Cessation of circulation and loss of muscle tone after death allows blood within vessels to ‘settle under gravity’, producing a pink/ purple colour in those areas of the body that are lowest, or ‘dependent’. In a body lying on its back, the back of the body show hypostatic discolouration, save for those areas compressed by direct contact with a firm surface, such as the buttocks and shoulder blade regions, for example.
The forensic significance of hypostasis is overstated in many textbooks, and was historically used as a means of providing a timetable for the postmortem interval. Modern forensic pathology aims to be evidence-based, and to provide cautious opinions regarding such phenomena, which are so heavily influenced by factors which may not be capable of adequate investigation, such as the temperature of the microenvironment in which the body lies.
Hypostasis may never actually appear (for instance in infants, the elderly, or those who were clinically anemic in life), and Fechner et al (1984) found that there was no linear relationship between the time at which no further movement of blood under gravity could occur (and the hypostatis was described as being ‘fixed’) and the time since death. Suzutani et al (1978) found that blood was mobile, and could therefore move under gravity, for up to 3 days. Most forensic pathologists would agree that hypostasis was maximal/ fully developed after about 12 hours.
The pattern of hypostasis may provide useful investigative information; if the distribution of hypostasis is on the back of the body, but the body is discovered lying on its front, the body has been moved after a sufficient time has elapsed during which posterior hypostasis has developed and become ‘fixed’.
When a body has lain on its front, postmortem hypostasis will devlop on the front of the body. Blood settling in lax soft tissues of the face can cause petechiae, and sometimes larger coalescences of petechiae (vibices or ‘Tardieu spots’), perhaps with pallor due to compression around the mouth and nose. In such circumstances, the pattern of hypostasis may appear sinister, and suggest that compression of the mouth/ nose (suffocation) had occurred. Detailed pathological dissection of the face and neck in such cases will frequently be able to distinguish such scenarios, but pressure to the face/ neck may not always be capable of being excluded. – I. Keränen