Low-velocity penetrating brain injuries are relatively uncommon, far less frequently encountered than blunt traumatic brain injuries, and depending on the country more or less common than high-velocity penetrating brain injuries from gunshot wounds.
The demographics of affected individuals will vary greatly depending on the circumstances and the nature of penetrating object (e.g. knife vs tree branch). Stabbings will disproportionately affect young men in the setting of assault. Plant material may be the result of sporting injury (e.g. trail bike accidents) or gardening, which will affect a different demographic.
The incidence of low-velocity penetrating brain injuries will also vary greatly depending on patterns of crime.
The damage that results from objects penetrating the brain is dependent on the size and trajectory of the object but also the amount of kinetic energy they deposit as they traverse tissues. Total energy is proportional to the mass of the object times the square of its velocity (E = 1/2mv2). In other words, most of the kinetic energy available is due to velocity, explaining why a small bullet can do far more damage than a much larger object (e.g. knife blade) and why the patterns of injury for high-velocity objects is different to low-velocity ones.
As is the case with high-velocity penetrating brain injuries, CT and CT angiography are the cornerstones of emergent imaging of all patients with penetrating injuries. Plain films have largely been replaced by CT and MRI is usually not indicated until later in the admission if at all. Of particular importance is the possibility that the penetrating material is ferromagnetic precluding safe MRI.
A number of features need to be assessed in patients with low-velocity penetrating injuries, with care taken to carefully document findings as often these scans will be used in medicolegal proceedings.
For the sake of brevity and to avoid repetition, as most findings are similar to those requiring comment in high-velocity injuries, they are discussed in that article: high-velocity penetrating brain injuries.
One particular feature that is most relevant to low-velocity penetrating injuries is the identification of foreign bodies. Unlike high-velocity injuries which are most often metallic and thus markedly hyperdense on CT and usually easily identified, low-velocity objects are far more likely to be of lower attenuation and these can be easily overlooked on standard narrow CT brain windows. As such care should be taken to review multiple window settings, not only brain and bone windows but also soft tissue and lung settings.
Both wood and plastic, for example, will appear black and similar to air on brain window. Intermediate density non-metallic material (e.g. glass, dirt) may not cause streak artefact, appearing white and similar blood or bone on brain window.
Treatment and prognosis
A detailed discussion of the treatment of penetrating injuries is beyond the scope of this article, however, generally foreign bodies and bone fragments that can be removed safely usually are and wounds are debrided and dural closure performed.
Particular care should be taken, particularly with transorbital penetrating injuries, to identify and treat vascular injuries such as dissection, false aneurysms and dural arteriovenous fistulas.