In compressed gas diving, DCS happens if:

1-The elimination of the inert gas cannot adequately parallel the rate of reduction of external pressure (as in the case of fast ascents).
2-The amount of residual inert gas at the end of a particular dive (or dives) is high enough to create bubbles - supersaturation. (as in the case of missing a decompression stop).
3-Accumulation of the inert gas in the body due to repeated diving for a few days (inert gas load)

DCS is differentiated into 2 types:

Type I DCS can be presented in one of these forms:

1- Extreme fatigue.
2- Skin rash
3-Joint pain (limb bend or musculoskeletal pain-only symptoms)
4- Lymphatic Symptoms (where localized swelling and pain occurs in the area drained by involved lymph glands).

Type II DCS

Where the nervous system, the inner ear or cardio-pulmonary systems are involved producing serious signs and symptoms.
DCS can present itself in a variety of symptoms and signs, which pose diagnostic problems for the non-experienced officer. Amongst these symptoms:

Skin rash - Joint pain - Back pain - Itching - Dizziness - Chest pain - Girdle pain - Paralysis - Hearing problems like ringing in the ears - Tingling - Urination difficulty - Memory changes - Nausea, vomiting - Extreme fatigue - Visual disturbances - Balance problems - Speech problems - Shortness of breath - Cognitive deterioration.

It is important to establish whether symptoms are diving related, and a good procedure is to:
1. Refer back to the dive profiles of the last few dives (depth and exposure times), and consider any unusual events happening during these dives like: emergency or fast ascents, omitted stops, YO -YO like profiles, reversed profile(s), multiple ascents, cold temperatures or strenuous exposure etc.
2. Pay attention to Inert Gas Load (in case of repeated diving days).
3. Overview the individual factors involved such as:

sometimes misdiagnosed as DCS.
So encourage your divers to drink rehydration salts together with water.
II. Age: altered blood flow due to atherosclerosis associated with age increases the risk of DCS.
III. Sex: some literature suggests an increased incidence in females due to the redistribution of the body fluids due to hormonal changes during different stages of menstruation, but it is still under study.
IV. Obesity: is a point of controversy:
Some authors accuse fat tissue of being a contributing factor (having a great affinity to nitrogen thus increasing
the nitrogen load in case of repeat diving days), whilst others believe in its action as a good reservoir for nitrogen in cases of short, deep single exposures.
V. High Serum Cholesterol levels can enhance bubble formation by rendering the blood more viscous.
VI. Improper training of equipment usage (mainly the BCD).
VII. Overconfident attitude in a poorly educated or a well trained diver.
VIII. Any underlying condition that hampers the anatomy of the body (like scarring), can form a bubble trap due to the alteration in local tissue perfusion and gas elimination.
E.g. a previous joint, spine or brain injury or affection, also any previous episode of DCI.
IX. Increased carbon dioxide tension due to exertion or increased breathing resistance because of faulty equipment, which increase nitrogen loading.