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Postmortem Interval Estimation

Postmortem Interval Estimation Using Forensic Approaches

What Is Postmortem Interval (PMI)

Postmortem interval (PMI) is the period of time since death occurs till the discovery of dead bodies. It is most of the time an estimation rather than a determination. It is usually written as a summary of minimum period or with a range estimated and can be given over a period of hours, weeks, months or years. In the first 72 hours after death, the forensic pathologist is usually considered to be able to provide a reasonably accurate determination of the time of death based upon the condition of the body itself and such features as the fall in body temperature. Beyond this time, there is less medical information with which to correlate PMI. So another area of expertise is required to clarify time of death (Dorothy 2007).


Source: taphonomy.wikispaces.com
Picture of Human Remains at Scene of Crime

Time of death actually may come in three different forms. One is estimated form that is the best guess as determined by forensic pathologist. Another form is legal death where the time when the body was discovered or pronounced dead and also the time that’s recorded on the death certificate. The third one is physiological death where the time at which the victim’s vital functions actually ceased. For example, if a serial killer kills a victim in July, but the body isn’t discovered until October, the physiological death took place in July, but the legal death is marked in October. The pathologist’s estimated time of death could be July, or even June or August. Many deaths are not witnessed by anyone. Natural deaths may occur during sleep, and accidental deaths often occur when the victim is alone, and so do suicidal deaths. In homicides, the perpetrator typically is the only witness, and even if he checks his watch, he’s not likely to pass on that information to the investigators (Lyle 2004).

Method To Determine PMI In Dead Body

After death, bodies begin to decompose in regular pattern which are used to estimate physiological time of death, however, these changes occur in widely variable ways and no single factor accurately indicates PMI.


Source: ilbe.com
Relationship between cooling of body temperature with the decomposition rate, livor mortis dan rigor mortis

Body Temperature

Normal body temperature when we’re alive is 37°C, the body loses heat after death at a rate of about 1 degree Fahrenheit per hour until it reaches ambient temperature but its rate varies on different environment. Body temperature is taken rectally or using liver temperature to reflect true core body temperature. The sooner body found, the more accurately time of death can be assessed by this method before body reaches ambient temperature. Obesity, clothing, warm still air, exposure to direct sunlight or enclosed environment can slow down the heat loss. A thin children or emaciated elderly, unclothed corpse exposed to cold and moving air, water, shade or cold surfaces will lose heat faster. Heat loss is fraught with inaccuracies, yet with early and careful measurement of core body temperature and surrounding factors, the pathologist still can make a reasonably accurate estimate (Lyle 2004).


Source: skepticaljuror.com
Changes of core body temperature after death

Rigor Mortis

Rigor mortis refers to the stiffening and contraction of muscles caused by chemical reactions that take place in the muscle cells after death. The reason the body becomes rigid after death is the loss of adenosine triphosphate (ATP) from the muscles. ATP is the compound formed from oxygen and nutrients, serves as energy for muscular activity and without it muscles can’t contract. When ATP levels fall after death, muscles contract and stiffen, producing rigidity but subsequently flaccidity occurs when muscle tissues itself begin to decompose. Rigor mortis begins throughout the body at the same time in predictable pattern at room temperature. Beginning about 2 hours after death, rigor mortis is first detectable in small muscles of face and neck and progresses downward to larger muscles. The entire contracting process takes about 8 – 12 hours, after which the body is completely stiff and is fixed in the position of death. The body remains fixed for another 12 hours, a state that is called the rigid stage of rigor mortis. The process reverses itself, and rigidity is lost in the order from small muscles to larger ones. After another 12 hours, the muscles become relaxed in the flaccid stage of rigor mortis. In a nutshell, rigor mortis is useful only for estimating time of death during the first 36 – 48 hours after death. Sometimes rigor mortis comes very quickly after death when ATP levels have been reduced before death such as running from assailant, intense physical activity, obesity, heat stroke, strychnine poisoning, sepsis, pneumonia or febrile causing process. Whilst cold climate or immediate frozen condition may not develop rigor on body for days, perhaps not even until body is warmed or thawed. Hence, rigor is the least reliable methods for determining PMI due to it is extremely variable and is easily broken by bending or stretching the corpse as muscle fibers broken and rigor won’t return (Lyle 2004).


