INTRODUCTION
Mechanical asphyxia (MA) is one of the most common causes of death in forensic medicine. MA consists of at least 8 types based on the constriction sites on the respiratory tract and lethal tools used, for example, hanging, ligature strangulation (LS), manual strangulation (MS), choking, smothering, drowning, traumatic asphyxia and positional asphyxia. The types of MA have been reported in cases of suicide, homicide, and accident.[ 1–4 ] To discriminate MA from other causes of death, examination on both specific and nonspecific signs on the Victim’s body may be helpful.[ 5 ] In addition, the type-specific signs are important for identifying different types of MA, and for inferring manners of death and finding crime clues. Besides signs on a victim’s body, epidemiologic characteristics, such as gender, age, death scene, and lethal tools are also important for identifying the cause and manner of death.[ 6 ]
The incidence and type preferences of MA may also be connected with geographic location, economy, and demography. However, these epidemiological characteristics of MA have been rarely reported. Guangdong province tops the province-class economy and population sizes (both total and migrant workers) in China, and the cities of Guangzhou and Dongguan are ranked the first and third respectively in the municipal-class ranking in Guangdong province. To determine the epidemiological features of MA in these two cities, which can represent semitropic districts with developed economy, large population and high-proportioned migrant workers, we retrospectively analyzed the typical characteristics of MA cases in these two cities. The results in this study will be comparable and useful for research and forensic practice in other regions with the similar geographic location, economy, and demography to Guangzhou and Dongguan.
METHODS
All of the death cases recorded in the Postmortem Examination Databases (from January 2009 to March 2019) of Dongguan Public Security Bureau and Tianhe Branch of Guangzhou Public Security Bureau were reviewed, and cases of MA were selected. Since drowning is usually studied independently from other types of MA, it was not enrolled in the present study. Records including gender, age, birthplace, asphyxia types, death scene, cause of death, manner of death, tools, and autopsy findings were analyzed. Demography data in the concerned areas were obtained from the official statistical databases (http://tjj.dg.gov.cn/tjnj/index.html http://tjj.gz.gov.cn/tjgb/qtgb/ http://112.94.72.17/portal/queryInfo/statisticsYearbook/index
Statistical analyses were performed using SPSS v24.0 (IBM, Chicago, Illinois, USA). Comparisons between rates were analyzed using the Pearson Chi-squared test or Fisher’s exact probability method. Logistic regression analysis was used to study the relationship between gender and homicide rates.
RESULTS
General profiles
A total of 913 MA cases were enrolled in this study. As shown in Figure 1a , the majority of the cases were hanging (596 cases, 65.3%). LS and MS accounted for 12.7% and 11.2%, respectively. Together these showed that strangulation comprised 89.2% of the all cases (Strangulation includes hanging, LS and MS). Other types of MA, including choking, smothering, traumatic asphyxia, and positional asphyxia, comprised 2.5%, 2.5%, 0.8%, and 0.2% of the all cases, respectively. Sixteen of the cases (1.8%) were combined with other fatal injuries, and 28 (3.1%) were not defined regarding the types of asphyxia. Sixty-one of the cases had more than one type of MA: 40 cases with a combination of MS and smothering, 6 cases with a combination of LS and smothering, 10 cases with a combination of LS and MS, and 5 cases with a combination of LS, MS and smothering. The population statistical databases disclosed 453.68 thousand deaths in the two districts during the study, and MA contributed 2.01 per thousand deaths (Data not shown).
Figure 1: General profiles of the enrolled cases. (a) The distribution of MA types. LS and MS refer to ligature strangulation and manual strangulation, respectively. (b) Sex distribution (a P value refers to a paired comparison between 2 of the 3 groups) of strangulation victims. (c) Age distribution of strangulation victims. MA: Mechanical asphyxia, LS: Ligature strangulation, MS: Manual strangulation
The 814 strangulation cases included 596 cases of hanging (73.2%), 102 of LS (12.5%), and 116 of MS (14.3%) [Table 1 ]. Males accounted for 57% of the total strangulation cases, 71.0% of hanging cases, 26.5% of LS cases, and 12.1% of MS cases, respectively [Table 1 ]. Differences in gender distribution were statistically significant among the three strangulation subgroups [P < 0.016, Figure 1b ], while the age distribution among them showed a similar rise-fall pattern, with a peak at 20–29 years old. However, the number of hanging cases from 0 to 29 years raised much more dramatically than that of LS and MS [Table 1 and Figure 1c ]. In the 23 choking cases, 17 victims (73.9%) were males [Table 2 ], 15 victims aged from 20 to 60 years, 4 victims were infants and 2 victims were over 60 years old [Table 2 ]. The victims of the 23 smothering cases, most of which were females [78.3%, Table 2 ], were comprised of 15 young adults, 5 infants, a 4-year-old child, and 2 persons over the age of 60. In cases of asphyxia concomitant with mechanical injuries and cases with undefined MA type, males were less than females [Table 2 ].
