Fortnightly Review: Medical aspects of scuba diving

History of diving

People's obsession with the underwater world began long before recorded history. Diving is considered in Greek mythology. Plutarch tells of an extraordinary fishing competition, using divers, between Anthony and Cleopatra, and Pliny refers to the use of military swimmers.

As underwater endurance and depth are limited by the ability to hold breath, people have continually striven to extend their underwater capabilities by mechanical means. From the fourteenth century onwards various designs for equipment have been found, including some attributed to Leonardo da Vinci. The use of diving bells is well recorded, one of the earliest being by Alexander the Great during the siege of Tyre in about 332 BC. Edmund Halley designed and used a bell to dive in the Thames in 1690, achieving a depth of 10 fathoms (60 feet of sea water; 18 m of sea water) for about one and a half hours. Such systems were clearly the forerunners of modern bell diving systems.

In 1819 Augustus Siebe introduced the “open diving dress,” the antecedent of the well known copper helmeted “standard diver,” supplied with air under pressure from pumps operated at the surface. Developing technology improved pump performance and hence the depths that could be achieved. The apparent poor performance of military divers in the first decade of this century prompted the Admiralty to commission an investigation. Far from criticising the men, the report identified nitrogen narcosis and hypercarbia as the physiological limitations on ability.1 The investigations also described the first scientific approach to decompression modelling and provided diving tables.

The dream of being able to dive unencumbered by unwieldy equipment and not requiring air from the surface was not realised until the second world war with the development of the “human torpedo” diver by the Italian and British navies. Though considerable work of physiological importance was achieved at this time - notably the world of Donald2 - diving became possible for ordinary people only with the development of the demand valve regulator. Demand valves sense the pressure in the water and allow divers to breathe normally - on demand - by regulating the release of gas from a high pressure supply. The self contained underwater breathing apparatus (scuba) had arrived.

Underwater world

The underwater environment imposes considerable physiological burdens on those who venture into it. People who have never dived generally base their perceptions of diving on films - the James Bond type - or television programmes - those of Cousteau and others. Each reveals magnificent blue water with considerable visibility, exciting marine life, colourful flora, and the often mistaken delusion that the water is very warm. The truth is that visibility may be on more than a few metres at best and the water is far from thermoneutral.

Ideally, divers aim to be neutrally buoyant to avoid unnecessary physical exertion in the dense and viscous medium of surrounding water. In poor visibility sensory information is reduced to a minimum and amounts to a state of sensory deprivation. This carries with it a high risk of disorientation. There are few if any proprioceptive inputs to determine up from down, and no visual stimulus to fix a horizon.

Breathing apparatus imposes resistance to breathing that may result in retention of carbon dioxide. This alone may give rise to deeper, more rapid and laboured breathing in addition to the increased ventilation associated with exertion. The physical construction of the breathing apparatus may also restrict respiratory peak flows required in an emergency. Furthermore, in venturing underwater divers need to adjust rapidly to the raised pressure of the environment, most obviously by equalising the pressure difference across the tympanic membranes between the external and middle ears. Failure to achieve this, by the Valsalva manoeuvre, may result in perforated or damaged drums (aural barotrauma) and may also contribute to the sensation of disorientation through the mechanism of alternobaric vertigo.3

Though by no means comprehensive, the preceding paragraph describes the unusual environment of sports divers. The effects of pressure and immersion, the physiology of gases and respiration, and other effects of diving are described in more detail in specialist texts.^4,5 Doctors and other medical staff should be aware, however, that considerable time and effort is devoted to basic physiology in sports divers' training. Divers themselves may have extensive knowledge of the medical and physiological aspects of their sport and understand the complexities of mathematical modelling of the uptake of inert gases by tissues, as well as the theory behind decompression tables.

The application of the principles of diving medicine is of general interest. There are estimated to be more than 50 000 sports divers in the United Kingdom, ranging in age from teenagers to senior citizens. Most are members or were trained under the auspices of the major clubs - the British SubAqua Club, the SubAqua Association, and the Scottish SubAqua Club. They dive off the coast and in inland lakes and quarries, and they come from all walks of life. Most doctors will probably therefore have several divers as patients. As well as several specialists in the United Kingdom, other doctors have received training in diving medicine to fulfil their role as approved doctors under the diving operations at work regulations. Others may be advisers to the various clubs, but most general practitioners, accident and emergency officers, and other clinicians who encounter divers professionally may be unfamiliar with these principles.

Fitness to dive

All medical screening should have well defined objectives. Assessment of fitness to dive for pleasure is no exception. The primary objective should be to identify who is fit to dive. Secondary objectives should identify patients for whom a second opinion should be sought and those who are unfit to dive.

