The primary reason for mortality after penetrating neck trauma is uncontrollable hemorrhage due to major vascular injury [3, 4]. Because the head and neck region is densely occupied with vital structures in a relatively small volume, even the minimum motions of a penetrating bullet can simultaneously inflict severe damage to a major artery, vein, and nerve, leading to serious life-threatening conditions. This anatomical property is reflected in the high mortality rates associated with head and neck-related firearm injuries that reach up to 35–36% [5, 6]. Furthermore, in terms of the proportion, while the head and neck region accounts for 13.8–20% of all gunshot injuries [5–7], deaths from head and neck-related firearm injuries account for as high as 54–58% of all firearm-related deaths [5, 6].
The effects of projectiles on living tissues, termed “wound ballistics,” are regulated by many factors, both projectile- and tissue-related [8, 9]. Ballistic factors consist of the physical properties of a bullet including its mass, caliber, shape, and construction; the dynamic properties including bullet velocity, trajectory, pitch, and spin motion; the type and caliber of the barrel; and the distance traveled by the bullet [8–10]. Among these, the mass and velocity of the bullet are considered the most critical for determining tissue damage, because they constitute kinetic energy (KE) as being equal to one-half the mass (m) times velocity (v) squared (KE = 1/2mv2), which defines the maximum wounding potential [8–11]. As a result, the faster a bullet is fired, the more KE is generated, increasing the potential tissue damage. Because the muzzle velocity of the bullet is defined by the length of the barrel and the explosive quantity , handguns with short barrel lengths yield lower velocity (less than 609.6 m/sec), whereas rifles with longer barrel lengths produce a higher velocity (609.6 m/sec or faster) [8, 9, 11]. As for our patient, neither the type of weapon used nor the exact distance between the weapon and the body was officially confirmed because the assailant has not been arrested, and details of the incident under investigation are not allowed to be disclosed. However, the patient testified that he witnessed the assailant fire bullets using a .38-caliber handgun from a very close distance, suggesting that his firearm injuries were presumably generated by a low-velocity shot.
Wounds generated by bullets are determined via their direct and indirect interactions with living tissues (Fig. 4) [8, 10, 12]. “Direct damage”, also called “prompt damage”, occurs with the rapid distension and rupture of tissue generated by the leading edge of the bullet passing through [8, 10]. This effect creates a “crush cavity”, also referred to as a “permanent cavity”, corresponding to the central area of tissues disrupted along the projectile’s track [8, 9, 11, 12]. On the other hand, “indirect damage” develops without direct contact between the tissue and projectile through a pair of highly dynamic pressure phenomena referred to as “stretch cavity formation” (also termed “temporary cavitation”) and “shock wave” (also called “sonic pressure wave”) [8, 9, 11, 12]. Stretch refers to the radial stretching of the tissue around the bullet tunnel, which yields the surrounding area of indirectly injured tissues and creates negative pressure that may draw in foreign objects such as clothing material, sand, hair, and bacteria into the wound [9, 11, 12]. At the point of impact, a stress wave is generated and rapidly spreads ahead from the maximum pressure point at the leading edge of the bullet [8, 9, 12]. This shock wave penetrates through the body without actual tissue movement . Bullets shot at a higher velocity cause more intense indirect damage by producing greater pressure changes [8, 12], which suggests the necessity of careful detection of latent damage to the surrounding tissues around the projectile's track . Our patient seemed extremely fortunate to avoid injury to any vital organs in the neck, such as the carotid arteries, jugular veins, and upper aero-digestive tract including the pharynx, larynx, and trachea, even though the bullet had almost transversely passed through the neck structures to the contralateral side. Aside from minor crush injuries to the soft tissues along the bullet path, serious direct damage was seemingly limited to a comminuted fracture of the right clavicle and right brachial plexopathy, although the latter was not substantially visualized by diagnostic imaging. By contrast, indirect damage via stretch cavity formation and/or shock wave mechanisms was thought to correspond with a broad range of edema and air sacs found in the soft tissues around the bullet trajectory, with the former partly represented by mild mucosal edema in the left arytenoid.
The longitudinal axis of a fired bullet traveling “nose-on” tends to cyclically deviate from the tangent to its trajectory, a phenomenon known as “yawing”, and its complete turn beyond 90 degrees is referred to as “tumbling” (Fig. 5) [8, 12, 13]. The destabilizing effect of yawing is counteracted by a high-frequency spin motion that provides gyroscopic stabilization, which maintains the bullet’s “nose-on” orientation by decreasing the amplitude of yawing [8, 12]. This stabilizing spin force results in a complex spiral movement of the bullet’s nose, called “precession”, wherein the bullet rotates around its center of mass located behind its mid-portion (Fig. 5) [8, 9, 12, 13]. Once the bullet enters the body, it cannot retain its preceding orientation because the stabilizing effect of the spin is overcome by tissue density far higher than that of air , which explains one of the reasons why a projectile does not necessarily follow a straight path when it passes through tissue. In addition, the unconstrained yawing or tumbling of the bullet allows its greater or entire length to act as its crushing edge, thereby maximizing the energy transfer [8, 9]. According to the X-ray image of our patient, the bullet lying on the right humeral head appeared to retain the “nose-on” orientation, which suggests that it did not tumble when penetrating the body, presumably due to a low-velocity impact.
