Inclusion Cysts

Approximately 50% to 60% of all scalp and skull lesions found in pediatrics are classified as either dermoids or epidermoids (see Table 206-1).^1,2 These lesions involve the calvarial bone (especially the outer table), and in some cases the inner table is eroded and the outer leaf of the dura is involved. Dermoids occur more frequently, at a range of 15% to 58%, whereas epidermoids are noted in 3% to 5% of patients. Pathologic analysis differentiates strictly between the two lesions (Fig. 206-1). Epidermoids are composed of only keratin-producing epithelial cells and are characterized by cystic contents with white cheesy keratin debris. Dermoid cysts, in contrast, have walls that are more complex, including the cutaneous adnexal cells of the lower layers of the skin combined with sebaceous and sweat secretions produced by exocrine glands.

FIGURE 206-1 The epidermoid (A) lining is made of squamous epithelium, with the inner cyst being composed of keratin debris, which clinically appears to be cheesy and is also referred to as a pearly tumor. In contrast, the lining of a dermoid cyst (B) contains cutaneous adnexa and in this figure shows a corrugated pattern with more complex epithelium, but it may also contain a variety of keratin debris, along with sebaceous secretions from the skin adnexa.

Although inclusion cysts may be found anywhere in the scalp, dermoid cysts develop more frequently along the midline in the area covered by the anterior and posterior fontanelles, and epidermoids tend to occur laterally at suture sites between the pterion and the asterion along the squamosal suture.³

The etiology of these inclusion cysts appears to be misdirected epithelial and dermal tissue originating from the scalp that embeds itself downward into the skull. Cyst size may range from less than 1 cm up to 4 or 5 cm. These lesions tend to be immobile and rubbery, and sinus tracts might extend intracranially through the dura.

On plain radiographs, a lucent center is identifiable and accompanied by an eroding outer table or by involvement of the full thickness of the skull and a rim of hypersclerosis. Computed tomography (CT) will confirm that the lesion is extradural by displaying evidence of bone erosion. Magnetic resonance imaging (MRI) often shows the cyst to have a hypodense or hyperdense core, and the cyst wall may also enhance (Fig. 206-2).^4,5

FIGURE 206-2 On magnetic resonances images in the sagittal (A) and coronal (B) planes, a midline dermoid is identified on T1 weighting, with the signal being hypointense in comparison to brain tissue.

In managing these lesions surgically, a curvilinear incision around the lesion is preferable to an incision on the vertex of the lesion. A curvilinear incision minimizes the risk of incising into the cyst and thereby contaminating the surgical field. Ideally, the cyst should be excised without rupturing it. Although separating the cyst from the surrounding subcutaneous tissue and bone is usually quite simple, a small fibrous or vascular pedicle arising from the suture may require curettage or cauterization. These lesions can occasionally be manifested as a dumbbell configuration affixed to the underlying dura. If the bone defect is very large, reconstruction with split-thickness cranial bone may be necessary. Sharp and scalloped bony margins should be smoothed with a bur, drill, or curet. Incomplete removal of these lesions may lead to recurrence.^6–8

Langerhans Cell Histiocytosis

Historically, this entity has been referred to variously as histocytosis X or eosinophilic granuloma; Hand-Schüller-Christian disease and Letterer-Siwe disease are sometimes classified as systemic forms of histiocytosis related to Langerhans cell histiocytosis. Accounting for 7% to 10% of all reported skull lesions, this entity is a non-neoplastic lesion believed to be an inflammatory response to an unknown stimulus.⁹ It may be a solitary lesion classically referred to as an eosinophilic granuloma, or it may extend to multiple lesions, especially in children younger than 2 years. In its systemic manifestations, the process can disseminate into the central nervous system and through the viscera, thereby generating skin lesions, pulmonary infiltrates, hepatosplenomegaly, pancytopenia, and lymphadenopathy. An intermediate chronic form of this entity features lesions involving the calvaria in the granulomatous process, as well as exophthalmos and diabetes insipidus (obviously affecting the neurohypophysial structures). A recent study has associated degenerative central nervous system histiocytosis with this chronic process.¹⁰

The lesions are characterized by the presence of Langerhans cells, which are immunoreactive to cluster differentiating protein (CDIa) and S-100 protein. Tinctorial histologic stains reveal a mixture of pleomorphic histocytes, eosinophils, lymphocytes, and plasma cells.

Skull radiographs show an osteolytic lesion with unequal involvement of the outer and inner tables, accompanied by a hyperostotic or sclerotic rim (Fig. 206-3A). CT demonstrates this expansile lesion to best advantage, especially with reconstructed bone formats. Even though the lesion usually enhances moderately, it can have a hemorrhagic core that is hyperdense on non–contrast-enhanced images. Although MRI will confirm the aforementioned findings in soft tissue, it defines the bone changes less clearly than CT does (Fig. 206-3B).

FIGURE 206-3 A, Plain skull radiograph showing the punched-out appearance of eosinophilic granuloma and the margins lacking in sclerosis. This is further imaged on a computed tomography scan (B), where greater involvement of outer table than the inner table is appreciated.

