When you get the bends, nitrogen gas that dissolved into your tissues under pressure forms bubbles as you ascend too quickly. Those bubbles can lodge in joints, muscles, blood vessels, the spinal cord, and even the brain, triggering symptoms that range from deep joint pain to paralysis. The severity depends on where the bubbles form, how many there are, and how fast you surface.
How Bubbles Form in Your Body
As you descend underwater or enter any high-pressure environment, the increased pressure forces nitrogen from your breathing gas to dissolve into your blood and tissues. The deeper you go and the longer you stay, the more nitrogen saturates your body. This is normal and painless.
The problem starts when you come back up. As outside pressure drops, that dissolved nitrogen wants to become gas again. If you ascend slowly enough, the nitrogen travels through your bloodstream to your lungs, where you simply exhale it. But if pressure drops too fast, the nitrogen comes out of solution faster than your body can clear it. The result is supersaturation: your tissues contain more dissolved gas than the surrounding pressure can hold, and bubbles begin to form.
Think of it like opening a bottle of carbonated water. While the cap is on, the CO₂ stays dissolved under pressure. Crack the cap and the pressure drops, and bubbles appear throughout the liquid. Your tissues behave the same way during a rapid ascent. Bubbles tend to form around tiny imperfections in tissue surfaces rather than spontaneously in open fluid, which is why certain body structures are more vulnerable than others.
Joint and Muscle Pain
The most common symptom is a deep, aching pain in the large joints, particularly the shoulders, elbows, hips, and knees. This is the symptom that gave decompression sickness its nickname: divers instinctively bend over or curl around the painful joint.
Nitrogen bubbles form in the tissues surrounding joints and in muscles and tendons, where they trigger inflammation and swelling. The pain typically starts as a mild ache and builds in intensity over minutes to hours. Moving the joint doesn’t usually make it worse, but holding the limb in certain positions can offer some relief. This type of decompression sickness, sometimes called Type 1, is the least dangerous form, though it still requires treatment.
Skin Changes
Some people develop itching, mild rashes, or a distinctive mottled skin pattern called cutis marmorata. This pattern looks like a red-bluish “geographical map” spread across the skin, typically appearing 30 minutes to an hour after surfacing. The patches are flat (not raised), often itchy or tender to touch, and spread rapidly.
Skin mottling was once considered a minor symptom, but research has shifted that view significantly. Studies have found that nearly 100 percent of divers who develop cutis marmorata have a patent foramen ovale (PFO), a small hole between the upper chambers of the heart that about one in four people are born with. The current thinking is that this skin pattern actually reflects gas bubbles reaching the brainstem through that heart defect and disrupting the nervous system’s control of blood vessel dilation in the skin. That makes it a neurological sign, not just a cosmetic one, and it’s now treated as a serious warning.
Neurological Symptoms
When gas bubbles affect the spinal cord, brain, or peripheral nerves, the consequences are far more serious. These Type 2 symptoms can include numbness, tingling, muscle weakness, difficulty walking, loss of coordination, bladder control problems, confusion, and in severe cases, paralysis. The spinal cord is especially vulnerable because of its blood supply, and spinal decompression sickness is one of the most common serious presentations.
Inner ear involvement produces its own cluster of symptoms: ringing in the ears, hearing loss, intense vertigo, nausea, and vomiting. Roughly 80 percent of divers who develop isolated inner ear symptoms after diving have a large PFO, suggesting that bubbles crossed from the venous side of the circulation to the arterial side through that heart defect and traveled directly to the inner ear.
The Chokes
The rarest and most dangerous form involves the lungs. Known as “the chokes,” this happens when a massive number of gas bubbles flood the blood vessels in the lungs. Symptoms include a dry cough, chest pain behind the breastbone, and difficulty breathing, caused by fluid accumulating in the lungs. If enough bubbles overwhelm the pulmonary circulation, it can lead to circulatory collapse and death. This is uncommon but represents a true emergency.
Why Some Divers Are More Vulnerable
The overall risk of decompression sickness in recreational divers is about 3.6 cases per 10,000 dives. That’s relatively low, but certain factors raise the odds considerably.
Having a PFO is the biggest known risk multiplier. Divers with a PFO have 2.5 times the overall risk of decompression sickness and four times the risk of the neurological form specifically. Even with that increased risk, the absolute numbers remain fairly small: roughly 4.7 neurological cases per 10,000 dives in people with a PFO. Most people with a PFO never know they have one unless they run into trouble.
Other factors that increase risk include dehydration, fatigue, cold water, rapid ascent rates, deep or long dives, and flying too soon after diving. The recommended ascent rate is no faster than 10 meters (about 30 feet) per minute, slowing to 6 meters per minute near the surface and between safety stops.
How the Bends Are Treated
The standard treatment is recompression in a hyperbaric chamber. You’re placed inside a sealed chamber where the pressure is increased to simulate being back underwater, typically to the equivalent of 60 feet of seawater. At that pressure, the gas bubbles shrink and redissolve into your tissues. You breathe pure oxygen, which speeds nitrogen removal and delivers more oxygen to damaged tissues. The pressure is then reduced very gradually, following strict timetables that allow the gas to leave your body safely this time.
A standard treatment session takes several hours. The protocol can be extended with additional breathing periods at different pressures if symptoms persist. Some cases require multiple sessions over several days. The sooner treatment begins after symptoms appear, the better the outcomes. Breathing high-concentration oxygen on the surface while waiting for a chamber is a critical first step.
Recovery and Long-Term Effects
Recovery depends almost entirely on which type of decompression sickness you had. For joint pain or skin-only symptoms with uncomplicated recovery, the recommended minimum before returning to diving is 7 days after treatment. If symptoms came back and required a second round of recompression, that extends to 14 days.
Neurological cases take much longer. Mild neurological involvement without motor, visual, or cognitive effects requires at least 28 days off and clearance from a diving medicine specialist. Serious neurological symptoms, including the chokes, vestibular damage, or motor dysfunction, call for a minimum of 3 months. Cases that needed multiple treatments or left residual symptoms are evaluated individually, with three months as a general floor.
Pain-only decompression sickness rarely leaves lasting damage, though it may slightly increase the risk of dysbaric osteonecrosis, a condition where bone tissue deteriorates in areas that were affected. Neurological decompression sickness, on the other hand, can cause permanent injury. Divers who have residual neurological deficits from a previous episode may have less capacity to recover from a future one, which is why specialists are cautious about return-to-diving decisions after serious cases.