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Estimation of Postmortem Interval

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Estimation of Postmortem Interval
Importance of Postmortem Interval in an investigation
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After death, the body of every living organism undergoes certain changes and gets converted into inorganic matter. This whole process is dependent on time and hence changes undergone during this time forms the basis for estimating Postmortem Interval or time since death (TSD) of an individual. Postmortem interval is the duration between the death to the time when the dead body is found. 

The basic principle behind PMI estimation is the postmortem changes observed in the cadaver. All the postmortem changes are time dependent, therefore careful analysis of these changes can estimate the approximate time of death. PMI associates the accused to the particular moment of time which can prove his/her guilt or innocence and plays a key role in medicolegal investigation.

Postmortem changes depend on various factors like temperature, geographical area, cause of death, etc. and this creates a great challenge for the forensic pathologists for PMI estimation, especially in tropical countries. In India, PMI is mentioned in every postmortem report, issued by a forensic pathologist based on the observation of postmortem changes by naked eye.

The postmortem changes occur at various intervals of time and are categorized into three categories i.e.,

  • Immediate changes
  • Early changes
  • Late changes.

Immediate changes as the name suggests occur instantly after the death of an individual and lasts for about 1-2 hours after death.

Early changes start to occur within 1-2 hours and last for 12-24 hours after death. The early changes include- algor mortis, livor mortis, and rigor mortis. 

  • The algor mortis, which is the cooling of the body that starts within 6 hours of death and remains for 10-12 hours till the whole body surface gets cooled. 
  •  Livor mortis is the discoloration of the body due to gravitational flow of blood on the dependent parts. Postmortem Lividity starts within 1-3 hours after death and after 6-8 hours the lividity is fully developed. 
  • Algor mortis is the stiffening of body muscles which starts within 7-9 hours in the upper body and stiffens the whole body in 10-12 hours.

Late changes refers to the decomposition which begins after 24 hours of the death. 

  • Change in colour of the cadaver is commenced within 12-24 hours and is completed by 48 hours.
  • Foul smell from the cadaver is observed from 18 hours to 48 hours. Marbling becomes prominent within 36-48 hours after death. 
  • The putrefaction of internal organs begins within 24 hours and may last for one week or more.
  • Complete skeletonization of the dead body may result within 20-30 days after death.

Time Interval For Body Changes

The time interval for the postmortem change may vary depending on the following:

Time interval till body decomposition
Source: Medico Legal
  • Weather conditions of the place where the dead body is places affects the changes in the body. For example hot weather conditions helps in rapid decomposition of body as compared to cold weather.
  • Medium in which the body lies. As according to Casper’s dictum temperature being the same, the ratio of the rate of putrefaction of a dead body in air, water and buried in soil is 1:2:8 respectively. This means that the rate of putrefaction in air is twice as fast as in water and 8 times faster than in ground even if the temperature is same.
  • Cause of death also plays a major role in decomposition of body. Death from alcoholism can retard the putrefaction, death from septic diseases or kidney diseases can enhance the putrefaction rate, whereas deaths from plant poisons or barbiturates, cyanide may resist putrefaction.
  • Condition of the body i.e., fat and flabby body may hasten putrefaction, thin bodies with tight clothings may resist putrefaction and putrefaction is slow in newborn infants.
  • Presence of microorganisms on the body may increase the rate of putrefaction.
  • Scavengers or other animals that can eat dead flesh cause destruction to the dead body which can be mistaken as antemortem injuries.

Methods for Estimating Postmortem Interval

 The methods involved in PMI estimation are:

1. Examination of Postmortem Changes 

As discussed above, the postmortem changes give a key to estimate the approximate PMI. This requires the careful examination of the external as well as internal changes in the deceased. 

2. Forensic Entomology

Forensic entomology is the emerging field based upon the analysis of insects feeding on the cadaver. The PMI estimation from the entomological analysis is based on succession and development of insects on the dead flesh.

The succession approach is based on the environmental conditions which favour the growing of insects whereas developmental approach is based on the presence of various life stages of insect species on the body and the surrounding environment. The insects from the diptera family (flies) are the first one to invade the dead flesh which feed on soft tissues (early stage of decomposition).

The coleoptera insects (beetles) feed on the dried flesh (late decomposition). The various stages of the life cycle of insects attacking the body are great indicators of PMI.

3. Molecular Assessment

Advancement in molecular biology has proved their significance in PMI estimation. The evaluation of degenerating DNA, mRNA and proteins can estimate the PMI. RNA transcripts degenerate rapidly in comparison to DNA and proteins.

Conclusion

Postmortem interval is an important information giving direction to investigation in medicolegal cases. Various methods have been developed to estimate PMI but they have the limitation of elucidating only the approximate time, therefore more development is required in them to determine the exact time of death.

With the estimation of exact time of death the investigation procedure is strengthened and a strong link between the suspects and victim is established.

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