Mechanism of Injury and Co-Morbid Factors

freeway crash
Freeway accidents are likely to cause more injuries than accidents in a parking lot. (c) Robyn Gallagher

Mechanism of injury refers to the method by which damage (trauma) to skin, muscles, organs, and bones happens. Healthcare providers use mechanism of injury (MOI) to help determine how likely it is that a serious injury has occurred.

We all use mechanism of injury, even if we didn't know what to call it. There's an old joke that says "It's not the fall that kills you, it's the sudden stop at the end." In mechanism of injury terms, that's known as sudden deceleration.

Use of the Term

A low-speed fender-bender in a parking lot is much less likely to cause a life-threatening injury than a rollover accident on the freeway. A gunshot wound has more potential for serious injury than a fistfight.

Falling off a five foot deck and walking away is a funny video on YouTube, but falling off the top of a five story building and walking away makes it on the evening news. Why? Because we all instinctively know that you shouldn't survive (or barely survive) such a long fall.

Long falls are just one type of mechanism of injury that is used in emergency medicine. We use the term to distinguish the two examples above. The trouble with mechanism of injury is that it's not the same for everybody. A good rule of thumb is if you are a young adult and healthy, a fall from a distance more than 3 times your height is significant.

An old (and usually brittle) patient would be much more likely to be injured in a ground level fall (tripping).

Complications (Also Known as Co-Morbid Factors)

Not everyone is young and healthy. As you age, your skin gets more delicate and your bones get more brittle. Sudden deceleration like that of a fall or a car accident affects the very old and the very young more severely than a young, healthy adult.

Those differences are known as co-morbid factors and include a lot more than age. Heart disease may affect the patient's ability to compensate for shock. Liver or kidney disease can lead to thinner blood that doesn't clot as well. Residual weakness from stroke or other neurological diseases can turn a minor trip and fall (called a ground-level fall) into a life-threatening event. That's why falls in the elderly are so concerning.

Alcohol and Substances Affect Mechanism of Injury

Anything that gets you high, drunk or stoned changes your behavior. Injuries to the brain often cause similar changes in behavior. So, an intoxicated patient is difficult to assess for significant injury. Plus, alcohol specifically changes the chemistry in the blood, making it thinner and less likely to clot. Generally safe, low mechanism injuries like ground-level falls become serious life-threatening events.

Mechanism of injury is a moving target that isn't the same for every patient. Use your gut reaction. If the incident seems like it would be life-threatening, you're probably right. If the patient is old, pregnant, an infant, sick, drunk or otherwise compromised and it makes you more concerned than normal, that's good. You're probably right.

Trust your gut.

View Article Sources
  • Staudenmayer, K., Lin, F., Mackersie, R., Spain, D., & Hsia, R. (2014). Variability in California triage from 2005 to 2009. Journal Of Trauma And Acute Care Surgery76(4), 1041-1047. doi:10.1097/ta.0000000000000197