. One of the most
clinically relevant experimental models of SCI is the
contusion injury in rats, created by rapid impact of the
DM Basso, PT, EdD, Assistant Professor, Division of Physical Therapy, School of Allied Medical Professions, The Ohio State University, 306 SAMP,
1583 Perry St, Columbus, OH 43210 (USA) (basso.2@osu.edu).
Dr Basso provided concept/idea, writing, data collection and analysis, and fund procurement. Michael S Beattie, PhD, and Jacqueline C
Bresnahan, PhD, contributed to the experiments and to Dr Basso’s understanding of basic science research as it applies to spinal cord injury. Karen
Hutchinson, PT, PhD, Lesley Fisher, and Craig Voll Jr, PT, provided data collection and analysis for the flexor withdrawal study. Lesley Fisher also
provided clerical support.
The work was supported by NIH grant NS-32000, American Paralysis Association grant BB-2-9302-2, and USPHS grants NS-25095 and NS-10165.
Portions of this work were presented at the 1993–1996 Annual Meetings of the Society for Neuroscience and at the 1998 Neuroplasticity
Symposium of the Section on Research, American Physical Therapy Association.
Physical Therapy . Volume 80 . Number 8 . August 2000
Basso . 809
A
pproximately 10,000 people survive spinal
dorsal surface of the spinal cord using an electromag-
netic device
12,13
or a weight-drop device.
14 –16
These
devices create a central core lesion and a peripheral rim
of spared white matter similar to that reported in
humans. The area of the cord with the greatest extent of
damage is referred to as the “lesion epicenter.” This
review will focus on data gathered from contusion mod-
els of SCI primarily in rats.
prevent the secondary lesion cascade, it is likely that the
incidence of functionally complete lesions will decline.
Relationship of Lesion Size and Behavioral
Outcome
Sensitive Anatomical and Behavioral Measures
Lesion severity can be measured in 2 ways: size of the
injury and severity of residual deficits in function. In
order to determine the relationship between lesion
severity and motor deficits, sensitive, reliable, and quan-
tifiable measures of neuropathology in the spinal cord
and gross behavioral performance are needed. Several
researchers
12,15,31–35
have developed standardized mea-
sures of lesion size in experimental animals. Once the
region of spinal cord containing the lesion has been
recovered and histologically prepared, it is cut serially
into transverse cross sections, mounted on slides, and
stained. In my laboratory, we use Luxol fast blue to stain
myelin in the peripheral rim of spared tissue and mea-
sure those areas that stain blue. Through light micros-
copy and computer processing, we calculate the percent-
age of spared tissue per cross-sectional area at the lesion
epicenter (for details, see Behrmann et al
12
).
Lesion development in SCI progresses over time and in
neuroanatomical distribution. The primary phase of
injury occurs during the first 18 hours and is typified by
necrotic death of neurons and axons that were directly
disrupted by the trauma.
17–20
The secondary phase of
tissue injury can last several weeks and progresses in
rostral and caudal directions, away from the lesion
epicenter. Recent work suggests that the immune system
plays a primary role in initiating cellular cascades that
contribute to the expansion of the lesion during the
secondary phase of lesion development.
21–23
In the sec-
ondary phase, immune system cells such as monocytes
and macrophages are thought to emit chemical signals
such as cytokines and chemokines.
22
These substances
act on neurons and oligodendrocytes and appear to
trigger apoptosis or programmed cell death in which
DNA in the nucleus of the cell is systematically broken
down into fragments.
24 –26
Thus, intracellular processes
in neurons and oligodendrocytes lead to destruction of
their nuclei, causing cell death, despite no direct trauma
to the cell. Apoptosis is known to occur after contusive
SCI, especially in white matter tracts great distances away
from the lesion (at least 4 spinal segments above and
below the lesion epicenter).
26,27
Oligodendrocytes
appear to undergo programmed cell death, which
results in demyelination of axons many segments away
from the lesion epicenter.
28
Thus, early after SCI, the
injury is small, focal, and localized to the point of
impact, but over time the lesion spreads to distant sites
and may preferentially affect myelination of axons that
are otherwise intact.
