Decompression Sickness (DCS): Causes, Symptoms, Prevention

What decompression sickness is

Decompression sickness (DCS), the bends, is an injury caused by dissolved inert gas, mostly nitrogen, coming out of solution as bubbles when you ascend. It's also called caisson disease, after the 19th-century bridge and tunnel workers who first suffered it coming up from pressurised caissons, long before anyone was diving for fun.

DCS is one half of a wider umbrella called decompression illness (DCI). The other half is arterial gas embolism, where gas from a burst lung is forced into the arteries. They arrive by different routes but get treated the same way, urgently, which is why the umbrella term is useful.

At depth, the pressure around you forces extra gas to dissolve into your blood and tissues. That's fine while you stay down. The danger is the ascent: if pressure drops faster than your tissues can release the gas, it comes out as bubbles instead of staying dissolved.

Why it happens

Every tissue absorbs and releases gas at its own rate. Your computer tracks this with the Bühlmann model and limits how much supersaturation you carry using gradient factors. Push past that limit, usually by ascending too fast or skipping stops, and bubbles form.

Here's the part that surprises people: bubbles are normal. Almost every dive sheds small "silent" bubbles into the venous blood (venous gas emboli), and your lungs filter most of them out harmlessly. They're graded 0 to 4 on Doppler (more on bubbles here). DCS is what happens when there are too many, or when they end up somewhere they shouldn't, like across a PFO into the arteries.

When bubbles do cause trouble, they do it two ways at once: mechanically, by blocking small vessels and pressing on tissue, and chemically, by triggering inflammation along the vessel lining. Where they lodge decides how bad it gets.

Type I vs Type II

Clinicians split DCS into two buckets, and the difference matters because it changes how urgently you act.

Type I, in more detail

• Musculoskeletal ("pain-only" bends): a deep, dull ache, classically in a shoulder or elbow, that's hard to pin to one spot and doesn't change much when you move the joint. The most common presentation.
• Skin (cutaneous) DCS: itching, or a mottled, marbled purple rash called cutis marmorata. It looks minor, but a marbled rash can be the visible edge of a heavy bubble load, so it's a reason to watch closely, not shrug off.
• Lymphatic: localised swelling and a heavy, puffy feeling as bubbles block lymph drainage.
• Unusual fatigue: wipe-out tiredness beyond what the dive should cause. Normal post-dive tiredness is covered in post-dive fatigue; the concern is when it's out of proportion or comes with anything else on this list.

Type II, in more detail

• Spinal cord DCS: the most common serious form in divers. Low-back or girdling pain, then numbness, tingling, leg weakness, an unsteady walk, and sometimes bladder trouble. Easy to dismiss early, dangerous to ignore.
• Cerebral DCS: headache, confusion, personality change, visual disturbance, or one-sided weakness.
• Inner-ear (vestibular) DCS: vertigo, nausea, ringing, or hearing loss. More common after deeper, mixed-gas dives and around helium-to-nitrox switches. It can be mistaken for inner-ear barotrauma, which needs different treatment.
• Cardiopulmonary ("the chokes"): chest pain, a dry cough, and breathlessness when a large bubble load hits the lungs. Rare, but a genuine emergency.

Symptoms and when they show

• Joint or limb pain, often the shoulder or elbow
• Unusual, heavy fatigue out of proportion to the dive
• Skin itching or a marbled (cutis marmorata) rash
• Tingling, numbness, or weakness
• Dizziness, vertigo, or hearing changes
• Difficulty walking, confusion, or trouble breathing

Onset is usually quick: around half of cases show within the first hour or two, and the large majority within 24 hours of surfacing. Delayed onset happens, and flying too soon can trigger or worsen it, which is why no-fly times exist. Anything that starts or worsens after a dive deserves suspicion, especially anything focal or neurological.

How it's diagnosed

There's no blood test or scan that confirms DCS. It's a clinical diagnosis: the doctor weighs your dive profile, the timing, and the symptoms. Because there's no confirmatory test and the cost of waiting is high, the rule is to treat on reasonable suspicion rather than hold out for certainty. The main thing to tell apart is arterial gas embolism, which comes on within minutes of surfacing with stroke-like signs, often after a breath-hold ascent. The first aid is the same either way.

First aid and treatment

• Oxygen is the single most useful field treatment. Breathing 100% O₂ steepens the gradient that pulls inert gas out of the bubbles and speeds their shrinking, and it oxygenates tissue starved by blocked vessels. Give it continuously, even if symptoms ease.
• Fluids help, since immersion and diving leave most divers mildly dehydrated, which thickens the blood around bubbles.
• Recompression is the definitive treatment: in a hyperbaric chamber the diver is taken back "down" under pressure to crush the bubbles, then slowly brought up while breathing oxygen. Serious cases usually run on US Navy Treatment Table 6.
• Recovery is good for most when treatment is prompt, but not guaranteed. Some divers need several chamber sessions, and a minority are left with residual numbness, weakness, or fatigue. Speed of recompression is the biggest lever on the outcome.

Who's most at risk

Some risk is in your hands and some isn't. The controllable factors dominate:

• Profile and ascent: fast ascents and missed stops are the biggest, most preventable causes. Deep, long, and repetitive multi-day dives stack gas load.
• Body and conditions: cold and hard exertion (especially on the bottom or during ascent), dehydration, and poor fitness all raise risk.
• Fixed factors: a right-to-left shunt such as a PFO, older age, and on recent data female sex.
• After the dive: flying or driving to altitude too soon.

The full breakdown is in predisposing factors for DCS and fit to dive.

How divers lower the risk

You can't reduce the risk to zero, only stack the odds in your favour:

• Ascend slowly and do your safety and decompression stops. Safe ascents covers flying the stop.
• Use conservative gradient factors, and watch GF99 and Surface GF on the way up.
• Hydrate, stay warm, and skip the hard workout right after a dive.
• Leave generous surface intervals, and respect no-fly times: at least 12 hours after a single no-stop dive, 18 hours or more after repetitive, multi-day, or decompression diving.
• Build aerobic fitness and dive sensible profiles, not aggressive ones.

How common, and how serious

DCS is rare in recreational diving, on the order of a few cases per 10,000 dives, and most treated cases recover fully. The risk that matters is real Type II DCS hitting the spinal cord, brain, inner ear, or cardiopulmonary system, which can leave lasting injury or, rarely, be fatal. The single biggest lever on the outcome is how fast it's recognised and recompressed, which is why prompt oxygen and a call for help beat waiting it out.

Reading your computer, planning conservative ascents, and knowing the warning signs are core skills, not trivia. Our technical and CCR training builds them properly. Get in touch and tell us where you're at.

References

1. Vann RD, Butler FK, Mitchell SJ, Moon RE. Decompression illness. Lancet. 2011;377(9760):153–164.
2. Divers Alert Network (DAN). Decompression Sickness, oxygen first aid, and flying-after-diving guidance.
3. Pollock NW, Buteau D. Updates in Decompression Illness. Emerg Med Clin North Am. 2017.
4. Moon RE (ed). Adjunctive and supportive treatment of decompression illness / UHMS guidance.
5. Bühlmann AA. Decompression–Decompression Sickness. Berlin: Springer-Verlag, 1984.