Relation between interatrial shunts and decompression sickness in divers

Decompression sickness of the inner ear and relationship with a patent oval foramen: a study of 61 cases

OBJECTIVE

To discuss the link between inner ear decompression sickness and patent foramen ovale.

MATERIALS AND METHODS

Monocentric and retrospective study on decompression sickness of the inner ear requiring hyperbaric chamber treatment, from 2014 to 2021.

RESULTS

Sixty-one patients of inner ear decompression sickness were included in this study. Twenty-four patients had vestibular injuries, 28 cochlear injuries and 9 cochleo-vestibular injuries. Compression chamber treatment was given, using an oxygen-helium mixture with oxygen partial pressure (PIO2) limited to 2.8 atmosphere absolute (ATA). All vestibular accidents completely recovered without clinical sequelae. For cochlear accident only 10 out of 37 patients (27%) recovered completely. A right-left shunt (patent foramen oval or intra-pulmonary shunt) was found in 31.1% of patients with inner ear decompression sickness (p > 0.05).

CONCLUSION

The presence of patent foramen oval in patients with inner ear decompression was not statistically significant in our study. Understanding of the pathophysiology of decompression illness and the physiology and anatomy of the labyrinth would suggest a mechanism of supersaturation with degassing in intra-labyrinthine liquids.

Pulmonary Physiology and Medicine of Diving

Pulmonary physiology is significantly altered during underwater exposure, as immersion of the body and increased ambient pressure elicit profound effects on both the cardiovascular and respiratory systems. Thoracic blood pooling, increased breathing gas pressures, and variations in gas volumes alongside ambient pressure changes put the heart and lungs under stress. Normal physiologic function and fitness of the cardiovascular and respiratory systems are prerequisites to safely cope with the challenges of the underwater environment when freediving, or diving with underwater breathing apparatus. Few physicians are trained to understand the physiology and medicine of diving and how to recognize or manage diving injuries. This article provides an overview of the physiologic challenges to the respiratory system during diving, with or without breathing apparatus, and outlines possible health risks and hazards unique to the underwater environment. The underlying pathologic mechanisms of dive-related injuries are reviewed, with an emphasis on pulmonary physiology and pathophysiology.

Right-to-Left Shunt in Divers with Neurological Decompression Sickness: A Systematic Review and Meta-Analysis

OBJECTIVE

The aim of this study was to assess the association between the presence of a right-to-left shunt (RLS) and neurological decompression sickness (NDCS) and asymptomatic brain lesions among otherwise healthy divers.

BACKGROUND

Next to drowning, NDCS is the most severe phenotype of diving-related disease and may cause permanent damage to the brain and spinal cord. Several observational reports have described the presence of an RLS as a significant risk factor for neurological complications in divers, ranging from asymptomatic brain lesions to NDCS.

METHODS

We systematically reviewed the MEDLINE, Embase, and CENTRAL databases from inception until November 2021. A random-effects model was used to compute odds ratios.

RESULTS

Nine observational studies consisting of 1830 divers (neurological DCS: 954; healthy divers: 876) were included. RLS was significantly more prevalent in divers with NDCS compared to those without (62.6% vs. 27.3%; odds ratio (OR): 3.83; 95% CI: 2.79-5.27). Regarding RLS size, high-grade RLS was more prevalent in the NDCS group than the no NDCS group (57.8% versus 18.4%; OR: 4.98; 95% CI: 2.86-8.67). Further subgroup analysis revealed a stronger association with the inner ear (OR: 12.13; 95% CI: 8.10-18.17) compared to cerebral (OR: 4.96; 95% CI: 2.43-10.12) and spinal cord (OR: 2.47; 95% CI: 2.74-7.42) DCS. RLS was more prevalent in divers with asymptomatic ischemic brain lesions than those without any lesions (46.0% vs. 38.0%); however, this was not statistically significant (OR: 1.53; 95% CI: 0.80-2.91).

CONCLUSIONS

RLS, particularly high-grade RLS, is associated with greater risk of NDCS. No statistically significant association between RLS and asymptomatic brain lesions was found.

S2k guideline for diving accidents

For the purposes of this guideline, a diving accident is defined as an event that is either potentially life-threatening or hazardous to health as a result of a reduction in ambient pressure while diving or in other hyperbaric atmospheres with and without diving equipment. This national consensus-based guideline (development grade S2k) presents the current state of knowledge and recommendations on the diagnosis and treatment of diving accident victims. The treatment of a breath-hold diver as well as children and adolescents does not differ in principle. In this regard only unusual tiredness and itching without visible skin changes are mild symptoms. The key action statements: on-site 100% oxygen first aid treatment, immobilization/no unnecessary movement, fluid administration and telephone consultation with a diving medicine specialist are recommended. Hyperbaric oxygen therapy (HBOT) remains unchanged as the established treatment in severe cases, as there are no therapeutic alternatives. The basic treatment scheme recommended for diving accidents is hyperbaric oxygenation at 280 kPa.

Increased Risk of Decompression Sickness When Diving With a Right-to-Left Shunt: Results of a Prospective Single-Blinded Observational Study (The "Carotid Doppler" Study)

Introduction: Divers with a patent Foramen Ovale (PFO) have an increased risk for decompression sickness (DCS) when diving with compressed breathing gas. The relative risk increase, however, is difficult to establish as the PFO status of divers is usually only determined after a DCS occurrence.

Methods: This prospective, single-blinded, observational study was designed to collect DCS data from volunteer divers after screening for right-to-left shunt (RLS) using a Carotid Doppler test. Divers were blinded to the result of the test, but all received a standardized briefing on current scientific knowledge of diving physiology and “low-bubble” diving techniques; they were then allowed to dive without restrictions. After a mean interval of 8 years, a questionnaire was sent collecting data on their dives and cases of DCS (if any occurred).

Results: Data was collected on 148 divers totaling 66,859 dives. There was no significant difference in diving data between divers with or without RLS. Divers with RLS had a 3.02 times higher incidence of (confirmed) DCS than divers without RLS (p = 0.04). When all cases of (confirmed or possible DCS) were considered, the Relative Risk was 1.42 (p = 0.46). DCS occurred mainly in divers who did not dive according to “low-bubble” diving techniques, in both groups.

