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THE PHYSIOLOGY OF PAIN PERCEPTION:Phantom Limb Pain, Learning and Pain

<< THE PHENOMENON OF PAIN ITS NATURE AND TYPES:Perceiving Pain
ASSESSING PAIN:Self-Report Methods, Behavioral Assessment Approaches >>
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Lesson 24
THE PHYSIOLOGY OF PAIN PERCEPTION
To describe the physiology of perceiving pain; we will trace the bodily reaction to tissue damage, as when
the body receives a cut or burn. The noxious stimulation instantly triggers chemical activity at the site of
injury, releasing chemicals called algogenic substances that exist naturally in the tissue. These chemicals--
which include Serotonin, Histamine, and Bradykinin--promote immune system activity, cause
inflammation at the injured site, and activate endings of nerve fibers in the damaged region, signaling injury.
The signal of injury is transmitted by afferent neurons of the peripheral nervous system to the spinal cord,
which carries the signal to the brain. The afferent nerve endings in a damaged region of the body that
respond to pain stimuli and signal injury are called Nociceptors. These fibers have no special structure for
detecting injury; they are simply free nerve endings. They may be found in skin, blood vessels, subcutaneous
tissue, muscle, joints, and other structures. When activated, these end organs, like other receptors, generate
impulses that are transmitted along peripheral fibers to the central nervous system.
Pain signals are carried by afferent peripheral fibers of two types: A-delta and C fibers. A-delta fibers are
coated with myelin, a fatty substance that enables neurons to transmit impulses very quickly. These fibers
are associated with sharp, well-localized, and distinct pain experiences. C fibers transmit impulses more
slowly-- because they are not coated with myelin--and seem to be involved in experiences of diffuse dull,
burning or aching pain sensations.
Signals from A-delta and C fibers follow different paths when they reach the brain. A-delta signals, which
reflect sharp pain, pass through specific areas of the thalamus on their way to motor and sensory areas of
the. This suggests that signals of sharp pain receive special attention in our sensory awareness, probably so
that we can respond to them quickly. On the other hand, C fiber signals, which reflect burning or aching
pain, terminate mainly in the brainstem and lower portions of the forebrain, such as the limbic system,
thalamus, and hypothalamus. The remaining C fiber impulses spread to many areas of the brain by
connecting with a diffuse network of neurons. Signals of dull pain are less likely to command our immediate
attention than those of sharp pain, but are more likely to affect our mood, general emotional state, and
motivation.
So far, the description we have given of physiological reactions to tissue damage makes it seem as though
the process of perceiving pain is rather straightforward. But it actually isn't. One phenomenon that
complicates the picture is that pains originating from internal organs are often perceived as coming from
other parts of the body, usually near the surface of the skin. This is called referred pain. The pain people
often feel in a heart attack provides one of the most widely known examples of this phenomenon: the pain
is referred to the shoulders, pectoral area of the chest, and arms. Other examples of referred pain include:
Pain perceived to be in the shoulder that results from inflammation of the diaphragm.
Pain in the upper back originating in the stomach.
Pain in the ear or in the wrong area of the mouth that result from a toothache.
Referred pain results when sensory impulses from an internal organ and the skin use the same pathway in
the spinal cord. Because people are more familiar with sensations from the skin than from internal organs,
they tend to perceive the spinal cord impulses as coming from the skin. Another issue that complicates our
understanding of pain perception is that people feel pains that have no detectable physical basis, as the next
section discusses.
Pain without Detectable Body Damage
Some pains people experience are quite mysterious, since they occur with no detectable "reason"--for
instance, no noxious stimulus is present. Most of these pain experiences belong to one of three syndromes:
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neuralgia, causalgia, and phantom limb pain. These syndromes often begin with tissue damage, such as
from an injury, but the pain (1) persists long after healing is complete, (2) may spread and increase in
intensity, and (3) may become stronger than the pain experienced with the initial damage.
Neuralgia is an extremely painful syndrome in which the patient experiences recurrent episodes of intense
shooting or stabbing pain along the course of a nerve. In one form of this syndrome, called trigeminal
neuralgia, excruciating spasms of pain occur along the trigeminal nerve that projects throughout the face.
