Understanding Postmortem Interval (PMI)

Postmortem Interval (PMI) refers to the duration between the time of death and the discovery of a deceased body. It is an essential aspect of forensic investigations, although it is primarily an estimation rather than a definitive determination. PMI is typically presented as a range or a minimum period and can span from a few hours to several weeks, months, or even years. During the initial 72 hours after death, forensic pathologists can generally provide a reasonably accurate determination of the time of death by evaluating factors such as the body’s condition and the decline in body temperature. However, beyond this timeframe, the available medical information becomes less reliable for correlating PMI, necessitating expertise from other domains to establish a clearer time of death (Dorothy 2007).

Figure 1: Picture of Human Remains at Scene of Crime
(Source: taphonomy.wikispaces.com)

Time of death can be understood in three distinct forms. First, the estimated time is the best guess made by the forensic pathologist based on available evidence. Second, legal death refers to the time when the body was discovered or officially pronounced dead, as recorded on the death certificate. Finally, physiological death indicates the point at which the victim’s vital functions ceased. To illustrate, consider a scenario where a serial killer kills a victim in July, but the body is not discovered until October. In this case, the physiological death occurred in July, whereas the legal death is marked in October. The pathologist’s estimated time of death may vary and could be July, June, or even August. It is worth noting that many deaths are not witnessed by anyone, such as natural deaths during sleep, accidental deaths when the victim is alone, and suicidal deaths. In cases of homicides, the perpetrator is often the sole witness, and even if they check their watch, they are unlikely to divulge such information to investigators (Lyle 2004).

However, despite the limitations, various forensic methods can be employed to estimate PMI more accurately. These methods involve observing changes that occur in the body after death and interpreting the presence of certain indicators, such as rigor mortis and entomological succession. Additionally, the examination of the vitreous humor and body temperature can also provide valuable insights into PMI estimation.

Postmortem Interval (PMI) estimation plays a crucial role in forensic investigations, helping to establish timelines and direct the course of criminal inquiries. Although it remains an estimation, experts use multiple forensic approaches to make reasonably accurate estimates of PMI within specific timeframes. Understanding PMI not only aids in determining the time of death but also serves as a vital tool in narrowing down suspects and focusing investigations, thus contributing significantly to the pursuit of justice.

Methods to Determine Postmortem Interval (PMI)

Estimating the Postmortem Interval (PMI) is a critical task in forensic investigations. Several methods are employed to assess PMI, each with its own advantages and limitations.

Body Temperature

The body’s normal temperature during life is approximately 37°C (98.6°F). After death, the body loses heat at a rate of about 1 degree Fahrenheit per hour until it reaches the ambient temperature. However, this rate can vary based on the surrounding environment. Forensic experts often measure the core body temperature either rectally or using the liver’s temperature to obtain a more accurate reflection of the body’s true temperature.

Figure 2: Relationship between cooling of body temperature with the decomposition rate, livor mortis dan rigor mortis
(Source: ilbe.com)

Various factors can influence the accuracy of PMI estimation using body temperature. For instance, the sooner a body is found, the more accurately the time of death can be assessed before the body reaches ambient temperature. On the other hand, factors like obesity, clothing, warm still air, exposure to direct sunlight, or enclosed environments can slow down the heat loss process. Conversely, a thin child or emaciated elderly individual with an unclothed body exposed to cold and moving air, water, shade, or cold surfaces will lose heat more rapidly.

Figure 3: Changes of core body temperature after death
(Source: skepticaljuror.com)

Despite some inaccuracies, early and careful measurement of core body temperature and consideration of surrounding factors allow forensic pathologists to make reasonably accurate PMI estimates, particularly within the first few hours after death (Lyle 2004).

Rigor Mortis

Rigor mortis refers to the stiffening and contraction of muscles caused by chemical reactions in muscle cells after death. The loss of adenosine triphosphate (ATP) from the muscles is the reason behind the body becoming rigid after death. ATP serves as the energy source for muscular activity, and without it, muscles cannot contract.

Rigor mortis follows a predictable pattern and begins throughout the body at the same time at room temperature. It is first detectable in small muscles of the face and neck about 2 hours after death and then progresses downward to larger muscles. The entire process of rigor mortis, from the onset to the flaccid stage, takes approximately 36 to 48 hours after death.

Entomology

 
Figure 4: Assemblage of insects with importance of PMI estimation
(Source: willsavive.com)

By identifying the various insect species and studying their succession on the corpse, forensic entomologists can determine the probable length of time the person has been dead. The sequence of colonization by insects varies depending on factors such as burial, exposure on the soil surface, or submersion in water.

Figure 5: Life Cycle of Blow Fly
(Source: nlm.nih.gov)

While entomological analysis is a powerful tool in PMI estimation, it requires meticulous identification of insect specimens and careful observation of their life cycle. By relating the succession of decomposition fauna to specific stages of decay, experts can provide accurate estimates of PMI, especially when a large assemblage of insects is present on the body (Arnaldos et al. 2004).

Accumulated Degree Temperature Data

PMI can also be estimated by using accumulated degree temperature data at the crime scene and specific base temperatures corresponding to different insect species and life cycle stages. Insects are cold-blooded and rely on environmental temperatures for their growth and development.

The accumulated degree days or hours (ADD/ADH) can be calculated based on the difference between the crime scene temperature and the base temperature for insect growth. By understanding the growth rate of insects at different temperatures, forensic experts can use these data to estimate the time when the insects first laid eggs on the body until the discovery of the corpse.

