But who are my real friends?
Have they all got the bends?
Am I really sinking this low?
Radiohead - 'The Bends'
Decompression sickness (DCS) is one of the nastiest maladies that can affect the average scuba diver; its effects are multitudinous, and if you're lucky you'll die rather than be turned into a quadriplegic vegetable. Okay, so admittedly that's the worst-case scenario, but it does have a multitude of presentations, none of which are particularly pleasant, although some are better than others. It is precipitated by the inert nitrogen gas that gradually dissolves into body tissues when breathed at a greater than atmospheric pressure. If the body moves from a high pressure environment to a low pressure environment too quickly, the effect is similar to a opening a can of coke that's been put through a washing machine rinse cycle. Only this happens in your skin, muscle, joints, spinal cord, and brain. Nice, eh?
Other terms used for decompression sickness include: diver's disease, because it's most commonly seen in scuba divers; diver's paralysis, because it can cause paralysis in the aforementioned divers; caisson disease, named for the DCS seen in construction workers who'd been labouring down a caisson and experienced a rapid decompression on exiting it; and 'the bends', the origins of which are probably the fact that DCS of the joints often causes individuals to keep the affected joint bent to relieve discomfort, and workers coming up from caissons would often be bending every part of their body that they could possibly to bend to alleviate their symptoms. The slang term used by the diving community is 'getting bent'.
It is important to clarify at this point that there is a difference between decompression sickness and decompression illness (DCI): DSC is the result of nitrogen bubbles expanding within body tissues; DCI is the umbrella term used to describe both DCS and lung over-expansion injury, which are both serious potential hazards of scuba diving, and are both treated in the same way in terms of rescue and first aid.
Mechanism of absorption
Water can not be compressed so there is a linear pressure increase gradient associated with water depth: surface pressure is 1 ATM; at 10m depth pressure is equal to 2 ATM; at 20m depth is equal to 3 ATM; and so on and so forth. When someone is diving, their body is subject to the compression forces that high pressure exerts. However, as most body compartments are composed of water this has few noticeable effects at the pressures that recreational divers usually keep to - an advised maximum of 5 ATM (40m). However, gases are compressible, and so gas-filled spaces (such as your lungs, sinuses, middle ears and scuba tank) will become subject to changes in volume depending on water depth.
A scuba regulator is designed to deliver air to the diver at a pressure equal to the surrounding atmosphere. When breathing underwater, you inhale the same volume of gas that you would at the surface. However, more molecules of the gases are inhaled, as the increase in pressure means that a larger number of molecules occupy the same volume in comparison to surface pressure (similarly, oxygen canisters are required by mountain climbers because the decrease in atmospheric pressure at 8000m means there are less oxygen molecules per breath than at sea level). The increase in pressure also means that the partial pressures exerted by the oxygen, nitrogen and carbon dioxide (and others) in the mix of gases in the scuba tank are altered (Dalton's law). In combination with Henry's law, the result is the increased absorption of inert nitrogen gas by the body. The amount of nitrogen that dissolves into body tissues is dependant on the amount of pressure being exerted, and the amount of time spent at that pressure. Five minutes at a (relatively) high pressure can result in the same nitrogen saturation levels that would occur with fifty minutes at a (relatively) lower pressure.
Is this a bad thing?
Well, it's only a bad thing if you let it be a bad thing. If you dive safely, then your risk of DCS is very low. To dive safely, there are three things that you should do:
- Keep your training and experience up to date
- Never ascend at a rate in excess of 60ft / 18m per minute, preferably 33ft / 10m per minute
- Use a dive table or a dive computer
This is probably a good point to mention that it's not just divers that get DCS. It can also occur to people in an
unpressurised aircraft that flies too high; people in a plane at high altitude that
suddenly depressurises; people who fly within 24hrs of their last scuba dive; and (as with caisson disease) with people who have to work deep underground for a significant period of time.
What if I don't want to play safe?
Okay, just for shits and giggles, let just say you were on a dive, had been down for about 45 minutes at 30m, when you notice that it's getting harder to breathe. You look at your gauges, only to see that you've been a bit of a twat and run out of air. Your buddy isn't anywhere near you and you don't have a pony bottle. The only option in this situation would be an emergency rapid ascent. It doesn't take a genius to realise that this means that you're going to be going faster than 18m per minute; having had to do one myself I can tell you that when you need to, you can get to the surface from this depth in under twenty seconds. Probably faster, depending on how desperate you are for air. Because you've been so deep for such a long time, you don't stand a hope in hell's chance that you're not going to end up bent doing this.
