Gunshot-related nerve injuries of the upper extremities: clinical, electromyographic, and ultrasound features in 22 patients

Objectives Gunshot wounds of the upper extremities may cause permanent neurovascular injuries, leading to significant morbidity, chronic pain, functional loss, and disability. While there are many reports on the incidence and intraoperative findings in gunshot-related nerve injuries (GSNI) sustained during wars, there is a paucity of details pertaining to GSNI of the upper extremities in civilians. The goal of this paper is to provide the clinical, electrodiagnostic (EDX), and ultrasound (US) findings in 22 patients with GSNI of the upper extremities. Methods This is a retrospective study of patients referred for EDX studies to evaluate the presence of nerve injury after sustaining GSWs to the upper extremities. All patients underwent EDX studies, and 16 patients had US evaluations. Numerous metrics were documented including presenting symptoms, neurological abnormalities, EDX findings, and US features. Results The forearm was the most frequent location of injury (8 [36%] patients). The ulnar nerve was the most common injured nerve (10 [45%] patients), followed by the brachial plexus (7 [32%] patients). All patients complained of muscle weakness; the most frequently affected muscles were the first dorsal interosseous (FDI) (14 [64%] patients) and abductor pollicis brevis (APB) (11 [50%] patients). Muscle atrophy was noted in 19 (86%) patients, 15 of whom had atrophy of the FDI. Axonotmesis was the type of nerve injury in all patients based on EDX studies. Of the 16 patients who underwent US studies, a neuroma in continuity was noted in 4 (25%) patients and neurotmesis in 1 (6.2%) patient. Eleven (69%) patients had enlarged and/or hypoechoic nerves. Conclusions Axonotmesis of the ulnar nerve was the most common finding among patients sustaining gunshot injuries to the upper extremities. EDX and US studies provide valuable insight into the underlying pathophysiology and guidance for management of patients with GSNI of the upper extremities.


Introduction
According to the Centers for Disease Control and Prevention (CDC), more Americans died of gun-related injuries in 2021 than in any other year on record (1).Gun violence (unintentional, suicide, and homicide) is higher than motor vehicle accidents as the leading cause of trauma-related years of potential life lost (2).The burden of gun violence represents a dire public health issue that primarily affects the younger members of society (3).While gunshot wounds (GSWs) of the upper extremities are not commonly life threatening, they pose a high risk of long-term disability due to neurovascular injuries (4).
The mechanism of peripheral nerve injury caused by firearms involves either a direct transection of the nerve or indirect injury by producing thermal damage, shock waves, laceration secondary to fracture fragment displacement, or compression due to swelling or subacute scar formation (5)(6)(7)(8)(9).Traction injuries often  spontaneously recover without surgical intervention, however, rupture and avulsion injuries usually require surgery and have a worse prognosis.Patients who undergo surgery within 6 months of the injuries usually have a better outcome (6,7,(10)(11)(12).Additionally, vascular injury and fractures increase the risk of nerve injury after GSWs (9).Firearm injuries of peripheral nerves can cause neuropraxia, axonotmesis, and/or neurotmesis (Table 1) (12).Low-velocity projectiles often damage nerves by direct impact leading to neuropraxia or axonotmesis, and patients usually have significant return of function within several months.High-velocity injuries are caused by shock waves or cavitation effects with ensuing stretching and compression often outside the path of the projectile and across longer nerve segments.Significant return of function is infrequent with high-velocity projectiles.Unlike the military, most of the gunshot injuries in a community setting inflict low-energy penetrating trauma (9).
Clinical examination alone cannot differentiate between neuropraxia, axonotmesis, and neurotmesis (7).EDX can differentiate between neuropraxia and axonotmesis 1-2 weeks after the injury but cannot distinguish neurotmesis from axonotmesis; US evaluation can make that distinction (13).Combining EDX and US can provide critical information in planning management of patients with nerve injuries.Prompt identification of nerve laceration is crucial as it requires surgical repair, while there is controversy about treatment for nerves with neuroma in continuity (7).A total of 70% of nerves in continuity regain function after 3-9 months of observation according to Pannell et al. (7).
We report 22 patients with GSWs of the upper extremities leading to nerve injuries and were referred for EDX studies.Sixteen (73%) patients also had US studies.The presenting symptoms, clinical and EDX findings, and US features of these patients are highlighted.The value of EDX and US studies in the evaluation of firearm wounds of the upper extremities as well as management of these type of injuries are discussed.We also describe the impact of the COVID-19 pandemic on the increased incidence of firearm injuries both nationally and among patients seen in our facility.

