July 2001 newsletter

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Clinical governance

Words to strike fear in the heart! Clinical governance is here to stay however, and doctors require to prove their continuing competence in all clinical areas. This also applies to diving medicine– while there are no approved qualifications in sport diving medicine, certain measures of competence can be used.

Diving medical referees require to meet the basic entry standards which include recognised diving medicine experience (usually by attending courses, or publishing relevant research), and possessing a personal diving qualification. Such entry criteria cannot be regarded as a ticket for life however, and it is important to maintain up to date knowledge of the many developments in diving medicine. This is one of the reasons for publishing a regular newsletter. We hope to highlight relevant research and present an informed consensus opinion. The medical standards reflect this evidence, although diving medicine remains an area with sparse evidence, and considerable opinion. The medical standards are available on the website (www.uksdmc.co.uk) but if you do not have internet access then contact the secretary for an updated copy.

All new referees should receive the equivalent of a starter pack with copies of the medical standards, back copies of recent newsletters and a password for the restricted access area in the website. This allows referees to search a database of cases for previous committee decisions. The overall aim is to provide support to medical referees since it is not possible to have specialist knowledge of all areas relevant to diving medicine. We are able to liaise with the relevant experts to provide reasoned decisions and advice. Let us know what you think we can do to improve access to information and expertise.

What happens if a referee is felt to be under-performing? As in any area of clinical medicine, the safety of patients (or divers in this case) is paramount. If decisions are made that are likely to be harmful then action has to be taken. This should take the form of constructive feedback initially. If there are persistent problems then re-education or updating may be required, although this is something all of us should be undertaking regularly anyway. Finally, if despite best efforts there remain problems, then the final measure must be to remove that doctor from the approved referee list. The GMC would be informed in cases of medical negligence.

This is not an abstract concept. Unfortunately there have been problems in the past, and no doubt situations will arise in the future. We value the enthusiasm and expertise of the referees in the UKSDMC network. It is important that the standards remain high and we will continue to make efforts on your behalf to make sure of this.

The internet diving message boards are often a good measure of public opinion, and it is good to see that most divers perceive referees to be well qualified and supportive. Long may this remain the case.

Von Willebrand's disease

This is a form of haemophilia where activity of one of the coagulation factors (vWF) is lower than expected. There is a link with blood group O where levels of vWF tend to be at the lower end of the normal range. Many individuals are therefore labelled as having mild von Willebrand’s disease because of this although the presence of a bleeding history or family history of von Willebrand’s disease may be used to confirm the diagnosis.

Diving may still be permitted in patients although specialist advice should be obtained. It may be possible to show that the patient’s vWF level increases in response to physical exercise, or the administration of DDAVP. If this is the case then it is possible to use a nasal spray containing DDAVP to normalise vWF levels before diving. This effect usually lasts for around 24 hours. There have been two recent referrals and both divers were certified fit to dive after this form of assessment. Contact the committee for further details if required.

Provocation testing in asthmatics

The new guidelines are almost ready for publication from the British Thoracic Society aiming to provide expert guidance on assessing individuals with lung disease for fitness to dive.

Worldwide there is considerable variation in recommendations, with some countries banning asthmatics outright. Others insist on provocation testing.

A recent paper assessed the prevalence of bronchoconstriction following the inhalation of carbachol. They studied 76 subjects who had all been passed fit to dive by their personal physician. All had a history of previous asthma which was now inactive, or currently well controlled allergic asthma or rhinitis.

The results are difficult to interpret. Half of all subjects developed bronchoconstriction following the provocation test, but there was no difference in the degree of bronchoconstriction between the three groups (remote asthma, allergic rhinitis without bronchospasm, current asthma). Unfortunately there was no normal group to allow comparison.

Baseline spirometry was similar in subjects with and without a positive response to carbachol, including FEV1, FVC, FEV1/FVC ratios and plethysmographic measurement of airway resistance. There was no difference in terms of age or basic physical characteristics such as height. The authors conclude that subjects with a positive response should not be allowed to dive because of the risk of pulmonary barotrauma.

