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| 50th Annual Meeting of the American Headache Society |
Boston, Massachusetts June 26-29, 2008 |
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How Pain becomes Chronic Facilitation and
Amplification |
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BY MAURY M. BREECHER
Contributing Writer |
BOSTON — Doctors need to recognize the
possibility that medications can induce long-lasting
adaptive changes resulting in future pain without new
injury, said Dr. Frank Porreca on June 26 during the
50th Annual Meeting of the American Headache Society.
“Adaptive changes that occur within the nervous system
can occur in a setting of tissue injury or,
surprisingly, might occur in the absence of tissue
injury, perhaps as a consequence of medications that we
use to treat pain,” said Dr. Porreca of the University
of Arizona Health Sciences Center.
Dr. Porreca suggested that the adaptive changes do occur
after some types of initiating stimulus and can persist
for very long periods of time. Furthermore, these
adaptive changes “may also influence the responsiveness
that can take place to external stimuli.”
We can think of pain in a variety of ways, he continued.
Nociceptive pain is caused by noxious stimuli,
inflammatory pain is caused by inflammation, and
neuropathic pain is caused by neuronal damage. Even when
there is no noxious stimulus, inflammation, or neuronal
damage, pain can arise. He called that category of pain
“dysfunctional pain” and said it is common to migraine
and tension-type headaches.
“These different types of pain syndromes can be
differentiated especially on the degree or intensity of
the stimuli required to produce activation of the pain
transmission pathway,” said Dr. Porreca.
Nociceptive pain is physiological pain that is produced
by high-intensity stimuli that are capable of producing
damaged tissues. “It offers protective value as it
alerts us to avoid pain circumstances that can result in
physical damage,” said Dr. Porreca.
However, in inflammatory and neuropathic pain, pain can
result not just from high-threshold stimuli, but also
from low-threshold stimuli.
Low threshold stimuli can be “clinically very, very
meaningful,” he said. “The question that arises is what
has happened to the nervous system when a high-threshold
system becomes converted into something that can be
activated by low-threshold inputs?”
The phenomenon known as amplification is responsible for
the conversion of high-threshold nociceptive pain to low
threshold clinical pain.
Amplification processes reflect neuroplasticity
occurring at various levels of the nervous system.
Amplification processes, for instance, can occur at the
very first site at which pain can be detected—the
peripheral nociceptor.
“One of the greatest advances that has occurred in pain
biology is that we have identified molecular transducers
that we find localized at the endings of the peripheral
nociceptor that can detect different types stimuli from
the outside world,” said Dr. Porreca.
He said it has been discovered that there are specific
electric transducers for detection of high-energy
temperatures, noxious chemicals, and for other noxious
mechanical stimuli. These transducers allow pain signals
to enter the sodium and calcium nerve terminal,
resulting in membrane depolarization. So different types
of stimuli from the outside world can achieve the
activation of specific nociceptors. For instance, the
TRPV1 channel, the first of the electric transducers
identified, can be activated by noxious heat, acidic
conditions, or even by Capsaicin (an extract of chili
peppers). Consequently, a warm temperature may now be
perceived as painful.
Increases in expression and trafficking of these
molecular transducers can result in increases in
sensitivity. Over time, transcriptional regulation may
occur so that the nociceptor can be maintained in a
sensitized state for sustained periods of time.
These changes can also occur at other levels of the
nervous system and in both descending and ascending pain
facilitation cells.
“All of these represent processes in which the
nociceptive pain syndrome can become a chronic pain
syndrome; a contribution of functional and chemical
plasticity that may contribute to the chronification of
the pain condition,” said Dr. Porreca.
“In nociceptive pain we can see the duration of the
stimulus and the duration of the response correspond
quite well, but under conditions where we have
functional and chemical plasticity, the duration of the
response greatly outlasts the duration of the stimulus,”
he continued.
Referring to Dr. Howard Field’s lecture on “Descending
Modulation” and his reference to structural adaptations
that can occur, Dr. Porreca added, “There may be a
balance between facilitation and modulation and that
balance may be disturbed under some conditions, for
instance, injury or even by the use of certain
medications.”
An example of this could be the condition known as
medication overuse headache. |
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