CT scan is the first choice of imaging. This noninvasive examination method not only allows rapid assessment of lesion extent, missile trajectory, shrapnel residue, size of the hematoma, and type of cranial injury but also helps us to assess patient prognosis and provides visual imaging guidance for surgical planning [4,14-16,22]. However, metal artifacts in CT scans from intracranial foreign bodies may mask some of the injuries. Magnetic resonance imaging (MRI) is superior to CT in many fields. However, the pellets of airgun are mostly metallic, so MRI is not selected because it may lead the metal fragments to move [4]. The position, shape and number of foreign bodies such as metals can be easily obtained in the sagittal position with intraoperative X-ray, but it is difficult to obtain the position in the coronal and axial positions [2,17]. Transcranial sonography (TCS) was usually used in TBI because it’s noninvasive, convenient, and efficient [19]. Intraoperative TCS can clearly show the midline structures, ventricular system and other important structures in the brain (Fig. 3C). TCS can display the size, shape, depth, surrounding brain tissue structure and echogenic characteristics of the foreign body in real time intraoperatively (Fig. 3D). This is a unique advantage over the metal artifacts of CT and the movement of metallic foreign bodies of MRI. However, it is not easy to observe the foreign body by TCS when the foreign body is deep, when the foreign body has an obvious acoustic shadow, or when the foreign body is obscured by the skull.

In this case, the CT played an important role, as it directly clarified the displacement of the lead bullet and the location of the intracranial foreign body after the first operation. The using of X-rays only showed us a two-dimensional plane of information during the first operation. That made us difficult to locate the foreign body accurately. The use of intraoperative TCS can be helpful in the localization of foreign bodies sometimes.

Displacement of the intracranial foreign body is a rare phenomenon [14]. The displacement is related to the qualities of the foreign body itself (e.g., mass, shape, surface, etc.) and the initial intracranial location (e.g., the foreign body is not easily displaced when proximity to bone, blood vessels, meninges, etc.) [6,25]. There is still considerable controversy regarding the management of intracranial foreign bodies. Some scholars believe that the metallic foreign bodies can’t to remove when they are located in deep vital structures, with small in size or has life-threatening by operation. However, for lead bullets and cranial fragment bodies, they should be removed completely because of their increased toxicity and chance of infection compared with other foreign bodies [23]. In addition, intracranial shrapnel or projectiles >5 mm in diameter need to be completely removed because they are prone to foreign body displacement and can cause local brain tissue hyperplasia inducing epilepsy or toxic effects on brain tissue (especially lead and aluminum bullets) [12]. However, Karadas et al. [9] suggested that in cases without complications, where the foreign body is located deep and difficult to remove by surgery, the operation is not necessary and regular follow-up is sufficient.

However, intraoperative management of intracranial foreign body displacement has not been reported. There are no uniform criteria about the time to remove intracranial metallic foreign bodies. If the displacement of the intracranial foreign body occurs within a few hours of trauma, it is difficult to successfully remove it through surgery. For this reason, some investigators have suggested that intracranial foreign bodies should be accurately localized and removed 2 to 3 weeks after the injury. When they are easier to be removed as they are held in place by proliferating fibrous connective tissue or encapsulated by fibrous tissue [13]. However, intraoperative management of intracranial foreign body displacement has not been reported.

In this case, the intracranial metallic foreign body was easily displaced because of its large mass, spherical shape and smooth surface. We therefore suggest that intracranial displacement should be considered if the foreign body is difficult to locate during the operation, especially in the early stages of trauma. It is particularly important to identify the location of the foreign body at this time. When the location of the foreign body is not clearly identifiable during the surgery, blind exploration is avoided in order to reduce the trauma associated with surgery. The CT scan should be done to locate the foreign body. When the intracranial foreign body is confirmed to be displaced, symptomatic treatment such as anti-infection can be administered first, and then the patient can be considered for surgical removal depending on his condition after the foreign body has been immobilized by localized hyperplastic brain tissue.

PTE is an important complication affecting the quality of life of patients and can be divided into early (≥7 days) and late (>7 days) epilepsy. The incidence of PTE is 3.8% to 4.2% in all types of TBI [10]. There is no direct evidence as to whether PHI in combination with intracranial foreign bodies increases the incidence of PTE. Based on this case, we consider that the presence of an intracranial metallic foreign body may precipitate seizures. Because this patient had frequent seizures after the first surgery, this may have been due to the trauma itself or the retention of the intracranial metallic foreign body. In contrast, the patient’s seizures were effectively controlled after the second successful surgery to remove the intracranial metal foreign body.

The pathogenesis of PTE is unclear. Aarabi et al. [1] suggested by multivariate regression analysis that GCS scores and motor deficits are associated with the development of epilepsy. Recent studies have concluded that excitotoxicity, neuroinflammation, oxidative stress, and neurodegeneration are the main causative factors in PTE [18]. Towner et al. [20] reported that the retention of lead bullets in the body resulted in a sustained increase in lead levels and toxic effects, and that the lead levels decreased rapidly after surgical removal of the lead bullets and the symptoms were effectively relieved. In this case, the patient had a GCS score of 14 and no motor deficits, so it was considered that seizures might be related to the type of foreign body. In other words, the intracranial metallic foreign body may increase the probability of PTE, which may be related to the slow release of metallic toxic ions from metallic substances acting on local brain tissue and causing abnormal local electrical activity.

There are no clear protocols on how to effectively prevent PTE. But there is clinical practice recommend the administration of drugs such as phenytoin sodium and carbamazepine to control epilepsy within 1 week after injury [18]. Although there is evidence that the use of these prophylactic antiepileptic drugs may reduce early seizures, there is no evidence of benefit for late epilepsy [5]. For those with severe seizures, antiepileptic drug therapy is preferred, but the surgery can be tried after evaluation of the condition for those with well-localized and limited seizure foci [18].

In late epilepsy patients with intracranial retention of foreign bodies, it has not been investigated whether removal of the foreign body can improve epileptic symptoms. In this case, the patient applied sodium valproate for seizure prevention during the first hospitalization and did not experience seizures, which supports the effectiveness of early antiepileptic drug application. However, the patient had several seizures during the follow-up period, and after adjusting the medication prescription, seizures continued and was more serious than before. Therefor we decided to have a surgery for the patient again, and the intracranial foreign body was successfully removed. The patient underwent seizures at the fourteenth day after the reoperation, which may be related to the trauma of the reoperation. But we did not find typical seizure waves after monitoring the EEG. However, based on EEG topographic (Fig. 4) changes since the patient’s injury, we hypothesized that the patient may have a confined epileptic lesion in the right frontal lobe of the brain, but this epileptic lesion did not correspond to the location where the foreign body was located after its displacement. However, after removal of the intracranial foreign body, the patient’s seizures were significantly relieved compared with before (continued follow-up for 8 months without seizures). Therefore, we conjecture that the removal of the intracranial metallic foreign body may contribute to the control of PTE by reducing the release of toxins from the metallic foreign body.

In addition, the reduction of psychological stress after the intracranial foreign body was removed from the patient in this case may have contributed to the control of PTE also. As some studies have concluded that psychological stress is an important trigger and sometimes the cause of seizures [8]. However, the relationship between changes in psychological factors and the progression of PTE still needs to be confirmed by further clinical studies.