Study population and electrodiagnostic/ultrasound studies
This was a retrospective study under an Institutional Review Board (IRB)-approved protocol.Inclusion criteria included patients referred to our facility for EDX studies to evaluate for  the presence of a nerve injury after sustaining a GSW to the upper extremities during the 8-year (2016-2023) period.The patients underwent clinical neurological examination followed by nerve conduction and EMG studies.The EDX studies were performed in our American Association of Neuromuscular and Electrodiagnostic Medicine (AANEM)-accredited facility using standard protocol of our laboratory (14).Patients in whom EDX studies did not show evidence for nerve injuries were excluded from the study.The US studies were conducted using the GE Logiq E system and 12-18 MHz probe (six patients were seen before the US machine was available to us).Short axis views at, proximal to, and distal to the injury were obtained to evaluate the cross-sectional area (CSA), fascicular pattern, and altered echogenicity (15).Long axis views were also studied.Several  metrics were collected including the patients' gender and age, location of projectile entry, the specific nerve(s) injured, and the type of injury (neuropraxia, axonotmesis, or neurotmesis).

Ethical approval and informed consent
Informed consent was obtained from all patients.The IRB determined that our study was exempt according to 45 CFR 46.101(b) under Category 4. The IRB number is 22.1087.

Electrodiagnostic studies
Denervation of muscles as well as absence/decreased amplitude of compound muscle action potential (CMAP) and sensory nerve action potential (SNAP) suggested axonal injury in all patients.Distinction between axonotmesis and neurotmesis could not be made based on the EDX findings.

COVID-and GSWs
According to the Morbidity and Mortality Weekly Report, the firearm homicide rate in the United States increased by approximately 35% during the COVID-19 pandemic (16,17).Compared with 2019, the average number of weekly Emergency Department (ED) visits for firearm injuries was 27% higher in 2020, 36% higher in 2021, and 20% higher in 2022 based on data from the National Vital Statistics System (17).The highest rate was observed among individuals ages 15-24 years.Similarly, we observed a higher number of upper extremities GSW injuries following the COVID-19 initiation compared to before COVID-19 (13 vs. nine patients, respectively), with eight patients evaluated in 2021 alone.This finding of the largest number of cases in 2021 concurs with the substantial increase in ED visits for firearm injuries specifically in 2021 as reported by the National Vital Statistics System.In Risinger and colleagues' study (2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020)(2021) of the association between COVID-19 and gun violence in our same county, the years of potential life lost was higher for firearm fatalities than COVID-19 (3).These authors attributed the increase in gun violence during COVID-19 primarily due to stay-at-home orders and decreased access to mental health care.Additional psychodynamic factors include diminished security and safety (housing and financial insecurity, elevated exposure to violence, fear of illness, uncertainty about the future), and increased firearm purchases (17).Numerous measures have been proposed to address the risk of gunshot injuries, including enhancing community and street outreach programs, initiating implement hospital-based violence prevention programs, enriching community physical environments, encouraging secure storage of firearms, and boosting social and economic supports (17).

Common nerves injured
Previous studies have reported gunshot injuries of the upper extremities with consequent peripheral nerve injuries, specifically, brachial plexus (6,18,19), radial nerve (5,10,20), ulnar nerve (12,21), or a combination of upper extremity nerves (7,8,11,(22)(23)(24)(25).In Secer and colleagues' 40-year study of 2,106 patients with 2,210 peripheral nerve lesions of the upper and lower extremities caused by combat-related GSWs, the median nerve was the most frequently injured upper extremity nerve followed by the ulnar nerve, radial nerve, and brachial plexus (8).In Pannell and colleagues' study (2007-2014) of 41 patients (59 nerves) who sustained firearm injuries of the upper extremities, there was a 63% incidence of nerve dysfunction after upper extremity GSWs (7).A higher frequency of fractures, retained fragments, vascular injury, and compartment syndrome in patients with nerve palsies was noted (7).Additionally, patients with palsies were significantly more likely to have nerve lacerations intraoperatively.Of the 59 nerves studied, 37 palsies were identified, including 13 ulnar, 8 median, and 7 radial.Ten of the 37 (27%) palsies had lacerated nerves, whereas nerve lacerations were not observed in patients without palsies.Straszewski et al. studied GSWs of the upper extremities in an urban trauma center; among 126 injuries in 117 patients, 38 had a documented nerve deficit (9).The location of injury was 18 in the arm, 13 in the forearm, six in both the arm and forearm, and one in the hand.The most common nerve injured was the radial nerve, followed by the ulnar and median nerves.In patients with a GSW to the forearm, the most common nerve injured was the ulnar nerve.In our study the most frequent nerve injured was the ulnar nerve, and the site of projectile entry was three at the forearm, two at the elbow, two at the palm, one at the wrist, and one at the upper arm.Henriques et al.'s study also noted the high frequency of ulnar nerve involvement (21).It is possible that the superficial location of the nerve at the elbow and the forearm makes it more vulnerable to injury.
The brachial plexus was the second most common site of GSW in our series.The projectile entry location included supraclavicular, clavicular, and infraclavicular areas; in one patient the projectile entered in the upper arm and exited on the left side of neck.Most brachial plexus injuries are due to traction injuries caused by motor vehicle accidents, however, ∼3-12% result from GSWs (6,19).
The radial/PIN nerve was injured in six patients in our study; in three patients, additional nerves were injured.In Guo et al.'s study of patients who sustained a gunshot injury of the radial nerve, only 30% of firearm radial nerve injuries were associated with another nerve injury in the upper extremity (5).

