Introduction

Increasing swelling arising in a delayed manner weeks after head injury is an uncommon complication pediatric skull fractures. One common mechanism for this phenomenon is a posttraumatic leptomeningeal cyst, also called a “growing skull fracture.” These lesions occur most frequently in children, with an estimated incidence of 0.03–1.6 % following skull fracture [1–3]. Over 90 % of cases occur in children under 3 years of age, and greater than 50 % occur in patients under 1 year old [4]. The classic presentation in a young child includes a subgaleal fluid collection overlying a diastatic parietal skull fracture resulting from head trauma in the setting of progressive neurologic deficit and/or posttraumatic seizures [5, 6]. Plain skull films demonstrate smooth-edged ossification defects with scalloping and often sclerotic margins. Computed tomography (CT) shows similar skull abnormalities in addition to underlying parenchymal injury with cystic degeneration. A leptomeningeal cyst or brain tissue will often herniate through the skull defect [5].

Clinical presentation and imaging characteristics are generally sufficient to diagnose growing skull fractures. In this report, we describe the case of a 2-year-old boy with the appearance of a lytic skull lesion on CT that, despite presentation consistent with eosinophilic granuloma (EG), histologic findings confirmed the diagnosis of a healing skull fracture.

Case report

History and presentation

A 2-year-old healthy male presented by referral for a skull lesion. Two months prior, another child struck the patient on his left forehead. His mother noticed mild bruising in the area immediately following the incident. The family did not seek medical attention at the time and the bruising resolved within several days following the injury. No imaging of the skull was performed at the time of the trauma. Several weeks later, the child developed a bony step-off and palpable soft tissue mass in the general area. The patient had no history of seizure or neurologic deficits before or after the injury.

On physical examination, the patient was neurologically intact. There was a tender, intermittently pulsatile mass on the left forehead without overlying redness or swelling. The mass was palpable within a skull defect that had irregular borders.

Head CT revealed a soft tissue mass within a lytic skull defect with irregular eroded borders measuring 2 cm in diameter (Figs. 1 and 2). The lesion affected the outer table more so than the inner table, and had a beveled appearance. There was no evidence of surrounding sclerosis or periosteal reaction. Based on clinical presentation and imaging, the most likely diagnosis was thought to be a solitary calvarial EG.

Fig. 1
figure 1

Preoperative non-contrast head CTs, with axial bone (upper) and brain (lower) windows. Imaging demonstrates a lytic skull lesion with beveled borders, the typical appearance for EG. Note the fracture is non-diastatic, and there is no cyst or brain herniation through the defect

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Fig. 2
figure 2

Three-dimensional CT skull reconstructions. Skull defect is visible over the left frontal bone

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Operation

The patient was taken to the operating room for surgical resection. Intraoperatively, an area of abnormal skull was identified in the left frontal bone but no linear fractures were seen. A 2-cm circumferential craniectomy using a high-speed drill and rongeurs was performed around the lesion for complete resection. Abnormal bone and underlying fibrous tissue were sent to surgical pathology. There was no dural defect under the skull lesion, and the dura was not violated during the procedure. Bone dust was collected and used to fill the craniectomy site. An absorbable plate was cut and fashioned to repair the skull defect. The surgical wound was closed in the standard layered fashion, and a sterile dressing was applied.

Pathologic findings

Microscopic features was evaluated using hematoxylin and eosin (H&E) staining, and CD34 immunohistochemistry showed fragments of woven and lamellar bone with fibrosis and granulation tissue in marrow spaces consistent with healing fracture. Structural features included hemosiderin-laden macrophages, fibrous tissue, and reparative woven bone. Osteoblastic rimming was seen and is typical of bone repair, but not fibrous dysplasia. Eosinophils and Langerhans cell histiocytes, squamous epithelium, and adnexa were not present. Immunohistochemistry was negative for cluster designation 1A (CD1A), K903 cytokeratin, and S-100 protein staining; these findings ruled out EG, epidermoid cyst, and dermoid cyst (Fig. 3) [7].