So what happens? Well, while you were down there, the nitrogen that you were breathing in from your scuba tank was slowly but surely dissolving into the fatty tissues of your body because of the pressure; a process called saturation. Nitrogen is an inert gas, but is lipid soluble under the right pressure conditions. This means that it can be absorbed into all the areas of your body that contains fat; this is a whole variety of places, but the most important is your nervous system which is 60% pinguid. The nitrogen will only stay dissolved in your body tissues while you're at a high enough pressure to make it stay there. Remove yourself from that area of high pressure, and the nitrogen will come out of its dissolved state. Do this too quickly, and all the nitrogen is going to come out at once, forming a bubble in the tissue that it was dissolved in.
Now, the body can tolerate a certain amount of bubble formation without displaying any symptoms. The development of symptoms is dependent on several factors: the number of bubbles; their location; the presence of any other injuries (e.g., lung over-expansion injury); and on the anatomical, biological and physiological state of the diver themselves.
So, what symptoms can I expect?
Well, it's best to break this one down into the various body parts that are affects:
General - the general symptoms of DCS are weakness, apathy, tiredness, or a general feeling of malaise. The victim may also demonstrate personality and behavioural disturbances.
Cutaneous (skin) - Skin manifestations can range from being a generalised itch that quickly fades away; a vesicular rash that takes several hours to disappear; or, more seriously, a pale area of skin with a cyanotic marbling, a sign of systemic DCS.
Gastrointestinal (gut) - Mild symptoms are of anorexia, nausea, vomiting, retching, abdominal cramps and diarrhoea. Yeah, I know, those are the mild symptoms. Severe manifestations of DCS result in bowel ischaemia and infarction, with secondary haemorrhages (a piece of bowel effectively dies, which, without urgent treatment, will result in you dying.)
Musculoskeletal (joints) - This is the commonest area for DCS to present, although the mechanisms behind why this should be are not well understood. While gas can form in the joint spaces, this is not in itself painful. The pain associated with joint bends is thought to be caused by bubbles forming in the subperiosteal areas, tendons, ligaments, joint capsule, and the muscles surrounding the joint. Local pressure over the painful joint will often relieve symptoms. A crepitus may often be felt in the joint. Musculoskeletal DCS often starts as an ill-defined discomfort in a joint or muscular area, which progresses into a deep dull ache, then pain that can be sharp and throbbing in character. The shoulder is the most commonly affected joint in divers.
Cardiorespiratory (blood circulation and lungs) - Intravascular bubbles can form in the veins and arteries, which can lead to a local ischaemia in the organ that they appear in, e.g., kidneys, spleen, heart. If these bubbles form in the venous system, then they will collect in the right atrium of the heart, and then into the pulmonary venous system, where they will displace blood and block up the pulmonary vessels. The slang term used for lung involvement is 'the chokes'; the patient will experience chest pain that is worse on breathing in, an irritating cough, and an increased respiratory rate. More extreme cases can progress to circulatory collapse or right heart failure. A severe, rapid decompression (such as an emergency rapid ascent from 30m after a 45 minute dive time) can cause bubble formation in all the blood vessels of the body; this very quickly results in death.
Neurological (brain, spinal cord and peripheral nerves) - The neurological sequela of DCS is varied, with different symptoms depending on what system is affected. The shorter the time between surfacing and the onset of symptoms, the worse the prognosis.
- Cerebral: This most commonly happens in those who ascend too rapidly from a deep dive, and most commonly affects the frontal and parietal lobes. DCS can cause perivascular haemorrhages, oedema, and demyelination (stripping of the nerve's supportive cells) of nerves in the brain. Essentially, cerebral manifestations of DCS are similar to having a large number of small strokes; paralysis of the limbs, loss of vision, convulsions, sensory disturbances, migraine and confusional states. Not nice.
- Cerebellar: This is the part of the brain that co-ordinates movement, so damage results in difficulty walking, incoordination, tremor, loss of muscle tone, and difficulty speaking. It also causes one of my favourite neurological terminologies: dysdiadochokinesis. A clumsiness in performing rapidly alternating movements e.g., quickly tapping your left wrist with your right hand, alternating between the back of the hand and the palm for each tap. Try doing this when you're really drunk sometime; alcohol exerts an inhibitory effect on the cerebellum, which also why you can't walk or talk after one pint too many.
- Spinal: There may be pain occuring in the area served by a spinal root. This will then progress to a weakness or paralysis of the same area.
- Inner ear: The inner ear has a very important role in balance and coordination, and so a disturbance in its function will result in dizziness, vertigo, vomiting, tinnitus, and hearing loss ('the staggers'). The problem with this version of DCS is that it is easily mistaken for seasickness, and lack of prompt treatment can result in permanent damage.
- Vision: Bubbles can form in the lens of the eye, and in the ocular fluid within the eye; this will cause a blurring of vision. Retinal DCS can also occur, which will result in blindness; however, disturbances of vision and visual loss are usually due to cerebral DCS rather than retinal DCS.
