Forensic challenges in house fire deaths: when did the burns occur?

#forensics #burns #postmortem

Establishing whether a victim found in a house fire sustained their burn injuries ante- or post- mortem is often challenging. Indirect clues can be found from the multidisciplinary forensic examination of the surrounding scene, e.g. a suicide note could point towards self-immolation or an obvious arson device might point towards a crime, but these are only adjuncts to the post-mortem examination, which is where the definitive clues as to the timing of burns will be sought.

During the autopsy, there are three key areas that can help distinguish between and ante- and a post-mortem burn.

I. Smoke inhalation

Ante-mortem burns are accompanied by vital reactions. As a result, soot and heat damage will be encountered in the distal airways (i.e. below the vocal cords) or the stomach. This indicates the victim was alive and actively inhaling/ swallowing combustion products during the fire. The presence of soot is identified macroscopically. Previous techniques of using electron microscopy of the trachea to identify soot particles are considered anachronistic and no longer used widely. Absence of soot distally would point towards a post-mortem burn injury.

However, care should be taken during autopsy, as soot can be encountered in the distal airways, if the trachea is not structurally intact. In these cases, soot can advance distally due to ingress, even in post-mortem burns. Furthermore, some ante-mortem burns may not be accompanied by vital reactions (e.g. from cyanide poisoning, flashfire, oxygen depletion), which may further confound the examination. In these cases, secondary clues such as the distribution of burn injuries (e.g. pointing to sitting/ standing vs lying position of the victim) might be helpful.

Smoke inhalation in ante-mortem burns is also often accompanied by elevated levels of noxious gases, most notably carbon monoxide (CO) and cyanide. While cyanide is difficult to measure, carbon monoxide is a very useful marker of smoke inhalation. Macroscopically, it causes the skin, internal organs and hypostases to appear cherry red (careful as refrigeration can cause similar patterns). It can also be easily tested in the blood, where levels above 35% are indicative of vital reactions and suggestive of ante-mortem burns. Lower levels are not that useful, as smokers and people who live in areas with air pollution can have CO levels of between 5-15%.

II. Burns and thermal injuries

Burns cause blistering and skin splits. It is practically not possible to establish whether a blister was caused pre- or post-death and skin splits may also mask other injuries, such as gunshot or knife wounds. However, careful internal examination may be able to help distinguish between splits and trauma, with particular emphasis on the presence of bruising or bleeding within the tissues, which would suggest an ante-mortem injury. This distinction may also be helpful in trying to establish whether falling masonry may have caused post-death blunt trauma.

III. Other trauma

Stab and gunshot wounds in the context of a greater burn injury were covered above.

Other notable confounding factors in assessing burn injuries, are specific to the skull, where thermal injury may cause heat extra durals and skull fractures from blood boiling within the skull; this may obscure external force trauma to the head. Both the pattern of the extra dural and its CO content can help to establish its cause: extra durals caused post mortem by heat have a high CO content and are not accompanied by temporal bone fractures, like traditional extra durals. Heat skull fractures are caused by separation of the inner and outer tables at the diploic space. Therefore, they are more friable than blunt force fractures and tend to involve the outer table only.

In summary, differentiating between ante and post mortem burn injuries can be difficult and requires a structured and meticulous approach to the forensic examination of the scene and subsequent autopsy of the victim.

Bibliography

Saukko P, Knight B. Knight's forensic pathology fourth edition: CRC press; 2015

Spitz WU. Thermal injuries. Spitz WU, ed. Spitz and Fisher's Medicolegal Investigation of Death. 3rd ed. Springfield, Ill: Charles C Thomas; 1993. Chapter X