Episodes of neuralgia occur very suddenly and without any apparent cause. Curiously, attacks of neuralgia
can be provoked more readily by harmless stimuli than by noxious ones. For instance, drawing a cotton ball
across the skin can trigger an attack, but a pin prick does not.
Another mysterious pain syndrome is Causalgia, which is characterized by recurrent episodes of severe
burning pain. A patient with causalgia might report, for instance, that the pain feels like my arm is pressed
against a hot stove. In this syndrome, the pain feels as though it originates in a region of the body where the
patient had at some earlier time been seriously wounded, such as by a gunshot or stabbing. Curiously, only a
small minority of severely wounded patients develops causalgia--but for those who do, the pain persists
long after the wound has healed and damaged nerves have regenerated.
Episodes of causalgia often occur spontaneously and may take minutes or hours to subside, but may occur
repeatedly each day for years after the injury. The frequency and intensity of the spontaneous pain-attacks
may increase over the years, and the pain may even spread to distant areas of the body.
Phantom Limb Pain is an especially puzzling phenomenon because the patient--an amputee or someone
whose peripheral nervous system is irreparably damaged--feels pain in a limb that either is no longer there
or has no functioning nerves. After an amputation, for instance, most patients claim to have sensations of
their limb still being there--such as by feeling it "move"--and most of these individuals report feeling pain,
too. Phantom limb pain generally persists for months or years, can be quite severe, and sometimes
resembles the pain produced by the injury that required the amputation.
Although the pain tends to decrease over time, it sometimes gets worse. Individuals with phantom limb
pain may experience either recurrent or continuous pain and may describe it as shooting, burning, or
cramping. For example, many patients who feel pain in a phantom hand report sensing that the hand is
tightly clenched and its fingernails are digging into the palm.
Why do people feel pain when no noxious stimulation is present? Perhaps the answer relates to the neural
damage that precedes the development of causalgia and phantom limb pain--and perhaps even neuralgia
involves neural damage, even though of a less obvious nature, such as from infection. But then why is it
that the large majority of patients who suffer obvious neural damage do not develop these curious pain
syndromes? Although the puzzle is far from being solved, the explanation will almost surely involve both
physiological and psychological factors.
The Role of the "Meaning" of Pain
Some people evidently like pain--at least under some, usually sexual, circumstances--and are described as
masochists. For them, the meaning of pain seems to be different from what it is for most people. Some
psychologists believe individuals may come to like pain through classical conditioning, that is, by
participating in or viewing activities that associate pain with pleasure.
Most of the evidence for the view that the meaning of pain can change by its association with pleasure
comes from research with animals. For example, Ivan Pavlov (1927) demonstrated that the dogs' negative
reaction to aversive stimuli, such as electric shocks or skin pricks, chanced if the stimuli repeatedly preceded
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presentation of food. Eventually, the dogs would try to approach the aversive stimuli, which now signaled
that food, not danger, was coming.
Physician Henry Beecher (1956) described a dramatic example of how the meaning of pain affects people's
experience of it. During World War II, he had examined soldiers who had recently been very seriously
wounded and were in a field hospital for treatment. Of these men, only 49% claimed to be in `moderate' or
"severe" pain and only 32% requested medication when asked if they "wanted something to relieve it."
Some years later, Beecher conducted a similar examination--this time with civilian men who had just
undergone surgery. Although the surgical wounds were in the same body regions as those of the soldiers,
the soldiers' wounds had been more extensive. Nevertheless, 75% of the civilians claimed to be in
"moderate" or "severe" pain and 83% requested medication.
Why did the soldiers--who had more extensive wounds--perceive less pain than the civilians? Beecher
described the meaning the injuries had for the soldiers, who had been subjected to almost uninterrupted fire
for weeks. Notable in this group of soldiers was their optimistic, even cheerful, state of mind.... They
thought the war was over for them and that they would soon be well enough to be sent home. It is not
difficult to understand their relief on being delivered from this area of danger. The battlefield wound
marked the end of disaster for them.
For the civilian surgical patients, however, the wound marked the start of a personal disaster and their
condition represented a major disruption in their lives.