Table 1: Sex and age distributions in strangulation case
Table 2: Sex and age distributions in nonstrangulation mechanical asphyxia cases
Death scene information
About 75.0% of the top 5-type cases had a definite death scene [Table S1 ]. More than half of the death scenes (62.2%) were indoors or enclosed spaces, among which 94.8% (507) were living areas (home, dormitories, rentals, or hotels), and a small amount were work sites or schools. Five death scenes were reported in cars. The outdoor death scenes were mainly on the mountains, in the woods, by the roads or rivers, and in the parks. Five of the 7 traumatic asphyxia cases took place during working time, caused by machines (2 cases) or collapsed buildings (2 cases) (data not shown).
Manners of death and lethal tools
As shown in Table 3 , 576 out of the 596 hanging cases (96.6%) were suicides. Only 2 hanging cases were homicide [Table 3 ]. One victim was a 4-year-old girl, who was hanged by somebody at her home. The other was a male aged 86, he was robbed at home and hanged using a nylon rope. Ninety-five of the 102 LS cases (93.1%) and all of the MS cases were homicides. Seven hanging victims and 4 LS victims died from accidents. Sexual asphyxia was found in 7 (4 hanging cases and 3 LS cases, 63.6%) out of the 11 strangulation accidents. In the choking cases, accidents accounted for 73.9%, and suicides were not found [Table 3 ]. Among the 5 homicidal choking cases, a newborn female was murdered by cramming tissue paper into her mouth. When her body was found in a washroom, her mouth and pharynx were filled with lots of bloody tissue paper. Buoyancy test of double lungs showed positive results, indicating that she was liveborn. Eighteen of the smothering cases (78.3%) were homicides [Table 3 ]. It is uncommon for someone to choose choking or smothering to suicide. We noticed that a 76-year-old man smothered himself in his apartment using two plastic bags to cover his head and filled the bags with petroleum gas [Table S2 ]. According to the results from the death scene investigation, DNA test, and autopsy, the possibility of homicide in this case was excluded. Only 7 traumatic and 2 positional asphyxia cases were identified [Table S2 and Figure 1a ], the majority of their manners of death were accident. We investigated the correlation between gender and manners of death. As a result, females were much more than males in homicide, compared in nonhomicide, and much more than males in LS (P < 0.001, odds ratio [OR]= 21.14) and in undefined type of MA (P < 0.001, OR = 28). Moreover, the ratio of female: male was 101:15 in the homicidal MS, with none victims in nonhomicidal MS [Table 4 ].
Table 3: Manners of death in the mechanical asphyxia cases
Table 4: Distribution of homicide and nonhomicide in mechanical asphyxia by gender
Detailed information on ligature materials were recorded in 73.3% of hanging and 57.8% of LS cases [Table S3 ]. At least 17 ligature materials were identified as killing tools, and 13 of them were found in both hanging and LS cases. Nylon ropes were found in 37.2% of all cases and 50.8% of hanging cases. Nylon and jute ropes were much more commonly reported in hanging cases than in LS cases, while plastic ropes, rubber ropes, tapes, clothes, belts, and silk stockings were less common in hanging cases (P < 0.0027). Electric wires, one of the most popular tools, accounted for only 18.6% of LS cases, indicating that the tool choices in the LS cases were more evenly distributed than in the hanging cases. This result may suggest high-proportioned unpremeditated homicides in the LS cases. In smothering cases, 6 different kinds of tools were reported [Table S2 ] and hands were the most common (9/23) one, and the victims in all of the hand-smothering cases were females. Adhesive tapes were used in 5 cases, in 2 of which plastic bags were used together. Plastic bags were also used together with gas in one case, and used alone in another case. It seems that plastic bags were used usually together with other tools in smothering cases (3/4). Quilts appeared as killing tools in 4 cases, of which infant accidents were in the majority (3/4). A drunken man was lying prostrate and smothered by the ground. So did a captured and binded thief. In a homicide case, a towel was used to kill a female infant by her mother.