Fig. 3
figure 3

a Lesion in the dura shows large numbers of phagocytes with plump cytoplasm containing variable amounts of brown pigment (H&E stain; original magnification, ×20). b The pigment in the cytoplasm stains blue, a characteristic feature of hemosiderin. While some hemosiderin granules are scattered in fibrous tissue, hemosiderin is not seen in pink nuclei (Gomori stain for iron; original magnification, ×20). c Immunohistochemistry for CD68 marker stains macrophages red, but does not alter the natural brown color of hemosiderin granules. Hematoxylin stains collagen blue and nuclei dark blue (CD68 with hematoxylin counterstain; original magnification, ×60). d Skull lesion shows no evidence of S100 protein, a marker for EG. Reparative woven bone lacks laminations of lamellar bone, and has hypertrophic osteocytes in lacunae. Reactive fibrosis and macrophages contain a few granules of hemosiderin (S100 protein with hematoxylin counterstain; original magnification, ×20). e There is no evidence of CD1a, another marker for EG. Reactive fibrosis and macrophages contain many granules of hemosiderin (CD1a with hematoxylin counterstain; original magnification, ×20). f Cytokeratin marker CK903 for epidermoid/dermoid cysts is negative (CK903 with hematoxylin counterstain; original magnification, ×20)

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Other diagnoses are associated with bone repair, but few show a focal lytic skull lesion in this age group. For example, active Paget disease makes woven bone, but it does not affect children. Fractures associated with unicameral (simple) bone cysts are rare in the skull. Features of simple bone cyst fracture include bone repair. In this 2-year-old patient, the injury preceded growth of the lesion, consistent with a growing skull fracture. Aneurysmal bone cysts have a rim of bone repair, but this case does not have other features of an aneurysmal bone cyst, such as spaces with serum or blood, septa with fibro-osteoid, or calcified matrix. Slides were reviewed by two pathologists with both coming to the same diagnosis of a healing skull fracture.

Postoperative course

Patient was discharged following surgery without complication. The incision healed well without underlying mass or bony deformity. He remained neurologically intact throughout with no seizures. Head CT at 6-month follow-up showed a healing skull defect (Fig. 4).

Fig. 4
figure 4

Postoperative non-contrast head CTs, with axial bone (left) and brain (right) windows. Skull defect edges demonstrate new bone formation. Absorbable plate can be visualized over the defect

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Discussion

To our knowledge, this is the first description of a healing skull fracture mimicking an EG in a pediatric patient. The literature contains two reports of adults with a distant history of head trauma (20 and 50 years) with growing skull fractures mimicking skull tumors [8, 9]. Our case is unique because of its atypical presentation for a growing skull fracture with multiple radiographic and clinical findings consistent with EG.

In some cases, the phenomenon of a “growing skull fracture” is thought to arise from a tear in the dura underlying a skull fracture that results in herniation of the arachnoid membrane. This herniation produces a leptomeningeal cyst that, through cerebrospinal fluid pulsations and continued pressure on the overlying fracture, leads to erosion of bone edges [10]. More recent evidence implicates the herniation of brain parenchyma itself through the skull defect. Continued underlying tissue injury causes brain necrosis with reactive gliosis leading to cystic degeneration [11–13]. This pathophysiology is more consistent with the progressive neurologic deficits and seizures often seen in these patients [4, 6].

Our patient was neurologically intact and had no dural defect, leptomeningeal cyst, or brain herniation through the skull fracture. The skull defect was non-diastatic with a “punched-out” lytic appearance and beveled edges located in the frontal bone, an uncommon location for growing skull fractures [5, 6]. These imaging features are typical for EG [14]. Patients diagnosed with EG have a history of trauma to the area over the tumor in 33–50 % of cases [15–18]. The strong association between local head trauma and EG has led authors to question this as coincidental [19]. These findings indicate that a history of trauma does not significantly aide in establishing a diagnosis of growing skull fracture versus EG.

EG is a local manifestation of Langerhans cell histiocytosis (LCH), often referred to as unifocal LCH. EG constitutes 21–32 % of all primary pediatric skull tumors with epidermoid/dermoid cysts, fibrous dysplasia, and vascular anomalies being other common diagnoses [20, 21]. Our case exemplifies the fact that EG may be mistakenly diagnosed, given the greater incidence of EG compared to growing skull fractures, even in the setting of previous head trauma.

We present this case as an example of an atypical presentation of a healing skull fracture and as a cautionary note on the early surgical management of presumed EG in patients with preceding head trauma. Watchful waiting is an acceptable treatment option in solitary calvarial EG and growing skull fractures. Spontaneous resolution of both lesions has been described [22–24]. A period of observation may be indicated to make a definitive diagnosis and establish appropriate treatment.