Conclusion: This prospective study confirms that DCS is more frequent in divers with RLS (such as a PFO), with a Relative Risk of 1.42 (all DCS) to 3.02 (confirmed DCS). It appears this risk is linked to diving behavior, more specifically diving to the limits of the adopted decompression procedures.

European position paper on the management of patients with patent foramen ovale. Part II - Decompression sickness, migraine, arterial deoxygenation syndromes and select high-risk clinical conditions

Patent foramen ovale (PFO) is implicated in the pathogenesis of a number of medical conditions but to date only one official position paper related to left circulation thromboembolism has been published. This interdisciplinary paper, prepared with the involvement of eight European scientific societies, reviews the available evidence and proposes a rationale for decision making for other PFO-related clinical conditions. In order to guarantee a strict evidence-based process, we used a modified grading of recommendations, assessment, development, and evaluation (GRADE) methodology. A critical qualitative and quantitative evaluation of diagnostic and therapeutic procedures was performed, including assessment of the risk/benefit ratio. The level of evidence and the strength of the position statements were weighed and graded according to predefined scales. Despite being based on limited and observational or low-certainty randomised data, a number of position statements were made to frame PFO management in different clinical settings, along with suggestions for new research avenues. This interdisciplinary position paper, recognising the low or very low certainty of existing evidence, provides the first approach to several PFO-related clinical scenarios beyond left circulation thromboembolism and strongly stresses the need for fresh high-quality evidence on these topics.

Going to Extremes of Lung Physiology-Deep Breath-Hold Diving

Breath-hold diving involves environmental challenges, such as water immersion, hydrostatic pressure, and asphyxia, that put the respiratory system under stress. While training and inherent individual factors may increase tolerance to these challenges, the limits of human respiratory physiology will be reached quickly during deep breath-hold dives. Nonetheless, world records in deep breath-hold diving of more than 214 m of seawater have considerably exceeded predictions from human physiology. Investigations of elite breath-hold divers and their achievements revised our understanding of possible physiological adaptations in humans and revealed techniques such as glossopharyngeal breathing as being essential to achieve extremes in breath-hold diving performance. These techniques allow elite athletes to increase total lung capacity and minimize residual volume, thereby reducing thoracic squeeze. However, the inability of human lungs to collapse early during descent enables respiratory gas exchange to continue at greater depths, forcing nitrogen (N₂) out of the alveolar space to dissolve in body tissues. This will increase risk of N₂ narcosis and decompression stress. Clinical cases of stroke-like syndromes after single deep breath-hold dives point to possible mechanisms of decompression stress, caused by N₂ entering the vasculature upon ascent from these deep dives. Mechanisms of neurological injury and inert gas narcosis during deep breath-hold dives are still incompletely understood. This review addresses possible hypotheses and elucidates factors that may contribute to pathophysiology of deep freediving accidents. Awareness of the unique challenges to pulmonary physiology at depth is paramount to assess medical risks of deep breath-hold diving.

Echocardiography - techniques and pitfalls whilst diagnosing persistent (patent) foramen ovale as a risk factor in divers with a history of decompression sickness

The case of a diver with a history of decompression sickness (DCS) after recreational scuba diving is presented. Cutis marmorata, a subtype of cutaneous DCS, has been consistently associated with the presence of a persistent (patent) foramen ovale (PFO) as a risk factor. Diagnostic uncertainty arose when transthoracic echocardiography with antecubital injection of agitated saline bubbles (ASBs) did not show any significant shunt, but the presence of a large Eustachian valve was counteracted by intra-femoral injection of ASBs, showing a large PFO with spontaneous shunting. The importance of proper echocardiography techniques prior to resorting to intra-femoral injection of ASBs to counteract the haemodynamic effects of the Eustachian valve is emphasised.

Does persistent (patent) foramen ovale closure reduce the risk of recurrent decompression sickness in scuba divers?

INTRODUCTION

Interatrial communication is associated with an increased risk of decompression sickness (DCS) in scuba diving. It has been proposed that there would be a decreased risk of DCS after closure of the interatrial communication, i.e., persistent (patent) foramen ovale (PFO). However, the clinical evidence supporting this is limited.

METHODS

Medical records were reviewed to identify Swedish scuba divers with a history of DCS and catheter closure of an interatrial communication. Thereafter, phone interviews were conducted with questions regarding diving and DCS. All Swedish divers who had had catheter-based PFO-closure because of DCS were followed up, assessing post-closure diving habits and recurrent DCS.

RESULTS

Nine divers, all with a PFO, were included. Eight were diving post-closure. These divers had performed 6,835 dives (median 410, range 140-2,200) before closure, and 4,708 dives (median 413, range 11-2,000) after closure. Seven cases with mild and 10 with serious DCS symptoms were reported before the PFO closure. One diver with a small residual shunt suffered serious DCS post-closure; however, that dive was performed with a provocative diving profile.

CONCLUSION

Divers with PFO and DCS continue to dive after PFO closure and this seems to be fairly safe. Our study suggests a conservative diving profile when there is a residual shunt after PFO closure, to prevent recurrent DCS events.

A review of diving practices and outcomes following the diagnosis of a persistent (patent) foramen ovale in compressed air divers with a documented episode of decompression sickness

INTRODUCTION

The presence of a persistent (patent) foramen ovale (PFO) increases the risk of decompression sickness (DCS) whilst diving with pressurised air. After the diagnosis of a PFO, divers will be offered a number of options for risk mitigation. The aim of this study was to review the management choices and modifications to diving practices following PFO diagnosis in the era preceding the 2015 joint position statement (JPS) on PFO and diving.

METHODS

A retrospective study was conducted of divers sourced from both the Alfred Hospital, Melbourne and the Divers Alert Network Asia-Pacific during the period 2005-2015. Divers were contacted via a combination of phone, text, mail and email. Data collected included: diving habits (years, style and depths); DCS symptoms, signs and treatment; return to diving and modifications of dive practices; history of migraine and echocardiography (ECHO) pre- and post-intervention; ECHO technique(s) used, and success or failure of PFO closure (PFOC). Analyses were performed to compare the incidence of DCS pre- and post-PFO diagnosis.