Overall there is little supporting evidence to perform inhalational provocation testing in all asthmatics before diving. It will be interesting to see what the BTS guidelines summarise from the limited evidence available, but it is unlikely to be substantially different from the current UKSDMC medical standard. Like many areas of diving medicine there are many strongly held opinions and only limited supporting data. The way forward here would be to prospectively collect information by performing a careful case control study, although given the current estimate of decompression risk (approx 0.0034% of dives) this would require to be a huge study and as such it is unlikely to be performed.

Reference
Value of bronchial challenge in SCUBA diving candidates. Badier M, Guillot C, Delpierre S, Fornaris E, Jacquin M. J Asthma 2000;37(8):661-5

Hereditary haemorrhagic telangiectasia

HHT is a fairly rare condition although this may reflect underdiagnosis. There are several important considerations if assessing subjects with HHT and these are described below. In general HHT should be regarded as a contraindication to diving.

There are two main features of the condition which can cause problems– telangiectasia are focal dilatations of the postcapillary venules and may be very large and convoluted. In some cases they may connect directly to dilated arterioles. Such arteriovenous malformations are common.

The specific areas at risk when diving include-

• Nose– epistaxis is the most common initial presentation in subjects with HHT. Frequently bleeding may be severe enough to require transfusion despite local treatment such as cautery, oestrogen therapy or even catheter thromboembolism. A tendency to massive epistaxis precludes diving because of the direct risk of haemorrhage, the indirect effect on vision if the diving mask fills with blood, and the possibility of diagnostic confusion since both epistaxis and pulmonary barotrauma can present as haemoptysis.
• Lungs– pulmonary arteriovenous malformations are present in around 10% of individuals with HHT. Conversely, 60% of subjects with lung A-V malformations have HHT so this is the most common associated condition. The risk of AV malformations is the possibility of paradoxical gas embolism since this may increase the chance of decompression sickness. Pulmonary telangiectasia may also cause haemorrhage.
• Brain– 5% of subjects with HHT have A-V malformations in the brain detectable by CT scanning. The true incidence is likely to be very much higher since CT is not a particularly sensitive detection technique. Future studies with MRI will be needed to confirm this.

Although it would be possible to perform a detailed and expensive series of investigations to examine for A-V malformations in the lungs, brain and spinal cord (using MRI angiography for example) this would not exclude smaller malformations or telangiectasia which could still cause haemorrhage or lead to arteriovenous shunting. The risks of recreational diving are excessive and individuals with HHT should be advised against diving.

Respiratory atopy at 50m

Continuing the asthma theme– a recent study examined the effects of a wet chamber dive to 50 metres on airway function in nine sport divers with hay fever and nine matched healthy controls.

The diagnosis of hay fever was based on a positive skin pin prick reaction to at least one common allergen particle. All divers had been passed their sport medical and had similar baseline characteristics including spirometry and detailed lung function tests.

The chamber dive was conducted at the same time of day for every subject to eliminate the effect of diurnal variation. Detailed measurements of pulmonary function were made before, three hours and 24 hours after the dive. In addition, methacholine bronchial challenge provocation was performed four weeks after the dive.

The results of the study are as follows- FVC (forced vital capacity), FEV1 (forced expiratory volume in one second), transfer factor and MEF50 (mid expiratory flow at 50% of vital capacity) fell after diving. Residual volume increased in the atopics after diving compared with the controls where this fell at 24 hours. A measurement of specific airways conductance (sGaw) fell in the atopics compared with a rise in the normals.

The authors conclude that atopics are more susceptible to the effects of diving exposure on lung function than normals. There was no significant difference between the two groups in peak flow measurements however, and the authors concede that any changes “are subtle and not clinically apparent”.

This is an interesting study which has several unique design attributes. Although the dive was simulated, it did involve immersion in water and the use of full diving equipment. This is probably the most realistic simulation currently available which still allows careful control of environmental factors to allow meaningful comparisons to be made with control groups.

The clinical implications of a fall in airways conductance are less clear however, and it is reassuring that more conventional measures of airway obstruction were either largely unchanged or not significantly different from the control group.

It is also interesting that the changes were not apparent at the 3 hour check and this suggests that the asthmatic divers were not at immediate risk. Quite how the changes at 24 hours translate into risk is uncertain. This is an area requiring more investigation.

