where it hurts

The first question is where it hurts, and the heart is the organ that lies when you ask it.

A man comes in with pain in his left arm. Not a sharp pain — an ache, a fullness, a sense that the arm has been bound too tight, running from the shoulder down past the elbow and sometimes into the two smallest fingers. Ask him to show you and he points to the arm, because that is where the pain is. He is not wrong about where the pain is. He is wrong about nearly everything else, because the arm is fine. The arm has no idea anything is happening. What’s happening is behind his breastbone, where a coronary artery has narrowed far enough that the muscle it feeds is starving, and the muscle is screaming, and the scream is coming out of his arm.

The wiring is the reason, and the wiring is almost embarrassingly simple. The nerves that carry pain from the heart enter the spinal cord at the upper thoracic levels — the first four or five segments down from the base of the neck. The nerves that carry sensation from the chest wall and the inner left arm enter at the same levels. And somewhere in the back of the cord, in the grey horn where the incoming fibers hand their signal to the next neuron up, the two streams arrive at the same cell. One wire from the heart, one wire from the arm, soldered to a single relay. The relay fires. It sends one signal up the cord to the brain, and the signal does not carry a return address. It only says: pain, from down here, on the left.

So the brain has to guess. And the brain is a creature of experience — it decides what a signal means by what that signal has meant ten thousand times before. It has localized pain in the skin of the arm all its life: every burn, every scrape, every slammed elbow taught it that this particular relay, lit this particular way, means the arm. It has almost never had pain from the heart. The heart is silent its whole life and then, once, perhaps only once, it speaks — and it speaks down a wire the brain has spent decades learning to read as something else. The brain does the reasonable thing. It bets on the familiar. It assigns the pain to the arm because the arm is where this feeling has always lived, and it is wrong, and it is wrong in exactly the way a good guesser is wrong: by trusting the past.

This is not a quirk of the heart. It is the standing arrangement of the body. The viscera are poorly mapped — densely felt and badly located, all of them refer their pain outward onto the body wall that happens to share their segment of cord. An inflamed diaphragm sends its pain up to the tip of the shoulder, because the nerve that drives the diaphragm comes off the cord at the neck, alongside the nerves for the shoulder skin, and the shoulder is where the brain has a map. A man can be bleeding under his diaphragm and feel it as a sore shoulder. The organ in trouble borrows the address of a neighbor who has one.

What the doctor learns — what takes years, what separates the one who catches it from the one who sends him home with a sling — is to not believe the finger. The patient is the most honest witness in the room and he is pointing at the wrong place, and both of those are true at once, and you have to hold them at once. The pain is real. The location is a lie. Not the patient’s lie — the body’s, told in good faith, by a system that was never built to report from the places it can’t see and so reports from the nearest place it can.

Because that is the rule underneath all of it, and it is a hard rule to keep in the front of the mind, because everything about pain argues against it. Pain feels like the most local thing there is. It feels like it is its location — the whole experience of it is here, this spot, this is where. And the spot is the one thing you cannot trust. The pain surfaces where there’s a map. It comes out at the readable place, the place with a thousand prior nerves and a thousand prior namings, the arm and the shoulder and the jaw — the legible body. The illegible thing, the heart, the diaphragm, the silent dense interior, has no language of its own loud enough to be heard, so it speaks in a borrowed one, and the borrowing is the whole danger. You go looking where it hurts. The thing that’s killing you is one wire over, in the dark, where nothing hurts at all.