46 - No.4:January 2002
J Physiol Pharmacol 2002;46 (4);
Pain measurement: a formidable task
SANHAY KUMAR *, O .P. TANDON ** AND R. MATHUR***
Departments of physiology,
*B.P.K.I.H.S., Dharan, Nepal,
**University College of Medical Science,
Shahdara, Delhi – 110 095
***All India Institute of Medical Sciences,
New Delhi – 110 029
(Received on July 26, 2001)
Abstract: Pain is defined as unpleasant sensory and emotional
experience, associated with actual or impending tissue damage.
It consists of multi-dimensional phenomenon having sensory discriminative,
cognitive-evaluative and effective motivational components. Though
the technology has advanced, still it is very difficult to objectively
assess all the attributes of pain, including alteration in cognitive
behaviour. However, subjective methods like Visual Analog Scale
rating (VAS) and preliminary objective methods like pain evoked
responses, behavioural monitoring and event related evoked potentials
for cognition are currently in vogue. It will take some more time
and effort to evolve yet other newer and sophisticated techniques
to measure all aspects of pain in human beings.
words : Pain,event related evoked potentials,Cognition,VAS,pain
committee of The international Association for the study of pain
has defined pain in humans as an unpleasant sensory and emotional
experience associated with actual or potential tissue damage,
or described in terms of such damage (1). Thus pain is more than
just a physiological transmission of nociceptive impulses. Therefore,
an ideal assessment should include measurement of the interplay
of different factors on the total experience of pain. Throughout
most of recorded history, pain was characterized as an effective
feeling state rather than a sensation (2). Emotions play key role
in painful experience. Aristotle described pain as a “passion
of soul”, distinct from the classic five senses (3). Melzack and
Wall (4) and Melzack and Casey (5) developed a model of pain in
which tissue damage concurrently activates sensory-discriminative,
cognitive-evaluative and affective-motivational components of
pain. The nature and severity of pain then become consequences
of affective and cognitive mechanisms as well as sensory events
deriving from tissue damage. Thus, pain is a complex physiological-behavioral
puzzle that requires assessment on different levels. At the same
time the measures of pain should be reliable and valid, otherwise
they will be little use to clinicians or researchers.
ideal pain measure should (6): Provide sensitive measurement free
of biases, provide immediate information about accuracy and reliability,
separate the sensory-discriminative aspects of pain from its hedonic
qualities, assess experimental and clinical pain with the same
scale and provide absolute scales that allow assessment of pain
between groups and within groups over time. A valid, reliable
and flexible measurement technology becomes a prerequisite for
evaluation of this. Progress in this domain has been extremely
slow but steady, because pain, as pointed out earlier, is a subjective
experience (7). However, in the recent past, technologies for
the non-invasive assessment of brain function have advanced at
a rapid pace. Amongst the techniques for measurement of brain
electrical activities and topographic mapping are quantitative
analysis of cortical power spectrum (CPS), as well as the computerised
averaging of Brain Evoked Potentials (EPs). The availability of
modern technology for imaging of brain and measurement of certain
processes in brain is a significant progress in science and also
for pain studies (8). These parameters can now be recorded non-invasively
from awake humans for correlating with conscious reporting of
pain and other subjective experiences. This includes Evoked potentials
(EP), Nuclear Magnetic Resonance (NMR), Position Emission Tomography
Pain measurement in humans
is both a somatic sensation and a powerful feeling state, which
evokes behaviours that minimize harm and promote healing (9).
Melzack and Dennis (10) considering the vague and diffuse feelings
associated with pain observed that “The effective dimension is
difficult to express in words such as exhausting, sickening, terrifying,
cruel, vicious and killing”. Individual differences in the experience
of pain (11) and variability among painful conditions (12) complicate
the task. Patients may not only have difficulty describing painful
experiences, but they may also be unwilling or reluctant to confide
these feelings of distress. Moreover, voluntary control can be
used to suppress or exaggerate the expression of pain (13).
measurement studies during past century are focused on the psychophysical
relationship between the extent of injury and perceived pain.
Excellent psychophysical power functions were generated. All studies
of pain measurement up to the time of publication of the gate
control theory of pain (4) concentrated exclusively on the measurement
of pain intensity. The gate control theory, together with the
increasing emphasis on pain as a major clinical problem (14, 15,
16), led to the recognition that pain rarely has a one-to-one
relationship to a stimulus. Acute pain is sometimes proportional
to the extent of injury, but the contribution of psychological
factors reveals complex relationship that are profoundly influenced
by fear, anxiety, cultural background and the meaning of the situation
to the person (17). Chronic pain presents an even greater problem
for the psychophysical concepts: backaches often occur without
any discernible organic cause; post herpetic neuralgia persists
long after peripheral nerve regeneration and healing of all tissues.
