There is insufficient evidence to support an evidence-based recommendation about whether patient characteristics (e.g., age, gender, military status) or injury characteristics (e.g., mechanism of injury, location of injury) impacts the accuracy of screening or diagnosis of pTBI.

In the absence of direct scientific evidence, EXPERT CONSENSUS concluded that:

• the screening test of choice to diagnose penetrating TBI is CT scan
• in austere environments where CT may not be available it can be difficult to determine dural penetration by a foreign or ballistic body, and therefore index of suspicion should remain high

No evidence or expert opinion supported distinct recommendations based on patient gender, age, wounding mechanism, or military vs. civilian context.

Despite the critical importance of timely diagnosis, there exists a significant gap in studies specifically targeting the influence of patient and/or injury characteristics on the accuracy of screening and diagnosis for penetrating traumatic brain injury (pTBI). Such characteristics may include, but are not limited to, patient demographics (e.g., age, gender, military status) and the specifics of the injury itself (e.g., mechanism and location of injury). Although identifying patients with pTBI might seem straightforward, various factors can hinder the immediate identification of wounds, especially in austere or conflicted environments, mass casualty settings, or in patients with multisystem trauma. Other less obvious wounds may be concealed under hair, helmets, or a bleeding scalp wound. pTBI entrance wounds can be deceptively small, and in cases of non-ballistic injuries, such as knife wounds that enter and exit, there may be almost no visible signs of penetrating trauma to the casual observer. Patient characteristics, including genetic and phenotypic differences, may contribute to the difficulty of accurately diagnosing pTBI, and the mechanism of injury would seem to strongly influence screening and diagnostic accuracy. The initial exploration of this topic revealed a stark lack of studies directly addressing how these characteristics might affect the screening process for pTBI. The review of existing evidence, encompassing both formally recognized studies and notable investigations that did not meet strict inclusion criteria, underscores the complexity of this issue. These non-randomized studies, primarily conducted in civilian populations across various countries, did not report diagnostic accuracy data, highlighting the challenges in drawing concrete conclusions from the available research.

Previous versions of the guidelines did not address the diagnostic accuracy of screening for pTBI based on patient characteristics or injury characteristics.

No studies meeting inclusion criteria provided information on how diagnostic accuracy of screening for pTBI differed based on patient or injury characteristics. Six studies provide information on the rate of characteristics associated with pTBI and non-penetrating head injuries.

Six non-randomized studies, not meeting inclusion criteria because they did not report diagnostic accuracy data, are reported here for completeness. Studies provided information on rates of characteristics associated with pTBIs and blunt (or closed) head injury in patients with head injuries.1-6 The studies ranged in sample size from 56 to 2,637; four studies (66.6%) included over 600 patients and the other two included 561 and 853 patients. All but one study was conducted in civilian populations;2 four in the United States, and one each in Turkey and Austria. The study conducted in Austria (N=85), which spanned 16 years and included patients with gunshot injuries to the head at a university hospital (indicating gunshot injuries were very infrequent), was an outlier compared with the other five studies, and reported a reverse relationship between age, gender, self-inflicted injury, and pTBI.

Four studies (N=2,244) report mixed results on differences in ages between those diagnosed with pTBI and blunt injuries. Two studies (N=1,364) reported younger mean ages in patients with pTBI compared with patients with blunt injuries (military population, mean age, pTBI vs. blunt: 25 vs. 27 years, p=0.0352 and civilian population, mean age pTBI vs. blunt: 29 vs. 40 years, p<0.001), whereas the study in Austria (N=85) reported older mean ages with penetrating injury (mean age, pTBI vs. blunt: 52.7 vs. 37.9 years).3 One study (N=795) reported percentages by age ranges, with younger patients more likely to have a penetrating injury (age, pTBI vs. blunt:16-29 years: 66.4% vs. 38.1%; 30-49 years: 23.6% vs. 38.1%; ≥50 years: 10% vs. 23.8%).4 One study (N=760) that reported mean ages, also categorized patient ages as less than 55 years versus 55 years and older and found significantly more patients with pTBI were younger than 55 years compared with blunt injuries (age, pTBI vs. blunt: ≤55 years: 93% vs. 79%; >55 years: 7% vs. 21%, p<0.001).6

