Case report

A 47 year old woman driving a 1997 registered Rover 414 at an estimated 19 mph was involved in a head-on collision with a 13 year old Opel Ascona driven at approximately 27 mph. There was structural damage to the front end and bonnet of the Rover but no substantial intrusion into the passenger compartment and the windscreen remained intact. The driver of the Rover had been wearing her seat belt and her airbag was deployed. She appeared to be unconscious in the vehicle and the emergency services were activated by a passer-by. The driver of the Opel was a 21 year old male. His vehicle was not equipped with an airbag, but he had been wearing his seat belt. He was uninjured apart from a cut lip and was ambulant at scene. A paramedic crew arrived approximately 10 minutes after the crash. The patient was unresponsive during extrication and a Glasgow coma score of 3 was recorded at scene (no eye opening, no motor response to painful stimuli, and no vocalisation). She was noted to have Cheyne-Stokes respirations and a palpable radial pulse. She was intubated and intravenous access was established. During transport to hospital the patient became asystolic and advanced cardiac life support was initiated according to standard protocols.⁵

On arrival in the emergency department there were no signs of life and asystole was apparent on the monitor. Bruising to the abdomen and sternal area, and bleeding from both ears was noted. Resuscitation was continued according to protocol for a further five minutes without success and the patient was declared dead.

A postmortem examination was carried out the next day. There was horizontal bruising of the lower abdomen, abrasions of the left anterior iliac region, and bruising from the central sternum to the left iliac region, all consistent with a seat belt mark. In addition there was an 8 cm diameter bruise in the middle of the sternal region. There was blood in both external ear canals and bruising of the nasal bridge. No other major facial injuries were noted. On internal examination there was a fracture of the sternum at the junction of manubrium and body, and fractures of the fourth to seventh left ribs laterally. In the skull there was a large ring fracture beginning just behind the pituitary fossa and extending through the right temporal bone posteriorly to the midline 8 cm behind the foramen magnum and anteriorly through the roof of the left temporal bone to the origin. The anterior and middle cranial fossae on both sides were freely mobile around this fracture. The brain showed diffuse swelling with blood in the subarachnoid space and at the base of the brain. The spinal column and nerve cord were normal. The immediate cause of death was deemed to have been a severe head injury.

The subject of this report was a middle aged woman of stout build, height 5` 7", weight 92 kg. Crash investigators have postulated that the unexpectedly severe injuries sustained were caused by her sitting too far forward when the airbag was deployed. As a result, the bag expanded suddenly into her chest and up towards the face, rather than the body impacting with the fully expanded bag as intended by design. This lead to sudden forced hyperextension of the head and neck resulting in the base of skull ring fracture as described.

Discussion

HISTORY AND DEVELOPMENT OF AIRBAGS

An airbag is a deflated balloon contained in the steering wheel or dashboard, which inflates rapidly on crash impact to cushion the occupant from injury within the car. Inflation is triggered by a deceleration sensor, which can be situated in various sites such as the front bumper or engine firewall. Triggering of the sensor activates a pyrotechnic device containing sodium azide that ignites producing a large volume of nitrogen gas, which then inflates the airbag to a volume of about 30 litres (Eurobag design) or about 70 litres (US full size) within 50 msec. The airbag then deflates almost immediately through vents in its lining. The timing of airbag inflation is crucial. If it fires too soon, it will already be deflating when the driver hits it, and will fail to provide any protection. If it fires too late, the driver's head will have already hit the steering wheel or will impact with the ballooning bag.

The first automobile airbag patent was filed in 1949 and issued in 1953. Since then airbag design and operation has been continually modified and improved. Design alterations have focused on such aspects as constituent materials, optimum bag size, inflation speed, and deployment threshold. However, it is only in the last decade that airbags have entered the UK car fleet in any significant numbers, and the proportion of vehicles equipped with airbags still lags well behind that in North America. Figures from the US show that in 1995 over 70% of US made new cars had airbags installed compared with 7% in 1989. Since 1998 car manufacturers in the US have been obliged to fit dual airbags in all new cars. The National Highway Traffic Safety Administration (NHTSA), a division of the US Department of Transportation, estimates that between 1986 and October 1999 airbags have saved 4758 lives (4011 drivers and 747 front passengers). This effect has occurred mainly in the last few years.¹

AIRBAG ASSOCIATED INJURY

With the increasing prevalence of airbags in North America, reports began to appear in the medical literature there in the early 1990s of injuries related to airbag deployment.⁶ The different constituents of the airbag, propellant capsule⁷ and chemicals,^{8, 9} airbag module cover,¹⁰ and the actual bag itself, have all been implicated in different injuries. However, most of the more serious injuries are caused by the act of deployment and are secondary to the shearing forces produced by the “punch-out” of the bag.^11–13