Victims’ birthplace in strangulation cases
We investigated the distribution of victims’ birthplace in the strangulation cases. Of those with known birthplace, 36.3% were from Guangdong province located in South China, and 38.9% were from other provinces in South China [Table 5 ]. The victims whose birthplace were further away from Guangdong were found to be fewer. We observed that there was a significantly higher proportion of hanging cases in the victims with a native place of Guangdong (84.8%) than in those with other birthplaces (Guangdong-excluded South China: 60.6%, Southwest China: 58.7%, and Eastern China: 53.3%, P < 0.05).
Table 5: Birthplaces of the victims in the strangulation cases
Characteristics of ligature marks in strangulation cases
The differences in ligature marks between hanging and LS cases were analyzed in this study. As shown in Table S4 , ligature marks could be found to be above, below or at the position of the thyroid cartilage in both hanging and LS cases, but over half of the known positions were above (80.43% and 56.67%, respectively). The proportion of marks above the thyroid cartilage in LS cases was fewer, compared with that in hanging cases (P = 0.003, OR = 0.32). The U shaped mark was found in over half of the hanging cases (51.01%) but none in LS cases. LS cases showed more completely encircling and discontinuous marks than hanging cases (P < 0.001, OR≥5.37), while the incidence of incompletely encircling marks had no significant difference between the two types (P = 0.086). Compared to hanging, more LS victims showed ≥two ligature marks (P < 0.001, OR=10.17). Exfoliation and subcutaneous hemorrhage were much more common in LS cases than in hanging (61.76% vs. 14.77%, P = 0.004 and 43.14% vs. 7.21%, P < 0.001), while blister marks showed an opposite result (4.90% vs. 12.00%, P < 0.001).
Autopsy findings in strangulation cases
As shown in Table 6 , hemorrhagic spots on the face were discovered in 40% of LS and 35.35% of MS cases, while in only 4.51% of hanging cases (OR = 14.29 and 11.11, respectively). About three-quarters of LS cases and one-quarter of MS and hanging cases exhibited conjunctival petechiae. The incidence of buccal petechiae in MS cases was higher (42.42%) than that in LS (18.89%) and hanging (4.15%) cases. Taken together, these results indicated that the typical signs of MA on the corpse surface are not always positive in strangulation cases, or even can be moderately rare in hanging cases. Thyroid hemorrhage was found in 12.93% of MS and 3.16% of LS cases, respectively. However, this corpse sign was absent in hanging cases. Neck muscle hemorrhages were the most common autopsy findings in all of the three MA types. The incidences of hyoid fracture in the three MA types were similar to each other (from 21.05% to 28.13%). MS cases showed a higher incidence of epiglottis hemorrhage than either hanging or LS cases. Similarly, the occurrence rates of pulmonary edema/emphysema, and hemorrhagic spots on the lung and the epicardium in LS and MS cases were much higher than that in hanging cases.
Table 6: Autopsy findings in the strangulation cases
DISCUSSION
MA has been classified into eight different types according to the pressed sites on the body and pressing manners.[ 7 , 8 ] Strangulation injuries are the primary cause of MA.[ 8 , 9 ] This study showed that strangulation accounted for up to 89.2% of the MA cases from the year 2009 to 2019 in the cities of Dongguan and Guangzhou, China.