The extent of tissue damage is also affected directly by deformation and fragmentation of the bullet, depending largely on the bullet design. Non-jacketed bullets, as well as semi-jacketed bullets (soft-point or hollow-point bullets) designed such that their tip is exposed from the jacket, can easily deform upon impact into a mushroom shape, thus transferring the KE more efficiently over a greater area of the target [8, 9, 12]. The abovementioned bullet designs are also aimed at causing fragmentation, in part or whole, upon impact. The resultant fragments, as well as any bone fragments generated by the bullet, can act as individual secondary projectiles, thereby increasing wound severity [8, 9, 13]. In contrast, full metal-jacketed bullets are completely armored with copper and do not deform or fragment upon hitting a target, although they can penetrate the target easily [8, 9, 12]. Thus, awareness of bullet deformation and fragmentation helps us predict the degree of tissue damage. Concerning our patient, the bullet removed from the body was a full metal-jacketed bullet with neither deformation nor fragmentation (Fig. 3a, b), as presumed by the X-ray and CT images (Fig. 2a, b), which also partly explained why the patient avoided lethal damage to the vital organs in the neck.
A bullet passing through the body is generally thought to travel in a straight line. However, the dynamics of a bullet can explain the possible nonlinear trajectories in a body. First, as mentioned previously, when a bullet with yawing and precession enters the much denser living tissue, the loss of stabilizing spin motion amplifies its yawing and resultant tumbling, which makes the “nose-on” orientation unstable and helps the bullet deviate from a straight path . Second, if the bullet hits bone, especially at lower velocity, it may deflect and turn direction, inevitably resulting in a twisted trajectory. Third, when passing through heterogeneous soft tissue structures, including artificial implanted materials, a bullet at lower velocity may be guided through a route of interstructural spaces where it encounters the least resistance , resulting in a curved or winding path. The bullet trajectory was thought to be nonlinear in our patient because of the localization of soft tissue edema and air sacs in the deep tissue structures, as well as the positional relationship between the bullet’s entrance and arrival point. The bullet’s route was estimated as follows (Fig. 6a, b). First hitting the left sternocleidomastoid muscle, the bullet passed through a very narrow gap between the thyroid cartilage and the left common carotid artery. Then, it penetrated the retropharyngeal space behind the hypopharynx diagonally downward from the left to the right, where it traveled in a curved line along the fascial planes and/or rebounded off the vertebral body. The bullet was assumed to pass behind the right carotid sheath, hit the right clavicle and deflect inferolaterally, and finally land on the front aspect of the right humeral head. If the bullet ran in a straight line to reach the aforementioned destination, the major blood vessels and the larynx and/or trachea would be directly perforated; however, they were unscathed in our case (Fig. 7a, b). Instead, the bullet was thought to penetrate the interstructural spaces, encountering the least resistance during its intrabody movement.
To our knowledge, this is the first report of a patient with a gunshot injury wherein the bullet almost transversely penetrated the entire neck, across its deep structures to the contralateral side of the shoulder, notably via a nonlinear path, traveling anterior to the left carotid sheath and posterior to the right carotid sheath without damaging any vital organs. Although various patients with atypical head and neck gunshot injuries who survived without fatal organ damage have been reported [10, 14–22], those with a true nonlinear bullet trajectory are limited to a few reports [10, 15, 21], where trajectories were implied to match the fascial planes with the least tissue resistance. Unlike our case, those reports suggested that the bullet penetrated the face from the masseteric region through the parapharyngeal and retropharyngeal spaces to the contralateral prestyloid space , or longitudinally passed through the unilateral side of the neck to the face from the caudal to the cranial side [15, 21]. Intriguingly, a couple of other case reports showed an unexpected route of a bullet that entered from the face, penetrated the facial cranium, passed through the nasopharynx, and ended up in the gastrointestinal tract by involuntary swallowing [16, 19]. These reports indicate that, in extremely rare cases, neither the bullet nor its exit hole can be identified by physical examination and local imaging tests. Accordingly, our experience and these previous observations strongly suggest the importance of realizing the unpredictable nature of a bullet trajectory in a body, regardless of its entrance. Appropriate understanding of the wound ballistics can help us in anticipating the severity of tissue injury, detecting any latent damage to internal tissues, and estimating and interpreting the bullet trajectory, particularly in cases with an unclear track, thereby enabling optimal management for victims of gunshot wounds.