RESULTS

Seventy-three divers were interviewed. Sixty-eight of these returned to diving following the diagnosis of PFO. Thirty-eight underwent PFOC and chose to adopt conservative diving practices (CDPs); 15 chose PFOC with no modification to practices; 15 adopted CDPs alone; and five have discontinued diving. The incidence of DCS decreased significantly following PFOC and/or adoption of conservative diving practices. Of interest, migraine with aura resolved in almost all those who underwent PFOC.

CONCLUSIONS

Many divers had already adopted practices consistent with the 2015 JPS permitting the resumption of scuba diving with a lowering of the incidence of DCS to that of the general diving population. These results support the recommendations of the JPS.

Patent Foramen Ovale Closure: State of the Art

Patent foramen ovale (PFO) is a common abnormality affecting between 20% and 34% of the adult population. For most people, it is a benign finding; however, in some people, the PFO can open widely to enable paradoxical embolus to transit from the venous to arterial circulation, which is associated with stroke and systemic embolisation. Percutaneous closure of the PFO in patients with cryptogenic stroke has been undertaken for a number of years, and a number of purpose-specific septal occluders have been marketed. Recent randomised control trials have demonstrated that closure of PFO in patients with cryptogenic stroke is associated with reduced rates of recurrent stroke. After a brief overview of the anatomy of a PFO, this article considers the evidence for PFO closure in cryptogenic stroke. The article also addresses other potential indications for closure, including systemic arterial embolisation, decompression sickness, platypnoea–orthodeoxia syndrome and migraine with aura. The article lays out the pre-procedural investigations and preparation for the procedure. Finally, the article gives an overview of the procedure itself, including discussion of closure devices.

Patent Foramen Ovale in Fetal Life, Infancy and Childhood

A patent foramen ovale (PFO) is a common, incidental echocardiographic finding in otherwise healthy and asymptomatic infants and children. However, a variety of clinical conditions have been ascribed to the presence of a PFO in childhood, such as cryptogenic stroke, platypnea-orthodeoxia syndrome, decompression sickness and migraine, although the data on these are controversial and sometimes contradictory. This review discusses embryology and correlation with post-natal anatomy, anatomical variations of the atrial septum, diagnostic modalities in special circumstances of PFO associated clinical syndromes, and the role of PFO in congenital heart disease, pulmonary hypertension, dilated cardiomyopathy and heart failure in children who require an extracorporeal membrane oxygenator or ventricular assist device as life support.

Patent Foramen Ovale, the Role of Antiplatelet Therapy Alone or Anticoagulant Therapy Alone Versus Device Closure for Cryptogenic Stroke: A Review of the Literature and Current Recommendations

Cryptogenic stroke and its relation to the Patent Foramen Ovale (PFO) is a long-debated topic. Recent clinical trials have unequivocally established the relationship between cryptogenic strokes and paradoxical embolism across the PFO. This slit-like communication exists in everyone before birth, but most often closes shortly after birth. PFO may persist as a narrow channel of communication between the right and left atria in approximately 25-27% of adults. In this review, we examine the clinical relevance of the PFO with analysis of the latest trials evaluating catheter-based closure of PFO's for cryptogenic stroke. We also review the current evidence examining the use of antiplatelet medications versus anticoagulants for stroke prevention in those patients with PFO who do not qualify for closure per current guidelines.

DCS or DCI? The difference and why it matters

There are few issues that generate as much confusion in diving medicine as the nomenclature of bubble-induced dysbaric disease. Prior to the late 1980s, the diagnosis 'decompression sickness' (DCS) was invoked for symptoms presumed to arise as a consequence of bubble formation from dissolved inert gas during or after decompression. These bubbles were known to form within tissues, and also to appear in the venous blood (presumably after forming in tissue capillaries). A second diagnosis, 'arterial gas embolism' (AGE) was invoked for symptoms presumed to arise when bubbles were introduced directly to the arterial circulation as a consequence of pulmonary barotrauma.

This approach was predicated on an assumption that the underlying pathophysiology could usually be inferred from the nature and tempo of resulting symptoms. DCS was considered to exhibit a slower more progressive onset, symptoms were protean (including pain, rash, paraesthesias, subcutaneous swelling, and neurological symptoms), and the neurological manifestations were mainly attributable to spinal cord or inner ear involvement. In contrast, AGE was considered to exhibit a more precipitous onset (often immediately on surfacing), and the principal manifestation was stroke-like focal neurological impairment suggestive of cerebral involvement.

In 1989 an association between a large persistent ('patent') foramen ovale (PFO) and serious neurological DCS was independently reported by two groups, and subsequently corroborated for neurological, inner ear, and cutaneous DCS by multiple studies. The assumed pathophysiological role of a PFO in this setting was to allow bubbles formed from inert gas in the venous blood to avoid removal in the pulmonary circulation and to enter the arterial circulation. These bubbles could then pass to the microcirculation of vulnerable target tissues where inward diffusion of supersaturated inert gas from the surrounding tissue could cause them to grow.

This emergence of 'arterialisation' of venous bubbles as an important vector of harm in some forms of DCS resulted in a challenge to the use of traditional 'DCS/AGE' terminology. It was suggested that very early onset of cerebral symptoms after diving could be explained not only by arterial bubbles introduced by pulmonary barotrauma, but also by venous bubbles crossing a PFO into the arterial circulation. Moreover, once venous bubbles had entered the arterial circulation they were then technically 'arterial gas emboli'; thus creating confusion with arterial gas emboli from pulmonary barotrauma. To many commentators, it made little sense to use diagnostic labels (DCS and AGE) that implied a particular pathophysiology when the two disorders might be difficult to tell apart, and had mechanistic processes in common.

An alternative approach derived at a UHMS workshop in 1991 was to shift from nomenclature that implied a particular pathophysiology, to a descriptive system that lumped both DCS and AGE together under the label "decompression illness" (DCI). Using this system, terms to describe the organ system(s) involved and the progression of symptoms were applied. For example, a diver with worsening upper arm pain after a dive could be suffering 'progressive musculoskeletal DCI'; and a diver who lost consciousness immediately on surfacing but regained consciousness minutes later would be considered to be suffering 'remitting cerebral DCI'. Classifying cases in this manner made considerable sense at a clinical level, particularly given that there was an emerging consensus that manifestations of DCS and AGE that potentially overlapped did not require different approaches to recompression treatment.

