probably plays an important role in causing
severe disability.
128,131
The paramedian tha-
lamic and upper brainstem structures are
specifically vulnerable to injury during periods
of acute cerebral edema produced by trau-
matic brain injuries, infarctions, hemorrhages,
infections, and brain tumors, as reviewed in
Chapters 3 and 4.
Recent studies suggest that slowly develop-
ing structural remodeling may be a potential
source of late recovery following severe brain
injury. Voss and associates
132
longitudinally
characterized brain structural connectivity and
resting metabolism in a 40-year-old man who
recovered expressive and receptive language
after remaining in MCS for 19 years after a
traumatic brain injury. The patient continued
to improve over the next 2 years. MRI revealed
extensive cerebral and subcortical atrophy par-
ticularly affecting the brainstem and frontal
lobes; there was marked volume loss through-
out the brain with ventricular dilation (Figure
9–11A). Diffusion tensor imaging (DTI) data
revealed severe diffuse axonal injury, as indi-
cated by volume loss in the medial corpus
callosum (Figure 9–11B, C). In contrast to the
overall severe reduction of brain connectivity
demonstrated by DTI fractional anisotropy
maps, measurements also revealed large re-
gions of increased connectivity in posterior
brain white matter not seen in 20 normal sub-
jects (Figure 9–11B). These large, bilateral
regions of posterior white matter anisotropy
were reduced in directionality when mea-
sured in a second DTI study 18 months later
(Figure 9–11C). At this time, repeat imaging
identified significant increases in anisotropy
within the midline cerebellar white matter that
correlated with significant clinical improve-
ments in motor control over the intervening
time period.
132
Figure 9–11D shows the quan-
titative changes in an index of fractional an-
isotropy and left-right fiber directions; the
open circle shows measurements at the time
of the first scan (Figure 9–11B), and the open
square shows the marked increase in frac-
tional anisotropy corresponding to the in-
creased intensity of the red region within the
midline cerebellum (Figure 9–11C). These
findings suggest the possibility of structural
changes within the patients’ white matter play-
ing a role in their functional recovery. Recent
experimental studies provide some support
for such a mechanism of late remodeling of
white matter connections after structural in-
juries
133,134
and in normal human adults.
135
Although suggestive and fascinating, indi-
vidual case studies of this sort must be inter-
preted cautiously. Nevertheless, such findings
indicate the need for larger prospective studies
examining whether slow structural changes do
arise in the setting of severe traumatic brain
injuries and, if present, whether they influence
functional outcomes.
The Potential Role of the
Metabolic ‘‘Baseline’’ in
Recovery of Cognitive Function
As discussed on page 370 and illustrated in
the example shown in Figure 9–10, the ab-
normal response to speech reversal in some
MCS patients provides a potentially impor-
tant clue to the mechanisms underlying their
profound cognitive impairment. Control sub-
jects were instructed to listen passively to the
sounds; however, the time-reversed narratives
elicited an involuntary attempt to decode the
speech. The failure of the time-reversed nar-
ratives to activate the large-scale language-
responsive networks identified by the forward
presentations in MCS patients suggests a sig-
nificant difference in the resting state of the
brain in MCS patients and control subjects.
The recruitment of a normal network activa-
tion pattern suggests that MCS patients may
require very salient stimuli to activate these
language systems (e.g., clear human speech, fa-
miliar voice, emotional content, etc.).
Raichle and colleagues have proposed
that the normal human brain has a ‘‘baseline’’
state of metabolic activation (as reflected
by oxygen uptake) reflecting ‘‘default self-
monitoring.’’
136–138
Specific areas of brain that
are active at rest (e.g., posterior cingulate cor-
tex and ventral anterior cingulate cortex
139
)
form a network that deactivates during tasks
that activate other areas of the brain. Data
obtained from these investigators provide
some evidence supporting a functional role of
a resting state of monitoring environmental
factors and an internal state that might be
sensitive to salient events such as emotionally
meaningful human speech.
138–141
Loss of sig-
nal in these regions is a common finding in
VS and partial recovery of this metabolic signal
is seen in MCS.