After fitness has been established and baseline information recorded the frequency and scope of subsequent examinations need to be considered. For commercial divers these factors are defined within the legislative framework of the industry. An arbitrary approach must be taken with sports and recreational divers. General guidelines for examination or screening must take into account age and its possible effects, particularly on cardiac and respiratory reserves; illness developing since the previous examination; the impact of long term drug treatment to maintain health such as for hypertension; and the consequences of a diving incident or accident, especially if the central nervous system were affected. The first box lists the factors to be taken into account. In reaching a decision it should be remembered that diving is not a solitary activity. Divers generally dive in pairs and operate a “buddy” system of care and mutual assistance. Therefore individual divers must not be a potential source of risk to their buddy, and they must be able to cope with any underwater emergency occurring to their companion. If either of these conditions cannot be met a person may reasonably be assumed to be unfit to dive.

Reduced exercise tolerance

Diving is physically demanding and participants must be fit not only to dive but also to cope with activities associated with diving, including handling a boat or heavy equipment and rescuing wounded people who might be incapable of helping themselves. Though sports divers do not need to have an olympian aerobic capacity, they need a reasonable degree of aerobic fitness, physical stamina, and mobility. Reduced exercise tolerance may be due to simple lack of fitness, obesity, cardiac or respiratory disease, and a wide range of musculoskeletal disorders. These must be viewed in context. Physical disability need not preclude sports diving, but great care must be taken in specifying the circumstances in which a disabled person may dive.

Vulnerability to immersion

Short term immersion in water affects the cardiovascular system, eliciting the diving reflex and altering venous return, for example. Longer exposures induce hormonal changes, the diuresis of immersion being the most obvious. Any reduction in ability to adapt to these changes should raise doubts about the fitness to dive. Maintaining consciousness is also fundamental to survival underwater. Conditions that may result in loss of consciousness such as epilepsy and diabetes mellitus are usually contraindications to diving. The British SubAqua Club, however, has recently relaxed its stance on diabetes allowing diabetic subjects to dive if conditions are met in terms of control of the disease, blood glucose estimations before a dive, and awareness by diving supervisors and buddies. Whether this proves workable remains to be seen.

Finally, the problems of long term immersion in water, often of dubious purity, should be borne in mind when considering the potential problems associated with disease of the external ear and skin. Skin disease may be avoided by the use of a dry suit, but allergy to rubber and occlusion in an impervious garment may exacerbate chronic skin disorders.

Vulnerability to pressure changes

Hollow and gas containing organs and spaces, principally the lungs, sinuses, and middle ear, need to be examined for vulnerability to pressure changes. Gas in these structures behaves in accordance with Boyle's law, which describes an inversely proportional relation between pressure and volume. An inability to compensate for pressure changes results in tissue damage, collectively referred to as barotrauma.

Good health and function of the lungs are crucial to safe ascent. The exclusion of any form of lung disease that may give rise to gas trapping, even with a normal breathing pattern, is important. Barotrauma of ascent is not uncommon and may result in pneumothorax, pneumomediastinum, and surgical emphysema of the neck. The life threatening cerebral arterial gas embolism may also occur, giving rise to a wide range of neurological manifestations. These will be considered in more detail later.

Vulnerability to cold

The waters around the United Kingdom range in temperature from about 4°C in winter to 15°C in late summer. Throughout the year special protective garments must be worn. Despite well designed and effective modern equipment a patient's vulnerability to cold needs to be considered as part of the medical examination. While divers are exercising they are reasonably well protected from central hypothermia so the effectiveness of peripheral blood flow needs to be determined. Subjects with peripheral vascular disease, Raynaud's disease, and previous clinically significant cold injury such as frostbite or non-freezing cold injury should be dissuaded from diving. Cold may also exacerbate or precipitate airways disease such as asthma. The effects of drugs such as β blockers on peripheral circulation should also be considered. Finally, cold induced pulmonary oedema has been described in divers.6 It is rare and may not be predicted or preventable.

Vulnerability to decompression

Vulnerability to the decompression disorders is less easily measured in a physical examination, although elderly, obese, and unfit divers may be more prone to them. Certain specific medical conditions such as patent foramen ovale have been implicated as causes of decompression illness to explain its occurrence when it may not otherwise be expected.^7,8 Although the theoretical mechanism which allows arterialisation of venous inert gas bubbles through a patent foramen ovale is plausible, the proposition remains controversial.9

Vulnerability to reduced sensory input

Abnormality of the five senses should be considered in practical terms. Hence the sense of smell and taste are comparatively unimportant functionally, whereas sight has implications for safety both above and below the surface. Normal function of the inner ear, especially the vestibular elements is essential. Hearing loss is acceptable but it must be realised by divers that additional hearing loss may occur as a manifestation of barotrauma and the decompression disorders. An intact nervous system with normal proprioception, coordination, and balance is obligatory in coping with the underwater environment.

