Over the past 20 years sport diving has become extremely popular, both at home and abroad.

However, diving is not without its dangers and it is vitally important that divers become suitably qualified to undertake any proposed dives.

Qualifications can be obtained by joining a club and undergoing a training course or by attending a recognised diving school for training, and subsequently making sure that you keep your skills up to date.

Diving must be planned and carried out in a responsible manner, making sure that first aid equipment and relevant telephone numbers are at hand should an accident take place.

Finally, it is important to know the signs of decompression sickness and to be able to give first aid to an affected diver.

What is decompression sickness?

Decompression sickness, also called the bends, is caused by nitrogen bubbles forming in the bloodstream and tissues of the body. The bubbles occur if you move from deep water towards the surface (where the surrounding pressure is lower) in too short a space of time.

Symptoms occur soon after the dive has finished and, in the most serious cases, it can lead to unconsciousness or death.

If you suspect decompression sickness, stop the dive, initiate first aid, and summon assistance from a specialist in divers' medicine. Treatment is 100 per cent oxygen on site and during transportation, followed by treatment in a decompression chamber.

What are the symptoms?

The symptoms of decompression sickness vary because the nitrogen bubbles can form in different parts of the body.

The diver may complain of headache or vertigo, unusual tiredness or fatigue. He or she may have a rash, pain in one or more joints, tingling in the arms or legs, muscular weakness or paralysis. Less often, breathing difficulties, shock, unconsciousness or death may be seen.

The symptoms generally appear in a relatively short period after completing the dive. Almost 50 per cent of divers develop symptoms within the first hour after the dive, 90 per cent within six hours and 98 per cent within the first 24 hours.

In practice this means symptoms that appear more than 24 hours after the dive are probably not decompression sickness.

An exception is if the diver has travelled in an aircraft or has been travelling in the mountains. Under these circumstances, low pressure can still trigger decompression sickness more than 24 hours after the last dive. As a result, it is wise not to fly within 24 hours of a deep dive.

Why does it happen?

Nitrogen makes up 79 per cent of the air we breathe (in the air around us and in our diving bottles). During a dive, large amounts of nitrogen are taken into the body's tissues. This is because the diver is breathing air at a higher pressure than if they were at the surface.

The quantities of dissolved nitrogen depend on the depth and duration of the dive. The deeper and longer the dive, the more nitrogen is taken up by the body. This does not present a problem as long as the diver remains under pressure.

As the diver begins to ascend to the surface, the surrounding pressure falls, and nitrogen is released from the body via the lungs when the diver breathes out.

If the rate of ascent exceeds that at which nitrogen can be released, it forms bubbles in the blood and tissues (similar to opening a bottle of fizzy drink too quickly).

To minimise the risk of bubbles forming and divers developing decompression sickness, various tables have been drawn up that show the relationship between a given depth of water and the time a diver can stay down.

Decompression Tables

In addition, divers are advised to make a safety stop every 5m, and not to ascend at a pace of more than 10m a minute. If the dive has been deep or of long duration, it may be necessary to stop one or more times on the way up, making so-called decompression stops.

However, following the advice of the tables is no guarantee of avoiding decompression sickness. This is because the risk of developing decompression sickness is not only determined by the depth and length of the dive, but also by any safety/decompression stops. Factors such as cold, current, exertion and lack of fluid also play a part.

Personal characteristics such as age, sex, percentage of body fat and physical condition must also be considered. Women are more at risk of decompression sickness than men. Similarly, the risk becomes greater the older the diver and also depends on the level of physical fitness.

How is it diagnosed?

In most cases, the diving history (ie information on the number of dives, diving depth, dive time, rate of ascent and decompressions) as well as information on contributory factors such as cold, current, work and the diver's physical condition will give some indication as to whether it could be decompression sickness.

After a thorough examination, which includes investigating balance, coordination, sense of touch, reflexes and muscular strength, the doctor can build up a complete picture to evaluate whether decompression sickness is likely.

The doctor will also decide if the diver requires treatment in a decompression chamber (also called a hyperbaric or recompression chamber).

What measures can be taken to avoid decompression sickness?

• Dive within the limits set out in the diving tables.
• Keep your rate of ascent to a maximum 10m/min.
• Don't plan any dives that need a decompression stop in the water.
• Make a 3 minute safety stop at a depth of 5m, (or 1 minute at 3m).
• Don't dive more than three times in one day.
• If you plan more than one dive in one day, start by making the deepest dive first.
• If you are diving for several days in a row, have a dive-free day after two to three days.
• Don't do any hard work before or after diving.
• Drink lots of liquid before diving. Lack of fluid due to heat or excess alcohol is dangerous.
• Make sure you are in good physical condition and well rested. Have regular medical checkups.
• Make sure there is an interval of at least 24 hours between diving and travel by air or climbing up mountains. If you have had decompression treatment, the recommended interval before the next dive is at least 48 hours.

Recovery after decompression sickness

Mild forms of decompression sickness can resolve themselves without treatment or by breathing 100 per cent oxygen at the site of the accident.

However, if there is any suspicion of decompression sickness, the diver must be examined by a doctor. This is because although it might not seem serious at the time, the condition may deteriorate.

If the diver receives treatment at an early stage, the chances of avoiding permanent injury are good. The longer that treatment is delayed, the greater the risk of serious consequences.

You should take a rest from diving after treatment for decompression sickness. The length of this rest depends on the severity of the decompression sickness and the effects of treatment, and should be discussed with a specialist in divers' medicine.

How is decompression sickness treated?

There is no medicine that is used as a matter of routine in treating decompression sickness.

At the dive site and during transport

• Administer 100 % oxygen.
• Give diver plenty of fluids if conscious.
• Give first aid if appropriate.
• Prevent the casualty from exerting himself or getting cold.

In hospital and specialised centres

A decompression chamber is a steel tank that can be pressurised.

There are decompression chambers in various places in the UK - some of these are situated at naval centres.

The pressure in a decompression chamber is increased by closing the doors and pumping air in.

During treatment for decompression sickness, pressure is increased to correspond to the pressure found 18m under water. In some cases, the pressure in the chamber is set at 50m.

The casualty breathes pure oxygen through a mask, which improves exhalation of nitrogen.

At depths in excess of 18m, and also after adequate intervals, the mask can be removed in the chamber. Pressure in the chamber is reduced gradually until the diver reaches surface pressure again.

Treatment typically lasts between five and six hours.

Throughout treatment a specially trained helper stays with the diver in the chamber. The diver's condition is closely monitored by further examination of coordination and balance, sense of touch, etc.

If necessary, the diver's medical specialist can join the diver in the chamber, but otherwise takes charge of the treatment outside the chamber in co-operation with the specially trained helper.

After treatment, the diver will be kept for 24 hours for observation in case his condition deteriorates.

In most instances one course of treatment is adequate, but occasionally several treatments may be needed.

After treatment for decompression sickness, a diver should take a rest from diving. The length of this rest should be discussed with a specialist in divers' medicine.