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Loss of Consciousness

When diving, it is essential to prevent the loss of consciousness underwater at all costs. Learn what causes to look out for in order to manage and minimise the risk of incapacitation.
Humans are not designed for living underwater. Diving is, and remains, a very unnatural activity. The only way to engage in, and return safely from, prolonged submersion is by learning and following certain procedures and by mastering the use of relatively complex equipment. All this requires mental alertness and agility in order to adapt to a hostile, alien environment; the physical ability to react quickly and appropriately to potentially life-threatening emergencies; and training to guide the process by means of over-rehearsed procedures and effective patterns of action. From the classroom to confined water, and ultimately to the open sea, there is a continuum of learning and response that requires both intelligence and constant situational awareness. This is exemplified by the golden rule which is taught in virtually all diving training programmes: stop, think, breathe, plan and act. To do so, a diver must be in complete control of both body and mind.
It stands to reason, therefore, that any condition that may impair one’s attention, awareness, or the proper evaluation of risk while underwater, let alone consciousness itself, may have catastrophic results and the prospect of death by drowning. One could even say that the loss of consciousness underwater (LOCU) equals death, unless the person is very lucky indeed. This could also apply to situations where effective decision-making is lost, which is a state of functional unconsciousness leading to impulsive, instinctive and ultimately involuntary actions by the diver. This cascade of catastrophe is usually described as panic. Typically, the diver will eventually make a frantic ascent to the surface in search of air. They forget that in scuba diving there is a better, safer solution which is available from their buddy. Unfortunately, this does not always materialise, either due to an irrepressible urge to surface, or due to a buddy system failure. The latter may be the result of separation by physical distance, lack of mutual awareness, or abortive efforts at air-sharing. The net result is a familiar acceleration to the surface where barotrauma, decompression illness (DCI) and death may await them. Of those who reach the surface, many lose consciousness shortly before or after their arrival, only to sink back to the bottom, unwitnessed. Here, they are usually only discovered much later when they are well beyond any hope of recovery.

To illustrate the very real, practical ways in which mental impairment may affect a diver’s safety, listed below are some real-life examples:
  • A diver suffering from petit mal epilepsy, leading to moments of mental absence, performed a dive to 30 m. As a result of an episode of mental absence, the diver omitted several minutes of decompression and made an uncontrolled ascent due to an over-inflated buoyancy compensator, of which he was oblivious at the time. The result was that he suffered serious DCI.
  • Another diver, suffering from diabetes for which he was taking insulin, developed a seizure underwater due to low blood sugar. Miraculously, the diver was rescued and taken to the surface, but because he had not disclosed the fact that he was a diabetic for fear of being precluded from diving, this was not suspected as a cause. The diver was taken to a chamber for presumed DCI and suffered permanent brain damage as a result of his prolonged and uncorrected blood sugar level.
  • A double fatality resulted from a serious case of nitrogen narcosis. Two divers performed an air dive to 66 m. One became completely disorientated and started swimming toward the ocean floor. The other, also presumably affected by nitrogen narcosis, tried to retrieve his buddy. Both lost consciousness and drowned with more than 70 bar left in their scuba diving cylinders.
  • Another fatality occurred recently in a rebreather diver. As a result of a relatively rapid ascent from 90 m, the diver experienced dilution hypoxia at 18 m. The gas he was breathing at depth became unable to sustain his consciousness closer to the surface, and the device was unable to adequately enrich the breathing mixture with oxygen due to an overinflated counter-lung. Hours later, he was discovered dead on the ocean floor. 
These are but some examples to illustrate that loss or even a temporary lapse in consciousness underwater may result in injury or death. Importantly, unlike unconsciousness on land, where we are at least able to continue breathing, water does not afford us the same luxury.