This study showed the percentage of males in the hanging victims was 2.4 folds of females [Table 1 ], consistent with the findings reported in other studies.[ 6 , 10 ] Almost all of the hanging cases [96.6%, Table 3 ] were suicides. In this study, only 2 out of the 596 hanging cases were homicide, of which the victims (a young girl and an old man) were not strong. These data indicated that about 70% of the victims in hanging suicides were males. It remains to study such a gender preference is displayed whether in gross suicide or just in MA suicide. In consideration of that males were 1.86 folds of females in the gross suicide in Guangzhou in 2021 (data not shown), it seems males were more likely to suicide, and hanging was more likely to be chosen when they suicided. The male: female ratios in LS and MS cases were about 1:3 and 1:7, respectively [Table 4 ]. When combining the LS and MS cases, we found the male: female ratio was 1:4.3, lower than the ratio of 1:2.6 reported in a previous study using the data from 2001 to 2014 in Shanghai (a developed city in East China).[ 6 ] We noticed the incidence of hanging increased dramatically in the age group of 20–29 years, compared to younger groups [Figure 1c ]. Our study showed that victims aged between 20 and 29 were the most susceptible to strangulation [Figure 1c ], consistent with a previous study, which enrolled 1746 medicolegal autopsies.[ 9 ] However, our finding is moderately different from the Shanghai study indicating the 30–39 age interval was the most susceptible.[ 6 ] The incidence of hanging cases (73.2% of total strangulation cases) [Table 1 and Figure 1a ], in this study was different from the result in Shanghai city (44.2%).[ 6 ] Besides, this study showed a wider range of susceptible ages (from 20 to 89 years old) in hanging cases than that in LS and MS cases [Table 1 ], contrary to the Shanghai study.[ 6 ] Around three-fourths of choking victims in this study were males [Table 1 ], moderately lower than 12/13 reported in another study.[ 4 ] One-fifth of smothering victims were males [Table 1 ], lower than 51.7% reported in the previous study.[ 4 ] The age distribution in choking and smothering victims was discrete (ranging from infant to over 60 years old), [Table 1 ]. The manner of death in all of the MA types showed a central tendency: the most popular manner accounted for over 73.9% in each type [Table 3 ]. Furthermore, we found that more females than males were subjected to homicides [Table 4 ], indicating that females were vulnerable to MA when they suffered from crimes.
The killers would make efforts to choose a tool in a premeditated crime or suicide, resulting in an aggregated distribution pattern of certain tools. For example, 50.8% of hanging tools in this study were nylon ropes [Table S3 ]. The tools choices were evenly distributed in LS cases, suggesting that impulsive murder might be predominant in these cases. Except for using bare hands, adhesive tapes and plastic bags seemed as preferred smothering tools. In suicidal smothering cases, plastic bags were predominantly used.[ 11–14 ] While in autoerotic asphyxia, plastic bags were rare.[ 13 ] Special gases, such as CO, NO, CO2 , and other inert gases, were usually used with plastic bags together in suicides in recent years.[ 15–19 ] Petroleum gas was used in one of the included cases in this study. In this case, a 76-year-old man with depressive disorder killed himself in the toilet by putting his head in a plastic bag inflated with petroleum gas. He used a hose to deliver the gas from a petroleum tank. Petroleum gas was rarely reported as an atypical killing tool in asphyxia cases. A viewpoint is that the use of an atypical tool reveals the victim’s determination to commit suicide.[ 20 ] And mental diseases intensify the determination significantly.[ 20 , 21 ] In this study, diagnosis of schizophrenia or depressive disorder was recorded in 22 of the suicide cases, suggesting the importance of careful and particular management of patients with psychiatric or cognitive disorders.[ 20 ]
Although hanging accounted for most (≥53.3%) of the strangulation cases, not associated with birthplace, migrant workers to Guangdong province were more prone to suffer from LS and MS than local provincial residents, which also meant that migrant workers were more prone to suffer from homicide [Table 5 ]. Migrant workers being less wealthy might be a factor for this tendency, while further investigations are in need to support this speculation.
Typical signs of asphyxia on corpses are important for identifying MA in forensic practices. However, proven again in this study, these signs do not always be present in a MA case.[ 9 , 10 , 22 ] Moreover, many of these signs are not asphyxia specific, leading to more difficulties in identifying the causes of death in MA cases. Therefore, comprehensive analyses using all accessible information and postmortem findings are essential. In line with the previous studies,[ 6 , 8 , 23 ] incidences of some corpse signs in different types of MA are distinct. For instance, the proportion of hemorrhagic spots on the face in hanging cases was near or <1/10 to that in LS and MS cases, while the incidences of neck muscle hemorrhage were similar among the three MA types [Table 6 ]. These large differences are useful for distinguishing death causes and perhaps manners of death.
Tianhe District has been at the top of economy and has the third largest population in Guangzhou city. Guangzhou ranked first in economy with the largest population in Guangdong province, and third in economy with the sixth largest population in China. Dongguan has been ranking third in both economy and population lists in Guangdong province. Migrant workers in the both regions occupied more than one-third of their populations. Therefore, our findings in this study may reflect similar epidemical features of MA in other parts of China or countries with developed economy, large population and high-proportion of migrant workers. These features will be helpful for determining the types of MA cases in medicolegal death investigation.
Financial support and sponsorship
This study was supported by the Natural Science Foundation of Guangdong Province under Grant No. 2018A030313254; Guangzhou Science and Technology Program under Grant No. 2019030015.
Conflicts of interest
There are no conflicts of interest.
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