The measurement of pain intensity is essential to determine the
initial intensities, perceptual qualities and time-course of the
pain so that the differences among different pain syndromes can
be ascertained and investigated. Measurement of these variables
provides valuable clues that help in the differential diagnosis
of the underlying causes of the pain. They also help determine
the most effective treatment necessary to control pain and essential
to evaluate the relative effectiveness of different therapies.
pain threshold can be determined by the classical (18) methods
of limits (ascending and descending trials). Simple category scales
such as the four-point ‘none, mild, moderate and severe’ (Verbal
categorical scale) or 1-10 numerical scale can be scored in several
ways. The simplest, the method of equal appearing intervals, assigns
successive integers to verbal categories or uses numerical categories
directly (19). The three most frequently considered aspects of
pain are the subjective (measured by self-report), the behavioural
(measured by sampling of physiological or electric potentials
and assaying body fluids or other biological responses). Self-report
measures, when they can be obtained should be regarded as the
‘Gold standard Indeed, the International Association for the study
of pain emphasizes that pain is always subjective.
techniques for the psychological assessment of the pain patients
have improved to the extent that emotional states such as anxiety,
depression and defensive personality styles can now be identified.
People experiencing pain are able to report separately on the
sensory and affective dimensions and emotional qualities differ
dramatically across different forms of clinical pain and within
individuals over time (20). Currently evidence indicates that
pharmaceutical and psychological interventions have different
effects on either sensory, affective or both qualities of the
experience. The narcotic Fentanyl reduces the sensory intensity
but not the unpleasantness of painful tooth pulp sensations (21).
In contrast, anxiolytics such as Diazepam reduces affective discomfort
rather than sensory-intensity qualities of the experience (22).
Similarly, placebo medication has an impact on unpleasantness
rather than on sensory qualities of painful events. The new emphasis
on the varieties of clinical pain and their variability have led
to new concepts of pain measurement.
Subjective pain assessment
Visual analogue scale (VAS) is a simple measure of subjective
pain. It consists of 10 cm horizontal (23) or vertical (24) line
with the two endpoints labeled ‘no pain’ and “worst pain ever’.
The subject is required to place a mark on the 10 cm line at a
point, which corresponds to the level
of pain intensity he or she presently feels. The distance in centimeters
from the lower end of the VAS to the patient’s mark is used as
a numerical index of the severity of pain. The VAS is sensitive
to pharmacological and non-pharmacological procedures that alter
the experience of pain (25, 26) and correlates highly with pain
measured on verbal and numerical rating scales (27, 28). The ease
of administration and scoring has contributed to the popularity
of this method. A major advantage of the VAS as a measure of sensory
pain intensity is its ratio scale properties (29). Other advantages
include minimal intrusiveness and conceptual simplicity (23, 30).
The major disadvantage of the VA is its assumption that pain is
a one-dimensional experience (30).
and Togerson (31) developed a questionnaire to specify the qualities
of pain. The three major classes were words that describe sensory
qualities (temporal, spatial, pressure, thermal and other properties),
affective qualities (tension, fear and autonomic properties) and
evaluative qualities. The questionnaire, which is known as the
‘McGill Pain Questionnaire (MPQ)’ (30), has become a widely used
clinical and research tool (8, 32, 33). MPQ has been translated
into several languages. The most important requirement of a measure
is that it should be valid, reliable, consistent and, above all,
useful. The MPQ appears to meet all of these requirements (7,
31, 32, 33) and provides a relatively rapid way of measuring subjective
pain experience (30).
et al (34) and Holroyd et al (35) evaluated the theoretical structure
of the MPQ. Turk et al (34) concluded that the three-factor structure
of the MPQ (sensory, affective and evaluate) is strongly supported
by the analysis. However, these authors argue that the factors
measured by the MPQ are highly intercorrelated, they are therefore
not distinct. It is evident, however, that the discriminative
capacity of the MPQ has limits. High level of anxiety and other
psychological disturbances, which may produce high affective score,
may obscure the discriminative capacity. Moreover, certain key
words that discriminate among specific syndromes may be absent
and Physiological measures
subjective experiences of pain and pain behaviours are, presumably,
reflections of the same underlying neural processes. However,
the complexity of the human brain indicates that although experience
and behaviour are usually highly correlated, they are far from
identical (26). Mistrust of verbal judgment has motivated the
development of physiological and behavioural objective measures
of pain. These are relatively insensitive to biasing factors and
the psychological demands associated with requests for introspective
reports (37). In addition, these methods are the only measures
available for pain assessment in animals and in infants or in
adults with poorly developed language skills. The behavioural
measures to assess magnitude of stimulus-evoked pain sensation
include the naturally occurring reactions (Grimace, vocalization,
licking, limping, and rubbing) and trained operant behaviours
(manipulating a bar to escape a painful stimulus). Recently facial
expression evoked by experimental stimulation (38) or analysis
of pain expression from photographs (39) has also been used.