Personal and Social Experiences and Pain
Imagine this scene: little Steve is a year old and is in the pediatrician's office to receive a standard
immunization shot, as he has done before, As the physician approaches with the needle, Steve starts to cry
and tries to kick the doctor. He is reacting in anticipation of pain--something he learned through classical
conditioning when he had received vaccinations before.
Learning and Pain
We learn to associate pain with antecedent cues and its consequences, especially if the pain is severe and
repeated, as it usually is with chronic pain. Many individuals who suffer from migraine headaches, for
example, often can tell when headaches are on the way because they experience symptoms, such as
dizziness, that precede the pain. These symptoms become conditioned stimuli that tend to produce distress,
a conditioned response, and may heighten the perception of pain when it arrives. Also, words or concepts
that describe the pain people have experienced can become conditioned stimuli and produce conditioned
responses.
A study of people who do and do not have migraine headaches measured their physiological arousal in
response to pain-related words, such as "throbbing", "sickening," "stabbing," "scalding," and "itching."
Migraine sufferers displayed much stronger physiological reactions to these words--especially the words
that described their own experience with pain--than those without migraines did. Other findings indicate
that people who suffer from chronic pain, such as headaches, show lower discomfort thresholds for pain
and non-pain stimuli than others do. Perhaps they learn to notice and react more strongly to low levels of
discomfort.
Learning also influences the way people behave when they are in pain. People in pain behave in
characteristic ways--they may moan, grimace, or limp, for instance. These actions are called pain behaviors,
and generally, they can be classified into four types:
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Facial or audible expression of distress, as when people clench their teeth, moan, or grimace.
Distorted ambulation or posture, such as moving in a guarded or protective fashion, stooping while
walking, or rubbing or holding the painful area.
Negative affect, such as being irritable.
Avoidance of activity, as when people lie down frequently during the day, stay home from work, or refrain
from motor or strenuous behavior.
Pain behaviors are a part of the sick role, and people in pain may begin to exaggerate these behaviors
because, they think, "No one believes me". Regardless of why the behaviors start, they are often
strengthened or maintained by reinforcement in operant conditioning, as Wilbert Fordyce has pointed out
(1976; Fordyce). When pain persists and becomes chronic, these behaviors often become part of the
person's habits and lifestyle. People with entrenched patterns of pain behavior usually feel powerless to
change.
How Pain Behaviors are Reinforced?
Although being sick or in pain is unpleasant, it sometimes has benefits, or "secondary gains." Someone who
is in pain may be relieved of certain chores around the house or of going to work, for instance. Also, when a
person has a painful condition that flares up in certain circumstances, such as when lifting heavy objects, he
or she may begin to avoid these activities. In both of these situations, pain behavior is reinforced if the
person does not like doing these activities in the first place: getting out of doing them is rewarding.
Another way pain behavior and other sick-role behaviors may be reinforced is if the person receives
disability payments. Studies of injured or ill patients who differ in the financial compensation they receive
have found that those with greater compensation tend to remain hospitalized and miss work longer, report
more chronic pain, and show less success from pain treatments.
Placebos and Pain (Role of Cognitions)
You have probably heard of physicians prescribing a medicine that actually consisted of "sugar pills" when
they could not find a physical cause for a patient's complaints or did not know of any medication that
would help. You may also have heard that this treatment sometimes works--the patient claims the
symptoms are reduced. An inert substance or procedure that produces an effect is called a placebo. Studies
have shown that placebos can often be effective in treating a wide variety of ailments, including coughs,
nausea, and hypertension, at least on a temporary basis.
Placebos can also be effective in treating pain. They do not always work, but they seem to produce
substantial relief in about half as many patients as do real drugs, such as aspirin or morphine. The effect of
placebos depends on the patient's belief that they will work--for instance, they are more effective:
With large doses--such as more capsules or larger ones--than with smaller doses.
When injected than when taken orally.
When the practitioner indicates explicitly and strongly that they will work.
Unfortunately, however, the effectiveness of placebos in treating pain tends to decline with repeated use.