In controlled environments, where temperature fluctuations are minimal, larval growth in length can also be used to determine PMI. Diagrams such as isomegalen and isomorphen are used to correlate the length of larvae to the time since hatching or the emergence of the adult (eclosion) (Dorothy 2007).

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).

Figure 6: Vitreous humor
(Source: stlukseye.com)

The Significance of PMI in Criminal Investigations

Estimating the Postmortem Interval (PMI) plays a crucial role in criminal investigations, aiding in the process of narrowing down suspects and directing the focus of the investigation. By accurately determining the time of death, investigators can eliminate some potential suspects and concentrate their attention on others, helping to bring justice to the victims and their families.

For example, consider a case where a husband claims to have left home for a business meeting at 2 p.m. and returned at 8 p.m. only to find his wife deceased. He asserts that she was alive and well when he left in the morning. If the forensic pathologist determines the time of death to be between 10 a.m. and noon, the husband will come under scrutiny and need to provide a credible explanation for his whereabouts during that time. On the other hand, if the PMI estimation reveals that the death occurred between 4 and 6 p.m., and the husband can produce a reliable alibi for that specific period, the investigation will move in a different direction.

Figure 7: Decay process and faunal investigation timeliness
(Source: visual-evidence.com)

Determining Primary and Secondary Crime Scenes

In addition to assisting with suspect identification, PMI estimation can also provide valuable insights into whether a body has been moved from its original location, indicating the presence of a secondary crime scene.

The specific assemblage of insects found on a corpse is a critical indicator of potential body relocation. Certain insect species may be more characteristic of different habitats or geographic regions. If an unexpected fly species is present on the body, it could suggest that the body was moved from one place to another, potentially from indoors to outdoors or vice versa. Determining this requires extensive knowledge of the local fauna and consultation of peer-reviewed journals that provide information about insect species in specific areas. The accuracy and reliability of such conclusions are essential for presenting evidence in a court of law.

Furthermore, the process of livor mortis, postmortem hypostasis, or lividity, is another tool used to estimate PMI and assess whether the body has been moved. Lividity is the settling of blood in the lowest parts of the body after death, resulting in a dark purplish discoloration. It typically appears within 30 minutes to a maximum of 8 – 12 hours after death. During the initial 6 hours, the position of the body can affect the location of livor mortis, as the blood vessels break down, and the blood settles in the surrounding tissues. After 6 – 8 hours, the lividity becomes fixed.

Figure 8: Livor Mortis occurred on a dead body
(Source: ghefley.com)

Challenges in Postmortem Interval (PMI) Estimation

Estimating the PMI is a complex process that involves considering various factors, and several challenges can affect the accuracy of the results. Some of the key challenges in the PMI approach include:

1. Indoor / Outdoor Sites:
   The location where the body is found can significantly impact the rate of decomposition and the colonization of insects. Indoor environments tend to have a delay of 3 – 4 days before flies begin to colonize the body, compared to outdoor sites where insect activity is usually quicker. This difference in colonization rates must be taken into account during the PMI estimation process.

2. Weather:
   Weather conditions, especially temperature, can have a profound effect on the decomposition process and insect activity. Fluctuations in temperature can lead to variations in the rate of decomposition and the growth of insects. Accurate and consistent temperature data from the crime scene is essential to make reliable PMI estimates. However, inconsistencies in temperature recordings, such as variations between meteorological station data before the body’s discovery and records in the days or weeks after, can pose challenges and require careful consideration.

3. Injuries:
   Injuries on the body can accelerate the decomposition process, leading to a faster colonization of flies and the progression of decomposition fauna. Trauma and wounds can affect the rate at which insects infest the body and may need to be factored into the PMI calculation to ensure accuracy.

4. Temperature of the Maggot Mass:
   In cases where there is a significant maggot mass on the body, the temperature of the mass may differ from the ambient temperature. It is crucial to use the appropriate temperature of the maggot mass for larval development, especially for late second instar or third instar larvae, to achieve more precise PMI estimations.

5. Puparia and Soil Temperature:
   If puparia (pupal cases of insects) are present on the body, the crime scene soil temperature at different depths (e.g., 5, 10, and 20 cm) should be considered. This helps adjust the estimated crime scene air temperatures to reflect the time when the insects entered the pupal stage, further refining the PMI estimation.

6. Temperature Fluctuations:
   Using average (mean) daily temperatures alone when calculating accumulated degree data may not account for temperature fluctuations (maxima and minima) that occur during the day. These fluctuations can impact insect development rates, and it is important to consider them for more accurate PMI estimates.

Conclusion

Postmortem Interval (PMI) estimation plays a significant role in forensic investigations. PMI refers to the time since death until the discovery of a dead body and is crucial in establishing timelines for criminal inquiries. Forensic experts use multiple approaches to make reasonably accurate estimates of PMI within specific timeframes. Understanding PMI helps determine the time of death, narrows down suspects, and focuses investigations, contributing to the pursuit of justice.

PMI estimation involves various forensic methods, each with its limitations, but combining techniques and considering different factors can lead to reliable estimates. Challenges in PMI estimation include indoor/outdoor sites, weather conditions, injuries on the body, and temperature fluctuations, which must be carefully addressed for accurate results. Though PMI is essential in criminal investigations, it remains an educated estimation, not an exact science. Therefore, forensic experts must exercise caution and interdisciplinary expertise to ensure the reliability and admissibility of PMI evidence in legal proceedings.

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