Rethinking airbag safety: airbag injury causing bilateral blindness.

A healthy 40-year-old man, restrained in the front passenger seat, suffered visually disabling blunt ocular trauma following spontaneous release of the passenger side air-bag module, during vehicular deceleration, without an automobile crash. Though the driver-side airbag was also released, the driver was unharmed. The passenger suffered bilateral hyphema, bilateral vitreous hemorrhage and suspected posterior scleral rupture in the left eye and also had an eyebrow laceration, from impact with the dashboard panel covering the air-bag module, which was detached by the force of airbag deployment. This is the first reported case from West Africa and the first case in which part of the airbag module detached to cause additional trauma. This report adds to the growing burden of evidence world-wide, for a review of the safety aspects of the automobile airbag. This case clearly illustrates that although airbags reduce mortality, they carry a high risk of ocular morbidity, even with seat belt restraint.

INTRODUCTION

The use of airbags to provide a cushioning effect in the event of a collision was first researched in the 1950s, approval for widespread commercial installation of airbags in passenger vehicles was given in the late 1980s and by the late 1990s, they became a regular safety feature in almost all cars, in aircrafts and even on the space shuttle.1 Airbags reduce fatalities in a crash by 28-50%.234 The risk of death and serious injuries, from airbag deployment modules was first recognized in the early 1990s when reports showed that certain groups of patients including small children, short statured women and persons seated in close proximity to a rapidly inflating airbag during the early phase of deployment, were vulnerable.25 Mechanical and chemical eye injuries have also been reported.23456 The majority of airbag-related eye injuries, appear to be of mild to moderate severity and are often unilateral.24 However, bilateral severe eye injuries have also been reported with poor long-term visual outcomes.7 This case reports a visually devastating outcome of airbag-related ocular injury.

CASE REPORT

A healthy 40-year-old male passenger seated in the front seat presented with a 6-h history of sudden bilateral vision loss following impact with the airbag module, which had suddenly deployed as the vehicle was decelerating. While the driver remained unharmed, the patient sustained bilateral severe blunt ocular trauma. He had been wearing his seatbelt, but was not wearing any eyewear. He had been sleeping until the impact of the airbag woke him. At presentation, visual acuity was no light perception (NLP) in the right eye and light perception with accurate light projection on the left. He had bilateral periorbital edema and mechanical ptosis [Figure 1] with an oblique laceration of the right brow measuring 4 mm. The orbital walls were intact. However, the right conjunctiva was injected with marked chemosis inferiorly, while there was left subconjunctival hemorrhage inferiorly and temporally (posterior limit seen). Both corneas were intact. There was a blackball hyphema in the right anterior chamber obscuring further view [Figure 2] and 30% hyphema with clots in the left eye [Figure 3]. The left pupil was round, mid-dilated and unreactive and the lens was clear. However, there was no view of the fundus in either eye. B-scan ultrasonography confirmed bilateral vitreous hemorrhage.

Figure 1

Marked bilateral periorbital edema, facial abrasions, right brow laceration (covered), right subconjunctival hemorrhage and chemosis at presentation

Figure 2

Marked lid edema, chemosis and subconjunctival hemorrhage, with black ball hyphema in the right eye

Figure 3

Hyphema clots and periorbital swelling of the left eye

He had right brow laceration repair with bilateral anterior chamber paracentesis and irrigation. The sclera underlying the subconjunctival hemorrhage in the right eye was also explored to identify and repair any scleral laceration, but none was found as far as the equator. Post-operatively, he received topical steroids, cycloplegics and prophylactic antibiotics. On the 10th day post-operatively, he remained blind with NLP in the right eye and could barely read 6/60 on the Snellen chart after refraction in the left eye. Examination of the posterior segment remained obscured by a dark reflex in the right eye and revealed commotio retinae on the left. He was discharged and is undergoing counseling and rehabilitation through the out-patient clinic. At his last follow-up visit, vision remained NLP in the right eye and 6/60 in the left eye.