Source: skepticaljuror.com

Entomology

PMI estimation can be done using succession of insect species on the corpse and their respective life cycle. Investigating PMI for a period of at least 3 or more months may mean that there is a large assemblage of flies, beetles and other insects present on the body. The duration of this stage, in relation to the particular stage of decay, gives an accurate measure of the probable length of time the person has been dead and may be the best estimate that is available. This methods requires that first of all every specimen is identified to family and then relate to the colonization and succession of the decomposition fauna (Arnaldos et al. 2004). Goff (1993) points out, some of the insects present on the corpse are only reflective of that location or habitat, whilst others are present on the corpse because of a direct relationship to stage of corpse decomposition and so can be used for determining the PMI. The sequence of colonization by insects is different, depending upon whether the body is buried, left on the soil surface or has been rinsed into water. The diversity of fauna colonizing buried corpses is smaller than those colonizing a corpse exposed above ground. Blowflies tend to be absent from buried corpses and the presence of coffin flies, Conicera tibialis revealed that the bodies buried at crime scene. Their presence on the soil surface could indicate the location of a body buried for at least a year and possibly longer. Flies can pass through a number of generations below ground before emerging; hence they are an indicator of buried bodies but not necessarily of the duration of the burial. Hakbijl (2000) relates the presence of beetle Rhizophagus paralellocollis (Gyllenhal) as inhabiting corpses buried for 2 or more years and feeing on fungi present. Successional changes in insect fauna on the corpse above ground have been related to the stages of decomposition through which the body passes. In general and under normal circumstances, if the entomologist finds only eggs of blowfly, death likely occurred less than 24 hours earlier. If fully grown maggots but no pupae, death occurred less than 10 days earlier. Finding pupae indicates that 6 – 10 or more days have passed, and the presence of mature hatchlings indicates that death occurred 2 – 3 weeks earlier

 
Source: willsavive.com
Assemblage of insects with importance of PMI estimation


Life Cycle
Source: nlm.nih.gov


Blowflies
Source: entomology.unl.edu


Conicera tibialis
Source: thecoffinfly.com


Rhizophagus paralellocollis
Source: coleoptera.ksib.pl

Furthermore, PMI can be estimated using the accumulated degree temperature data at crime scene and their specific base temperature according to different species and different stages of life cycle comparable to the available experimental growth rate at certain temperature (Klotzbach et al. 2004). The start of PMI is considered to coincide with the point when the fly first laid its eggs on the body, and its end to be the discovery of the body, depending on the recognition of life stage of the oldest colonizing species infesting it. Insects are cold-blooded (poikilothermic) and cannot control their body temperatures, so they use the environment as source of warmth. Insects use a proportion of the environmental energy (thermal units) to grow and develop. The overall energy budget to achieve life stages can be specifically calculated. The thermal units are called degree days (°D) / hours (°H) and can be added together to reflect periods of development in which called accumulated degree days (ADD) or accumulated degree hours (ADH) based on the corrected crime scene temperature using regression of meteorological data and temperature at scene after death, and then minus by base temperature. The minimal temperature for growth (base temperature) is worked out in the laboratory from the insect’s growth rate at set experimental temperatures by plotting temperature against 1/(total days to develop) whereby the cooler the temperature, the slower the insect develops by using linear approximation estimation method (Dorothy 2007).

Under a controlled or indoors environment where the temperature is not fluctuating, larval growth in length can be used to determine PMI by sketching isomegalen and isomorphen diagrams. The length of the larva, when killed in the standard way by immersing in boiling water, can be related to the time since the larva hatched. The time since hatch can then be read directly off the isomegalen diagrams on the basis of the length of the individual larvae collected from the crime scene. Another type of graph called isomorphen diagrams in which is derived where life cycle stages from egg hatch through to the emergence of the adult (eclosion) have been plotted against time at specific temperatures. Each line in the diagram indicates a change in the life cycle from one stage to the following stages (Dorothy 2007).


Source: scielo.br
Experimental Data

 
Source: scielo.br
Isomegalen Graph


Source: peerj.com
Isomorphen Graph

Vitreous Humor

Last but not least, looking into the eyes will give some clues on PMI as well. After death, corneas (clear covering over pupils) become cloudy and opaque but may take only a few hours if eye were open at death or up to 24 hours if closed. The concentration of potassium within vitreous humor (thick jellylike substance fills the eyeballs) increases slowly during first few days. This is independent of ambient temperature. However, any PMI determination from vitreous potassium clouding isn’t very accurate and is useful only within first 3 or 4 days (Lyle 2004).


Source: stlukseye.com
Vitreous humor


Source: anilaggrawal.com
Potassium level

How Can PMI Help In Investigation

In criminal cases, an accurate determination of the time of death eliminates some suspects and focuses attention on others. For instance, a husband says that he left home for a business meeting at 2 p.m. and returned at 8 p.m. to find his wife dead. He says that he was at home all morning and that she was alive and well when he left. If the pathologist determines the time of death was between 10 a.m. and noon, the husband has a great deal of explaining to do. On the other hand, if the estimation reveals that the death occurred between 4 and 6 p.m., and he has a reliable alibi for that time period, the investigation will move in a different direction. However, estimating the time of death is an inexact action. In order to make a best-guess estimate, the pathologist uses each and every means available; from witness statement to body temperature to bugs on the body as mentioned earlier (Lyle 2004).


