Taking a mechanism of injury history

A reliable account of the mechanism can be useful in guiding further investigation and index of suspicion for certain injury patterns. However, witness accounts should be treated with some caution if the sole source of information on mechanism⁸.

The following questions should establish mechanism of injury:

1. Where was the patient?

• Was the patient a front seat passenger, a pedestrian on the pavement, a child in rear, on a bike seat carrier etc.?
• The majority (76%) of passenger vehicle occupants killed in 2020 were drivers⁹.
• Pedestrians, cyclists and motorcyclists are vulnerable road users as their mortality is much higher if involved in a RTC¹⁰.

2. Where did the energy force originate, and at what speed was force applied?

• How fast was each party going? This can be in mph/kph if known (the speed limit of the road can guide this) but can be descriptive such as travelling downhill on a road bike.
• What did they hit/hit them? For example:  travelling at 50 mph and ejected from vehicle after hitting lamppost.
• In falls: What was the height & landing surface? Did they land on concrete patio or grassy area? Was there any debris or protrusions where they landed (e.g., railings or tree stumps)? Was the fall broken?

Vehicle speed

Speed of vehicles involved is helpful but often gets confused and drivers or witness reports of speed may not be wholly reliable. If speed of the vehicles are not known then the speed limit of the road can give an indication of speeds that may have been involved.

People often refer to Combined speed when handing over patients involved in RTCs, however it is not a useful term and is better to describe the individual speeds of the vehicles involved in descriptive terms.

For example: this patient was the driver of a car travelling at 60mph which collided head on with a HGV pulling out of a junction. This gives more information and describes the forces better than RTC with combined speed of 70 mph.

In RTCs: Rural roads have a much higher average speed than urban roads. Rural roads can prove more challenging in nature with blind bends, dips and other distractions. Accidents at lower speeds on urban roads are less likely to result in serious injuries or fatalities¹¹.

3. What protection did the patient have?

In motor vehicle collisions, specific points to note:

• Age & type of vehicle? Vehicles produced in the last 20 years have much better built-in protection than older models. Safety features include airbags, crumple zones, and anti-lock braking systems. Small/light vehicles have less structure to absorb crash energy so forces on occupants will consequently be higher¹².
• Were seatbelts worn by all occupants? Seat belts generally lock when a car is involved in a significant RTC. The fire service will be able to tell you if patients were wearing seatbelts if they have been extricated before your arrival. Document your own observations also i.e. has the belt been cut to extricate?
• There is good evidence that wearing seatbelts decreases the overall risk of major road traffic injuries¹³. There is also evidence that there is an almost 5-fold increase in risk of death for occupants of a car wearing a belt where there are other unrestrained occupants¹⁴.
• Were children in car seat? Proper car seat use has been shown to reduce the risk of injury or hospitalisation by >70% when compared with seat belts or no restraints¹⁵.
• Did airbags deploy? Typically, airbags deploy if the car has an impact above 16 mph in the UK.

For pedal bikes/motorcyclists/horse riders:

• Was protective clothing such as helmet or back protection worn? There is evidence that helmets are protective against brain injury⁹ so the lack of a helmet should increase our index of suspicion for a major head injury.
• What is the damage to that protective equipment e.g. is the helmet split?

4. What injuries have others sustained/other vehicles? 

• Are there other severely injured or dead passengers? Whilst another occupant of the same compartment being dead as a result of their injuries doesnt necessarily increase the chances of severe injuries in your patient¹⁶, it demonstrates the high energy forces transmitted through the occupants during a crash.
• Rollover: A vehicle (and therefore the occupants) can undergo several impacts at many different angles during a rollover. This increases the potential for serious injury, as well as the possible extrusion of body parts through apertures, resulting in momentary crush damage or complete ejection.
• Intrusion: if there is significant intrusion into the vehicle, the likelihood of entrapment increases.
• Entrapment /if patient could self-extricate: Trapped patients have been shown to have a higher rate of significant injury and mortality¹⁷.
• Windscreen Bullseye: The windscreen is the most commonly struck area by pedestrians when hit by a car. Pedestrians or unrestrained occupants of a car who bullseye the windscreen would have hit it with force and as such have a risk of serious intracranial and spinal injury.

Documentation

• For prehospital teams this may mean photos of the scene and injuries that have been reduced or covered. It can be much easier to understand the extent of a mechanism when looking at a photo of the scene as a whole rather than just a verbal description. Be mindful that any images taken may be disclosable in future trials.
• History provided by the patient and witnesses should be clearly documented in the notes.

As the initial assessing clinician, we have unique access to information about mechanism of injury. It is important to use the correct terms in documentation of any injuries found as these could be crucial in any future legal proceedings related to injuries sustained. For example, a laceration describes only a wound created by blunt force, whereas wounds caused by cutting or stabbing actions are incisions/incised wounds. Describing a wound incorrectly could imply a blade was used when it wasnt, and vice versa.

If you are unsure of the causation, it is better to use general descriptive terms such as injury, and wound rather than incorrect terminology.

A wound is legally defined as a break in the continuity of the skin or mucosal membrane and an Injury would refer to things like bruises or swelling [18]

**Where patients are conscious, consent should be gained for any information recorded in the patient record where possible. Whether treating patients in best interests or with their consent, all documentation should follow GMC guidance, ensuring to respect patients privacy and dignity, and using only secure devices if electronically stored¹⁹.