This descriptive classification of bubble-induced dysbaric disease gained substantial traction in the community, though not always with a full appreciation by users of the intended nuances of its application. Indeed, it became increasingly common over time to see the terms DCS and DCI used interchangeably; for example, authors using the term DCI to specifically infer the consequences of bubble formation from dissolved gas. This highlights one of the shortcomings of the DCI terminology: it becomes confusing when discussing dysbaric disease at a theoretical or experimental level when the nature of the insult is known or there is a specific intent to discuss bubble formation either from dissolved gas or from pulmonary barotrauma.

The potential for confusion between mechanisms and manifestations of DCS and AGE as one of the principle drivers for adopting the DCI terminology deserves further discussion. It is tempting to suggest that if venous bubbles cross a PFO into the arterial blood then any resulting symptoms should be considered a manifestation of 'AGE'. However, there seems little sense in re-naming the primary pathophysiological event (DCS caused by bubble formation from inert gas) just because the bubbles have distributed elsewhere; especially using a name that commonly infers a completely different primary event (bubble formation from pulmonary barotrauma). Moreover, there are grounds for suggesting that these two processes may not be as difficult to distinguish as previously believed. Venous inert gas bubbles are small, and of a similar size distribution to those used as bubble contrast during PFO testing. Decades of experience in testing thousands of divers (and other patients) for PFO using bubble-contrast echocardiograpy have shown that even when strongly positive (that is, large showers of bubbles enter the arterial circulation), symptoms of any sort are very rare. There are sporadic reports of evanescent visual or cerebral symptoms, but (to this author's knowledge) reports of the focal or multifocal cerebral infarctions that can be caused by large arterial bubbles introduced iatrogenically or by pulmonary barotrauma are lacking. One could argue that in the context of PFO testing the brain is not supersaturated with inert gas (which might cause small arterial bubbles to grow), but being such a 'fast tissue' nor is it likely to be after diving. Thus, while sustained showers of small inert gas bubbles crossing a PFO after diving appeal as a plausible cause of transient visual symptoms or dysexecutive syndromes after diving, they are less likely to be the cause of dramatic stroke-like events occurring early after surfacing.

In the final edition of Bennett and Elliott it was suggested that one editorial approach to the terminology conundrum would be to utilise the traditional terminology (DCS and AGE) when referring specifically to the pathophysiology and manifestations of bubble formation from dissolved inert gas or pulmonary barotrauma respectively, and to utilise the descriptive (DCI) terminology in clinical discussions when a collective term is useful, or when discussing individual patients where there is either ambiguity about pathophysiology or no need to attempt a distinction. Diving and Hyperbaric Medicine recommends a similar approach. The journal is reluctant to attempt to generate or apply hard 'rules' in relation to terminology of bubble-induced dysbaric disease, but we strongly discourage use of the term 'arterial gas emboli(ism)' to characterise venous inert gas bubbles that cross a right-to-left shunt such as a PFO. The pathophysiological consequences of bubble formation from dissolved inert gas should be regarded as decompression sickness (DCS). There is an expectation that authors are cognisant of the above issues and attempt to adopt terminology that reflects these considerations and best suits the circumstances of their manuscript.

Static Metabolic Bubbles as Precursors of Vascular Gas Emboli During Divers' Decompression: A Hypothesis Explaining Bubbling Variability

Introduction

The risk for decompression sickness (DCS) after hyperbaric exposures (such as SCUBA diving) has been linked to the presence and quantity of vascular gas emboli (VGE) after surfacing from the dive. These VGE can be semi-quantified by ultrasound Doppler and quantified via precordial echocardiography. However, for an identical dive, VGE monitoring of divers shows variations related to individual susceptibility, and, for a same diver, dive-to-dive variations which may be influenced by pre-dive pre-conditioning. These variations are not explained by currently used algorithms. In this paper, we present a new hypothesis: individual metabolic processes, through the oxygen window (OW) or Inherent Unsaturation of tissues, modulate the presence and volume of static metabolic bubbles (SMB) that in turn act as precursors of circulating VGE after a dive.

Methods

We derive a coherent system of assumptions to describe static gas bubbles, located on the vessel endothelium at hydrophobic sites, that would be activated during decompression and become the source of VGE. We first refer to the OW and show that it creates a local tissue unsaturation that can generate and stabilize static gas phases in the diver at the surface. We then use Non-extensive thermodynamics to derive an equilibrium equation that avoids any geometrical description. The final equation links the SMB volume directly to the metabolism.

Results and Discussion

Our model introduces a stable population of small gas pockets of an intermediate size between the nanobubbles nucleating on the active sites and the VGE detected in the venous blood. The resulting equation, when checked against our own previously published data and the relevant scientific literature, supports both individual variation and the induced differences observed in pre-conditioning experiments. It also explains the variability in VGE counts based on age, fitness, type and frequency of physical activities. Finally, it fits into the general scheme of the arterial bubble assumption for the description of the DCS risk.

Conclusion

Metabolism characterization of the pre-dive SMB population opens new possibilities for decompression algorithms by considering the diver's individual susceptibility and recent history (life style, exercise) to predict the level of VGE during and after decompression.

The effectiveness of risk mitigation interventions in divers with persistent (patent) foramen ovale

INTRODUCTION

Persistent (patent) foramen ovale (PFO) is a recognized risk for decompression sickness (DCS) in divers, which may be mitigated by conservative diving or by PFO closure. Our study aimed to compare the effectiveness of these two risk mitigation interventions.

METHODS

This was a prospective study on divers who tested positive for PFO or an atrial septal defect (ASD) and either decided to continue diving without closure ('conservative group'), or to close their PFO/ASD and continue diving ('closure group'). Divers' characteristics, medical history, history of diving and history of DCS were reported at enrollment and annually after that. The outcome measures were the incidence rate of DCS, frequency and intensity of diving activities, and adverse events of closure.

RESULTS

Divers in both groups dived less and had a lower incidence rate of confirmed DCS than before the intervention. In the closure group (n = 42) the incidence rate of confirmed DCS decreased significantly. Divers with a large PFO experienced the greatest reduction in total DCS. In the conservative group (n = 23), the post-intervention decrease in confirmed DCS incidence rate was not significant. Of note, not all divers returned to diving after closure. Seven subjects reported mild adverse events associated with closure; one subject reported a serious adverse event.