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Social Processes and Pain
People who suffer with pain also receive attention, care, and affection from family and friends, which can
provide social reinforcement for pain behavior. Researchers have demonstrated this relationship with both
child and adult patients. Karen Gil and her colleagues (1988) conducted a study of parents' reactions to the
pain behavior of their children who had a chronic skin disorder with severe itching that should not be
scratched since it can cause peeling and infection. The researchers videotaped the behavior of each child
and his or her parent in the child's hospital room.
As you might expect, the parents paid attention to the scratching, perhaps because of the harm it can do.
But what effect did the attention have? An analysis revealed that parent attention appeared to increase the
children's scratching, rather than decrease it, and paying attention to the children when they were not
scratching seemed to reduce their scratching behavior.
Research has examined how family members' reactions affect pain behavior. Studies have used
questionnaires to assess how patients' pain behaviors relate to their receipt of social rewards, such as being
able to avoid disliked social activities or getting from their spouses considerate care, that is, high levels of
help and attention. Receiving higher levels of social reward was associated with patients reporting more pain
and showing more disability and less activity, such as in visiting friends or going shopping.
Research findings on parents' and spouses' reactions to chronic pain behavior and the social climate within
the family system illustrate how each family member's behavior impacts on the behavior of the others.
When families lack cohesion or the members are highly attentive to pain behavior without encouraging the
patient to become active, they are likely to promote sick-role behavior. These conditions can develop into a
vicious circle--for example; solicitousness may lead to more pain behavior, which elicits more
solicitousness, and so on.
Showing care and concern when people are in pain is, of course, important and constructive. But the
patient's diminished activity may then lead to physical deterioration, such as through muscle atrophy, and
lead to progressively more pain and less activity. These social processes in the family system of pain-patients
are gradual and insidious--they tend to increase the patients' dependency and decrease their self-efficacy
and self-esteem. Self-efficacy is important because people who believe they cannot control their pain very
well experience more pain and use more medication than those who believe they can control it.
Gender, Socio-cultural Factors and Pain
Studies have found gender and sociocultural differences in the experience of pain. Men and women appear
to differ in the types of pain they experience and reactions to pain, Women have higher incidence rates of
pain from arthritis, migraine headache, myofacial neuralgia, and causalgia; but men have a greater incidence
of back pain and cardiac pain.
Women tend to report more than men that pain interfered with their daily activities. Surveys of adults in
different countries who suffer from chronic low back pain revealed greater work and social impairments
among Americans, followed by Italians and New Zealanders, and then by Japanese, Colombian, and
Mexican individuals (Sanders et al.. 1992). Research on the pain experienced after dental surgery by people
from different ethnic groups in the United States found that blacks reported more pain than people of
European, Asian, or Hispanic backgrounds, and women in each group reported more pain than men. The
reasons for these gender and sociocultural differences are not clear, but they may include differences in the
social support and financial consequences these people receive for being sick.
Emotions, Coping Processes and Pain
People in chronic pain experience high levels of anger, fear, and sadness. Pain and emotion are intimately
linked, and cognitive processes mediate this link, in a study of these relationships, Gerry Kent (1985) had
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dental patients fill out a brief dental anxiety scale while waiting for their appointments. Then they rated the
pain they expected in their visits. After the appointment, the patients rated the pain they actually
experienced, and rated it again by mail 3 months later. The results revealed that anxiety played a role in their
expectations of pain and in their memories of it 3 months later. The patients with high dental anxiety
expected and later remembered four times as much pain as they experienced. In contrast, the low-anxiety
patients expected and remembered less than twice as much pain as they experienced. These findings suggest
that high-anxiety patients' memories of pain are determined more by what they expect than by what they
feel.
Does Emotion Affect Pain?
A study of emotion and pain compared the anxiety and stress levels of children who suffered from migraine
headache with those of their best friends, and then had the migraine sufferers keep diaries of their
headaches over the next 4 months. Although the scores on tests of anxiety and stress were about the same
for the two groups and were within the normal range, migraine sufferers with high levels of anxiety had
more frequent and severe headaches than those with lower anxiety. Other investigations using self-report
methods have found that migraine and muscle-contraction headaches tend to occur after periods of
heightened stress and that Type A individuals have more frequent chronic headaches than others do. These
studies clearly indicate that stress and headache are related. Has any research shown that stress causes
headaches?