For more information:
Respiratory effects of a single dive to 50 metres in sport divers with asymptomatic respiratory atopy. Tetzlaff K, Staschen C,-M, Struck N, Mutzbauer TS. Int J Sports Med 2001;22:85-89

Surface temperature and decompression sickness

The effects of ambient temperature on decompression risk are complex and recently were highlighted during the US Navy salvage operation to recover the wreckage of TWA Flight 800. An excess of decompression incidents developed after the navy switched from wetsuits to hot water suits and this required an increase in the decompression time.

A recent paper outlines the issues involved and includes a retrospective analysis of two large series of surface decompression dives (North Sea commercial off-shore air diving, and a much earlier series from 1951 from the Experimental Diving Unit of the US Navy).

There are some facts of historical interest here including the incidence of decompression sickness in the US Navy series of 9.8% compared with 0.5% in the 1986 commercial series.

Overall the results confirm that an increased ambient water temperature is associated with an increased risk of decompression sickness. This effect is particularly pronounced for shorter dives.

There are many factors involved in this but the most important appears to be the effect of temperature on peripheral tissue perfusion. Divers who are warm at depth probably develop peripheral vasodilatation and this can lead to more rapid on-gassing. Compartments with rapid gas exchange will therefore become saturated more quickly. After the dive, if the ambient temperature is cold then off gassing will be slower and this is consistent with the observation that “divers were more likely to have DCS on cold and windy days particularly when the air is colder than the water”.

The message to divers must be that they should be aware of the huge number of factors influencing their individual risk of decompression risk. Adequate safety margins should always be incorporated in the dive profiles.

For more information:
Effect of ambient temperature on the risk of decompression sickness in surface decompression divers. Leffler CT. Aviation, Space & Environmental Medicine. 2001;72(5):477-83

Inner ear decompression sickness

<Inner ear decompression sickness (IEDCS) is thought to be relatively uncommon, occurring in an estimated 0.5% of divers. We have briefly described the features of this form of injury in previous newsletters. The concern is that inner ear DCS can result in permanent damage unless diagnosed and treated immediately. A recent paper from the Israel Naval Medical Institute describes their 12 year experience of the diagnosis, treatment and follow-up of 24 divers with inner ear barotrauma. These cases represent 26% of the cases of severe DCS treated by the institute during this period. 80% of cases were associated with breaches of the decompression schedule or rapid ascents. When identified and treated early, more than half of the cases made a full recovery. When presentation was delayed, the recovery rate was much lower.

It is important to distinguish between inner ear DCS, and inner ear barotrauma (IEB) which results in rupture of the round or oval window membranes and causes perilymphatic leakage into the middle ear. Recompression would be harmful for IEB, and treatment should consist of complete bed rest, decongestants, vestibular tranquilisers and eventually surgical repair of the perilymphatic fistula.

The authors summarise the factors which distinguish the two syndromes and these are directly quoted below:

“1. In IEB, the patient will report having had difficulty clearing his ears during pressure changes. In contrast in IEDCS there will have been no such problems.
2. Symptoms of IEB appear during the dive, whereas those of IEDCS appear after the dive.
3. Other forms of DCS may accompany IEDCS.
4. Signs of middle ear barotrauma will often accompany IEB. No such signs will accompany IEDCS.
5. In contrast to the improvement observed in IEDCS during hyperbaric treatment, IEB will be aggravated by treatment in the hyperbaric chamber."

For more information
Inner ear decompression sickness in sport compressed-air diving. Nachum Z, Shupak A, Spitzer O, Sharoni Z, Doweck I, Gordon CR. Laryngoscope 2001;111:851-6

Vestibular findings in professional divers

This study of 13 professional divers compared detailed measurements of audiometry and vestibular function with 12 healthy non-diver controls. The average pure tone hearing threshold was higher in the divers compared with controls and this is consistent with previously published studies. Individual results however were all within normal limits, and it was only when the population was analysed as a whole that the slight difference was apparent.

There was a difference however in the vestibulo-occular reflex measurements. These measure the eye movement response to head rotation. The authors conclude that the difference was likely to be an adaptive response to coping with head movement underwater.

It was reassuring that no evidence of silent vestibular disease was apparent in the divers.

For more information
Vestibular findings in professional divers. Sharoni Z, Shupak A, SpitzerO, Nachum Z, Gadoth N. Ann Otol Rhinol Laryngol 2001;110:127-131