Seat Belt Injury

Etiology

The use of restraints in vehicles prevents severe head injury and ejection from vehicles. By holding individuals in the seat, lives are saved at the expense of certain injury patterns specific to seat belt use.

The three-point adult harness rests on the shoulder, chest, and abdomen. At these sites of impact, the seat belt can transmit significant force depending on the mechanics of the MVC. Seatbelt injuries result from forces transmitted from the restraint to the passenger of a vehicle. Underlying structures can be damaged by the subsequent blunt force trauma delivered to the neck, chest, or abdomen.

Epidemiology

As of 2015, roughly 35,000 deaths occur in the United States annually due to MVC.[4] Estimated fatality from motor vehicle crashes has decreased from 16 per 100 million miles (in 1930) to 1.8 deaths per 100 million miles currently.[5] Seatbelts reduce fatal injuries by an estimated 43% according to the NHTSA.[3]

The literature has established different estimates of the incidence of seat belt-related injuries. Seat belt sign to the neck and chest appears to be less predictive of serious underlying injury, usually, present less than five percent of patients. Seat belt injuries to the abdomen (and to the lumbar spine) may occur slightly more common in those with external signs of injury, but data has not been consistent.

Pathophysiology

Injury in MVC trauma is related to change in velocity and dissipation of energy. High vehicle speed, vehicle rollover, passenger ejection, and loss of consciousness with a low Glasgow coma scale (GCS) are predictive of more force transmission and, therefore, severe injury.[6]

Seat belts are designed to transmit the energy to the clavicle, chest wall, and superior pelvis (anterior superior iliac spine [ASIS]). When a seatbelt rests on the neck or well above the ASIS, force is transmitted to the softer neck area and the more vulnerable abdominal viscera.

All injury patterns are explained by the transmission of physical force. The seat belt can act as a fulcrum, with a point of contact remaining fixed in the same location while the areas above and below continuing forward.

The "seat belt sign" was first described by Garrett and Braunstein in 1962 as an ecchymosis across the abdominal wall in the location of a lap belt. The "seat belt syndrome" referred to the musculoskeletal and visceral injuries associated with the force transmitted by the seat belt.[7]

Skin abrasions and bruising occur on the surface, while common internal injuries are bowel and mesenteric injuries, along with fractures of the lumbar spine. Solid-organ injuries such as liver and spleen laceration can occur in these patients. Likewise, injuries to the pancreas, kidneys, and any other visceral organ can occur.

When the seat belt impacts the neck, the clinician may diagnose the cervical vascular injury, cervical spine fracture, or clavicle fracture.

Thoracic injuries due to seat belt force include sternal fracture, rib fracture, pulmonary contusion, and more rarely myocardial contusion.

Histopathology

Damage to internal structures can occur in seat belt injuries. The histopathology of bony fractures is similar to any fracture due to blunt force trauma. Similarly, injuries to the mesentery or bowel are of a tearing nature, with many being undetectable on CT and potentially very small histologically.

History and Physical

A complete history of all patients in motor vehicle collisions should be obtained as soon as feasible. Use of restraints, airbag deployment, vehicle velocity, position in the vehicle, etiology of the collision, time of extrication, loss of consciousness, and injury level of others individuals in the accident are all valuable pieces of data in the evaluation of these patients.

Patients should be interviewed as to the presence of any pain with attention to the potential distracting nature of many injuries.

Complete past medical and surgical history, along with medications and drug allergies, are the high yield in these patients, as many will require an operation.

Evaluation

Airway (with cervical spine protection), breathing, and circulation should be assessed first in all patients. Neurologic disability should be briefly evaluated, and full exposure of the patient should take place to ensure no missed injuries. Vital signs and the shock index should be trended for deterioration.

Laboratory testing varies in sensitivity and specificity in MVC patients. Leukocytosis, low hemoglobin, elevated liver transaminases, elevated amylase and lipase, elevated lactate, and a high base deficit can all alert the clinician to the presence of serious injury.[8] A urinalysis may detect injury to the genitourinary system.

Plain x-rays of the chest and pelvis are typically performed in patients with a significant mechanism of trauma. Ultrasound of the lungs, heart, and viscera (FAST exam and eFAST exam) should be implemented in all unstable patients.

CT imaging plays a major role in the evaluation of blunt trauma patients. The pan-scan approach (CT of the head, cervical spine, chest, abdomen, and pelvis with thoracic and lumbar spine reconstitution) appears to save time, lead to less delay in diagnosis, and may add very little to costs and radiation.

Reassessment of the physical exam, laboratory values, and selected imaging may be implemented in critical patients, and in those in whom the occult injury is suspected.

Treatment / Management

Treatment of patients with seat belt injury depends entirely on the type of injury.[9] Patients with vascular, viscus or spinal injuries will be admitted to the hospital, preferably to a trauma center. Injuries to ribs, sternum, and soft tissues could be managed conservatively, possibly on an outpatient basis with pain control. The treatment of these injuries should follow local practice patterns and established standard-of-care guidelines.

Differential Diagnosis

The differential diagnosis is broad in MVC patients with a significant mechanism. Any area of the body can be injured, many in an occult manner. A take-home point is the index of suspicion one must maintain in cases with clear external signs of seat belt injury.

Prognosis

The prognosis of seat belt injuries depends largely on which type of injury is involved.[10]

• Damage to the mesentery or bowel can cause severe morbidity, especially if not detected early. However, with timely operative intervention, most patients will recover with no sequelae.
• Fractures of the cervical, thoracic, or lumbar spine will have prognostic features dependent upon the severity of bony disruption and the neurologic deficits present.
• For vascular injuries, again timely diagnosis is key. However, some carotid or vertebral injuries, even if diagnosed early, can lead to irreversible neurologic disability.