Seatbelts Determine Location of Brain Injury

Abstract

Introduction:

Crash testing research reveals that wearing a seatbelt greatly reduces the chance of brain injury during roll-over and frontal impact crashes; the restrained passenger will experience less overall body movement and is less likely to be ejected from the vehicle (Strother, 1984). In a 35 mph frontal barrier collision, the average displacement of a restrained passenger is 56, 40, and 30 cm for the head, pelvic, and chest areas. While it is difficult to estimate the displacement of a unrestrained passenger, during the same collision, the overall displacement would be measure in meters (Moffat, 1984). Due to greater overall movement by unrestrained passengers, one would expect an unrestrained participant to sustain a greater number of impact points to the head, resulting in a greater number of cortical lesions.

The purpose of this study was to analyze the effect of seatbelt usage on type and severity of TBI. It is commonly accepted that using a restraint while operating a motor vehicle decreases the likelihood of serious injury. This notion was supported. For example, only 21.4% of the head injury patients in our study came from the presumed 67% of seat belt wearers, while 78.6% came from the presumed 23% of unrestrained drivers. In the current investigation, belted and unbelted subjects who sustained moderate to severe TBI were compared. Specific effects of seatbelt usage were analyzed on two levels: type of brain injury sustained (i.e., the anatomical structures involved), and head injury severity, as measured by Glasgow Coma Scale (GCS), loss of consciousness (LOC) and length of hospitalization.

Methods

Subjects were included in the study only if their file contained a completed PHIP application, acute care medical records, and evidence of head injury as determined by the PHIP criteria. Approximately 100 subjects were excluded due to prior head injury, congenital defects, organic or degenerative brain disorders, vascular disorders, psychiatric history, or other prior physical, emotional, or cognitive disabilities, (in accordance with Schatz, 1995). In addition, approximately 229 subjects were excluded from the study because they did not sustain TBI as a motor vehicle occupant.

Procedures: Lesions specified on CT and MRI reports were coded for 27 distinct locations. Cortical lesions were grouped into anterior (frontal lobe) and posterior (parietal/occipital). Subcortical lesions were divided into subcortical (diencephalon and basal ganglia) and brainstem (mesencephalon and metencephalon) groups.

Results:

Location of Injury Incidence and percentage of lesions were calculated for cortical lobes and for subcortical regions for each group (belted vs. unbelted motor vehicle occupant).

The presence of lesions was collapsed into anterior and posterior regions. Chi² analysis reveals a significant effect [X²(1)=7.10, p<.01] of seatbelt usage on posterior lesions; subjects not wearing seatbelts were significantly more likely to sustain damage to the posterior region of the brain. Anterior lesions were common to both groups, as 59% of all restrained drivers and 61% of all unrestrained drivers sustained frontal lobe damage

Because roughly 60% of all participants sustained frontal lobe damage, analysis of cortical damage excluded this region. A two tailed t-test revealed significant effects of seatbelt usage on total number of lesions sustained in the parietal and occipital lobes [F=1.98, p<.002]. Those utilizing a seatbelt sustained significantly fewer posterior cortical lesions.

In addition, subcortical lesions were counted for each subject, ranging from (0) lesions to (4) lesions per individual. The average number of lesions for each group is displayed in Table 1. A statistical analysis of brain injury to the hypothalamus-thalamus region, basal ganglia and internal capsule revealed marked differences between groups. Participants wearing a seatbelt were much more likely to have positive findings in this group of structures (labeled "Subcortical"), at [F= 4.44, p < .001]. However, an analysis of only brainstem structures (pons, medulla, cerebellum, and fourth ventricle) did not reveal significant differences between groups.

Table 1 Lesions to the Interior of the Cerebrum

	SUBCORTICAL*		BRAINSTEM**
	Mean	(S.D.)	Mean	(S.D.)
Seatbelt	.37	(.82)	.16	(.37)
No Seatbelt	.12	(.39)	.17	(.40)
*-hypothalamus/thalamus, basal ganglia, internal capsule
**- pons, medulla, cerebellum, fourth ventricle

Discussion:

Lesion analysis allowed for several generalizations to be drawn about the types of brain injuries sustained during MVA. Subjects in both groups sustained an equal percentage of anterior lesions, which can be explained by deceleration conditions commonly experienced by all vehicle occupants (restrained or unrestrained) during MVAÕs. The present results show that subjects not wearing a seatbelt at the time of their TBI sustained significantly more focal and posterior lesions. It is probable that this is due to the greater displacement experienced by unbelted passengers. Furthermore, belted passengers sustained a greater number of subcortical lesions. This may be the result of head and neck flexion at impact.

By reducing severity of TBI, seatbelt utilization may have significant implications for rehabilitation prognosis. In addition, decreased acute care hospital stays with seatbelt use may translate to significantly higher residual funding for rehabilitation in the current managed-care climate. Furthermore, understanding the fundamental differences in brain injury between these groups may ultimately be useful in the intervention and rehabilitative planning for the brain injured patient.

In gaining a better understanding of the physical process of brain injury, different methods to protect the central nervous system can be developed. As this study points out, the physical conditions that the human body endures during an MVA might be quite different, depending upon the use of a safety restraint. A multidisciplinary approach to studying brain injury could offer specific answers about TBI by bringing physiology face to face with physics. For example, the best prophilaxis to brain injury may be to allow for increased movement by the head and neck, while preventing head/obstacle contact. The answers are not simple, yet by understanding exactly how brain damage occurs, engineers may develop the next generation of safety systems with the brain in mind.

Future Directions:

The seatbelt is only one factor accounting for the variance in brain injury severity. The authors of this investigation plan a prospective investigation into other factors believed to be contributing to patient injury and outcome. These factors include vehicle factors such as size and speed, external factors such as what is struck or what is striking the vehicle, as well within cab factors such as airbag deployment. There have been very few ecologically valid studies performed that investigate brain injury. As stated, most investigation into MVAÕs and passenger outcome focuses on survival rates and not specifically injuries.

References:

Jagger, J., Levine, J. I., Jane, J.A., & Rimel, R.W. (1992). Epidemiologic features of head injury in a predominantly rural population. Journal of Trauma, 24 (1), 40-44.

Kallieris, D., Mellander, H., Schmidt, G., Barz, J., Mattern, R., "Comparison between Frontal Impact Tests with Cadavers and Dummies in a Simulated True Car Environment", 26th Stapp Car Crash Conference, SAE, 1982.

Moffat, C., Moffat, E., Weiman, T., "Diagnosis of Seat Belt Usage in Accidents", SAE, 840396, March 1984.

Popular Press, Motor Trend (1993). Peterson Publishing Company, October, Vol. 45, No. 10, pg. 114, ISSN: 0027-2094.

Schatz, P. (1995). Predicting level of Independence following moderate and severe traumatic brain injury. Doctoral Dissertation, Drexel University.

Strother, c., Smith, G., James M., Warner, C., "Injury and Intrusion in Side Impacts and Rollovers", SAE, 840403, March 1984.