Convincing evidence that stress can cause headaches comes from a study with adults who suffered from
either chronic headache or only occasional headaches. Before testing a subject, researchers attached sensors
to the person's body to take several physiological measurements, such as of heart rate and electrical activity
of muscles. A researcher also told the subjects that they "might or might not" experience headache pain in
the procedures and that they would rate their perception of pain several times during the study. After sitting
quietly for 15 minutes, they were given a stressful task--calculating arithmetic problems, such as 349 + 229,
every 15 seconds for an hour--and told that a buzzer would sound if their performance fell below a norm.
Actually, the buzzer sounded periodically regardless of their performance. Then the subjects sat quietly for
10 minutes. How did they react to these conditions? More than two-thirds of the chronic headache sufferers
and only one fourth of the occasional sufferers reported developing headaches during the stress task.
Ratings of headache pain increased throughout the stress condition, and decreased later while they sat
quietly. The headaches tended to resemble tension-type headaches and be preceded by sustained
physiological arousal. These are important findings that indicate that stress can cause headaches.
Emotions are also related to other kinds of pain, but whether emotions cause the pain is still in question.
Research has demonstrated, for instance, that the amount of pain people with sickle cell disease report
increases with the amount of stress they experience each day and with increases in stress during the
preceding 2 days. But although people with recurrent low back pain report higher levels of anxiety and
tension than pain-free control subjects do, these mood states do not worsen in the day or so preceding pain
attacks.
Feelings of depression appear to result from pain people with chronic discomfort experience on previous
days, and lead to pain on subsequent days. Pain is itself very stressful, and many people with chronic pain
consider their discomfort--the actual pain and the physical limitations it produces--to be the most
prominent stressor in their lives. Health psychologists who work with pain patients often try to assess how
well they cope with their pain.
Coping with Pain
Part of the stress that chronic pain patients experience stems from their common belief that they have little
personal control over their pain, aside from avoiding activities they believe can trigger an attack or make it
worse. As a result, they tend to deal with their stress by using emotion-focused coping strategies, That is,
rather than trying to alter the problem itself, they try to regulate their emotional responses to it. Some of the
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more common coping methods adults and children with chronic pain use include hoping or praying the
pain will get better someday and diverting their attention, such as by counting numbers or running a song
through their heads. These approaches are not very effective in reducing chronic pain.
How effectively do people cope with pain? Studies that tested pain patients with the MMPI have found
some fairly consistent outcomes. These outcomes lead to three conclusions: First, individuals who suffer
from various types of chronic pain, such as severe headache and low back pain, show a characteristic MMPI
profile with extremely high scores on hypochondriasis, depression, and hysteria--the neurotic triad scales.
But their scores on the seven other MMPI scales tend to be well within the normal range. Second, this
pattern appears to hold regardless of whether their pain has a known organic source. In other words, people
whose pain might be classified as psychogenic by a physician tend to show similar problems of adjustment
on the MMPI as those whose pain has a clear organic basis. Third, individuals with acute pain, such as
patients recovering from injuries, sometimes have moderately elevated scores on the neurotic triad scales,
but these scores and those for the remaining MMPI scales are generally well within the normal range. These
findings make sense and reflect the differential psychological impact of pain that patients expect will end
soon versus pain they fear will never end. Keep In mind also that people with chronic-recurrent pain
conditions show worse psychological symptoms during pain episodes than during pain-free periods.
It is clear that being in frequent, severe discomfort is related to having high scores on the MMPI neurotic
triad scales, but does chronic pain cause maladjustment? One school of thought is that the causal sequence
may be the other way around--that is, chronic pain may be a symptom of a psychological disorder, such as
depression, that preceded the pain syndrome. But most current evidence points in the other direction--
indicating, for instance, that people in chronic pain become depressed because of the stress they experience
without being able to change their situations. They develop a sense of helplessness, which leads to
depression. One type of evidence indicating that pain leads to depression is that people whose pain has
ended show substantial reductions in various measures of psychological disturbance.