141,99
Consciousness, Mechanisms Underlying Outcomes, and Ethical Considerations
373
The very low overall resting cerebral met-
abolic rates in MCS patients generally in-
clude the posterior and ventroanterior cingu-
late regions associated by Raichle with self-
awareness. This may account for the failure to
engage functional network activation with pre-
sentation of time-reversed narratives (Figure
9–10). Specifically, a lack of a metabolically ex-
pensive ongoing self and environmental mon-
itoring process may leave the MCS brain stim-
ulus bound and limited to activations provoked
by extremely salient events. This interpretation
is supported by direct comparisons of changes
in cerebral metabolism, functional MRI signal
activation, and neuronal activity that indicate
a linear correlation of these measures.
142,143
The dissociation of low resting cerebral me-
tabolism and recruitable cerebral networks in
MCS invites speculation that patients who re-
main near the border of emergence from MCS
(see red line in Figure 9–1) may show fluctu-
ation of recruitment of these large-scale net-
works under varying internal conditions of
arousal and appearance of environmentally sa-
lient stimuli, leading to the occasional surpris-
ingly high level of response.
A further consideration is whether injuries
incurred by compression of the thalamus and
brainstem during acute herniation may un-
derlie the chronically low metabolic rates in
patients remaining in MCS despite connected
and recruitable cerebral networks (both MCS
patients studied
121
had herniated with mid-
brain signs of third nerve palsies during the
acute phase of their injuries). As discussed in
Chapter 1, the paramedian mesencephalon and
thalamus contain several interconnected brain
systems that interact closely with the brain-
stem arousal systems. Although these struc-
tures were originally identified as the primary
arousal systems, the thalamic intralaminar nu-
clei (ILN) (and paralaminar regions of the
thalamus rich in neurons that preferentially
project to layer I of the cerebral cortex), the
mesencephalic reticular formation (MRF),
and their connections with the thalamic retic-
ular nucleus appear to play a key role linking
arousal states to the control of moment-to-
moment intention or attentional gating (re-
viewed in
144
). These structures are well posi-
tioned to control interactions of the cerebral
cortex, basal ganglia, and thalamus through
their patterns of innervation within the cortex
as well as rich innervation from the brainstem
arousal systems.
145,146
Even incomplete in-
juries to these networks may produce unique
deficits in maintaining adequate cerebral ac-
tivation and patterns of brain dynamics neces-
sary to establish, maintain, and complete com-
plex behaviors.
The Potential Role of Regionally
Selective Injuries Producing
Widespread Effects on
Brain Function
At least three different mechanisms may lead
to marked alteration of integrative brain ac-
tivity following relatively focal or regionally
restricted brain lesions: (1) a form of passive
inhibition of a brain area following deaffer-
entation of remote but strongly connected ar-
eas, (2) active inhibitory phenomena resulting
from altered connectivity and neuronal func-
tion following injury, and (3) persistent or par-
oxysmal functional activity producing excess
excitation of distributed neuronal networks.
121
Whether such processes underlie partially re-
versible impairment of cognitive function in
severely disabled patients is unknown. It is
likely, however, that transient changes in dis-
tributed network function underlie the wide
fluctuations in cognitive performance in some
MCS patients and patients who emerge from
MCS. These phenomena are well known but
not frequently described in the medical liter-
ature.
91,127
We briefly discuss potentially rel-
evant sources of variations of brain dynamics
within the wakeful state of the injured brain.
A relatively common finding following focal
ischemia or traumatic brain injury is a reduc-
tion in cerebral metabolism in brain regions
remote from the site of injury. This transsy-
naptic (or ‘‘crossed’’) down-regulation of dis-
tant neuronal populations results from the loss
of excitatory inputs from the damaged re-
gions.
147
The clinical significance of these
changes is unclear, although electrophysiologic
correlates have been identified. A recent study
by Gold and Lauritzen
148
showed that al-
though changes in blood flow may be modest
in remote cortical regions, the transsynaptic
down-regulation produces dramatic decreases
in neuronal firing rates (e.g., a neuronal firing
rate decreased by 80% with only a 20% re-
duction in regional blood flow). Thus, stable
374
Plum and Posner’s Diagnosis of Stupor and Coma