- Peripheral nerves: This will cause a patchy sensory and / or motor disturbance in the area served by the affected peripheral nerve. It is important to distinguish this from spinal DCS, which has a more serious prognosis than peripheral nerve DCS.
Predisposing factors
Unsurprisingly, some people are more likely to experience DCS than others. Predisposing factors include:
- Exertion - those who experience a higher level of exertion while diving, e.g., swimming against a current, are more likely to experience DCS
- Physical fitness - the fitter you are, the lower your risk
- Temperature - the colder the water that you dive in, the higher the chances of DCS occuring
- Sex - females have a higher incidence of DCS than males, although the reasons for this aren't clear; it could possible be due to physiological differences, or because of social conditioning (women are less likely to have the same fitness levels of men, resistance to cold exposure, etc.) It may also be linked to the fact that women have a higher body fat percentage than men
- Age - the older you are, the higher your risks
- Obesity - the more tissue mass that you have, the more nitrogen you can absorb
- Dehydration - reduces the perfusion of body tissues, and therefore limits the rate that nitrogen can be eliminated
- Alcohol - may adversely influence judgement, but also causes dehydration
- Previous injuries - A previous joint injury or episode of DCS causes scarring and alteration to local tissue perfusion, predisposing to further episodes of DCS
- Diving itself - the deeper the dive, the higher the risk of DCS. Multiple ascents / descents in the same dive will cause a higher incidence of DCS. More than one dive in a day will increase the risk of developing DCS
Treatment
Since there is such a vast array of presentations, specific treatment will vary from case to case. It is important when diving to be ultra-alert to any unusual behaviour in any of your fellow divers, and to be ultra-conscious of any niggles that you yourself experience. DCS has a habit of impersonating a variety of illnesses. For instance, when diving abroad, it the diarrhoea that you experience at the end of a day's diving a consequence of DCS, or of the fish that you ate for lunch? Always have a high-index of suspicion and be prepared to seek medical treatment if symptoms either fail to improve or worsen.
Immediate treatment on suspecting DCS is to complete first aid measures (Airway, Breathing, Circulation) and give the patient high-flow oxygen. Whilst doing this, emergency help should be requested, such as local emergency telephone assistance, and the Divers Alert Network (DAN). When someone has DCS, the primary treatment will always be recompression. Whilst it may be tempting in an emergency situation, in-water recompression is a big no-no. It can be used as a first-aid measure in certain instances (usually by commercial divers), but it should only be done by people who know what they're doing, and even then, only as a stopgap. If you don't know what you're doing, you'll be doing the patient more of a disservice; it is best to wait for professional assistance.
The definitive treatment that is applicable to all instances of DCS is hyperbaric therapy; by placing the patient in a recompression chamber (or hyperbaric chamber) the patient can be taken 'back down' to a certain pressure level whilst breathing 100% oxygen. This results in the nitrogen bubbles in the body decreasing in size, thus improving the patient's symptoms. The patient can then be decompressed gradually, giving the nitrogen a chance to slowly diffuse out of the body tissues. Multiple hyperbaric oxygen treatments may be required, and further treatment may also be required, depending on the type of DCS.
Prevention
The most important thing to do to avoid getting bent is to ascend from depth at a safe rate (33ft / 10m per minute), to allow the nitrogen to diffuse out gradually. Depending on how long a diver spends at a certain depth, they may need to do 'decompression stops', where they stop ascending for 2-5 minutes at certain depths to allow extra time for nitrogen desaturation. These decompression stops are usually only performed by technical divers; recreational diving is designed with safety in mind, and so, in theory, a recreational diver who is properly trained, and adheres to the techniques that they have been taught should never have to do a decompression stop. However, it is advisable that all divers, technical or recreational, do a 'safety stop' at 5m for 3-5 minutes; it's not essential, but it gives you that extra margin of safety.
Dives should also always be planned using a dive table (less accurate) or a dive computer (more accurate, but also more expensive). There are different dive tables that are designed for different types of diving (military, commercial, technical and recreational), and recreational divers should use the 'no stop' decompression limit dive tables. These tables (or computers) are used to plan a dive so that a diver would know the maximum depth was that they could descend to, and how long they could spend there before having to come back up and NOT have to do a decompression stop. If you've splashed out on a dive computer, it will start beeping at you incessantly when you are approaching your limits.
To dodge being stuck in a hyperbaric chamber for months on end, you should make sure that you avoid all the relevant risk factors: don't drink alcohol for 24hrs before a dive; ensure you're adequately hydrated; maintain a good level of fitness; check the water conditions before you dive; make sure you plan the dive meticulously; don't deviate from the plan; and importantly, all females should remember to be male whilst diving.
Reference
- Edmonds C, Lowry C, Pennefather J, 1992, "Diving and Subaquatic Medicine", 3rd edition, Butterworth-Heinemann ltd