PMI Estimation 3.0

Why estimate PMI?

• To give police a preliminary idea on the time of an assault

• To check whether the time-since-death is consistent with the alibi of a suspect

• To order deaths and survivorships when more than one death occurs (i.e. Who died first?)

Sources for PMI estimations

• Evidence from the body of the deceased (postmortem changes, eg. stages of decomp, forensic entomology)

• Factors concerning the deceased’s habits (eg. arriving at work and day-day))

• Information from the environment in the vicinity of the body (eg. newspaper w/ date on it —> circumstantial)

Methods of highest scientific value

Quantitative, mathematical descriptions, clear data, precision proven on independent material & in field studies

Methods of lowest scientific value

Subjective descriptions, progression of postmortem changes is highly dependent on environment factors which cannot be considered quantitatively

General problems with PMI estimation

• Most methods only useful in first 1-2 days postmortem

• The actual case does not meet requirements of experimental investigation

• High ambient temp. is a problem (speeds up decomp)

• Experimental reference material w/ calculation of the margin of error is not available

• Experts unfamiliar with the scientific background, requirements, limitations of a method

Supravitality in muscles (First phase)

(Subjective, Dotzauer, 1955)

• Mechanical excitation of the muscle reveals a contraction of the whole muscle (propagated excitation). Can be seen up to 1.5-2.5 hrs postmortem

Supravitality in muscles (second phase)

(Subjective, Dotzauer, 1955)

• A strong and typically reversible idiomuscular pad (localized muscle contraction) develops. This phase maybe seen as long as 4-5 hours postmortem

Supravitality in muscles (third phase)

(Subjective, Dotzauer, 1955)

• A weak idiomuscular pad develops which may persist over a rather long period (up to 24 hrs). The weak idiomuscular pad can be seen in the time interval up to 8-12 hrs postmortem

Factors influencing PMI estimation via supravitality in muscles

• The duration of electrical excitability differs among diff muscles

• Gylcogen content at time of death

• Ambient temperature

Rigor mortis

• Begins at 2-6 hrs after death

• Persists for 24-84 hrs

• It is followed by gradual relaxation until the muscles become flaccid again

Livor mortis

• Most evident 2 hrs after death

• Dependent on body position, mass, contact areas

Algor mortis (rectal temperature)

• Influenced by accuracy of the device

• Influenced by insertion of the device (10-15 cm beyond the anus)

Algor mortis (brain temperature)

• Might be influenced by amount of hair

• Might be influenced by the region of the brain

Algor mortis (eye temperature)

• Not to be used in cases of open eyelids or face covered

Algor mortis (External auditory canal)

• Not to be used if the body is lying on its side w/ one ear in close contact w/ a surface

• Tested w/ living bodies w/ sensors, not probes

Vitreous humor

• Isolated topography

• Vitreous K+ concentration increases linearly with time since death and helps us estimate PMI

• More difficult to use than rectal temp; more resources needed

Factors influencing PMI estimation via vitreous humor

• Higher urea values = higher VH [K+]

• Eye disease

• Younger individuals = higher VH [K+] —> older individuals have a shriveled up VH, leading to lower VH [K+]

Protein degradation

• Usually happens in a specific timeframe, but there are so many proteins in the body that it’s a hard method to use

• An estimate of the time of death can be made:

  • When certain degradation processes have already occurred

  • When certain degradation processes are yet pending

• Affected by temp

Principles behind protein-based PMI estimation methods

• Identify suitable marker proteins

• Analyze particular postmortem alterations

• Correlate those alterations to specific PMI

• Consider influencing factors

RNA/DNA Degradation

• DNA easily degraded by environmental factors

• RNA better for estimating PMI, but it is expensive and time consuming + applied to recent deaths ≈ 24 hrs

Gastric contents

The material found in the stomach, including food, liquids, and digestive enzymes, which can provide insight into the time of death based on its digestion stage.

Forensic entomology

The correct identification of the insect species collected in association with the corpse and/or the surroundings is of utmost importance (diff. species have diff life cycles)

Radiocarbon dating

A method used to determine the age of organic materials by measuring the amount of carbon-14 isotopes remaining in the sample.

Fluorescence and chemiluminescence

• Have been used for skeletonized remains, results not bad

• “Fresher” bones show up purplish while older bones don’t show a difference

  • But standards of “strong” vs “weak” color depend on analyst

• Expected decrease in the intensity of chemiluminescence reaction with the increase of PMI

Bone weathering

• Six distinct stages of bone weathering

• Considers presence/absence of soft tissues, bleaching, bone cracking, delamination, and exposure of spongy bone