The Pain Chronicles (30 page)

WHAT IS PAIN?

What, finally, is pain?

Pain is an experience one is never in doubt about having. One might pause to wonder,
Am I in love?
but never,
Am I in pain?
Indeed, uncertainty in the face of this question answers it in the negative. As Wittgenstein observes, “if anyone said ‘I do not know if what I have got is a pain or something else,’ we should think something like, he does not know what the English word ‘pain’ means.” Why, then—in light of this extraordinary clarity—is pain so difficult to define?

Is pain sensation, emotion, or idea? Is it a product of biology or culture? If it is primarily a biological phenomenon, then why does it seem to vary so much from person to person and from culture to culture? If it is primarily a cultural one, then why does it seem so universal? After all, there is a word for
headache
in every language, ancient and modern. When the ancient Babylonian describes the headache that envelops like a garment, we know exactly what he means.

Or do we? The Thaipusam devotees possess the same tongues as the tourists, sensitized by the same nerves, but they do not appear to feel what we would expect them to feel when those tongues are pierced. Nevertheless, members of one culture can confidently torture someone from another, relying on the so-called universal language of pain. Or can they? A martyr may experience such torture quite differently from another type of victim.

Defining pain tests our understanding of the relationship between the body and the mind. For Hippocrates, pain was a physical sensation that emerged from discernible physical phenomena; for Aristotle, pain was what today we might call an emotion—a visceral reaction of the mind to physical or metaphysical stimuli, but one that might be overcome through reason’s dominance over all other functions of the mind and the body.

Neither definition suffices. If pain is simply a sensation, then why is it so upsetting? If it’s an emotion, then why does it feel like it necessarily entails a physicality not entailed by any other unpleasant emotion? One can feel mental pain without physical pain (the “pain” of betrayal), but the reverse is not true. Physical pain seems to always conjure distress. (Indeed, the blurring of the two is reflected by the word for pain in most languages. For example, the French
douleur
means not only physical pain but also grief and distress, and it is derived from the Latin word
dolor
, which means pain, suffering, and grief.) Pain that conjures no negative emotions would seem not to be painful. (A masochist might “enjoy” the feeling of pain, but the frisson of its aversiveness is part of what is enjoyable.) Dictionaries are vexingly circular on the topic, defining pain as “suffering, or distress” and then defining distress, in turn, as “great pain, anxiety, or sorrow” and suffering as “pain or distress”!

In the seventeenth century, philosopher René Descartes proposed a theory of pain as a simple physical sensation that is triggered when fire or other threats to the body are registered by a “delicate internal thread” that sends a message to the brain, causing it to create pain, the way pulling a rope sounds a bell. Descartes illustrated his idea with a sketch of a comely man-child whose foot touches a ball of fire. Through the boy’s transparent body, one can see a rope stretching from the foot to the brain, where it is ringing a bell in the pain center.

Even though one of Descartes’ primary intellectual contributions was the model of the body as machine, he saw problems with using such a simple mechanistic model for pain. He wrote of the perplexing relationship between pain and emotion, noting that “we may sometimes suffer pains with joy, and receive titillating sensations which displease us.” He was fascinated by phantom limb pain, which he felt demonstrated that perception “must sometimes be at fault and deceptive” and “the sense will be deceived.”

He was struck by the case of a young girl whose gangrenous lower arm had been amputated without her knowledge. After the operation, the girl complained of pain in her fingers, the physical absence of which was concealed from her beneath bandages. “And this clearly shows that the pain of the hand is not felt by the mind insofar as it is in the hand, but insofar as it is in the brain,” he concluded.

Although the part of Descartes’ writings on pain that became best known was the metaphor of the rope and the bell, according to which the brain’s role in registering pain is a passive one, his recognition that the pain of the hand actually exists only in the brain could be said to set the stage for the modern understanding of pain as a perception
actively generated
by the brain, like hunger or thirst.

The model of pain as a bell attached to ropes turns out to be wrong in many respects. There are no ropes directly connected to a bell in the brain that can reliably command pain. And, whereas a bell cannot determine how loudly it will ring, the brain
does
determine how neural signals are transformed into pain. While there are neural signals indicating tissue damage that come into the brain from the periphery, the brain may or may not pay any attention to them.

