Kidney Stones Are Excruciating, But the Source of Pain Is Surprising

woman in ER — As many as one in 10 Americans will have kidney stones, and many of them will end up in the emergency room in excruciating pain. Vicki Smith/Getty Images

Mark Byrd was 38 when he had his first kidney stones while living in Miami. The pain started in his back, like he had a slipped disc. Byrd was used to back pain, so he tried his standard routine of stretching, yoga and frequent trips to the chiropractor. But nothing helped.

Then one day he was sitting in his office chair when it felt like somebody sneaked up behind him and stabbed him in the back with a Bowie knife.

"The pain takes your breath away," says Byrd, now 48 and the veteran of two excruciating kidney stone episodes. "You're doubled over and thinking, 'Oh my God, I need to get to the hospital right now.'"

What Are Kidney Stones?

Every year, half a million people like Byrd visit the emergency rooms in the United States with kidney stones, and around one in 10 Americans will have a kidney stone at some point in their life.

You have two kidneys, and their main job is to filter waste products out of the blood. Normally, those waste products are flushed out of the kidney as urine. Your kidneys filter 50 gallons (189 liters) of blood every 24 hours and eliminate about 64 ounces (1.9 liters) of waste.

In some cases, however, there are excess waste products in the blood that aren't flushed out of the kidneys. Those leftover waste products can form tiny crystals that bunch together over time to form increasingly large "stones." Calcium oxalate stones are the most common, but there are others, too, including uric acid, struvite and cystine.

The real trouble starts when one of those stones leaves the kidney and enters the ureter, a narrow tube that transports urine from the kidney to the bladder. That's when it can feel like you've been stabbed in the back.

Why Are Kidney Stones So Painful?

Pain is a hard thing to quantify, but the discomfort of passing a kidney stone is routinely compared to childbirth. Or worse. But if you thought that the excruciating pain of kidney stones was caused by a jagged clump of crystals slowly passing through your urinary tract, you'd be wrong.

"The common belief is that the stone itself causes the pain," says Dr. Timothy Averch, a kidney stone specialist at Prisma Health Urology in Columbia, South Carolina. "Patients will frequently say, 'It must have a lot of rough edges or spikes, because this one hurts a lot.' Or it's really large and that's what hurts so badly. That actually has nothing to do with the pain."

If not the stone, what the heck is causing the searing pain? If a kidney stone is large enough, it will get stuck in the narrow passageway of the ureter, causing urine to build up behind it. With nowhere to go, the urine exerts increasing pressure on the narrow ureter and the kidney, causing the tissues to stretch like a balloon.

"And that stretching is what triggers the pain that people feel when they have kidney stones," says Averch. "You feel it in your back first and then it can radiate around the front and down to the groin. It's pretty excruciating for most."

The body has all types of nerves for sensing things like touch and temperature. The nerves in your kidneys and ureter detect distention or dilation, Averch explains, just like your intestines, bladder and bowels. An obstructed bowel is also very painful, but kidney stones are far more common.

The discomfort of kidney stones is described as "colic," since it comes in waves, says Averch. That's because the ureter uses peristalsis — involuntary, wavelike muscle movements — to move urine from the kidney to the bladder in packets or bunches. If the pathway is blocked, the pressure builds with every involuntary squeeze of the ureter. And so does the pain.

Doesn't It Also Hurt to Pass a Stone?

In both women and men, urine exits the bladder through the urethra and eventually from the body. Another common misconception, says Averch, is that the most painful part of having a kidney stone is passing it through the urethra. And that's certainly the part that Averch's male patients fear the most.

But those fears are largely unfounded, Averch says.

"The urethra, in men and women, is much wider than the ureter — almost twice the size," says Averch. "Patients will frequently say to me, 'Oh, I'll know when I pee it out.' Often, they actually don't."

Byrd, for his part, disagrees. Even though his kidney stones were internally "blasted" with shockwaves to break them into smaller particles, the largest fragments were still the size of BBs. Byrd says it felt like peeing out "shards of glass."

One of the many unpleasant surprises of Byrd's ordeal was that he had to have a stent inserted into his ureter to open the passageway from the kidney to the bladder, and the stent stayed in for two long weeks. Ureteral stents are 10- to 15-inch (25- to 38-centimeter) plastic tubes inserted through the urethra all the way up to the kidney to let urine flow to the bladder again.

In Byrd's case, and many kidney stone removals, it was used keep the ureter from becoming obstructed again by postoperative swelling. Insertion of the stent is done under heavy sedation, but removal isn't.

"The nurses try to use the element of surprise," says Byrd. "They pull it out like a ripcord."