Of course, this does not mean psychological factors cannot lead to physical pain--for instance, we've seen
that stress can cause headaches. One study examined this issue prospectively for 8 years and found support
for both causal directions. People who are depressed are somewhat more likely than others to develop a
chronic pain condition in the future, and people with chronic pain are much more likely than others to
become depressed. Pain and maladjustment involve interactive processes, with each feeding on the other
overtime, but chronic pain is more likely to lead to maladjustment than the other way around. Also keep in
mind that not all patients with severe chronic pain become maladjusted--many adapt to their conditions
much better than others do. Coping well with chronic pain is a struggle that unfolds over time, as this
arthritis patient noted: Over time I've figured out that I can do things to bring on the pain and things that
could limit it. I also figured out that my flares won't last forever, although while they're happening it seems
like forever. It took quite a while to figure that out.
To summarize, the process by which people perceive pain involves a complex chain of physiological and
neuro-chemical events. These events can be affected by psychosocial processes, such as people's beliefs
about whether a drug will reduce their discomfort.
Pain also affects and can be influenced by people's learning, cognition, social experiences, and emotion.
Although people can indicate through their behavior that they are feeling pain, the pain they perceive is
actually a private and subjective experience.
How can researchers and clinicians who work with patients who have painful symptoms assess the level and
type of pain these individuals perceive? We will be answering this question in our next lecture.
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Table of Contents:
  1. INTRODUCTION TO HEALTH PSYCHOLOGY:Health and Wellness Defined
  2. INTRODUCTION TO HEALTH PSYCHOLOGY:Early Cultures, The Middle Ages
  3. INTRODUCTION TO HEALTH PSYCHOLOGY:Psychosomatic Medicine
  4. INTRODUCTION TO HEALTH PSYCHOLOGY:The Background to Biomedical Model
  5. INTRODUCTION TO HEALTH PSYCHOLOGY:THE LIFE-SPAN PERSPECTIVE
  6. HEALTH RELATED CAREERS:Nurses and Physician Assistants, Physical Therapists
  7. THE FUNCTION OF NERVOUS SYSTEM:Prologue, The Central Nervous System
  8. THE FUNCTION OF NERVOUS SYSTEM AND ENDOCRINE GLANDS:Other Glands
  9. DIGESTIVE AND RENAL SYSTEMS:THE DIGESTIVE SYSTEM, Digesting Food
  10. THE RESPIRATORY SYSTEM:The Heart and Blood Vessels, Blood Pressure
  11. BLOOD COMPOSITION:Formed Elements, Plasma, THE IMMUNE SYSTEM
  12. SOLDIERS OF THE IMMUNE SYSTEM:Less-Than-Optimal Defenses
  13. THE PHENOMENON OF STRESS:Experiencing Stress in our Lives, Primary Appraisal
  14. FACTORS THAT LEAD TO STRESSFUL APPRAISALS:Dimensions of Stress
  15. PSYCHOSOCIAL ASPECTS OF STRESS:Cognition and Stress, Emotions and Stress
  16. SOURCES OF STRESS:Sources in the Family, An Addition to the Family
  17. MEASURING STRESS:Environmental Stress, Physiological Arousal
  18. PSYCHOSOCIAL FACTORS THAT CAN MODIFY THE IMPACT OF STRESS ON HEALTH
  19. HOW STRESS AFFECTS HEALTH:Stress, Behavior and Illness, Psychoneuroimmunology
  20. COPING WITH STRESS:Prologue, Functions of Coping, Distancing
  21. REDUCING THE POTENTIAL FOR STRESS:Enhancing Social Support
  22. STRESS MANAGEMENT:Medication, Behavioral and Cognitive Methods
  23. THE PHENOMENON OF PAIN ITS NATURE AND TYPES:Perceiving Pain
  24. THE PHYSIOLOGY OF PAIN PERCEPTION:Phantom Limb Pain, Learning and Pain
  25. ASSESSING PAIN:Self-Report Methods, Behavioral Assessment Approaches
  26. DEALING WITH PAIN:Acute Clinical Pain, Chronic Clinical Pain
  27. ADJUSTING TO CHRONIC ILLNESSES:Shock, Encounter, Retreat
  28. THE COPING PROCESS IN PATIENTS OF CHRONIC ILLNESS:Asthma
  29. IMPACT OF DIFFERENT CHRONIC CONDITIONS:Psychosocial Factors in Epilepsy