A more apt analogy might be that of a watchman in a tower who is charged with surveying the landscape of the body and sounding the alarm in the case of an attack. In theory, the watchman should sound the bell softly in the case of a small incursion or loudly in the case of an onslaught. But the watchman is far from an ideal employee. He is erratic, lazy, easily confused, fearful, a poor multitasker, and sometimes just deluded. Sometimes he responds to a threat helpfully, with a proportional pain alarm. But sometimes he’s preoccupied with a higher-priority task and does not sound the alarm at all. Other times he imagines a threat that isn’t there and rings the bell with no cause. And in the case of chronic pain, he rings the bell ever louder, driving everyone mad. Or in certain states of mind, such as during a religious rite, the watchman regards the incursion into the integrity of the body not as a threat but as a cause for celebration.

This is why the brain can generate an experience of pain without any nociception at all, or it can fail to generate pain when tissue damage has occurred. On the occasion of his 1981 shooting, Ronald Reagan was initially unaware that he had been shot through the chest. He was taken to the hospital because he was coughing up blood (his lung had been punctured), and that’s where the bullet wound was discovered. “I had never been shot before, except in the movies,” he commented later. “Then you always act as though it hurt. Now I know that does not always happen.” Perhaps he had imagined it would feel like a
shooting
pain. (In fact, being shot is often described as feeling like a thud followed by a burning sensation.)

Yet, a man who believes he has been shot when bullets have in fact missed him may start to wrench in agony. Once volunteers are repeatedly shocked by a painful electrical stimulus, their brains start to generate pain when they expect the stimulus to be applied again but
before
it is actually applied. The anticipation of pain is pain.

Pain is now understood to be neither sensation nor emotion alone, but rather an experience that draws upon both: the elusive intersection of three overlapping circles—cognition, sensation, and emotion. When any of these elements is missing, there is no pain. There is no such thing as being in pain without knowing you are. There is no such thing as being in pain without feeling the sensation of pain. And there is no such thing as pain that does not cause a salient emotional reaction.

“Pain is whatever the experiencing person says it is, existing whenever the person says it does,” declared Margo McCaffery, a leader in the field of pain-management nursing. Highlighting the radical subjectivity of pain, this definition (which has been widely used by clinicians in recent decades) suggests that trying to characterize the kind of thing a person refers to when he refers to his pain is futile. In 1979, the International Association for the Study of Pain came up with the definition of pain that is most accepted today: “An unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage.”

Among its virtues, this definition aptly expresses pain’s complex relationship to tissue damage: pain is a feeling that can be distinguished from other types of emotions and sensations by the way in which—accurately or not—
it connects that feeling to a sense of tissue damage.
Yet the definition makes explicit that the connection of pain to tissue damage is only by “association.” A note following the definition elaborates that “activity induced in the . . . nociceptive pathways by a noxious stimulus is not pain, which is always a psychological state.” In short, pain is not nociception, faithfully informing the brain about damage in the body the way a seismometer informs a scientist of the motion of tectonic plates. The relationship between pain and tissue damage can be compared to that between love and sex: it may or may not exist.

Pain is an aspect of bodily consciousness involving activation of the areas of the brain that process sensory information and those involving culture, memory, emotion, and association (the limbic system). The thick overlap between the limbic system and the other parts of the human brain is thought to explain the most perplexing aspect of our pain—its remarkable fluidity of meaning, from the agony of torture to the ecstatic pain of a sacred rite.

The contemporary paradigm of pain reconciles the ancient concept of pain as a spiritual signifier with the nineteenth-century conception of it as a biological function; analogously, the current understanding of dreaming reconciles the concept of dreaming as deeply meaningful (as signals from the gods or signals from the unconscious) with that of dreaming as random brain activity. Both reconciliations follow from the understanding that the brain’s activity draws upon its meaning-making parts.

The shift in paradigms of pain correlates with shifts in types of treatment. The rope/bell mechanical view of pain, according to which there are fixed pain pathways, rationalized the ineffective nineteenth- and early-twentieth-century practice of snipping nerves in order to destroy those pathways. The contemporary model can be described as a model in which pain is regarded as a perception shaped by biological, psychological, and sociological factors. This understanding has led to the invention of multidisciplinary pain programs and treatments that attempt to intervene in pain in all its aspects. The treatments of the future will focus on targeting the brain’s perception and modulation of pain.

THE DEMON IN THE MACHINE

For many years, researchers sought the demon in the machine—the “pain center” in the brain—in order to exorcise it. But there turned out to be none. Unlike senses such as vision and hearing that depend on activation of clustered portions of the brain, pain is a complex, adaptive network of neurons (a neuromatrix) involving roughly half a dozen areas of the brain that transmit information back and forth. As pain is one of evolution’s most important functions, it is distributed over many different areas, so that if one part of the brain is disabled through injury or disease, the system can continue to generate pain.