CONCLUSIONS

PFO closure should be considered on an individual basis. In particular, individuals who are healthy, have a significant DCS burden, a large PFO or seek to pursue advanced diving may benefit from closure.

Patent Foramen Ovale Closure in 2019

Patent foramen ovale (PFO) is a common abnormality affecting between 20% and 34% of the adult population. For most people it is a benign finding; however, in some the PFO can open widely, enabling a paradoxical embolus to transit from the venous to arterial circulation, which is associated with stroke and systemic embolisation. Percutaneous closure of PFO in patients with cryptogenic stroke has been undertaken for a number of years, and a number of purpose-specific septal occluders have been marketed. Recent randomised controlled trials have demonstrated that closure of PFO in patients with cryptogenic stroke is associated with reduced rates of recurrent stroke. After a brief overview of the anatomy of a PFO, this review considers the evidence for PFO closure in cryptogenic stroke. The review also addresses other potential indications for closure, including systemic embolisation, decompression sickness, platypnoea-orthodeoxia syndrome and migraine with aura. It lays out the pre-procedural investigations and preparation for the procedure. Finally, it gives an overview of the procedure itself, including discussion of closure devices.

Identification of candidates for PFO closure in the echocardiography laboratory

A patent foramen ovale (PFO) is implicated in several pathologic processes, including that of cryptogenic stroke (cCVA). Recent trials identify "high-risk" PFOs in patients with cCVA as likely to benefit from percutaneous closure. The younger the patient (<60 years old) the more likely a PFO may be attributable to the cCVA. The RoPE Score index helps determine the likelihood that an existing PFO is related to a cCVA. This may help guide the clinician and patient when contemplating percutaneous PFO closure. When evaluating a patient for possible percutaneous closure, one should identify the CVA as a typical ischemic type stroke. In order to "rule-out" other causes of CVA, imaging of the intracranial arteries, cervical, and aortic arch vessels should be performed. Small vessel disease or a lacunar-type infarct should be excluded. To rule out atrial fibrillation, prolonged monitoring should be performed. An index has been developed to determine the probability that a PFO is the causative etiology and calculates the risk of recurrence. This may help guide the clinician and patient in the decision for PFO closure. In addition, one should consider a work-up for a hypercoagulable state. We will obtain an ultrasound of the lower extremities or consider deep pelvic vein thrombosis (prolonged sitting or malignancy). If the closure is to be performed, the Food and Drug Administration (FDA) has approved the Amplatzer PFO Occluder and the GORE Cardioform Septal Occluder for percutaneous closure. These devices are both approved in patients predominately between ages 18 and 60 years with a cCVA due to presumed paradoxical embolism as verified by a neurologist and cardiologist and when other causes of ischemic CVA have been excluded. "High-risk" PFOs appear to achieve the most potential benefit from percutaneous closure.

Diving with pre-existing medical conditions

INTRODUCTION

This is the second report based on a survey of Divers Alert Network Asia-Pacific (DAN AP) members who dive with cardiovascular and respiratory conditions and diabetes. It examines the medical management of the divers' conditions, any diving modifications used to mitigate the risk and outcomes.

METHODOLOGY

An online cross-sectional survey was sent to 833 divers who had declared a targeted medical condition when applying for DAN AP membership between July 2009 and August 2013.

RESULTS

Two-hundred-and-sixty-eight respondents (32%) provided sufficient information on their conditions to be included in the analyses. These included ischaemic heart disease (31), arrhythmias (20), cardiac septal defects (31), other cardiac conditions (10), hypertension (127), diabetes (25), asthma (40) and pneumothorax (5). Forty-nine per cent had sought specialist diving medical advice about their condition and 23% reported modifying their diving practices to mitigate their risk. The cohort had completed 183,069 career dives, 57,822 of these since being diagnosed with their medical condition. There were 27 individuals who reported having decompression illness (25 of whom were subsequently diagnosed with a persistent foramen ovale), and two individuals who experienced an arrhythmia during diving.

CONCLUSIONS

Some DAN AP members are diving with medical conditions which could potentially impact the safety of their diving. A minority modified their diving practices to mitigate the risk of their condition and approximately half sought specialist diving medical advice. The incidence of diving-related problems precipitated by known and managed pre-existing health conditions seems low but further studies of larger cohorts and incorporating fatality data would be necessary to confirm this. These results are limited by the 32% response rate and potential for bias towards selection of those most careful with their health.

Spinal cord

We first present a brief historic review of developments in the understanding of spinal cord clinical neuroanatomy and neurophysiology over the past 200 years. We then discuss the technical aspects that apply to the examination of the human spinal cord giving details on the interrelations between the spinal cord and the overlying structures, including the meninges and vertebrae. The subsequent discussion focuses on diseases of the spinal cord. Diseases that affect the spinal cord are vascular disease, diseases of spinal column, trauma, developmental abnormalities, central nervous system degenerative disease, inflammatory disease, metabolic and nutritional myelopathies, and tumors. We summarize our knowledge regarding general reactions of spinal cord tissue to disease, in particular Wallerian degeneration of descending/ascending tracts and axonal reaction. Two categories of disease will be covered in depth: vascular disease of the spinal cord, including a review of normal vascular anatomy, and diseases of the vertebral column that can affect the cord secondarily.

Skin Lesions in Swine with Decompression Sickness: Clinical Appearance and Pathogenesis

Skin lesions are visual clinical manifestations of decompression sickness (DCS). Comprehensive knowledge of skin lesions would give simple but strong clinical evidence to help diagnose DCS. The aim of this study was to systematically depict skin lesions and explore their pathophysiological basis in a swine DCS model. Thirteen Bama swine underwent simulated diving in a hyperbaric animal chamber with the profile of 40 msw-35 min exposure, followed by decompression in 11 min. After decompression, chronological changes in the appearance of skin lesions, skin ultrasound, temperature, tissue nitric oxide (NO) levels, and histopathology were studied. Meanwhile bubbles and central nervous system (CNS) function were monitored. All animals developed skin lesions and two died abruptly possibly due to cardiopulmonary failure. A staging approach was developed to divide the appearance into six consecutive stages, which could help diagnosing the progress of skin lesions. Bubbles were only seen in right but not left heart chambers. There were strong correlations between bubble load, lesion area, latency to lesion appearance and existence of cutaneous lesions (P = 0.007, P = 0.002, P = 0.004, respectively). Even though local skin temperature did not change significantly, skin thickness increased, NO elevated and histological changes were observed. Increased vessel echo-reflectors in lesion areas were detected ultrasonically. No CNS dysfunction was detected by treadmill walking and evoked potential. The present results suggest skin lesions mainly result from local bubbles and not CNS injuries or arterial bubbles.

