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).
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
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).
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).
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).
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).
Challenges In PMI Approach
- Indoor / Outdoor Sites
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.
- 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.
- 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.
- Dorothy E.G. 2007. Calculating the Post Mortem Interval. 1st Ed. Forensic Entomology: An Introduction pg. 115 – 130. UK: John Wiley & Sons Ltd.
- Goff M.L. 1993. Estimation of post mortem interval using arthropod development and successional patterns. Forensic Science Review 5(2): 81 – 94.
- 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.
- 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.
- Lyle, D.P. 2004. Estimating the Time of Death. 1st Ed. Forensic for Dummies pg. 161 – 173. Indiana: Wiley Publishing Inc.
- 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|