DISCUSSION

The airbag module is a complex engineering system consisting of an inflatable bag, a trigger system, a crash sensor and compressed expandable gases including sodium azide, potassium nitrate, sodium dioxide and nitrogen.1248 The airbag system is delicately designed to ensure that it rapidly inflates at a rate of approximately 100-200 mph within a period of 30-50 ms, when the sensor detects a deceleration greater than 12-15 mph/s. However, spontaneous deployment has been reported at speeds below the deceleration threshold.1289 There are two main mechanisms of air bag related eye injuries. Mechanical forces may injure the face and eye directly or indirectly through the propulsion of hard objects, causing blunt or penetrating eye injury.2 This patient reported that the dashboard cover of the airbag module; was detached by the force of deployment and had hit his right eyebrow (causing the brow laceration). The other mechanism responsible for airbag related eye injury is chemical injury. Thermal burns to the cornea, lids and face occur as a result of heated gases released by the chemical reaction converting sodium azide to nitrogen gas within the bag. Pearlman et al.2 reviewed 127 eye injuries in 101 patients with airbag related trauma and reported corneal injuries in 57 patients (21.6%) while hyphema was the second most common injury in 45 cases (17%). Of the 57 patients with corneal injury, 9 (3.4% of the total number of cases) had chemical keratitis.2 Thus, mechanical trauma is by far the more common mechanism. Though airbags reportedly cause only 3% of all automobile crash related ocular injuries,5 these injuries are being more commonly reported since airbags became standard issue in most cars after 1997. Vehicle parts causing ocular injury in a crash include driver's side windshields, airbags and glass fragments.2 Bilateral airbag related eye injuries are relatively uncommon, especially in restrained passengers. Lee et al. reported five cases of bilateral airbag related eye injuries and reviewed a further 89 cases in the literature. Of the 94 cases reviewed, only 24 had bilateral eye injury and only 12 were passengers.3 Of 263 patients involved in 101 automobile accidents, Pearlman et al.2 recorded only 33 bilateral injuries. Most injuries involved the cornea and anterior segment (hyphema) with good visual outcomes.2 However, posterior segment injury, though less common, was associated with less favorable outcomes.2 Most cases of bilateral severe posterior segment airbag related injuries are individual case reports. Salman et al.7 reported bilateral globe rupture with poor visual outcome in a 75-year old restrained front seat passenger. Ball and Bouchard6 presented seven cases of airbag-related eye injury; only one of which was bilateral and in whom; the injury was limited to the anterior segment (bilateral hyphemas) with good visual outcome in both eyes. While good visual outcome is to be expected with airbag-related trauma to the anterior segment, when the injury involves the posterior segment as in the current case report, the injury is associated with a poor visual outcome with potentially permanent visual loss. Rao et al.10 have suggested that the airbag mitigates severe ocular trauma in a crash, when deployed in addition to a three-point restraint (seat-belt). Mechanical injuries caused by airbag inflation include periocular abrasions and lacerations, corneal abrasion, laceration, endothelial contusion, hyphema, lens dislocation, traumatic aniridia and in the long-term, angle recession glaucoma as a result of direct globe compression and rupture of blood vessels.

Penetrating ocular injuries often result from projectiles within the vehicle rather than the airbag itself.26 In this case, additional injury (a brow laceration) resulted from the plastic dashboard panel covering the airbag, which was detached by the force of the expanding bag. The code of federal regulations (CFR), CFR 173.166 specifies that an airbag, “if not completely enclosed by design, should be encased in a container or handling device which must be covered with plastic, fiberboard or metal” and that “the covering must be secured to the container by banding or other comparable methods.” It is therefore important for vehicle inspectors to ensure that the banding or other materials used to secure airbag module covers can withstand the explosive force of airbag deployment, over time. This is particularly important in older or second hand cars, which are common in developing countries. Nevertheless, the more significant injuries appear to have been a result of direct and forceful impact with the inflating airbag. This is often the case with low velocity crashes because airbag technology is designed for greater protection in high impact, high velocity crashes8 where the rapid inflation protects the occupant from contact with more rigid interior surfaces in the car at high speed, which could be fatal. The fact that the patient was sleeping at the time of impact, may also have contributed to the severity of his injuries, despite seatbelt use, because he may not have been aware to brace himself for impact with the bag, nor would he have been able to shield his face with his arms before impact, which would have been a natural protective response if the individual was awake. It is also possible that he was slumped forward as a result of his slumber and this may have put his face dangerously close to the dashboard and at a greater risk of ocular injury.4 Rao et al.10 observed posterior segment injuries only in patients who did not wear seatbelts. However, various reports of severe bilateral eye injuries in seatbelt wearers have been described.467 Stein et al. reviewed 97 airbag related eye injuries and found that 67% occurred at low speed (35 mph or less) and only 14% involved passengers.4 The fact that this case involved only the passenger and resulted in such severe and permanent bilateral visual loss is therefore pertinent. The bilateral hyphema and vitreous hemorrhage and suspected scleral rupture can be explained by horizontal expansile forces at the iris root, equator and vitreous base. Commotio retinae can be explained by a counter coup response to the anteriorly-located compressive blunt force.24 The design of airbags have evolved from the large, untethered, explosive first generation bags9 to “smart,” sensor-driven, depowered,9 tethered, venting airbags that are able to adjust their threshold and rate of deployment depending on the crash severity and can also vent quickly to ensure that the occupant does not strike a “hard” bag.1 However, this comes at an extra cost. In developing countries where the latest and safest technologies may be beyond reach, it is necessary to educate the public about the risks inherent in the older generation airbags. It may be necessary to apply regulations to restrict the use or recommend the replacement of first-generation airbags in second-hand vehicles purchased for resale in the less developed or resource poor nations as the prevention of these devastating injuries is more cost effective than attempting treatment.

CONCLUSION

The public should be enlightened to the risk of severe ocular injuries from airbag systems and should be advised to avoid sitting too close to the dashboard or sleeping while seated in the front passengers’ seat. The public should be encouraged to ensure regular vehicle and airbag module maintenance especially in older model automobiles, commonly found in the resource-constrained developing world. With this illustrative case, we hope that ophthalmologists will continue to advocate for safer airbag module designs, which address the vulnerability of the face and globe.