Frequency of decompression illness among recent and extinct mammals and "reptiles": a review

The frequency of decompression illness was high among the extinct marine "reptiles" and very low among the marine mammals. Signs of decompression illness are still found among turtles but whales and seals are unaffected. In humans, the risk of decompression illness is five times increased in individuals with Patent Foramen Ovale; this condition allows blood shunting from the venous circuit to the systemic circuit. This right-left shunt is characteristic of the "reptile" heart, and it is suggested that this could contribute to the high frequency of decompression illness in the extinct reptiles.

Potential Role of Patent Foramen Ovale in Exacerbating Hypoxemia in Chronic Pulmonary Disease

Patent foramen ovale has been associated with multiple pulmonary diseases, such as pulmonary hypertension, platypnea-orthodeoxia syndrome, and chronic obstructive pulmonary disease. A connection between patent foramen ovale and chronic pulmonary disease was first described more than 2 decades ago in case reports associating patent foramen ovale with more severe hypoxemia than that expected based on the severity of the primary pulmonary disease. It has been suggested that patients with both chronic pulmonary disease and patent foramen ovale are subject to severe hypoxemia because of the right-to-left shunt. Furthermore, investigators have reported improved systemic oxygenation after patent foramen ovale closure in some patients with chronic pulmonary disease. This review focuses on the association between chronic pulmonary disease and patent foramen ovale and on the dynamics of a right-to-left shunt, and it considers the potential benefit of patent foramen ovale closure in patients who have hypoxemia that is excessive in relation to the degree of their pulmonary disease.

4D flow MRI assessment of right atrial flow patterns in the normal heart - influence of caval vein arrangement and implications for the patent foramen ovale

AIM

To investigate atrial flow patterns in the normal adult heart, to explore whether caval vein arrangement and patency of the foramen ovale (PFO) may be associated with flow pattern.

MATERIALS AND METHODS

Time-resolved, three-dimensional velocity encoded magnetic resonance imaging (4D flow) was employed to assess atrial flow patterns in thirteen healthy subjects (6 male, 40 years, range 25-50) and thirteen subjects (6 male, 40 years, range 21-50) with cryptogenic stroke and patent foramen ovale (CS-PFO). Right atrial flow was defined as vortical, helico-vortical, helical and multiple vortices. Time-averaged and peak systolic and diastolic flows in the caval and pulmonary veins and their anatomical arrangement were compared.

RESULTS

A spectrum of right atrial flow was observed across the four defined categories. The right atrial flow patterns were strongly associated with the relative position of the caval veins. Right atrial flow patterns other than vortical were more common (p = 0.015) and the separation between the superior and inferior vena cava greater (10±5mm versus 3±3mm, p = 0.002) in the CS-PFO group. In the left atrium all subjects except one had counter-clockwise vortical flow. Vortex size varied and was associated with left lower pulmonary vein flow (systolic r = 0.61, p = 0.001, diastolic r = 0.63 p = 0.002). A diastolic vortex was less common and time-averaged left atrial velocity was greater in the CS-PFO group (17±2cm/sec versus 15±1, p = 0.048). One CS-PFO subject demonstrated vortical retrograde flow in the descending aortic arch; all other subjects had laminar descending aortic flow.

CONCLUSION

Right atrial flow patterns in the normal heart are heterogeneous and are associated with the relative position of the caval veins. Patterns, other than 'typical' vortical flow, are more prevalent in the right atrium of those with cryptogenic stroke in the context of PFO. Left atrial flow patterns are more homogenous in normal hearts and show a relationship with flow arising from the left pulmonary veins.

Patent foramen ovale: Unanswered questions

The foramen ovale is a remnant of the fetal circulation that remains patent in 20-25% of the adult population. Although long overlooked as a potential pathway that could produce pathologic conditions, the presence of a patent foramen ovale (PFO) has been associated with a higher than expected frequency in a variety of clinical syndromes including cryptogenic stroke, migraines, sleep apnea, platypnea-orthodeoxia, deep sea diving associated decompression illness, and high altitude pulmonary edema. A unifying hypothesis is that a chemical or particulate matter from the venous circulation crosses the PFO conduit between the right and left atria to produce a variety of clinical syndromes. Although observational studies suggest a therapeutic benefit of PFO closure compared to medical therapy alone in patients with cryptogenic stroke, 3 randomized controlled trials (RCTs) did not confirm the superiority of PFO closure for the secondary prevention of stroke. However, meta-analyses of these RCTs demonstrate a significant benefit of PFO closure over medical therapy alone. Similarly, observational studies provide support for PFO closure for symptomatic relief of migraines. But one controversial randomized study failed to replicate the results of the observational studies while another two demonstrated a partial benefit. The goal of this review is to discuss the clinical conditions associated with PFO and provide internists and primary care physicians with current data on PFO trials, and clinical insight to help guide their patients who are found to have a PFO on echocardiographic testing.

Microbubbles shunting via a patent foramen ovale impair endothelial function

OBJECTIVES

Exposure to intravascular microbubbles after diving and during medical procedures alters endothelial function. The aim of this study was to investigate whether a patent foramen ovale altered forearm endothelial function by facilitating microbubbles transfer.

DESIGN

Patients attended on two separate visits, at least seven days apart receiving agitated saline or no active intervention in random order. On both days, flow-mediated dilatation of the brachial artery was measured using vascular ultrasound. On the intervention visit, agitated saline was injected and the passage of microbubbles into the arterial circulation was confirmed by echocardiography. Serial flow-mediated dilatation measurements were made after agitated saline and at the same time points after no intervention.

SETTING

St Thomas' Hospital in London.

