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Increasing
Our Knowledge of Gilles de la Tourette Syndrome Through the Use of Electroencephalograms:
A
Summary of Findings
Gilles
de la Tourette Syndrome, or Tourette's Syndrome (TS), is a genetically
inherited, neurological disorder of the dopaminergic and serotinergic
systems, and is thought to be localized in both the basal ganglia (particularly
the caudate nucleus), and prefrontal cortex (Comings, 1990). While TS
was traditionally characterized by both motor and vocal "tics"
- involuntary twitches -- which would wax and wane in severity, it has
more recently been re-defined as a disorder of general disinhibition
(Comings, 1990). Normal inhibitory controls malfunction or fail, most
acutely under moments of stress, which lead to random movements and
sounds, obsessive thoughts, impulsive behaviour, attentional problems,
low frustration tolerance, or any combination of the above. While TS
was initially little more than an obscure and bizarre curiousity, this
new, more accurate, characterization of it as a subtype of a larger
"Generalized Disinhibition Disorder", and recognizing it's
comorbidity and overlap with many other more pevalent disorders, has
prompted more interest and research into this syndrome than ever before.
From the beginnings of their trade electroencephalographers seemed to
have established an interest in TS; literature in this domain can be
found as early as 1958 (Dolmierski & Klossowna, 1958). The electroencephalogram
(EEG) abnormalities found in TS'ers were far ranging, and seldom
agreed in severity, type, and prevalence (which ranged from 25-100%)
(Shapiro et. al., 1978). Kelman (1965) found that 42.1% of his 19
subjects had abnormal EEG patterns. He described slow rhythms, non-focal
spiking, and infectious cerebral changes suggestive of a temporal lobe
disorder. In 6 of his 7 patients, Feild reported bi- and triphasic sharp
activity in the Rolandic and Sylvian areas (Feild, 1966). 10 of 18 subjects
were found by Lucas and Rodin (1973) to have abnormal EEG's: 5 had
mildly dysrythmic, nonspecific theta activity, 2 experienced generalized
dysrythmia, and one had random waves in both the delta and theta frequencies,
maximized in the temporal and occipital areas. Other abnormalities documented
include occasioal diphasic sharp waves, bisynchronous over the parietal
region, and generalized short bursts of slow waves (Corbin et. al.,
1968), activity slower than the normal basic frequency in 12.5 %
of TS subjects (Krumholz, 1983), asymmetrical temporal rhythms and hypersynchronous
or paroxysmal discharges similar to epilepsy (Dalmierski and Kloss,
1962), excessive theta activity and bilateral fronto-temporal sharp
waves between episodes of tics and rushing thoughts (Logue, 1973), epileptiform
alterations found 5 to 7 times more frequently than in the normal population
(Verma, 1986), and spikes, polyspikes, or spike-like discharges with
tics or tic impulses in postencephalitic patients who had acquired Tourettism
(Sacks, 1982).
In an attempt at parsimony, Cohen, Bruun, and Leckman (1988) claimed
most researchers agree that minor, nonspecific abnormalities on the
EEG record occur more frequently among TS'ers than within the normal
population. While this was true, Shapiro (1978), in a comprehensive
review of the literature, stated that the majority of researchers do
not adequately describe the abnormalities seen. More importantly, he
noted that the data was not broken down by age; while Shapiro had found
that 48.4% of his 79 patients displayed abnormal EEG's, when
they were split by age, he found that 68.8% of subjects under the
age of 17 had abnormal readings, compared to only 26.7% of subjects
older than 17. It has been suggested that this age difference reflects
the detection of abnormalities associated with delayed maturation (Obeso,
1982).
In further attempts to "clean up" the research, other experimenters
began to account for whether subjects were currently taking medications
(typically dopamine antagonists) or not, and whether there existed any
confounding neurological impairments. Verma, Syrigou-Papavasiliou, and
LeWitt (1986) found that when an unmedicated, neurologically, and intellectually
intact TS sample was used, a mere 20% showed any unusual EEG patterns.