As a gross behavioral outcome measure for use with
animal models, my collaborators and I
31
recently devel-
oped the semiquantitative Basso, Beattie, and Bresnahan
(BBB) Locomotor Rating Scale to assess overground
locomotion in the open field. The measure is a sensi-
tive,
31
reliable
36
measure of overall locomotor perfor-
mance. The ratings of the BBB Locomotor Rating Scale
range from 0 to 21 and distinguish between locomotor
features such as flaccid paralysis, isolated hind-limb joint
movements, weight-supported plantar stepping, coordi-
nation, and fine details of locomotion (eg, toe clearance,
paw position).
Does Tissue Sparing Mediate Behavioral Recovery?
In a series of experiments, my collaborators and I
15,31
analyzed locomotor outcomes after mild, moderate, or
severe spinal cord contusion in rats with extensive
(
One critical issue in SCI research is whether lesion size
and tissue sparing are important determinants of final
functional outcome after contusion injury. If the size of
the lesion is an important determinant of final func-
tional outcome in patients with SCI, then interventions
that attempt to bridge the central lesion or limit the
secondary progression of the lesion would appear to be
quite beneficial. The action of methylprednisolone, the
standard treatment after SCI, is to reduce the inflamma-
tory response and inhibit the immune system.
29,30
Methylprednisolone, therefore, could limit lesion expan-
sion, which might explain the marked improvement in
functional outcome of those treated.
29,30
As other phar-
maceutical interventions are developed that limit or
40%), intermediate (15%– 40%), or minimal (1%–
14%) tissue sparing at the lesion epicenter. We found
differences in BBB Locomotor Rating Scale scores across
groups. In general, locomotor recovery was extensive
after mild SCI with extensive axonal sparing (
40%) but
was quite limited after severe SCI with little sparing (as
low as 1%–2%).
15
The BBB Locomotor Rating Scale
scores predicted the extent of underlying neuropathol-
ogy, as indicated by a positive correlation between BBB
Locomotor Rating Scale score and percentage of spared
tissue at the epicenter (replicated across studies:
r
2
.
.001). Although there was a causal
relationship between the extent of sparing and the
extent of recovery, correlational data were insufficient to
.79,
15
r
2
5
.88
31
;
P
,
810 . Basso
Physical Therapy . Volume 80 . Number 8 . August 2000
.
5
conclude that the spared axons directly mediated the
recovery. Perhaps spinal cord systems below the level of
the lesion were responsible for the locomotor recovery.
Therefore, we took 2 groups that had recovered for 9
weeks after moderate and severe SCIs and transected the
spared axons at the lesion epicenter.
31
If functional
recovery was due to segmental systems in the lumbar
spinal cord, then transection of the spared tissue would
have no behavioral effect. However, we found that
severing the spared axons eliminated the behavioral
recovery. The secondary transection resulted in a loss of
locomotion such that the animals with SCI and spinal
cord transection performed no differently within the
first 5 days after transection than animals with spinal
cord transection alone, based on BBB Locomotor Rating
Scale scores. These findings suggest that tissue sparing at
the lesion epicenter is responsible for behavioral recov-
ery after experimental spinal cord contusion.
transection,
41
we found further evidence that an incom-
plete complement of descending or ascending axons
induced marked reorganization below the level of the
lesion. The study was designed to examine the effects of
complete spinal cord transection made early in develop-
ment on adult motor function.
41
Opossums are born 12
days after conception, crawl into the mother’s pouch,
and continue developing.
42,43
Therefore, lesions can be
made extremely early in development without using
high-risk,
in utero
techniques. Previous work by Martin
and colleagues
44,45
established that, if the transection is
made at postnatal day 26 in development, then at least a
few axons from some supraspinal systems (primarily
rubrospinal axons) grow across the lesion and travel into
the lumbar spinal cord. However, in those studies, there
was no examination of the behavioral effects of these
axons.
Can Minimal Sparing Improve Motor Function?