Of four studies that reported differences in rates of males injured compared with females, three studies (N=2,159) report more males with head injury among pTBI patients compared with those with blunt injuries (% males, pTBI vs. blunt: 87.3% vs. 81.2%;4 37% vs. 18%, p=0.001;6 98.7% vs. 94.6%, p=0.010),2 while the study in Austria (N=85) reported more males in those with blunt injuries compared with pTBIs (% males, blunt vs. pTBI: 91.6% vs. 76%).3

One study reported that of those with intentional injuries (N=469), more had pTBIs compared with blunt injuries (intentional, pTBI vs. blunt: 63.5% vs, 37.1%, p value not reported),5 while the study in Austria (N=85) reported more self-inflected injuries were blunt (self-inflicted, blunt vs. pTBI: 53.6% vs. 46.4%).3 A third small study (N=30) reported a significantly higher rate of suicide in those with pTBIs compared with blunt injuries (suicide, pTBI vs. blunt: 33.3% vs. 3.8%, p=0.006).1

Two studies (N=851) reported more pTBIs in those sustaining injuries from firearms (firearms, pTBI vs. blunt: 93.6% vs. 0.7%),4 bullets (bullets, pTBI vs. blunt: 83.3% vs. 19.2%, p<0.001),1 and high-velocity weapons (high-velocity, pTBI vs. blunt: 73.3% vs. 0, p<0.001)1 compared with those with blunt injuries. As well as in those with lateral as opposed to anterior/posterior injury in one study (lateral injury, pTBI vs. blunt: 36.7% vs. 11.5%, p=0.03).

The evidence available is insufficient to support any strength of recommendation regarding the impact of patient or injury characteristics on the accuracy of pTBI screening or diagnosis. This indicates a significant gap in research, underscoring the need for more targeted studies in this area. The studies that are mentioned do not meet the inclusion criteria for providing diagnostic accuracy data but offer insights into the prevalence of certain characteristics among patients with pTBI versus those with blunt head injuries. The papers which are subject to high bias provided mixed results on age differences between patients diagnosed with pTBI versus blunt injuries, with some studies suggesting younger mean ages for pTBI patients and one study suggesting older mean ages.

The consensus among experts, in light of the current scarcity of direct evidence, identifies CT scanning as the definitive method for diagnosing penetrating traumatic brain injury (pTBI). Despite this, the lack of accessible CT technology in many settings presents significant challenges. Physical examinations can be misleading or obscured by factors such as hair or scalp lacerations. Moreover, small wounds or those resulting from non-ballistic objects can be easily overlooked or misclassified. Additionally, tangential injuries that do not immediately manifest with significant neurological dysfunction may be underestimated. This underscores the complexity of diagnosing pTBI, particularly when ideal diagnostic tools are unavailable, and highlights the necessity for heightened clinical vigilance and the development of alternative diagnostic strategies.

The important of rapidly and accurately identifying patients with (pTBI) cannot be understated and the varied influences of patient and injury characteristics on diagnostic accuracy, future research should prioritize several key areas. To address the current evidence gap, there is a need for large-scale, multicenter studies that specifically investigate or at the least include as preplanned analysis the impact of patient demographics and injury specifics on the diagnostic accuracy of pTBI screening and diagnosis. These studies should aim to include diverse populations and injury contexts to enhance generalizability. Considering the limitations of current imaging modalities in prehospital or austere settings, research should focus on the development and validation of portable, efficient, and effective diagnostic tools for pTBI. This could include portable imaging technologies, point-of-care testing devices, or blood-based biomarkers that could offer rapid and reliable diagnosis in the field. Given the potential role of genetic and phenotypic differences in the difficulty of diagnosing pTBI, future studies should explore how these factors may influence the presentation and detectability of penetrating head injuries. This could help tailor screening and diagnostic approaches to individual patient profiles. Comparative studies examining the effectiveness of various diagnostic modalities and tools, including emerging technologies, in the screening and diagnosis of pTBI are needed. Such research could help identify the most reliable methods for use in different settings, from the emergency department to the battlefield. More detailed research into how the mechanism and location of injury affect the diagnostic process and outcomes for pTBI patients is required. This could include studies on specific injury types, such as ballistic versus stab wounds, and their impact on diagnostic accuracy and patient outcomes. By addressing these research priorities, the medical community can move closer to improving the accuracy of pTBI screening and diagnosis, particularly in challenging environments and among diverse patient populations.