Amongst the many possible causes for the LOCU, the following are the most likely and frequent:

H - Hypos and hypers (lows and highs): Hypoglycaemia, hypotension, hypoxia,               hypocarbia, hypercarbia or high pressure nervous syndrome (HPNS);
E - Epilepsy or electrolyte disturbance;
A - Apoplexy (stroke), arrhythmias or animals (hazardous aquatic animal injuries); and
D - DCI, drugs or damage.
Hypos and hypers
  • Hypoglycaemia (low blood sugar) is a possible complication of insufficiently controlled diabetes. Refer to
  • Hypotension (low blood pressure) can lead to the insufficient circulation and oxygenation of the brain, with sudden LOCU. This is unlikely during immersion as the increased oxygen partial pressure (PO2) in the breathing gas at depth and the anti-gravity effect of immersion usually prevent blood from “going to the feet”. However, a hypotensive individual may suffer from the consequences of low blood pressure when emerging from or leaving the water.
  • Hypoxia (low oxygen concentration or PO2) leads to sudden blackout and can occur during ascent from a deep freedive, due to the malfunction in a rebreather or due to accidental gas switching during technical diving.
  • Hypercarbia (high carbon dioxide [CO2]) may occur with excessive skip-breathing or as a side-effect of insufficient gas exchange at depth due to greater gas density and is possibly combined with nitrogen narcosis. Eventual consequences may include precipitating panic or LOCU.
  • Hypocarbia (low CO2) may be caused by excessive ventilation due to panic and, at its extremes, can lead to the possible LOCU. Hypocarbia is infrequent underwater.
  • HPNS is rare and associated with deep commercial or military diving, but it may also affect the modern extreme recreational technical divers at depths in excess of 150 m. Its extreme results are tremors of the hands, muscle jerking, nausea, vomiting, dizziness, hallucinations, bouts of somnolence and the deterioration of mental and motor performance.
Epilepsy and electrolyte disturbance
  • Epilepsy and seizures in general (due to oxygen toxicity, hypoglycaemia or many other causes) result in the LOCU.
  • Electrolytes refer to the various salts and ions in our blood and body fluids that determine the proper functioning of our cells. Illnesses cause changes in body salts or the electrolytes in the body. Our bodies are particularly vulnerable to changes in sodium, potassium and calcium concentrations, each of which can lead to LOCU situations, frequently complicated by cardiac dysfunction. Diseases, conditions and even medication that may cause changes in electrolyte equilibrium should be carefully considered when assessing fitness to dive.
Apoplexy, arrhythmias and animals
  • Apoplexy is a term that is used to indicate sudden cerebral vascular accidents, such as a stroke, which cause immediate and serious cerebral consequences. Although loss of consciousness during a stroke is relatively rare, it may occur. More commonly, sudden weakness or paralysis from a stroke may jeopardise a diver’s safety. Individuals who are at a high risk for cerebrovascular incidents should seriously consider their fitness to dive. They should also consider the safety implications for their buddies who would be expected to respond to such an emergency.
  • Arrhythmias are alterations of the cardiac rhythm (heart rate). Some may cause unpleasant sensations of a pounding pulse, anxiety and even panic. Occasionally, certain arrhythmias may result in a partial or complete loss of consciousness. Any condition affecting the heart rate or rhythm should be evaluated thoroughly prior to diving. Some slow heart-rate-related arrhythmias are innocent and may actually indicate superior cardiovascular fitness. Such slow heart rates are quite common in highly-trained individuals or runners. However, a family history of sudden cardiac deaths in young individuals should prompt investigations for inherited heart muscle or cardiovascular risk factors. A dramatic slowing down of the heart rate may also occur due to the stimulation of certain areas within the body such as the neck below the angle of the jaw and behind the eyes. Manual pressure or tight-fitting equipment may cause extremely slow heart rates with the LOCU and even cardiac arrest in particularly susceptible individuals (vasovagal reactions). These individuals are normally disqualified from diving.
  • Animal lesions can cause immediate strong pain, fear reaction, panic and even rapid LOCU due to serious trauma (like sharks or stingrays), envenomation (like jellyfishes, sea snakes or stonefishes) or anaphylaxis, which is a severe allergic reaction.
Decompression illness, drugs and damage or trauma
  • DCI, particularly in the form of cerebral gas embolism, can cause loss of consciousness both during ascent and at the surface.
  • Drugs, such as sedatives or recreational drugs, can cause a loss of mental acuity, drowsiness, the enhancement of nitrogen narcosis, and even the LOCU when used during or in close association with diving activities.
  • Damage, referring to physical damage such as head trauma (like bumping the head on the bottom of a boat) even when not immediately causing the LOCU, can precipitate anxiety, panic, loss of control, uncontrolled ascent, pulmonary barotrauma, DCI or drowning.