The study of rats with SCI and spinal cord transection
31
yielded important information about reorganization
within the spinal cord. We were surprised by our obser-
vation that the rats with SCI and spinal cord transection
appeared to demonstrate some motor recovery 2 weeks
after complete transection of the spinal cord, a finding
that rarely or never occurs after complete transection
alone in adult rats.
12,31
Although the ability to locomote
remained lost, the rats with SCI and spinal cord transec-
tion demonstrated more extensive hind-limb joint move-
ments than the rats with transection alone, as measured
by the BBB Locomotor Rating Scale scores. On closer
examination of videotaped performance in the open
field, we found that the rats that recovered from SCI and
spinal cord transection performed more hind-limb
movements than the rats that had recovered from a
transection alone.
31
This is an important finding because
it shows that animals that recover in the presence of
relatively few spared axons (
In our study,
41
a group of opossums had midthoracic
transection on postnatal day 5 and grew to adulthood; a
procedure that results in only a partial complement of
axons growing across the lesion into the lumbar spinal
cord. Despite less input to the lumbar spinal cord, these
opossums developed nearly normal locomotion, as mea-
sured by the BBB Locomotor Rating Scale in adulthood.
Three animals from this group were then given spinal
cord retransection to determine whether supraspinal
axons reaching the lumbar spinal cord contributed to
the locomotor effects. At the end of the experiment, we
used one of the most sensitive silver staining methods
46
to detect axons as small as 1
2% in some cases) have
altered organization of systems below the lesion. This
reorganization, presumably of segmental systems, is evi-
dent when the lumbar spinal cord functions in isolation
of supraspinal systems after the transection. Reorganiza-
tion of caudal segments may be in the form of anatom-
ical changes such as synaptogenesis of primary afferent
fibers into vacated synaptic sites,
37
physiological changes
that lower the threshold of postsynaptic neurons and
render them more likely to produce an action poten-
tial,
38
or loss of axo-axonic presynaptic inhibition.
39,40
In
summary, tissue sparing directly mediates recovery of
function after experimental SCI, and sparing of as little
as 1% to 2% is sufficient to facilitate reorganization
within the lumbar spinal cord.
As observed in the study of rats with SCI and transec-
tion,
31
severing the partial complement of axons that
reach the lumbar spinal cord in the opossum eliminated
locomotion immediately and hind-limb movements were
similar to those exhibited by opossums that had spinal
cord transection in adulthood.
41
More importantly, the
opossums with retransection demonstrated dramatic
rebound in hind-limb motor function over time, a
pattern seen in rats with SCI and transection. By 6 weeks
following retransection, each of the animals was able to
take several bouts of consecutive weight-supported step-
ping with the hind limbs. The hind limbs often stepped
in a rhythmic, alternating manner. In contrast, it is well
documented that complete transection of the spinal
cord in adult animals (rat,
12,47
cat,
48,49
opossum,
50
review
51
) results in hind-limb paralysis during over-
ground locomotion. Thus, the only difference between
In another experiment, in which my collaborators and I
studied the locomotion of opossums after spinal cord
Physical Therapy . Volume 80 . Number 8 . August 2000
Basso . 811
m in diameter that may
have been spared after retransection. Based on our
post-mortem findings that the cut ends of the spinal cord
separated 5 to 10 mm and that no axons traversed the
lesion site, we confirmed that the retransection was
complete. Thus, any hind-limb motor function would be
mediated solely by the isolated lumbar spinal cord after
retransection.
m
,
opossums with spinal cord transection in adulthood and
those with retransection in adulthood is that the lumbar
spinal cord of the animals with retransection developed
in the presence of an incomplete complement of
descending systems. The remarkable stepping exhibited
by the animals with retransection is evidence of substan-
tial reorganization of lumbar systems during
development.
12)
in several other studies in our laboratory and present
these data as representative of animals with mild SCI
(unpublished observation). One week after mild SCI,
the stimulus intensity was slightly less than normal but
had returned to normal levels by 4 weeks after SCI. In
summary, animals with less sparing required a much
lower stimulus intensity to elicit the reflex than animals
with more tissue sparing and animals without lesions.