Source: visual-evidence.com

How Can PMI Determine The Primary & Secondary Crime Scene

The particular assemblage of insects present on a corpse is also an important indicator of whether or not the body has been moved from primary to secondary crime scene. If an unexpected fly species is present on the body in which is more characteristic of a different habitat or geographic region, hence this shows that the body may have been moved from one place to another or from indoor to outdoor. This again depends on the knowledge of the local fauna and peer reviewed journals that providing information about species in particular areas and creates the basis for conclusion to be acceptable in a court. Livor mortis, postmortem hypostasis or lividity is used to estimate PMI and to determine whether the body has moved where something the dead don’t do without assistance. Lividity appears within 30 minutes up to maximum 8 – 12 hours after death. The dark purplish discoloration can be shifted by rolling the body to different position during the first 6 hours after death and become fixed after 6 – 8 hours due to the blood vessels have broken down and blood settles in surrounding tissues (Dorothy 2007).


Source: ghefley.com
Livor Mortis occurred on a dead body

Challenges In PMI Approach

  • Indoor / Outdoor Sites
  • Weather
  • Injuries

 When calculating the PMI, a number of factors need to be taken into account. Indoor condition will have a delay of 3 – 4 days for the fly to start colonizing the body compared to outdoor sites. It is also important to consider using temperature of the maggot mass as the temperature for larval development if it is greater that the ambient temperature especially true for late second instar or third instar. If puparia are recorded, the crime scene soil temperature at 5, 10 and 20 cm depth should be used to adjust the estimated crime scene air temperatures to reflect the time insect was in pupariation. Concern about the accuracy of temperature predictions has been expressed whereby meteorological station temperature recordings are at variance before body was discovered compared to records for few days or weeks after body was discovered (Archer, 2004). If the weather conditions differ markedly within 3 – 5 days, the best way forward is to grow the insects through to the life stage recorded from the body, using the crime scene temperature. Wall (2004) expressed concern about using average (mean) daily temperatures when calculating accumulated degree data, rather than taking account of the temperature fluctuations (maxima and minima). Injuries on the body will also affect the PMI calculation as dead body will be faster exposing to colonization of fly and further succession of decomposition fauna. In short, PMI is always only a best guess to assist the investigation of crime and exclude the innocents.

References
  1. Archer M.S. 2004. The effect of time after body discovery on the accuracy of retrospective weather station ambient temperature corrections in forensic entomology. Journal of Forensic Sciences 49(3): 1 – 7.
  2. Arnaldos M.I., Garcia M.D., Romera E., Presa J.J. and Luna A. 2004. Estimation of Postmortem Interval in Real Cases Based on Experimentally obtained Entomological Evidence. Forensic Science International 149: 57 – 65.
  3. Dorothy E.G. 2007. Calculating the Post Mortem Interval. 1st Ed. Forensic Entomology: An Introduction pg. 115 – 130. UK: John Wiley & Sons Ltd.
  4. Goff M.L. 1993. Estimation of post mortem interval using arthropod development and successional patterns. Forensic Science Review 5(2): 81 – 94.
  5. Hakbijl T. 2000. Arthropod remains as indicators for taphonomic processes: an assemblage from 19th century burials, Broerenkerk, Zwolle, The Netherlands. In Huntley J.P. and Stallibrass S. (eds), Taphonomy and Interpretation. Symposia for the Association for Environmental Archaeology, No 14. Oxbow Books: Oxford; pp 95 – 96.
  6. Klotzbach H., Schroeder H., Augustin C. And Pueschel K. 2004. Information is Everything – A Case Report Demonstrating the Necessity of Entomological Knowledge at the Crime Scene. Aggrawal’s Internet Journal of Forensic Medicine and Toxicology 5(1): 19 – 21.
  7. Lyle, D.P. 2004. Estimating the Time of Death. 1st Ed. Forensic for Dummies  pg. 161 – 173. Indiana: Wiley Publishing Inc.
  8. Wall R. 2004. Daily temperature fluctuation and the accumulation of the day-degrees. Proceedings of the European Association for Forensic Entomology, 29 – 30 March 2004, London.

 

Last Reviewed : 8 September 2015
Writer : Lai Poh Soon
Accreditor : Dr. Siew Shueue Feng