Читать книгу Why It Hurts - Dr. Aneesh Singla - Страница 8

The body uses physical pain to get our attention when something is amiss. Someone’s jaw hurts; the dentist discovers an infected tooth and pulls it. Someone’s abdomen hurts; the family doctor diagnoses appendicitis and orders an emergency appendectomy. These pains are acute. Acute pain develops immediately after an injury or another distinct event. By contrast, chronic pain develops over time and generally lasts for months or longer.

Acute and chronic physical pain can be further classified as inflammatory, nociceptive, or pathological. Rheumatoid arthritis and osteoarthritis are two familiar kinds of inflammatory pain. Inflammation occurs when our immune system responds to an injury by sending an army of infection-fighting cells to destroy invaders in our bodies. This response results in warmth, swelling, and hypersensitivity, along with pain. In the case of an autoimmune disease like rheumatoid arthritis, the body mounts an immune response to harmless tissue that is misinterpreted as dangerous. This results in infection-fighting cells, which produce antibodies, to attack the cushioning and shock-absorbing cartilage in our joints.

“Doc,” one patient with arthritis said, “my hip feels like I have a constant toothache.” His pain was chronic and inflammatory.

Nociceptive pain results from physical trauma such as a skin laceration or a burn from a hot stove. It’s a response by the nervous system to a physical event that damages our body. This is generally a sharp, stabbing, or cutting sensation in the area of the injury, depending on the type of damage inflicted. Acute, nociceptive pain is what I experienced when I bent down to clean the garage floor and discovered the gash on my calf.

Pathological, sometimes called neuropathic, pain has no adaptive purpose. In fact, it is often referred to as maladaptive pain because it provides no specific protective function. From an evolutionary perspective, it does not confer a survival advantage. Typically, this type of pain is due to nerve injury or nervous system dysfunction. Depending on the type of nerve involved, a patient suffering from pathological pain might feel a burning, stabbing, or electrical sensation with no injury to account for it. As you can imagine, this kind of pain presents unique challenges for diagnosis and management because it is more difficult to identify its underlying cause.

Unlike neuropathic pain, inflammatory and nociceptive pains are considered to be adaptive. Think of a smoke detector. Working properly, it sounds an alarm whenever there is enough smoke in the air to indicate a fire. Thus, it serves a vital adaptive function: alerting you to evacuate before your house burns down. Adaptive pain is the body’s smoke detector. It signals an alarm—a pain sensation—whenever pain receptors are triggered by damage to your cells. It’s the body’s way of saying, “Take your hand off that stove before the burn gets any worse.” That is a classic example of nociceptive pain.

When you do get a burn, on your finger for instance, the pain you feel for days afterward is inflammatory. This is the pain from the inflammation response at the site of the healing tissue. The body’s smoke alarm is still serving its purpose by alerting you that the cells are busy healing injured tissue. Even though it’s annoying, the inflammatory pain is there for a very good reason: to tell you that your finger is not ready for the next task at hand. Achy, tired joints and muscles while fighting off the flu are another example of adaptive inflammatory pain. In this case, the pain suggests you rest and let the body fight off the virus.

Of course, smoke detectors aren’t always right. Sometimes, they go off when there’s a bit of smoke but no danger of a house fire. For example, I recently overdid it searing some salmon for dinner. Off goes the ear-piercing smoke alarm, and up the ladder I go to wave the smoke away. In the body, this kind of false alarm is known as pathological pain and, unfortunately, waving a towel around it isn’t going to help.

Pathological pain is a kind of hurt that occurs when an acute injury hasn’t occurred, a maladaptive pain. For example, people with trigeminal neuralgia, also known as tic douloureux, experience severe jaw pain. There is nothing physically wrong with the jaw, but the pain alarm sounds loudly and ceaselessly, without serving any adaptive or protective purpose.

When someone presents with any type of physical pain, one of the first steps in evaluating the problem is to ask a series of diagnostic questions: Where does it hurt? Does it radiate? Would you describe it as sharp or dull? Shooting? Can you give a number to your pain, with zero being no pain and ten being the worst pain imaginable?

Doctors may order a barrage of tests: X-rays, MRIs, blood work, and so on. They then try to piece it all together to form a diagnosis. When a cause is detected, the patient is either sent to a specialist (e.g., a urologist to treat a kidney stone) or treated on the spot (e.g., with antibiotics for a urinary tract infection). In cases of pathological pain, however, test results may show nothing abnormal. Such perplexing cases usually require the intervention of a pain specialist like me.

Beyond inflammatory, nociceptive, and pathological pain, there is the realm of psychological pain. Both physical and psychological pains require a diagnostic workup to get to the underlying cause. There are parallels in the diagnostic processes of a psychologist or psychiatrist: “I see you are anxious and depressed. Can you describe the sensation of your anxiety? Do you know what kinds of situations trigger your depression?”