is a search for a physiological pain measure more objective than
verbal report. Profound physiological changes often accompany
the experience of pain, especially if the injury or noxious stimulus
is acute (40). Autonomic measures such as heart rate, skin conductance
and temperature have been correlated with pain stimulation. Although
influenced by painful stimulation, these responses habituate quickly
and respond nonspecifically (41). Measures of cortical-evoked
potentials have been studied extensively and correlate with both
stimulus intensity and verbal report. Cortical activity has also
been assessed recently by analysis of noncontingent electroencephalogram
(EEG) (42) and by nuclear magnetic resonance (NMR) techniques
(43). Willer and others (44, 45) have used measures of reflex
activity (e.g. blink, H and nociceptive reflexes) as objective
measures of pain sensation. Neurophysiological recording methods
commonly employed in animal research have been used to investigate
peripheral mechanisms in unanaesthetized normal volunteers (46).
Human microneurography is powerful tool. This technique can identify
all classes of primary afferent fibers and have verified the association
of specific sensations with type of fiber stimulated (47). More
effective pain measurement may ultimately result from an approach
that integrates information from these separate, yet complementary
sources of information (48, 49).
quantification of effects of experimental pain upon the spontaneous
EEG of health volunteers by analysis of power spectral density
(PSD) after Fourier transformation has been performed by many
workers (42). The quantitative methods in the study EEG have
shown great increase in the use of research and clinical evaluation
of pain in patients. The use of experimental pain stimulus in
association with EEG recording permits non-invasive investigation
of the functional integrity of the nociceptive system in animals
and humans. It has a great utility during pain, analgesic therapy
and anesthesia. By this technique noxious stimulus of both brief
phasic pain activation and longer diffuse type of tonic pain activation
have been examined. The heightened delta activity may reflect
the stress component of human pain responsivity and that beta
activity reflects the vigilance scanning of pain processes. After
intracutaneous noxious electrical stimulation, the stimulus-induced
increase of power is mainly in low frequencies, delta and theta
(42). Luque et al (50) studied power spectrum estimates of brain
activity in chronic upper extremity pain patients and their result
showed a diffuse asymmetry and cerebral activity for one half
of the patients during intense, unilateral pain in contrast to
non-pain states. We have also conducted a series of power spectral
studies (PSA) in normal controls, frostbite and chronic low backache
patients. Our preliminary and unpublished results also indicate
a heightened delta activity during pain state.
reviewing the result of several studies in the CPS analysis of
experimental pain and clinical pain patients, the EEG spectral
power of low frequencies in delta and theta activities is often
found to be associated with human pain. Some of the experimental
studies also indicate that these power densities are sensitive
to analgesic modulations (51). The difference among these studies
may be related to different pain stimuli and pain duration, as
well as in the classification of the bandwidths. But the striking
similarity was a decrease in alpha and increase in beta activity
after pain. The second consistent effect of tonic experimental
pain of EEG power spectra is an increase of beta activity. It
may directly be related to the resynchronization of alpha being
replaced by faster rhythms but the nature of EEG changes in clinical
pain states is very difficult to relate perceptual, emotional
or cognitive processes linked to pain itself.
Pain evoked potentials
potentials (EPs) are stimulus or event related electrical signals
of brain activity that may be recorded from the scalp when precisely
controlled discrete stimuli are delivered significant correlation
exists between late. EP components evoked by experimental pain
stimuli and the induced pain sensation. EPs can serve as indicators
of perception at multiple levels. Pain related brain evoked potentials
are recognized as an objective and quantitative test for evaluation
of peripheral and central spinothalamic tract, thalamocortical
projections and cortico-cortical circuit for pain processing.
