Osmium impregnation detection of pulmonary intravascular fat in sudden death: A study of 65 cases

Pulmonary fat embolism is widely recognised in forensic pathology. Pulmonary fat embolism requires mobilisation of free fat, entry of free fat into the circulation and lodging of fat globules in fine venous capillaries. This paradigm of fat embolisation has been used to support the evidence of antemortem fat depot disruption when the presence of intravascular fat is confirmed at autopsy. However, sporadic reports of intravascular fat in various medical conditions, which contradict the above mechanism, have opened questions about the alternative pathogenesis. In this study, 65 cases of sudden deaths were examined for the presence of pulmonary intravascular fat (PIF) by osmium impregnation. Cases were selected based on the criteria that were designed to eliminate the possible confounding effect from medical intervention or postmortem changes. Slides were graded based on their ease of search and only the fat droplets confined by the blood vessel or capillary wall were considered as a positive finding. The results show surprisingly high PIF incidences of varying degrees in all the categories of sudden deaths. Further study is needed to devise criteria for diagnosis of fatal fat embolism since the histological appearance of the high-grade PIF in natural sudden death may not be easily distinguishable from the traumatic fat embolism.