Krumholz et. al. (1983) controlled for medication, neurological dysfunction,
and mental retardation, and found that only 5 of 40 patients exhibited
any EEG abnormalities; these were generally described to be excessive
nonspecific slow activity (excessive slow background activity, centro-parietal
spikes, and sharp waves). From this, Krumholz et. al. then concluded
that eliminating these extraneous variables greatly reduced the frequency
of abnormal EEG readings in the TS population.
The picture may not be that simple, however. Shapiro (1978) claimed
that when TS'ers with minimal brain dysfunction (MBD) were compared
to a sample with only MBD, the former group had more moderately abnormal
and markedly abnormal EEG's. It is possible that factors such as
MBD interact with TS, and are not simply confounding variables. Another
problem with simply factoring out other neurological problems is that
there is still considerable debate concerning how exactly TS is to be
defined; depending on one's interpretation of the disorder, samples
may or may not include individuals who also have obsessions, attentional
problems, or even seizures (Comings, 1990). Indeed, Shapiro et. al.
observed that a higher incidence of minimal neurological dysfunction
exists among the TS population; until this correlation can be explained,
one must be cautious in partialling out EEG abnormalities due to this
associated problem.
Three variables which seem to make little difference are sex, tic severity,
and state of consciousness. Verma (1986) found that the ratio of males
to females with abnormal EEG readings was not significantly different
from the ratio of males to females in the sample. In most studies reviewed,
more men were used than women - this is to be expected, however, since
TS is recognized to be three times more prevalent in men than women.
Regarding severity of tics, Krumholz's five patients who exhibited
excessive nonspecific slow activity after the effects of medications,
neurological dysfunction, and MR had been factored out ranged from mildly
Tourettic (n=2) to severe (n=1) (Krumholz, 1983) Age of onset and duration
of tics were also inconsequential. Finally, state of consciousness (awake
versus asleep) yielded nothing of significance (Krumholz, 1983).
Moving away from the general EEG recording, some researchers have concentrated
their attentions on unusual activity surrounding ticking. Obeso claimed
that there was no evidence
of paroxysmal activity time-locked to tics (Obeso, 1982). However, he
did find that the usual pre-movement EEG potentials, typically recorded
over the sensory motor cortex contralateral to the body movement, were
absent in six Touretters when ticking. Instead of the expected slow
potential, four of the six exhibited unusual negative, abrupt potentials
100 ms prior to the movement onset: Obeso thought this to be a motor
potential, possibly a discharge of pyramidal tract neurons (Obeso, 1982).
This pattern had nothing to do with the movements in and of themselves;
Obeso ascertained this by asking the Touretters to voluntarily mimic
those same tics. When they did so, the usual pre-movement potentials
appeared (Obeso, 1982). As the usual pre-movement EEG changes are believed
to take place only in preparation before willed, vluntary movemnts,
Obeso concluded that the tics in TS or not psychologically [cortically]
generated (Obeso, 1982). He also concluded that, since pre-movement
potentials were not being detected at the onset of a "true"
TS tic, the structures responsible for TS are most likely buried deep
within the brain. This is supported by the majority of the literature
today that points to the basal ganglia (Comings, 1990), and somewhat
agrees with Corbin et. al.'s belief that, while cortical neuronal
populations are involved in TS, they are subject to the influence of
deeper structures (they suggested the reticular activation system).
Corbin et. al. supported their hypothesis through the observations that
ticking decreases when subjects are asked to problem-solve, and increases
when subjected to auditory and proprioceptive stimuli (Corbin et. al.,
1968). Lastly, post-movement potentials of the tics have not ever been
found to significantly differ from other planned post-movement potentials
(Obeso, 1982).
Another area of concentration has been in Evoked Responses (ER's).
Krumholz studied Visual, Auditory, and Somatosensory ER's in 17
TS patients, and found no ER differences between TS'ers and controls.
Obeso (1982) also studied ER's: by stimulating the median nerve
he found no abnormalities in the induced potentials. Of his 127 patients,
however, he only tested three. Weate et. al. (1993) claimed that while
short latency Evoked Potenials (EP's) were normal in TS'ers,
long latency Event Related Potentials (ERP's) show some differences.