PARTICIPANTS

Patients with a patent foramen ovale (PFO+n = 14, 9 male, mean ± SD age 42.2 ± 10.5 years) and patients without a patent foramen ovale (PFO- n = 10, 7 male, mean ± SD age 49.4 ± 18.4 years) were recruited.

MAIN OUTCOME MEASURES

Change in brachial artery flow-mediated dilatation.

RESULTS

In patent foramen ovale + patients, flow-mediated dilatation did not change significantly on the control day but after agitated saline reduced by 2.3 ± 0.3%, 20 minutes after bubble injection (P < 0.005 vs. corresponding change in flow-mediated dilatation during control study). There was no significant change in flow-mediated dilatation for patent foramen ovale- patients at either visit.

CONCLUSION

These results suggest that the presence of a patent foramen ovale facilitated impairment of endothelial function acutely by the transfer of microbubbles into the arterial circulation. As a patent foramen ovale is a common condition, this may be relevant to microbubbles exposure in medical procedures and in decompression illness.

Bubble size on detachment from the luminal aspect of ovine large blood vessels after decompression: The effect of mechanical disturbance

Bubbles nucleate and develop after decompression at active spots on the luminal aspect of ovine large blood vessels. Series of bubbles were shown to detach from the active spot with a mean diameter of 0.7-1.0mm in calm conditions. The effect of mechanical disturbance (striking the bowl containing the vessel or tangential flow) was studied on ovine blood vessels stretched on microscope slides and photographed after hyperbaric exposure. Diameter on detachment after a heavy blow to the bowl was 0.87 ± 0.43 mm (mean ± SD), no different from bubbles which detached without striking the bowl (0.86 ± 0.28 mm). Bubble diameter on detachment during pulsatile tangential flow at 234 cm/min, 0.99 ± 0.36 mm, was not smaller than that seen in the same blood vessels in calm conditions (0.81 ± 0.34 mm). The active spots were stained for lipids, proving their hydrophobicity. The most abundant active spots, which produced only a few bubbles, did not stain for lipids thereafter. The possibility that phospholipids were removed along with detached bubbles may correlate with acclimation to diving. The finding of bubble production at the active spots matches observed phenomena in divers: variable sensitivity to decompression, acclimation to diving, the effect of elevated gas load on increased bubble formation, a higher bubble score in the second dive on the same day, and unexplained neurological symptoms after decompression. Large bubbles released from the arterial circulation give serious cause for concern.

Patent foramen ovale influences the presentation of decompression illness in SCUBA divers

BACKGROUND

Few have examined the influence of patent foramen ovale (PFO) on the phenotype of decompression illness (DCI) in affected divers.

METHODOLOGY

A retrospective review of our database was performed for 75 SCUBA divers over a 10-year period.

RESULTS

Overall 4,945 bubble studies were performed at our institution during the study period. Divers with DCI were more likely to have positive bubble studies than other indications (p<0.001). Major DCI was observed significantly more commonly in divers with PFO than those without (18/1,000 v.s. 3/1,000, p=0.02). Divers affected by DCI were also more likely to require a longer course of hyperbaric oxygen therapy (HBOT) if PFO was present (p=0.038). If the patient experienced one or more major DCI symptoms, the odds ratio of PFO being present on a transoesophageal echocardiogram was 3.2 (p=0.02) compared to those who reported no major DCI symptoms.

CONCLUSION

PFO is highly prevalent in selected SCUBA divers with DCI, and is associated with a more severe DCI phenotype and longer duration of HBOT. Patients with unexpected DCI with one or more major DCI symptoms should be offered PFO screening if they choose to continue diving, as it may have considerable prognostic and therapeutic implications.

Patent foramen ovale and scuba diving: a practical guide for physicians on when to refer for screening

Divers are taught some basic physiology during their training. There is therefore some underlying knowledge and understandable concern in the diving community about the presence of a patent foramen ovale (PFO) as a cause of decompression illness (DCI). There is an agreement that PFO screening should not be done routinely on all divers; however, when to screen selected divers is not clear. We present the basic physiology and current existing guidelines for doctors, advice on the management and identify which groups of divers should be referred for consideration of PFO screening. Venous bubbles after diving and right to left shunts are common, but DCI is rare. Why this is the case is not clear, but the divers look to doctors for guidance on PFO screening and closure; both of which are not without risks. Ideally, we should advise and apply guidelines that are consistent and based on best available evidence. We hope this guideline and flow chart helps address these issues with regard to PFOs and diving.

[Neurological decompression illness in a Japanese breath-held diver: a case report]

We report a Japanese breath-hold diver (Ama) who presented neurological disorders after diving. He repeated diving into 25-30 meters depth in the sea for 6 hours. After diving, he felt dizziness and unsteady gait. Neurological examination showed left quadrant hemianopia, bilateral limb ataxia and ataxic gait. Head CT revealed gas bubbles in the left parietal lobe. In CT scan on 3 days after onset, gas bubbles disappeared and low density areas were observed in the bilateral parietal lobes. Brain imaging (DWI, T(2)WI and FLAIR) demonstrated high intensity in the parieto-occipital lobes. Neither pulmonary barotrauma nor intracardiac shunt was detected. He was diagnosed as having neurological decompression illness and therefore underwent hyperbaric oxygen therapy. The pathogenesis of this case was considered to be microbubbles induced by decompression. The present case suggests that repetitive rapid surfacing from the deep sea causes neurological decompression illness even in the breath-hold diver.

The pathogenesis of atrial and atrioventricular septal defects with special emphasis on the role of the dorsal mesenchymal protrusion

Partitioning of the four-chambered heart requires the proper formation, interaction and fusion of several mesenchymal tissues derived from different precursor populations that together form the atrioventricular mesenchymal complex. This includes the major endocardial cushions and the mesenchymal cap of the septum primum, which are of endocardial origin, and the dorsal mesenchymal protrusion (DMP), which is derived from the Second Heart Field. Failure of these structures to develop and/or fully mature results in atrial septal defects (ASDs) and atrioventricular septal defects (AVSD). AVSDs are congenital malformations in which the atria are permitted to communicate due to defective septation between the inferior margin of the septum primum and the atrial surface of the common atrioventricular valve. The clinical presentation of AVSDs is variable and depends on both the size and/or type of defect; less severe defects may be asymptomatic while the most severe defect, if untreated, results in infantile heart failure. For many years, maldevelopment of the endocardial cushions was thought to be the sole etiology of AVSDs. More recent work, however, has demonstrated that perturbation of DMP development also results in AVSD. Here, we discuss in detail the formation of the DMP, its contribution to cardiac septation and describe the morphological features as well as potential etiologies of ASDs and AVSDs.