In summary, the rat and opossum studies present dra-
matic evidence in both adult and developing animals
that a great deal of reorganization of lumbar circuits
takes place in the presence of a partial complement of
descending systems. In terms of clinical application, it
remains to be seen whether this reorganization can be
shaped or molded through therapeutic interventions
such as exercise training or constraint-induced move-
ment of the affected extremities.
The reflex response was more robust after moderate SCI
but not after mild SCI, as was evident by: (1) incorpo-
ration of pronounced trunk flexion and rotation,
(2) rapid, repetitive hind-limb flexion to a single stimu-
lus, and (3) brisk, full extension of the unstimulated
hind limb. These qualitative observations suggest that
hyperreflexia may have developed after moderate SCI,
an interpretation also supported by precise quantitative
measurements of hind-limb movement. Although flexor
excursion of the ankle was unchanged for the animals
with mild and moderate SCI, we found that movement
speed and timing were different between groups. There
was a trend that peak angular velocity of the ankle was
consistently higher for animals with moderate SCI than
for animals with mild SCI at 2, 3, and 4 weeks after SCI.
We also found that the animals with moderate SCI had
faster movement times and reached peak flexion sooner
than the animals with mild SCI (Figure). Our data
suggest that pronounced hyperreflexia of segmental
systems in the lumbar spinal cord developed when the
lesion was extensive and tissue sparing was minimal.
Are Segmental Reflexes Modified According to the
Amount of Tissue Sparing?
In our studies, we determined that tissue sparing facili-
tates recovery of locomotion after contusion but we did
not know whether the amount of sparing would differ-
entially affect other motor behaviors such as reflexes. In
another set of experiments, my collaborators and I
52
compared the performance of the flexor withdrawal
reflex in animals with substantial sparing (mild SCI) or
minimal sparing (moderate SCI). The flexor withdrawal
reflex was elicited by pinching the intrinsic muscles of
the paw with our fingers, which elicited rapid flexion of
the hip, knee, and ankle. By using our fingers rather
than an implement, we were able to release the paw
immediately at the onset of movement so that the
motion was not impeded. We rated the pressure of the
pinch on a scale from 1 to 3, with 1 being almost no
pressure, 2 being the pressure necessary to elicit a
response in an animal without a lesion, and 3 being a
large amount of pressure. In order to assess the move-
ment characteristics of the hind limb after stimulation,
we videotaped the performance and used frame-by-
frame kinematic analysis. We focused our analysis on the
ankle because it appeared to have greater deficits than
the other joints during locomotion.
In summary, tissue sparing after contusion-type SCI
appears to directly mediate recovery of function in
experimental models. Although the extent of locomotor
recovery is related to the amount of tissue sparing, even
minimal sparing induces marked reorganization of neu-
ral systems below the level of the lesion. This reorgani-
zation appears to increase segmental reflex responses
and may or may not be sufficient to facilitate locomotor
recovery in animals. Given the relative dependence of
recovery of motor function on tissue sparing, it seems
important to identify which supraspinal systems are
spared after spinal cord contusion and to determine
whether training in motor skill development after SCI
will promote even more extensive recovery.
Examination of stimulus threshold for the animals with
moderate SCI showed an initial increase above normal 1
week after SCI, which decreased to below-normal levels
by 4 weeks after SCI. Over time, very little pinch pressure
was required to elicit a robust flexor withdrawal
response. Given the manner in which we measured
stimulus intensity, we have recently replicated these
findings in animals with moderate SCI under blinded
conditions and found a decrease in stimulus threshold
compared with animals with laminectomy that served as
controls.
53
Identification of CNS Systems Spared After SCI
To date, the focus of research in my laboratory has been
on identifying the source and extent of sparing in
descending systems after spinal cord contusion in the
rat. Using anatomical techniques, my collaborators and
Due to technological problems in which
812 . Basso
Physical Therapy . Volume 80 . Number 8 . August 2000
verbal statements of stimulus intensity recorded on
videotape were erased, no stimulus data could be recov-
ered for the animals with mild SCI. Therefore, we
quantified the stimulus intensity after mild SCI (n
5
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