Each of these regions contributes to the experience of pain. The nociceptors that detect tissue damage send a pain signal up the spinal cord through two major pathways. Dull pain runs slowly along one track, and sharp pain runs quickly along the other. The signal continues through the brain stem, the primitive part of the brain that controls autonomic nervous and various homeostatic systems (such as those regulating breathing, heart rate, and sleeping), and activates norepinephrine to create a feeling of arousal and vigilance. It then feeds into the thalamus—a region in the brain that acts as a way station for other senses, such as hearing and vision. The thalamus relays pain signals to several areas: the limbic system, the somatosensory cortex, and the prefrontal cortex. The limbic system (the collective name for a group of areas in the brain associated with memory, emotion, and attention) produces feelings of sadness and unpleasantness; circuitry shared by the brain stem and the limbic system is activated to produce a sense of anxiety. The somatosensory cortex locates the pain; signals that originate from the foot, for example, are registered in the part of the homunculus (the brain’s internal map of the body) that represents the foot. Finally, the prefrontal cortex (an area associated with consciousness and cognition) ascertains the cause of the pain and formulates a strategy to stop it.

Disease processes or damage can alter or interrupt the flow of information among the parts. People with lesions on their somatosensory cortices still experience pain, but they are no longer able to identify where it is coming from. Patients who had cingulotomies—a radical surgical treatment once used to treat pain or mental illness that severed nerves in part of the limbic system known as the rostral anterior cingulate cortex (rACC)—reported that they were still aware of pain but they didn’t “mind” it as much. Their emotional response had receded, and so their pain was diminished. (This was also true of patients who had the older treatment of lobotomy, a similar though more extensive procedure in the prefrontal cortex.) Similarly, when a rat’s rACC is lesioned, he will cease to avoid a painful stimulus; it no longer pains him. Cingulotomies are rarely, if ever, performed now because, among other reasons, over time people’s brains tend to rewire the pain circuitry around the damaged part, and their pain returns.

Most of the time, damage to the pain circuitry results in greater pain. For example, one of the most intractable, terrible pain syndromes—phantom limb pain—arises from neural reorganization in the somatosensory cortex. In 1871, the Civil War surgeon Silas Weir Mitchell coined the phrase “phantom limb,” noting “thousands of spirit limbs haunting as many good soldiers, and every now and then tormenting them.” The great majority of amputees suffer from phantom limb sensations, and many of them are excruciating. The missing limb (or teeth, eyes, internal organs, or breasts) may feel as if it is clenched or crooked or cramped or—oddly—too short. (Most people’s only experience of phantom sensation is “phantom lip”—the peculiar feeling caused by local anesthesia at the dentist that the lip is not only numb but has suddenly grown uncomfortably fat.)

Historically, phantom limb pain was believed to arise from neuromas—injured nerve fibers at the tip of the stump that grow back irregularly and begin sending aberrant messages that translate into pain. Yet when surgeons tried shortening the stump to remove the neuromas through a second amputation, pain only worsened. Cutting the sensory nerves where they attach to the spinal cord also failed to alleviate pain. Moreover, the neuroma theory failed to explain why some people who lack limbs owing to congenital birth defects also suffer from phantom limb pain. Even more tellingly, people who are paralyzed by spinal cord injuries that prevent any pain signals from crossing the spinal cord into the brain can suffer from phantom pain that they experience as coming from the bodies they can no longer feel.

In recent years, brain imaging has shown that although neuromas can contribute, the primary source of phantom pain derives from pathological changes in the representation of the body that forms the homunculus. When a body part is lost, the sensory information normally transmitted to its corresponding neural area in the homunculus abruptly ceases. Although it is not understood why this causes pain, it has been theorized that the neurons register alarm at the absence of normal input, and this alarm translates into pain.

Moreover, for unknown reasons, the neurons of adjacent areas of the homunculus grow into the area corresponding to the missing limb. For example, one of the quirks of the homunculus is that the arm and hand areas are adjacent to the face area. When a person loses her arm, the facial neurons of the homunculus begin to grow pathways into the adjacent arm area. Because of that, stroking the face (particularly the lips) of an amputee who has lost a hand may trigger the sensation of being stroked on the phantom hand. It is not known how this cross wiring causes pain, but the greater the degree of cross wiring in the homunculus, the more severe pain the amputee suffers.