Potential confusion with diving related illnesses

Diving illnesses have a protean range of symptoms and signs and therefore may be confused with other, perhaps more common, medical conditions. For example, the various presentations of migraine in a diver - severe headache, visual and other sensory disturbances, and possibly paretic symptoms and signs - will be a diagnostic dilemma, resolvable only when the patient undergoes recompression. Few doctors would take that option routinely, but the consequences for an untreated diver might be permanent paralysis, the need for rehabilitation, and costly long term care. For the doctor there may be medicolegal consequences. Several other conditions may, under certain circumstances, be difficult to differentiate from decompression illness. These include the porphyrias, sickle cell disease, and neurological and musculoskeletal diseases.

Contentious issues

Some conditions present diving medical authorities with considerable difficulty in relation to fitness to dive, including head injury, asthma and other respiratory disease, diabetes, and pregnancy. The risk after head injury is post-traumatic epilepsy, which might present underwater or be associated with diving. The risk reduces with time, but head injury associated with a lengthy period of unconsciousness, amnesia, and temporary neurological abnormalities should preclude diving for leisure or reward. Asthma is more difficult to assess as the diagnosis is often misused or misunderstood by doctors and patients. Patients need to have adequate respiratory function with minimal risk of gas trapping. Basic spirometric results should be considered in cases of doubt. Patients with more sensitive airways who require drugs to control their asthma are best advised not to dive. Issues unique to female divers have been reviewed.10

Decompression illness

The decompression disorders are surrounded by myth, misunderstanding, and confusion. This is not surprising as the number of patients requiring recompression annually is small, usually 150-200.11 Many more seek medical advice (T J R Francis, personal communication).

The symptoms and signs of decompression sickness were first described in 1843 in caisson workers returning to atmospheric pressure. Clinical descriptions and a classification (summarised in the box) were provided by Golding et al and Elliott et al.^12,13 To use this classification system, however, requires previous knowledge and understanding as the terms used (types I and II decompression sickness) convey no inherent information. Cerebral arterial gas embolism after pulmonary barotrauma is distinguished from cerebral decompression sickness on an assessment of probability based on the history of the dive, ascent rate, and onset of symptoms. The accepted dogma is that types I and II decompression sickness have subtly different presentations. Based on the traditional assumptions, Francis et al studied the presentation in 1070 cases of cerebral decompression sickness and concluded that there was considerable overlap, especially in the time to onset of symptoms and signs.14 This observation cast doubt on the veracity of accepted diagnostic criteria and the degree of concordance of diagnosis that might be expected in epidemiological studies. In the study of Smith et al 47 diving physicians applied these diagnostic criteria to 50 selected case records.15 The results indicated poor agreement despite the presentation of identical information on which to base diagnoses. An alternative approach to the classification of the decompression illnesses was proposed after a symposium to define and agree recognised criteria, principally to enable epidemiological study of diving related disorders.16

The new concept removes the necessity to assume a pathological mechanism and relies solely on a physician's observing, describing, and recording the clinical manifestations of the disease. The system requires no previous knowledge or training in diving medicine; it uses terms familiar to physicians, and it permits accurate and concise exchange of clinical information.

Alternative descriptive approach to decompression illnesses

The descriptive approach was developed to handle epidemiological information without recourse to complex definitions. Its aim is to collect essential information that requires minimal interpretation for entry on a database. It relies on two key items of information: the evolution and a summary clinical description of the manifestations. The system is dynamic - the description of the presentation may alter dramatically but detailed information on the natural course of the disease is available up to the time of treatment. The third box summarises the principal terms used.

Evolution

Most diseases develop in a general way that may be described using the simple terms progressive, static, spontaneously improving, and relapsing.

Progressive applies when a patient's condition deteriorates, with development of an increasing number of new symptoms and signs, or when the symptoms and signs at presentation worsen and become more severe.

Static applies when the progression of symptoms and signs plateaus.

Spontaneously improving applies when the patient's condition seems to improve on its own. Complete resolution of symptoms and signs may occur in mild cases, which may give the physician a false sense of security.

Relapsing applies to the recurrence of symptoms and signs after a period of spontaneous improvement. Neurological symptoms and signs are often relapsing. The term should be reserved for cases in which the deterioration has been preceded by either a static or an improving profile of development.