The most commonly observed injuries are minor bruises and abrasions, mainly to the face, neck, and upper limbs.¹⁴ These are often referred to as “bag slap” injuries. Some of the first reports were of eye injuries.⁶ The eye may be injured by the fully deployed airbag causing corneal abrasions, but more serious injuries such as retinal detachment and orbital blow out fractures can be sustained from an actively deploying airbag.^15–17 These injuries may be worse if the patient is wearing spectacles.¹⁸ Also, an alkaline chemical keratitis has been reported from the combustion byproducts of the conversion of sodium azide to nitrogen.⁸ Superficial burns of the upper extremities, face, and neck are well documented,^{19, 20} and full thickness burns can also occur.²¹ Temporary hearing impairment and tinnitus have been reported on a number of occasions.^{22, 23} Upper limb injuries occur due to the proximity of the forearm to the deploying airbag and are caused either by direct contact with the activated bag, or by flinging the limb into interior car structures.^24–26 Fractures of the forearm are rare and are usually due to direct impact with the opening of the airbag module cover.¹⁰

Minor chest injuries and rib fractures have been reported, and more serious thoracic injuries have occurred in comparatively low velocity crashes. These include rupture of the right atrium²⁷ and aortic dissection.²⁸ The aerosols generated by airbag deployment have also induced exacerbations of asthma in susceptible individuals.⁹

UNEXPECTED DEATHS

In recent years there have been a number of reports of serious and fatal injuries occurring in low velocity crashes where the forces involved would not normally be expected to cause serious harm.^{4, 12, 13} Also, the particular dangers passenger airbags pose to children placed in the front seat have been well documented.^{29, 30} The NHTSA Special Crash Investigation Program has identified 146 airbag related deaths to 1 October 1999, including 84 children, 18 of whom were infants in rear-facing infant seats.¹ When infants are restrained in rear-facing child seats in the front passenger seat, the head and neck are close to the airbag housing and traumatic brain injuries occur after a direct blow from the rapidly inflating airbag. Children who are unrestrained or poorly restrained will often sit or stand too close to the airbag when it deploys. Similarly, such children can be thrown forwards toward the airbag in pre-impact braking, and will be in front of the deploying airbag at impact.

Serious and fatal injury attributed to airbags has previously been reported mainly from North American centres. Thus far, few airbag related injuries have been reported from the UK and all have been relatively minor. Apart from the lower prevalence of airbags (which will change), there are a number of other reasons for this. Firstly, in the UK seat belt usage is mandatory and uptake is therefore much greater than in some US states. As a result, fewer British vehicle occupants rely solely on the protection afforded by the airbag. Secondly, airbags here are somewhat different. European airbags are designed primarily to protect belted drivers from making head and face contact with the steering wheel. Consequently, our airbags are smaller (average volume 30–40 litres) and deploy with much less force than the average US airbag (about 70 litres). The US airbag is designed to protect unbelted occupants, and inflation to 70 litres occurs within the same time frame (about 50 msec)—a much more violent deployment. A final point is that traffic laws have been in place here for some time which prohibit children from travelling unrestrained in the front seat.

When an adult driver or passenger is in an atypical position there is a change in the pattern of deployment of the airbag.³¹ Rather than preventing injury, the airbag can cause injury. This occurs primarily when the occupant is in the “deployment zone” of the airbag. Specifically, if the occupant is too near the steering wheel, the lower part of expansion of the airbag is restricted by the chest leading not only to excessive forces being applied to the thorax but also to a greater upward expansion of the bag. This can cause severe head and facial injuries with hyperextension of the neck and cervical spine injury. Reasons for being within the deployment zone include not wearing a seat belt (the commonest cause), sitting too close to the airbag housing (children and adults of short stature), and where deployment has occurred after pre-crash braking (the occupant having already been thrown forward before the airbag was deployed). The occupant could also be within the deployment zone if for some reason the airbag should fire late in the crash sequence. Basilar skull fractures as seen in our patient may not be uncommon in this context as illustrated by a recent report from Denmark of two similar cases.³² However, in both of these cases the drivers were unbelted. Our patient was wearing a seat belt, and the exact contribution of individual factors (such as initial seating position and pre-crash braking) that were responsible for placing her too close to the expanding airbag are not certain. All of these cases reinforce the need for emergency physicians to be alert to the possibility of significant head and cervical spine injuries in accidents of apparently moderate impact, particularly if the patient is unconscious.

RECOMMENDATIONS AND FUTURE DEVELOPMENTS

Most airbag related adult deaths are preventable if the occupants use seat belts, and keep at least a 10 inch distance between the airbag cover and the front of the chest. The vast majority of drivers (including those of shorter stature) should be able to maintain this distance, and the NHTSA in the US now asks drivers to measure the distance from the sternum to the middle of steering wheel. If this distance is less than 10 inches then the driver can apply to have the bag disconnected or an on/off switch installed. The 10 inch distance is 3 inches for initial activation, 5 inches of seat belt “give”, and an extra 2 inches.

Airbag technology continues to evolve. Future advances include the development of “smart” airbags that will adjust to seat position and passenger weight and height. Dual stage inflation systems (that is an initial reduced charge of propellant fires the airbag only to be augmented milliseconds later if a more severe deceleration is detected), side airbags, and driver feet protection airbags are all being developed.

Conclusions

The reduction in morbidity and mortality since airbags became widely available in new vehicles is well established. This report, the first fatal injury attributed to airbags reported in the UK, highlights the need to be aware of potential hazards associated with their use. Vehicle occupants should always wear seat belts and place children in approved child restraints in the rear of the vehicle. Drivers and passengers, who are unable to maintain the recommended 10 inches from the airbag cover, should consult with vehicle manufacturers over potential solutions including disabling of the airbag mechanism.