EDX studies
EDX studies play a valuable role in the investigation of gunshot injuries by detecting nerve injuries and recovery.They localize the site of the lesion, identify the type and severity of the lesion, and provide prognostic details pre-and postoperatively (6,19,26).EDX studies are crucial in differentiating conduction bock from axonal injury; they are also useful in differentiating total from partial injuries and to detect reinnervation.Serial EMGs are beneficial in analyzing the progression of recovery by assessing the presence of volitionally-recruited motor units and the quantity of fibrillation potentials (19).An important limitation of EDX is its inability to distinguish total axonotmesis from neurotmesis; combining EDX and US studies can potentially circumvent this limitation and lead to prompt surgical repair without delay in cases of neurotmesis.
In this series, the findings suggested axonotmesis in all patients.The EDX studies were performed in most patients 3 months after injury (ranging from 2 months to 12 months).Injuries of the brachial plexus were partial and involved trunks/cords.

Ultrasound study
An MRI is the gold standard for imaging nerve injuries of the upper extremities due to its multiplanar images and highcontrast resolution in soft tissues (6).However, this modality may be contraindicated in GSWs due to the presence of metal fragments which may shift, generate heat, and may cause imaging artifact.The readily available and cost-effective US may be a more useful technique to detail the anatomy of the injury without being limited by metal fragments.US also detects both nerve and vascular injury as well as the presence of neuroma formation and nerve structural integrity following firearm injuries (6,27).In Fagan and colleagues' study of 17 patients who sustained a GSW of the upper or lower extremities and subsequently underwent neuromuscular US, all patients had either a nerve transection or neuroma in continuity of a major upper or lower extremity nerve at the site of the GSW (27).A total of 13 (76%) patients had significant morphological changes in distal segments of the injured nerve, and 12 (71%) had changes in other nearby distal nerves.Enlarged nerve crosssectional area, enlarged fascicles, and hypoechogenicity were also frequently observed.These authors surmised that ballistic trauma of firearm injuries can result in concussive damage that disrupts the normal architecture of distal nerves (27).In Nwawka and colleagues' case series of 3 patients who sustained brachial plexus injuries due to firearms, the US revealed nerve abnormalities that complemented the EDX and intraoperative findings (6).Two of these patients experienced nerve transection of the median and ulnar nerves detected by US, one of whom also had a radial nerve transection.In this series four patients with brachial plexus injury showed hypoechoic enlarged trunks, perineural thickening and hyperechoic scar tissue surrounding the neural structures.Of the 16 patients who underwent US in the present study, a neuroma in continuity was observed in four patients, while 11 patients had enlarged and/or hypoechoic nerves.In one patient neurotmesis was noted (Table 4).
Among the advantages of US in comparison to MRI, dynamic evaluation can be useful for confirming neurotmesis and monitoring the recovery process.Additionally, US may detect compression by a hematoma and entrapment by scar tissue formation (28,29).

Strengths and limitations
The strength of our study is the use of both EDX and US studies in evaluating a large cohort of gunshot-related nerve injuries of the upper extremities in a non-combat setting.This allowed us to correlate the clinical examination findings with the EDX and US features in these patients.Limitations of our study include its retrospective nature and lack of follow-up of patients after their EDX evaluation which has precluded our ability to assess the longterm outcome.Additional limitations include the lack of measuring the cross-sectional area of the different nerves using comparative scanning, the lack of color/power Doppler assessment of the pathological peripheral nerves, and the lack of sonographic followup.

Conclusion
Physicians should be cognizant of the potential for nerve injuries following GSWs and look for the clinical signs, diagnostic of injury to specific nerves.EDX and US studies should be utilized to determine the location and the type of nerve injury so that appropriate management can be initiated promptly.

FIGURE
FIGURE Patient # : (A) Entry in palm (black arrow) and exit at base of hypothenar eminence (white arrow).(B) Ultrasound: Long axis view shows hypoechoic enlargement of the ulnar nerve (neuroma in continuity) (box).Arrows outline the ulnar nerve.M: Abductor digiti minimi muscle.

FIGURE
FIGURE Patient # : (A) Clawing of ulnar digits of right hand.(B) Atrophy of FDI (arrow).(C) Long axis view at the elbow showing neuroma in continuity (arrows).The black rectangle denotes the position of the US probe.(D) Short axis view at the elbow showing the neuroma (arrow); the nerve is hypoechoic with epineural thickening.Note the transition from fascicular edema (on the left) to irregular echotexture (to the right).M: Flexor carpi ulnaris muscle.

FIGURE
FIGUREPatient # : Ultrasound: short axis view at distal upper arm.Arrow points to the median nerve with hypoechoic enlarged fascicles and perineural thickening.The brachial artery is in orange color.M: Biceps muscle.

FIGURE
FIGURE Patient # sustained a radial nerve injury (neurotmesis) at the upper arm.Figure shows terminal neuromas in the proximal and the distal portions of the injured nerve.H, Humerus.
TABLE Clinical findings of patients with gunshot-related nerve injuries to the upper extremities.