A 10 year old boy presents to your emergency department following a high speed MVA – car vs tree. He was seated in the rear middle seat. On arrival he is noted to have significant bruising across his lower abdomen from the seat belt but otherwise appears well.
What is the significance of the seat belt sign?
The “seat belt sign” – defined as an “area of ecchymoses, erythema, or abrasions sustained secondary to seat belt use” – has been associated with an increased risk of both intra-abdominal injury and lumbar spine injury. Presence of a seat belt sign in children is important because:
- It is associated with an absolute risk of gastrointestinal injury of 11-25%, this represents relative risk increase of ~9.4 – 12.8 compared to children without a seat belt sign.
- Small mesenteric tears and perforation are the most common hollow viscus injury.
- Mesenteric tears are thought to be due to direct compression of tissue between the belt and the spinal column, whereas intestinal perforations are thought to be caused by an increase in intraluminal pressure combined with compression of a short segment of bowel.
- The most common site of intestinal injury is the jejunum, followed by the duodenum, then ilium and caecum.
- It is associated with an absolute risk of solid organ injury of 9-21% however, two large prospective trials have found no significant increase relative risk for solid organ injury between matched patients with or without a seat belt sign.
- It is associated with an absolute risk of spinal injury of up to 50%. Where spinal fractures occur, around 18% are associated with paraplegia.
- It is associated with increased (~RR=5.5) requirement for acute interventions such as therapeutic laparotomy, angiographic embolization, blood transfusion for intra-abdominal haemorrhage, administration of IV fluids for two or more nights in patients with pancreatic or gastrointestinal injuries, as well as an increased risk of death.
- Even where no pain or tenderness is associated with the seat belt sign, 2% of injured children go on to require acute surgical intervention.
The combination of a seat belt sign, intra-abdominal injury and spinal injury is known as “seat belt syndrome”.
How long has seat belt syndrome been recognised?
Coined in the 1960s, seat belt syndrome was first described shortly after an increase in the use of seat belts in drivers and front seat passengers. As mentioned above, the triad of injuries it refers to are abdominal wall bruising, intra-abdominal injuries and lumbar spine injuries. The syndrome was originally described in adults, however as seat belts (in particular lap belts) were introduced to rear seats of cars during the 1980s and 1990s, an increasing body of literature noted the presence of this syndrome in children.
Why does this pattern of injury occur?
Seat belts prevent injury overall in a number of ways:
- they prevent ejection from the vehicle – which is associated with greater injury
- they help decelerate the occupant over a period of time (rather than the sudden deceleration associated with striking the windscreen or other object).
- they are designed to distribute the forces involved over a large area of the bony skeleton (clavicle, sternum, iliac crests) rather than soft tissues.
However, the lap component of seat belts – although designed to sit over the anterior superior iliac spine – can easily ride up onto the abdomen or even lower chest during a crash. Rapid deceleration then leads to the child hyperflexing their torso over the belt, leading to compression of the abdominal contents, with flexion-distraction forces typically acting on the lumbar spine.
What’s the pathophysiology?
Damage occurs to intra-abdominal contents such as the mesentery and bowel due to shearing and compressive type forces acting on the soft tissues. Injuries to these structures may be small and thus undetectable on early CT imaging. The injury to the spinal column is due to hyperflexion of the spine. When the effective fulcrum for this hyperflexion is anterior to the spine, there is a distraction injury across all three columns which leads to a them failing in tension (i.e. they are pulled apart). Where the effective fulcrum is posterior to the anterior edge of the vertebral body, a combination of anterior compression injuries and posterior distraction injury may be seen.
Why are children still at risk?
In modern cars “lap belts”(2-point harnesses) have been replaced with “lap and shoulder belts” (3-point harnesses). Whilst “lap and shoulder” belts reduce the risk of seat belt syndrome, they do not completely prevent it, and children remain at risk for a number of reasons.
Firstly, if inappropriately placed in an adult belt, children may find the shoulder component sits uncomfortably across their neck. They are subsequently tempted to ride with the shoulder component of the belt tucked under their arm or behind their back, effectively converting their 3-point harness into a 2 point harness. The tendency of children to scoot forward on the car seat to enable their knees to naturally bend over the edge of the seat, exacerbates any poor fit by the addition of a “slouch” factor.
Secondly, the immature pelvis has less well developed anterior superior iliac spines. This increases the ease at which the lap component of the belt can ride up over the abdomen of children. Furthermore, the abdomens of children have thinner muscles and subcutaneous tissue than adults, so offer less protection to their intra-abdominal organs.
Thirdly, the combination of increased head size (leading to greater flexion around the belt) and a smaller AP diameter of children (leading to shorter distance over which the deceleration force is applied) contribute to severity of injury secondary compression of the intra-abdominal organs between the seat belt and spinal column.
How to manage children with a “seat belt sign”
The presence of the seat belt sign identifies children with an increased risk of both intra-abdominal and spinal injury. During the primary survey the paramount concern is identification and management of life threats. The well looking child with a seat belt sign and abdominal tenderness should be strongly considered for an abdominal CT scan. The well child with a seat belt sign but without abdominal tenderness remains at increased risk for intra-abdominal injury compared to the child without a seat belt sign. There is a lack of evidence about how best to manage these patients, but the non-trivial rate requiring intervention (2%) suggests that observation and serial examination for developing signs of intra-abdominal injury be employed.[i]
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