Anxiety, with or without pain, can be adaptive. For example, if you’re worried about an upcoming test, your anxiety may push you to study. As you firm up your knowledge of the test material, your anxiety should fade. Anxiety can also be maladaptive. If you suffer from an anxiety disorder like agoraphobia, the irrational fear of open spaces, it can prevent you from even holding a job that requires leaving the house. This would require proper psychiatric treatment to regain normal function. (We discuss psychological pain further in Chapter 3.)

Pain is unpleasant; it needs to be. If it weren’t, we’d ignore it. Think about that time the low-fuel indicator came on in your car and you kept driving. (How did you like that walk to the gas station in 90-degree heat?) Pain is painful precisely because it needs to jolt us into action.

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Let’s return to the garage on that Sunday afternoon when I was moving beer into the fridge. The gash on my leg was starting to throb and burn. I tried unsuccessfully to bandage it; it was bleeding too profusely. It looked like stitches would be necessary, given the size and depth of the laceration. Reluctantly, I told my wife and daughter that I’d be missing the family trip to the neighborhood pool. Instead, I hopped in the car, saw and ignored the low fuel indicator, and drove to the nearest emergency room.

“Insurance card and ID please?”

After filling out the forms, feeling a bit sheepish considering how minor my injury was compared to those I saw around me, I buried myself in an outdated magazine and tried to get comfortable.

Physical pain helps us survive life-threatening situations by activating our fight-or-flight system. When it detects danger, our body releases adrenaline and other chemicals to help us run faster, jump higher, and focus more clearly on the threat at hand—our survival depends on it. So when the smoke detector is going off for no reason, as it does with maladaptive pain, it can’t simply be ignored. Chronic pain gnaws away at you because millions of years of evolution have designed pain to command your attention. This wears you down quickly and adds a component of psychological pain to the picture.

As a pain specialist, I seek to diagnose the root cause of adaptive or maladaptive pain from a constellation of symptoms. I order laboratory tests, perform clinical examinations, and use every other tool at my disposal to confirm or refute a list of possible diagnoses until I’ve narrowed it down to the most likely culprit. While there’s no doubt that pain exists when a patient reports it, I have to keep the subjective experience of the patient in mind as I look for a possible physical source.

When the source of pain is hard to identify, people become desperate. This is natural, but their despair can actually amplify their pain sensation. No matter how unpleasant the pain, it isn’t wise to seek to turn the sensation off entirely. One of my patients, Mark, suffered from severe complex regional pain syndrome (CRPS), a painful condition of sharp, shooting, burning pain in the arms or legs. This syndrome can occur after an injury. The nerves get stuck in a circular feedback loop: The pain produces inflammation, then the inflammation produces more pain. The sympathetic nervous system, which controls a person’s fight-or-flight response, facilitates something we call wind-up. The pain in the affected area worsens and worsens.

To understand this phenomenon, think of the volume control on your stereo. You turn the knob up and the “volume” of the music is increased—and each note is amplified many times higher. In the nervous system, the pain signal is progressively turned up many times higher through wind-up. In a pain circular feedback loop, the nervous system continues to wind up, so the intensity of the pain increases over time.

By the time of his appointment, Mark was experiencing extraordinarily severe pain in his leg.

“Can’t you just cut the leg off?” Mark asked. That question should give you some sense of the desperation chronic pain patients experience.

Mark isn’t my only patient with nerve damage to have asked about surgically removing a limb. I empathize deeply with anyone whose suffering has reached the point where amputation seems like a feasible alternative. Unfortunately, as I tell my patients, severing the limb in question would actually make the pain much worse due to central sensitization, a phenomenon where pain sensation actually increases due to the nervous system’s sensitivity going into overdrive. Think of central sensitization as the end result of wind-up. It is the new set point for your nervous system. Going back to the volume control example, your centrally sensitized nervous system is now where the volume has been turned up on your body’s stereo.

To give you a sense of how bad pathological pain can feel, here’s an excerpt from The Story of Pain by Joanna Bourke, quoting physician Valentine Mott:

I have seen the most heroic and stout-hearted men shed tears like a child, when enduring the agony of neuralgia. As in a powerful engine when the director turns some little key, and the monster is at once aroused, and plunges along the pathway, screaming and breathing forth flames in the majesty of his power, so the hero of a hundred battles, if perchance a filament of nerve is compressed, is seized with spasms, and struggles to escape the unendurable agony.

Mott puts it more poetically than I ever could, but his description resonates with what I’ve witnessed as a pain specialist.