Chen et al (42) identified the current sources dipoles in noxious
the early components of somatosensory evoked responses reflect
contralateral activation. However, the pain related late components
(52) and ultralate components are distributed bilaterally in both
hemispheres and exhibit maximum amplitude in the vertex. The late
components are assumed to reflect secondary mechanisms of information
processing, such as stimulus recognition, localization, estimation
of stimulus intensity and painfulness and initiation of motor
movements. Further analysis by Chen et al (42) on the consistency
and reliability of the pain-related sources revealed that too
pain related components are consistently found a negatively at
145 msecs and a positivity at 225 msecs. The initial components,
correlates with stimulus intensity and appear to code information
about intensity of the external stimulation. The latter components
are closely related to the subjective estimation of pain intensity
and seem to reflect association processes involved in evaluation
of noxious stimulus. Findings of Chen et al (42) clearly support
the hypothesis that the EP may serve as a measure of pain experience
in humans. Their finding suggests that the late components of
the EP waveform may reflect the cognitive aspects of dental pain
perception in laboratory models. When analgesic interventions
are introduced, subjective reports of pain intensity is reduced
and EP components diminish accordingly. Zaslansky et al (53) suggested
that the pain-EP reflects the emotional-motivational response
to pain rather than the sensory-discriminative. Thus, it provides
a useful neurophysiological tool for studying the emotions associated
related potentials (ERP)
are late potential, which occur in response to a task. P300 is
one such candidate. It is a late endogenous positive event related
potential, occurring approximately 300 msecs after the onset of
novel stimulus or a stimulus that is perceived as important. The
P300 has been demonstrated to be related to information processing
of the stimulus. The changes in amplitude and the latency are
dependent on presentation qualities of the stimulus. The recoding
of long latency event related cerebral EPs have been used as an
objective measure of cognitive functions and perception. These
potentials are valuable and useful parameters for assessing a
variety of cognitive abilities (54) and psychological process
including expectancy, attention, search, discrimination, decision-making
and memory. P300 is a valuable tool and our preliminary work
have shown that it can evaluate the cognitive and effective components
of pain (55, 56, 57, 58). It is generally believed that P300 components
is immune to the effect of stimulus factors because of its endogenous
origin (59). But Papanicolaou et al (60) have shown that P300
latency, but not amplitude, decreased as stimulus intensity was
increased for both the standard and target stimuli. Further Polich
(61) reported changes in components wave forms with manipulation
of frequency and intensity parameters. Sugg and Polich (62) found
that P300 amplitude increase and peak latency decreased as stimulus
intensity increased with stimulus frequency also affecting these
outcomes. Thus, both P300 amplitude and latency are affected variation
in stimulus factors, although virtually no systemic studies of
intensive effects have been reported (63). Our unpublished work
has shown a change P300 with increasing strength of stimulation
in healthy subjects (64). We (65) have shown a significant increase
in P300 latency patients suffering from pain as compared to age
and sex matched controls. Our findings suggest that there are
cognitive changes in chronic pain states. The cognitive blunting
is reversible with analgesic intervention (66).
negative variation (CNV) a surface-negative slow potential recorded
from human subjects during a fixed foreperiod of a warned reaction-time
task (67). This electrical phenomenon of the brain has drawn the
interest of many psychophysiologists because it reflects some
psychological processes such as expectancy motivate attention
and arousal. The CNV waveform depends primarily on psychological
and to a lesser extent on physical proportion of the stimulus
presentation. CNV can be used to evaluate the affective-motivational
components pain. It is still unclear how brain processes emotionality
and pain, what are interaction mechanisms of emotion and pain.
We have shown that the slow brain potentials in the contingent
negative variation (CNV) are found to be larger in the pain condition
than that of the control (68, 69). When patients with chronic
pain were studied (70) in the CNV paradigm, the appearance of
contingent negative variation response was more marked. It was
concluded that slow evoked potentials are sensitive to anxiety
level and to pain perception in man. Rizo (71) have also shown
the usefulness of this measure in the investigation of pain as
a complex sensation. A number of recent studies have assessed
the influence of baseline or induced mood on subjective and psychological
responses to experimental stimulation. Baseline (72) and induced
(73) anxiety have been shown to increase pain ratings to thermal
or pressure pain ratings, while an experimentally induced depressive
mood (induced by presentation of text with depressing themes)
decreased tolerance to cold pressor pain (74). Pain memory processes
have been investigated using experimental painful stimulation
(75). These studies provide experimental examples of how the experience
of chronic pain can exert subtle influence on cognitive processes
and mood. Pain itself can also impair cognitive and psychomotor
is a personal, subjective experience influenced by cultural learning,
the meaning of the situation, attention and other psychological
variables. Approaches to the measurement of pain include verbal
and numeric self-rating scales, behavioural observation scales
and physiological responses. The complex nature of the experience
of pain suggests that measurements from these domains may not
always show high concordance. Further development and refinement
of pain measurement techniques will be able to meet the challenge
and lead to increasingly accurate tools with greater predictive
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