In general, unconsciousness is a state of cerebral dysfunction due to poor oxygen or glucose reaching the brain cells.

Less commonly, toxic effects of metabolites, gases and drugs can also lead to loss of consciousness.

To know roughly the possible cause of unconsciousness in a recovering scuba diver, you have to answer the following questions:

1. When did the diver loose his/her consciousness? During the dive? During ascent? Immediately after the dive or sometime after the dive?

2. Any symptoms that the diver developed before losing consciousness?

3. Sudden loss of consciousness, or gradual onset?

4. Any environmental hazards?

5. Equipment used (closed, semi closed or open circuit), and maintenance condition?

As the subject is very big with many unexplained cases of unconscious divers, we will consider here the most common causes.

Hypoxia

It is the most common cause in closed and semiclosed circuit equipment. Unconsciousness can happen during any phase of the dive, either with a gradual onset if it is due to a slow gas flow rate or can happen suddenly when oxygen is depleted in the breathing apparatus.

Oxygen toxicity

 It can occur suddenly due to a used faulty mixture at the wrong depth, a well known example is oxygen toxicity on breathing the inappropriate nitrox mixture at depth. Prodromal symptoms can be experienced by the diver as muscle twitches, tinnitus or narrowing of the field of vision (tubular vision).

Hypercapnia ( carbon dioxide accumulation )

Is unlikely to cause a sudden loss of consciousness, and if it happens, it will be preceded by prodromal symptoms such as dizziness, bitemporal headache, confusion, and a sense of breathlessness. (Some of these symptoms are experienced by divers who skip breathing to improve their air consumption!).

Retaining carbon dioxide can happen when effort is exerted at depth,

(E.g. installing buoys, lifting objects etc,) or faulty equipment that impose resistance on expiration.

 This can lead to sudden unconsciousness due to carbon dioxide toxicity either alone or combined with oxygen toxicity as the susceptibility to oxygen toxicity increases with hypercapnia.

Pulmonary barotruma with cerebral arterial gas embolism (CAGE)

It is the first diagnosis in a diver who ascends to the surface and rapidly looses consciousness. It can happen with all lung overexpansion injuries due either to faulty or nonexperienced divers holding their breath during ascent or with minimal air trapping in chronic smokers and patients of obstructive lung disease (asthma and bronchitis).

Serious decompression sickness (DCS)

Rapid rate of ascents from deep dives or inadequate decompression time especially with repetitive decompression dives can cause neurological DCS which is inert gas bubbles formation in the central nervous system which are namely the brain and the spinal cord). A common inborn heart condition known as patent foramen oval (PFO) which is a hole between both atria of the heart can under some conditions cause a neurological DCS case to develop into a serious case of CAGE when the inert gas bubbles formed on the venous side of the circulation (right atrium) to pass to the arterial side (left atrium). Brain DCS being more serious and faster to develop than spinal DCS and is often associated with loss of consciousness. 

Other important causes

Mechanical hypoxia when oxygen supply to the brain is interfered with as in cases of a tight wet suit collar squeezing the main arteries supplying the brain especially upon exertion. Cardiac dysfunction caused by over exertion (diving against current), cold anxiety, and some antihypertensive drugs. Hypoglycemia (low blood sugar) due to physical exertion, fasting, or antidiabetic medication with inadequate carbohydrate intake.

Management

1. Support respiration and circulation.

2. Administer 100% oxygen.

3. Rapid recompression therapy in a case of CAGE or severe DCS

Prognosis

* Cases of oxygen toxicity and hypoxia due to faulty equipment and cases of carbon dioxide toxicity, victims usually recover quickly when they return to the surface.

*Delayed consciousness recovery suggests underlying serious illness requiring prompt both intensive care and diving medicine consultation.

                                                               Wishing you all safe dives

                                                                    Dr. Hanaa M. Nessim