Apart from the obvious causes listed previously, there are many other conditions that may pose a risk of the LOCU. A previous head trauma with loss of consciousness or memory for more than 30 minutes carries a significant risk of epilepsy, even months later. Similar risks are incurred following brain surgery. Even minor head injuries should prompt a delay of six weeks before returning to diving.

Epilepsy is generally considered an absolute contraindication to diving. While underwater, a diver may be exposed to possible triggering stimuli for seizures, including glare, flickering lights, sensory deprivation, hyperventilation and an increased oxygen partial pressure. Having a convulsion underwater often involves breath-holding during the fit, making pulmonary barotrauma a high risk, as well as drowning.

These are some existing recommendations on the subject:
  • British Sub-Aqua Club (BSAC) medical committee: “An epileptic can be permitted to dive after five years free from fits and off medication. Where the fits were exclusively nocturnal, this can be reduced to three years.”
  • Underwater and Hyperbaric Medical Society diving committee: “Individuals with epilepsy, who have been seizure-free for five years and take no medication, who choose to dive should be advised to avoid hyperventilation and cautioned that elevated pressures of oxygen may precipitate seizures. Individuals with controlled epilepsy (taking medication and seizure-free for two years) are advised not to dive.”
Even individuals who may meet these recommendations should bear in mind that they may still be at a significantly high risk for developing seizures. They should advise their buddies to this effect and keep within close contact during diving.

Diabetic divers who are well controlled can usually dive without seriously increased risks. However, they should bear in mind that they remain at risk for hypoglycaemia and therefore the LOCU. Attention should be given to minimising the risk, for example, by eating before diving.

Second only to the loss of consciousness, panic remains the most common cause of diving fatalities. In a review of 12 149 recreational divers by Dr David Colvard consisting of
2 916 females and 9 233 males, he discovered that 57% of the females and 45% of themales who reported an episode of panic underwater had experienced prior episodes of panic on land. He concluded that a history of prior panic corresponded to a two-fold increased relative risk of panic while diving.

When providing emergency first aid or assistance, always remember that the first attention should go to the safety of the rescuer. It is undesirable to increase the number of victims by adding the rescuer to the list. Yet, the urge to help often blinds rescuers to the dangers and this also becomes a form of functional loss of consciousness. A frequently-ignored factor is the risk of developing DCI. In fact, it is not uncommon to end up treating the rescuer for DCI after a heroic retrieval of their dead buddy from the ocean floor. While contentious in all its aspects, a victim found on the ocean floor after an unwitnessed descent, with the regulator out of the mouth, is likely to be beyond the hope of recovery. In this situation, the victim, if this can be undertaken safely, should probably be allowed to make an independent, buoyant ascent to the surface. Otherwise, the body should be attached to a buoy line or have the air removed from the buoyancy compensator (BC) to limit drifting and thereby improving the chances of recovery during a subsequent search. Clearly, this situation is different to being confronted by a breathing but anxious or incapacitated diver. For the rescuer this then becomes a moral decision. The rescuer will risk their health for the sake of another’s life – risk today or guilt tomorrow. For the victim, the risk of developing DCI is certainly more acceptable than the prospect of death by drowning.

Loss of consciousness, whether real or functional, remains one of the biggest threats to a diver’s safety underwater. It is incumbent upon instructors and dive leaders to be attuned to these risks. They need to be vigilant and wary of those divers who may be particularly susceptible and to identify them in advance and, hopefully, to protect them from harm.

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Russ - January 23rd, 2017 at 10:08am

LOCU; I enjoyed reading this article very much. Dr Cronje diplomatically avoided another cause of LOCU, i.e., Stupidity in the case of the diabetic diver, and again in the case of the divers who did an air dive to 66 m (I'm assuming they were recreational divers), which blows the bottom out of all standard dive tables. The individual with diabetes was not mature or considerate enough to realise that he posed a serious threat to other divers in his group; this also makes a good argument for mandatory medical examinations prior to a diving course. With the growing popularity of Free Diving and Spearfishing, I wonder if Dan could promote an article on LOCU specific to these dive activities as well. Thank you


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