Rigor Mortis (Cadaveric Rigidity)
Definition
- Rigor mortis (Cadaveric Rigidity) is the postmortem stiffening/rigidity of the muscles in a dead body following Primary flaccidity.
Mechanism
The physico-chemical basis of rigor mortis is complex.
- The changes occurring under these phenomena are mainly due to the irreversible fusion of two contractile elements, the essential proteins, namely actin and myosin filament of muscle fibres into
a dehydrated stiff gel, making them remain in a rigid inextensible state. This will persist till the actin and myosin filaments undergo autolysis. - Rigor mortis is basically due to the depletion of adenosine triphosphate (ATP) reserve from the muscle. It is reported that when the ATP level falls to 85 per cent of normal level, rigor mortis is initiated and it will be
maximum when this level is 15-20 per cent of normal. - ATP is the main source of energy for muscle contraction.
Muscle needs a continuous supply of ATP to contract because the amount of ATP present in muscles is sufficient to sustain muscle contractions for only a few seconds. - After death, generation of ATP stops, though consumption continues. With fall of ATP levels, actin and myosin filaments become permanently complexed (fused) into a dehydrated gel and with this rigor mortis sets in.
Any violent muscular exertion prior to death will produce a decrease in ATP levels. This will speed up the onset of rigor mortis, since no ATP is produced after death. - Some of the factors that can cause depletion of ATP prior to death are violent or heavy exercise, severe convulsions and high body temperatures. All these factors in turn can bring about rapid onset of rigor
mortis within minutes in some cases, and in rare instances, instantaneously, which is thus called as ‘instantaneous rigor’ or ‘cadaveric spasm’. - Rigor mortis disappears with onset of decomposition. Cold and/or freezing will delay onset of rigor mortis as well as prolong its presence. Rigor mortis, when it develops, it involves all the muscles at the same time and at same rate. However, it becomes most evident in the smaller muscles. Thus rigor mortis is said to appear first in the smaller muscles, such as the muscles of eyelids, face, jaw and then gradually spread to larger muscle groups.
Order of appearance
- The classical presentation of rigor mortis in order of appearance is – jaw, upper extremities, and lower extremities in a proximo-distal distribution. It passes off also in the same order.
Testing for Rigor in Cadaver
- Testing for rigor in cadaver is done by trying to open eyelids, depressing the jaw, gently bending the neck and various joints of the body and noting the degree (complete, partial or absent) and distribution.
Onset of Rigor
- Gradually muscles begin to stiffen. It first occurs in the involuntary muscles and then in the voluntary muscles.
Time Taken for Onset of Rigor
- Upper part of body—stiffens by about 7 to 9 hours of death.
- Whole body—stiffens by 11 to 12 hours of death.
Secondary Flaccidity
- When the molecular death has occurred in each of the actin and myosin filaments, the muscular stiffness that had developed earlier in the previous stage will begin to disappear slowly and gradually, relaxing the body back to the original state [however, now it shows no response to any stimulus].
- Sequence of this relaxation commences in the same order as that of commencement of the stiffening, starting from eyelids downwards.
- Just as for developing complete rigor mortis 12 hours time is required, passing off of rigor also require another 12 hours.
Position Opted on Complete Development of Rigor
- Rigor mortis does not cause any significant contraction of muscles. However, if a person opts a particular position at death and remains like that, rigor mortis will be established in same position. This can provide a clue on position opted by the deceased at the time of death.
Postmortem Interval and Rigor
- In general, rigor mortis sets on within 1 to 2 hours after death, and is well developed from head to toes in about 12 hours.
- It is then maintained so far about 12 hours and passes off in about another 12 hours subsequently with onset of putrefaction. This is also known as ‘March of rigor’ or ‘Rule of 12’. However, when rigor mortis sets in early, it will tend to pass off quickly (vice versa of this is also true).
