Forensic Medicine: Case 3

Emergency services were called at 10:14 on the 20th August when a group of divers was seen in difficulty in the Sound of Kerrera. A coastguard helicopter, local coastguard teams and the Oban RNLI lifeboat were mobilised as part of a rescue operation.

The body of a 50-year-old man was recovered from the water.

Following their inquiries, you have been provided with a copy of the police report.

Police Sudden Death Report:

• The deceased’s unconscious body was recovered from the surface of the water approximately 30 minutes after fellow divers reported him missing
• The deceased was taken to a nearby hospital but died en route. Cardiopulmonary resuscitation was attempted.
• The deceased was an amateur diver.
• The dive profile downloaded from the dive computer showed the deceased ascended 29 meters in 20 seconds. The equipment was tested and no faults were identified.

The patient has undergone a post mortem CT scan of his chest, abdomen and pelvis. The main findings are summarised below:

• Gas in the left ventricle and right middle cerebral artery
• Mediastinal emphysema
• Overdistended lungs with bullae
• Antero-lateral rib fractures
• No other injuries

Routine tissue samples for histology and a sample of ilio-femoral blood, urine and vitreous humour for toxicological analysis are all negative.

Diving related deaths are complex. The cause of death is due to drowning in the majority of cases but the underlying aetiology of this is varied and includes fatigue, panic and reduced level of consciousness secondary to decompression illness, trauma, equipment malfunction or underlying natural disease.

Decompression illness encompasses arterial gas embolism and decompression sickness; these two entities present similarly but have distinct mechanisms. In decompression sickness, metabolically inert gases are dissolved in plasma and body tissues during descent as the pressure increases, as governed by Henrys law (the solubility of a gas in a liquid is proportional to its partial pressure) and if the diver is to return to normal atmospheric pressure too quickly, the dissolved gas will come out of its solution and gas bubbles can appear in the circulation, potentially causing gas emboli and give rise to infarction, this is sometimes referred to as ‘off-gassing’ and may occur passively post mortem. The clinical presentation of this is varied.

A more physical damage is that of volume changes when a sudden or rapid decompression occurs, known as barotrauma. Barotrauma is a consequence of pressure changes on the body and can occur on descent or ascent. According to Boyles law; pressure and volume of gas are inversely proportional at a constant temperature. With increased pressure during descent, gas volume in air containing body cavities such as the lungs is diminished. If the diver ascends too quickly or without equalising the pressure, the rapid decrease in ambient pressure increases gas volume and in the lungs the risk of overdistension can cause rupture of alveoli. In this case, air can leak into the arterial circulation (arterial gas embolism; AGE) or pass into different anatomical spaces causing pneumothorax, pneumomediastinum or subcutaneous emphysema. In arterial gas embolism (AGE), gas bubbles can reach organs via the systemic circulation and cause ischaemia, the most sensitive to this is the central nervous system and cerebral air embolism can be fatal.

Investigation into Scuba diving fatalities requires a multifaceted approach, with inquiry into a number of areas including; past medical and diving history, the circumstances of the dive (water conditions, dive profile, depth etc), equipment, events before and after the fatal incident as witnessed and the findings at post mortem. Pathologists play a central role in this and should be aware of the outcome of investigation into these areas. Difficulties encountered in the investigation of diving deaths include prolonged post mortem interval, inconsistent diving profile information and witness statements and equipment loss or damage.

In addition, body decomposition, predation damage and resuscitation related injuries make autopsy interpretation of decompression illness problematic and can cause false positive results on post mortem imaging.

The general consensus amongst pathologists is to perform a post mortem CT scan (PMCT) as soon as possible prior to undertaking an autopsy in order to look for intra and extra vascular air, which can be practically difficult to assess during autopsy.

Whilst drowning may be reported most commonly as the cause of death in divers, this is ultimately a terminal event and any investigation needs to identify underlying or precipitating  causes. AGE and pulmonary barotrauma represents the next largest group of fatalities after drowning. The history of the diver coming to the surface and quickly losing consciousness is classic, because of the loss of consciousness, evidence of drowning is often seen as an agonal event.

Intravascular gas, subcutaneous emphysema and pneumothorax can be seen in cases where resuscitation has been performed. Although CPR rarely results in cerebral air embolism, pulmonary vessels may rupture and findings at autopsy may include air bubbles present in the epicardial and cerebral vessels.

In our case, the history was suggestive of arterial gas embolism and the findings at on CT scan and post mortem also support this. Unfortunately, most cases are not this convincing and there may be no specific signs. It is important to look for underlying pathology as this may be a more significant contributing factor to the death.

• The cause of death is due to drowning in the majority of cases

• Underlying health problems need to be ruled out

• Arterial gas embolism is difficult to diagnose but post mortem CT scanning can help

• Resuscitation injuries and decomposition can give false positive results