DETERMINING TIME SINCE DEATH (POSTMORTEM INTERVAL)
Following death, numerous changes occur that ultimately lead to the dissolution of all soft tissues. The importance of these changes to the forensic pathologist is that they provide a sequence of events that may allow an estimate of time since death ensued. This determination is based on the principle of sequential changes called the postmortem clock. The evaluation may include the following phenomena:
• Changes evident upon external examination of the body, such as temperature, livor, rigor, and decomposition;
• Chemical changes in bodyluids or tissues;
• Physiological changes with progression rates, such as digestion;
• Survival after injuries, based on the nature, severity and other factors such as blood loss. This hypothetical “postmortem clock” factors in various phenomena, such as stiff ness of the joints (rigor mortis), settling of blood on the skin surface (livor mortis), and decrease in body temperature (algor mortis), with additional laboratory findings, such as chemical measurements of body fluids and physiological changes such as digestion, to provide an estimate of how long it has been since the person died. Any attempt at this type of evaluation is best described as difficult, imprecise, and often not possible (DiMaio, 1999). As the postmortem interval increases, all these estimates become increasingly more inaccurate. Having started with that precautionary note, we must say that these observations are an integral part of the autopsy and can, in individual cases, provide valuable information. Because of the variation inherent in each of these processes, an initial time range of death is established and modified as more information becomes available. This initial time range is the interval prior to which it may be asserted with some evidence that the victim was alive, based on witness sighting, signed documents, or other established events. This initial time range is then modified by various methods of evaluating postmortem changes. Postmortem cooling (algor mortis) is often evaluated by various “rules of thumb” that state various degrees of cooling per hour. The best comment on these rules is by the eminent British forensic pathologist Bernard Knight who states, “The only thing that can be said about these rules is that, if they happen to be right it is by chance and not science” (Saukko and Knight, 2004). The eyes are also an indicator of postmortem changes. Because the circulation of blood ceases, blood settles in the innermost corners of the eyes. If the eyes remain open, a thin film forms on the surface within minutes and clouds over in 2–3 h; if they are closed, it may take longer for this film (an hour or more) and cloudiness (24 h) to develop. Postmortem lividity can be seen as early as 20 min after death, peaking in about 3–4 h. In its early stages, lividity will blanch when pressed (nonfixed), but in advanced stages it will not change under pressure (fixed). If the pattern of lividity does not match the position of the body as it was found (refer to Figure 7.7), it indicates that the body has been moved after death. In more advanced stages, the eventual pressure will burst the skin capillaries, causing petechiae.
Stomach contents may be helpful in the determination of time since death. This determination is based on the assumption that the stomach empties at a known rate, which speeds or slows with the various types of food in it. Light meals last in the stomach for 1.5–2.0 h, with heavier meals or meals mixed with alcohol taking a longer period of time. Food moves from the stomach in small amounts, after having been chewed, swallowed, and ground into tiny pieces. A meal eaten hurriedly or gulped will last longer because it hasn’t been properly chewed. Alcoholic beverages also delay the stomach’s evacuation. The range of variation is quite large, and estimates must be taken with caution. Decomposition of the body begins almost immediately after death and consists of two parallel processes:
• Autolysis, the disintegration of the body by enzymes released by dying cells; and
• Putrefaction, the disintegration of the body by the action of microorganisms, such as bacteria. The body passes through four main stages of decomposition: fresh, bloated (as the gaseous by-products of bacterial action build up in the body cavity), decay (ranging from wet to mushy to liquid), and dry. These changes depend in large part on the environmental factors surrounding the decedent, such as geographical location, seasonality, clothing, sun exposure, and animals and insects in the area. Insect activity, when present, greatly assists the decomposition process.