Paradoxical gas embolism after SCUBA diving: hemodynamic changes studied by echocardiography

Hemodynamic changes induced by self-contained underwater breathing apparatus diving were investigated using Doppler echocardiography. We detected circulating bubbles in both right and left cavities of the heart and in the cerebral circulation in two divers with a large patent foramen ovale. A reduction in the left ventricular preload was suggested by echocardiographic measurements. The decreased cardiac preload was paralleled to a lower stroke volume and cardiac output. These findings were also observed in divers with no evidence of circulating bubbles. In these subjects, pulmonary vascular resistances remained unchanged while an increase was observed in the two divers with arterial bubbles. This increase could promote right-to-left shunting.

Venous gas embolism as a predictive tool for improving CNS decompression safety

A key process in the pathophysiological steps leading to decompression sickness (DCS) is the formation of inert gas bubbles. The adverse effects of decompression are still not fully understood, but it seems reasonable to suggest that the formation of venous gas emboli (VGE) and their effects on the endothelium may be the central mechanism leading to central nervous system (CNS) damage. Hence, VGE might also have impact on the long-term health effects of diving. In the present review, we highlight the findings from our laboratory related to the hypothesis that VGE formation is the main mechanism behind serious decompression injuries. In recent studies, we have determined the impact of VGE on endothelial function in both laboratory animals and in humans. We observed that the damage to the endothelium due to VGE was dose dependent, and that the amount of VGE can be affected both by aerobic exercise and exogenous nitric oxide (NO) intervention prior to a dive. We observed that NO reduced VGE during decompression, and pharmacological blocking of NO production increased VGE formation following a dive. The importance of micro-nuclei for the formation of VGE and how it can be possible to manipulate the formation of VGE are discussed together with the effects of VGE on the organism. In the last part of the review we introduce our thoughts for the future, and how the enigma of DCS should be approached.

Determinants of arterial gas embolism after scuba diving

Scuba diving is associated with breathing gas at increased pressure, which often leads to tissue gas supersaturation during ascent and the formation of venous gas emboli (VGE). VGE crossover to systemic arteries (arterialization), mostly through the patent foramen ovale, has been implicated in various diving-related pathologies. Since recent research has shown that arterializations frequently occur in the absence of cardiac septal defects, our aim was to investigate the mechanisms responsible for these events. Divers who tested negative for patent foramen ovale were subjected to laboratory testing where agitated saline contrast bubbles were injected in the cubital vein at rest and exercise. The individual propensity for transpulmonary bubble passage was evaluated echocardiographically. The same subjects performed a standard air dive followed by an echosonographic assessment of VGE generation (graded on a scale of 0–5) and distribution. Twenty-three of thirty-four subjects allowed the transpulmonary passage of saline contrast bubbles in the laboratory at rest or after a mild/moderate exercise, and nine of them arterialized after a field dive. All subjects with postdive arterialization had bubble loads reaching or exceeding grade 4B in the right heart. In individuals without transpulmonary passage of saline contrast bubbles, injected either at rest or after an exercise bout, no postdive arterialization was detected. Therefore, postdive VGE arterialization occurs in subjects that meet two criteria: 1) transpulmonary shunting of contrast bubbles at rest or at mild/moderate exercise and 2) VGE generation after a dive reaches the threshold grade. These findings may represent a novel concept in approach to diving, where diving routines will be tailored individually.

Decompression illness

Decompression illness is caused by intravascular or extravascular bubbles that are formed as a result of reduction in environmental pressure (decompression). The term covers both arterial gas embolism, in which alveolar gas or venous gas emboli (via cardiac shunts or via pulmonary vessels) are introduced into the arterial circulation, and decompression sickness, which is caused by in-situ bubble formation from dissolved inert gas. Both syndromes can occur in divers, compressed air workers, aviators, and astronauts, but arterial gas embolism also arises from iatrogenic causes unrelated to decompression. Risk of decompression illness is affected by immersion, exercise, and heat or cold. Manifestations range from itching and minor pain to neurological symptoms, cardiac collapse, and death. First-aid treatment is 100% oxygen and definitive treatment is recompression to increased pressure, breathing 100% oxygen. Adjunctive treatment, including fluid administration and prophylaxis against venous thromboembolism in paralysed patients, is also recommended. Treatment is, in most cases, effective although residual deficits can remain in serious cases, even after several recompressions.

High incidence of venous and arterial gas emboli at rest after trimix diving without protocol violations

SCUBA diving is associated with generation of gas emboli due to gas release from the supersaturated tissues during decompression. Gas emboli arise mostly on the venous side of circulation, and they are usually eliminated as they pass through the lung vessels. Arterialization of venous gas emboli (VGE) is seldom reported, and it is potentially related to neurological damage and development of decompression sickness. The goal of the present study was to evaluate the generation of VGE in a group of divers using a mixture of compressed oxygen, helium, and nitrogen (trimix) and to probe for their potential appearance in arterial circulation. Seven experienced male divers performed three dives in consecutive days according to trimix diving and decompression protocols generated by V-planner, a software program based on the Varying Permeability Model. The occurrence of VGE was monitored ultrasonographically for up to 90 min after surfacing, and the images were graded on a scale from 0 to 5. The performed diving activities resulted in a substantial amount of VGE detected in the right cardiac chambers and their frequent passage to the arterial side, in 9 of 21 total dives (42%) and in 5 of 7 divers (71%). Concomitant measurement of mean pulmonary artery pressure revealed a nearly twofold augmentation, from 13.6 ± 2.8, 19.2 ± 9.2, and 14.7 ± 3.3 mmHg assessed before the first, second, and the third dive, respectively, to 26.1 ± 5.4, 27.5 ± 7.3, and 27.4 ± 5.9 mmHg detected after surfacing. No acute decompression-related disorders were identified. The observed high gas bubble loads and repeated microemboli in systemic circulation raise questions about the possibility of long-term adverse effects and warrant further investigation.