Finally, Domino (1982) studied 5 medication-free TS'ers and 5 TS'ers
on haloperidol and concluded that Visual ER's are normal only if
unmedicated; Domino claimed that haloperidol acted to slow visual impulse
transmission, which prolonged the ER latencies (Domino, 1982).
In the nineties, EEG abnormalities in TS are considered rare (Weate,
1993), and neither EEG readings nor ER findings are considered helpful
in the diagnosis or therapy of TS (Krumholz, 1983). Of latest interest
are abnormalities in Contingent Negative Variations (CNV's) in TS.
CNV's are slow, negative, brain potentials which occur after a particular
stimulus in anticipation of a second stimulus associated with the first.
CNV's are thought to reflect the levels of the neurotransmitter
dopamine in the central areas of the brain. The study of CNV's in
TS was prompted by the fact that dopamine is implicated in TS.
Weate et. al. (1993) tested 12 patients (10 male) between the ages of
10 and 21. All were unmedicated. Fz, Pz, and Cz sites were used. They
found that the CNV amplitude was significantly higher (p<.01), and
that Post-Imperative Negative Variation (PINV) was significantly more
often present among TS patients than among controls (Weate et. al.,
1993). These findings are similar to those associated with individuals
suffering from depression, schizophrenia, and Parkinson's patients
undergoing dopamine replacement therapy (Weate et. al., 1993). Frontal
PINV has also been associated with obsessive thoughts, and distractibility
problems - problems previously cited to be prevalent in TS (Tecce &
Cattanach).
As a TS'er himself, the author of this paper has often described
TS as being "stuck in a rut" - whether it be a motoric rut
(i.e. being unable to stop a tic), a cognitive rut (i.e. being unable
to stop a thought), or an emotional rut (i.e. being unable to pull oneself
from a particular mood), the sensations are similar: an irrational,
pressing need or "itch" to repeat these movements, sounds,
thoughts, or feelings. If CNV can be thought of as a type of "classical
conditioning" at the neuronal level, then this exaggerated CNV
in TS'ers is indicative of a kind of "hyper-association"
of different stimuli. Assuming then that the problem in TS is that associative
links between stimuli are too strongly made, one would expect that associations
among TS'ers would be formed easier and with less trials, and perhaps
associations which are too weak to be made in a "normal" population
would be forged in a Tourettic population. Based on these premises,
it is easy to see why and how tics, obsessive thoughts, and other rituals
become generalized and widespread.
Consider an illustration; if a "normal" person was to ever,
by chance, blink his eyes while walking under an archway, this incidental
pairing would be too random or inconsequential (or too rare an occurrence)
for strong associations to be tied between walking under an archway
and blinking. If, however, TS'ers are hypersensitive to any associations,
no matter how unimportant, the next time a TS'er walks beneath an
archway (s)he will trigger a powerful negative preparatory variation
in anticipation of blinking. Psychologically, this would be experienced
as an irrational urge or "itch" to blink. By this logic, one
would expect the urge to be "satiated" once one blinked, and
one would also expect that association to be strengthened even more,
so that the next time that individual walks under an archway, the "itch"
would be even more impossible to ignore.
There is certainly considerable evidence which shows that tics can be
"traced" to their origins (Sacks, 94). For example, the author
of this paper has had for years a "head tic", which involves
a whipping of his head to the left. This tic began years ago when the
author grew the bangs of his hair so long that he needed to occasionally
"toss" his hair out of his eyes. The fact that the tic involves
throwing one's head to the left is explained by the fact that the
author used to part his hair on the right side, hence the bangs hung
over his left eye. Finally, there exists evidence which claims that
CNV is decreased by distraction methods (Tecce & Cattanach). Distraction
is also a key strategy for decreasing tics, and for getting a TS'er
past obsessive thoughts.
In conclusion, the history of EEG recordings on TS'ers is quite
voluminous. Much of the early work was not focused, and a confusing
variety of findings were cited. Later researchers began controlling
for such variables as age, sex, medications, and MBD, which resulted
in a much smaller percentage of abnormal EEG readings in the TS population.
Particular areas of interest have been EEG activity surrounding tics,
Evoked Responses, and Contingent Negative Variations.
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