Manifestations

The manifestations of decompression illness may occur singly or in any combination. The objective is to describe the presentation as seen.

Pain generally affects the larger joints. Described as dull or boring at first, it is often transient and difficult to locate, flitting from joint to joint. Later the pain tends to localise and become progressively more severe. In severe cases “guarding” may be seen and limitation of movement may mask local neurological effects. Normal signs of inflammation will be absent. The onset of an ill defined ache or constricting pain around the loins and waist, so called girdle pain, may be described immediately preceding the onset of neurological symptoms and signs. Patients may not volunteer that they have girdle pain if they are coping with other symptoms, but examining doctors should always ask about it.

Neurological manifestations may be the most common form of presentation.^17,18 Symptoms and signs may suggest disorder of any function of the central or peripheral nervous system. Vulnerable higher functions include mood, behaviour, affect, intellectual processes, speech and expression, and the level of consciousness. Cranial nerves may also be affected. Eyes, ears, coordination, proprioception, and balance may be disturbed. Motor and sensory disturbances vary in severity from subjective appreciation of abnormality to complete loss of function with associated physical signs. The distribution of these disturbances may be patchy, follow the pathways of peripheral nerve or nerve roots, or have a stocking or glove distribution. Loss of sphincter control, especially the bladder, may also occur. Manifestations may be referred anatomitically to the brain or spinal cord, often to both, and may suggest that more than one lesion or site is affected. At the early stage of diagnosis an accurate description of manifestations is more valuable than an informed guess about the principal site of abnormality.

Audiovestibular manifestations are recognised as a special case and may be caused by two distinct pathological processes that are difficult to distinguish clinically. Symptoms and signs include nausea, vomiting, vertigo, tinnitus, nystagmus, and loss of hearing. Barotrauma may give rise to rupture of the round window. In addition, the cochlear and the eighth cranial nerve and its associated cerebellar and cerebral pathways are vulnerable to tissue damage caused by bubbles in a dissolved gas.

Pulmonary manifestations may also result from distinctly different pathological causes that are difficult to distinguish clinically. These mechanisms are barotrauma and inert gas embolism. Symptoms often include chest pain, cough, dyspnoea, haemoptysis, cyanosis, and cardiorespiratory collapse. Clinical signs will vary depending on the mechanism. In pulmonary barotrauma a pneumothorax may be evident, as may the abnormal presence of gas in the tissues of the neck and mediastinum.

Lymphatic manifestations - Lymphoedema and lymphadenopathy have been described in association with decompression illness, mainly on the chest and torso.

Cutaneous manifestations - Barotrauma to the skin occurs when folds of skin become trapped in creases of a dry suit during compression. This gives rise to a typical dark, linear reticulate pattern of skin marking appropriate to the means of its creation. It is called suit squeeze. Itching is sometimes described, usually over the upper torso. An erythematous rash may also be seen in association with itching. The rash may become cyanotic and marbled in appearance and will ultimately resolve without recourse to treatment.

Constitutional manifestations are less specific symptoms often associated with decompression illness, including anorexia, malaise, fatigue, and headache. Patients may feel fatigue greater than that expected from the physical exertion of diving but be aware of this only retrospectively after treatment - their recovered state contrasting sharply with their condition before treatment.

Recognition of decompression illnesses

An accurate and clinically useful description of decompression illness is incomplete without other information if it is to be used for epidemiological purposes.16 A comprehensive review of the pathophysiology has been published recently,19 together with a much more comprehensive review of the clinical manifestations.20

A presumptive diagnosis of decompression illness requires a high index of suspicion. This should be provided by the patient from a history which includes recent diving or exposure to raised environmental pressure. Even if the clinical picture suggests an alternative diagnosis specialist advice should be sought (box). Emergency services are listed in the appendix.

First aid treatment

The most important first aid measures to be taken before evacuation to a specialist treatment unit are the administration of oxygen and hydration of the patient. Patients are usually free of chronic respiratory disease and therefore oxygen can be given in high concentrations. Ideally, 100% oxygen should be provided from a demand system. The use of lower concentrations is of little clinical value. Oral fluid can be given to fully conscious patients whose airway is not at risk. Alternatively, an intravenous infusion of crystalloid solution should be given. Patients may be obviously dehydrated during the acute phase of decompression illnesses because of insensible losses to the environment. The disease also causes haemoconcentration, which should be corrected.

Other supportive treatment will be determined by the condition of the patient. The need for bladder catheterisation should be considered. The use of drugs should be kept to a minimum consistent with maintaining the patient's immediate condition. Analgesics should be avoided; a mixture of nitrous oxide and oxygen is positively contraindicated.