down-regulation of cortical, thalamic, or basal
ganglia neuronal populations through passive
inhibition secondary to deafferentation is a
possible source of functionally reversible al-
teration of cerebral network function. Intrinsic
neuronal membrane properties allow nonlin-
ear state changes on the basis of small devia-
tions in excitation. In vivo experimental studies
demonstrate that the loss of excitatory drive
to neuronal populations as a result of transsy-
naptic down-regulation produces a powerful
form of inhibition that hyperpolarizes the neu-
ronal membrane potential.
149
In cerebral cor-
tex
150
and basal ganglia,
151
up and down states
have been identified in in vitro studies com-
parable to burst and tonic mode firing in the
thalamus (Chapter 1). The potential interplay
of these mechanisms in the setting of brain
injury remains to be unraveled, but the ob-
servations suggest mechanisms by which large
connected networks of potentially functional
systems might remain dormant despite a bal-
ance of neuromodulators producing a wakeful
EEG and arousal pattern.
152
Other types of alteration of the balance of
excitation and inhibition, particularly hypersyn-
chronous discharges, may play a key role. Ex-
perimental studies have shown increased excit-
ability following even modest brain trauma that
may promote epileptiform activity in both cor-
tical and subcortical regions.
153,154
Hypersyn-
chronous activity within relatively restricted
networks may underlie several different clinical
phenomena following structural brain injuries.
For example, a patient fluctuating from classic
akinetic mutism to interactive awareness fol-
lowing an encephalitic injury
155
had epilepti-
form activity in the thalamus that appeared only
as surface slow waves in the EEG. Such a mech-
anism might also explain a reported case of
episodic remission of akinetic mutism.
91
A 52-
year-old man remained in an akinetic mute state
following the rupture of a basilar artery aneu-
rysm with infarcts in the thalamus and basal
ganglia. This behavioral state persisted without
change for 17 months, at which time a sponta-
neous fluctuation in behavioral state occurred,
described as a return to his ‘‘premorbid state,
with full return of his demeanor and affect.’’
The patient’s functional recovery lasted 1 day
and then he relapsed. One year after this event,
the patient had a second ‘‘awakening’’ following
a grand mal seizure. Electroconvulsive therapy,
tried empirically, also reproduced the change.
A related mechanism may explain the late
emergence from MCS reported by Clauss and
colleagues.
127
A 28-year-old man suffered a
diffuse axonal injury (presumably grade III
with subcortical hemorrhages in the basal gan-
glia, thalamus, and brainstem). Spontaneous
eye opening with a GCS of 9 persisted for 3
years following injury until 10 mg of zolpidem
(a GABA
A
potentiator that binds to many of
the same sites as benzodiazepines) was admin-
istered. Within 15 minutes of administration,
the patient began to speak and was able to
respond to questions with ‘‘yes or no’’ answers
and ultimately demonstrated intact remote and
immediate memory. Temporary remission of
chronic aphasia in a 52-year-old woman 3 years
following administration of zolpidem has also
been reported.
156
In this patient, regional ce-
rebral blood flow (CBF) measurements using
SPECT demonstrated a 35% to 40% increase
in the medial frontal cortex bilaterally, and left
middle frontal and supramarginal gyri (Broca’s
area) 30 minutes after zolpidem ingestion. Sim-
ilar mechanisms most likely underlie the well-
publicized cases of Gary Dockery (‘‘The Coma
Cop’’) and Donald Herbert, a fireman who
made international headlines in 2005 with a
marked recovery of speech and cognitive func-
tion after 9 years of remaining in MCS fol-
lowing traumatic brain injury.
Injury to the paramedian thalamus (intra-
laminar and related thalamic nuclei) and upper
brainstem alone can produce widespread hemi-
spheric transsynaptic down-regulation,
157,158
as well as a variety of paroxysmal disturbances.
Most common among the types of paroxysmal
alterations in brain dynamics following injury
to the paramedian thalamus are generalized
epileptic seizures, typically variations of the
3/s spike-and-wave form.
90,159
Other less well-
known phenomena, such as oculogyric crises,
are also associated with injuries to this region.
160
Hypersynchronous discharges restricted to the
thalamostriatal system might also account for
forms of catatonia
161,162
and the obsessive-
compulsive disorder infrequently observed
after brain injuries.
163
Thus, damage to the up-
per brainstem and medial thalamus, in combi-
nation with other cerebral injuries, may lead to
a variety of partially reversible mechanisms of
dysfunction that could contribute to a reduced
baseline activity in severely disabled patients
and provide a structural basis for wide varia-
tion in functional performance. Overreliance
Consciousness, Mechanisms Underlying Outcomes, and Ethical Considerations
375