Management of these lesions involves wide surgical resection to normal bone margins (Fig. 206-4). The skull lesions can be attached to the outer leaf of the dura or to subcutaneous tissue, from which the lesion can usually be safely removed by curettage. These friable lesions are quite vascular and expansile within the outer and inner tables of the calvarial bone. Despite occasional location over the venous sinuses, vascular invasion is not seen in radiologic studies of these lesions. Persistence or recurrence of these lesions may require further treatment, including low-dose chemotherapy or, rarely, low-dose radiation therapy.

FIGURE 206-4 A, Intraoperative photograph demonstrating the appearance of an eosinophilic granuloma before surgical resection but after the scalp has been peeled back. B, The surgeon is now peeling the lesion away from the skull, where a clear border is identified between the granulomatous tissue and the lytic skull margin.

Hemangioma

Hemangiomas account for approximately 3% to 20% of reported skull and scalp lesions. They are classified into capillary and cavernous forms. An unusual osseous form that has been noted in adults is rarely seen in children. Hemangiomas tend to occur in the midline but may arise in the diploic spaces, where they extend outward and rarely involve the inner table of the skull, unless their blood supply originates from the dura.¹¹

These lesions may initially be noted primarily as a simple cosmetic deformity or secondarily after trauma as a source of bleeding (Fig. 206-5). In some cases, compression, necrosis, and infection have been identified. In older children, hemangiomas have been accompanied by focal headaches, presumably caused by stretching of the surrounding pericranium.

FIGURE 206-5 This photograph of a child shows the midline location of an anterior fontanelle scalp hemangioma, with the appearance of varices coming through in a very superficial fashion on the outer layer of the scalp.

Scalp hemangiomas are best imaged with MRI, which should confirm that the lesions are confined extradurally, especially when they are located in the area of the anterior fontanelle (Fig. 206-6). Either plain radiographs or CT scans provide the clearest images of the bony form of these lesions with sunburst-radiating trabeculae. The vertical orientation of this trabecular honeycombed arrangement of the skull is especially well visualized on tangential cross sections (Fig. 206-7).

FIGURE 206-6 Sagittal T1-weighted magnetic resonance image demonstrating full-thickness involvement of the scalp by this anterior fontanelle hemangioma.

FIGURE 206-7 This histologic section of a scalp hemangioma demonstrates the vascular lining of this malformation. The lack of muscularis in this vascular lesion suggests that it is dominantly a venous abnormality, and its ability to extend and compress is readily appreciated.

Treatment requires en bloc circumferential excision to margins of normal bone and pericranium. This surgical approach not only minimizes blood loss but also prevents recurrence of the lesion. When the lesion involves the superficial aspect of the scalp, the operation may require excising the involved skin area and reapproximating the skin margins. Hemangiomas occurring in the scalp alone frequently involute spontaneously, and plastic surgical consultation in such cases is often helpful.

Occult Meningoceles

A wide spectrum of congenital cranial dysraphic abnormalities (from overt encephalocele to simple cutis aplasia, also known as a sequestered/occult meningocele) can be manifested as scalp lesions. Retention of the heterotopic brain tissue associated with these lesions and even direct central nervous system connections may occur to varying degrees.

Cutis aplasia tends to be found over the cranial vertex along the sutural lines (Fig. 206-8). It may vary from less than 1 cm in size to very large scalp lesions (>10 cm). The resulting scalp defect may include the absence of bone formation and even a dural defect. Although smaller lesions (<3 cm) will spontaneously re-epithelialize, larger lesions (especially those with bone and dural defects) will require surgical reconstruction. Involvement of a plastic surgeon for skin rotation flaps, skin grafts, or tissue expanders is highly recommended for these larger lesions.

FIGURE 206-8 In this photograph, the scalp has been shaved and an area of epithelial abnormality over an occult meningocele is appreciated. In the unshaven state there was minimal hair directly over this abnormal epithelium. Adjacent to this area were heterotopic meninges without glial structures.

Cephalohematomas

As the frequency of high forceps deliveries has decreased, the occurrence of cephalohematomas has markedly diminished. This birth injury, usually in the parietal prominence, is more frequently encountered in babies with large birth weight. It tends to occur unilaterally. Unless there is an associated skull fracture, a cephalohematoma spreads along the subperiosteal interface and is limited by sutural lines. Although a cephalohematoma can easily be mistaken for a growing skull fracture and a skull defect can be suspected on palpation, CT or a plain skull radiograph will verify the integrity of the underlying calvaria.

This lesion is usually resorbed in 4 to 6 weeks, but occasionally it may progress to ossification wherein the outer layers of the cephalohematoma calcify around the fluid-filled central cavity of lysed blood cells. Even in cases of ossification, the cranium will continue to remodel as skull growth progresses (Fig. 206-9). In extremely rare cases, severe deformity might require reconstruction of the cranium either by simply removing the outer table of calcification or by performing autologous bone cranioplasties.

FIGURE 206-9 This computed tomography scan of an old calcified cephalohematoma has the appearance of separation of what appears to be an outer table, which is actually calcification over the cephalohematoma.

In some cases, cephalohematomas have been associated with infections, and a significant scalp abscess can occur in the subgaleal space (Fig. 206-10). These abscesses might have developed loculations and cannot simply be aspirated. In such cases, incision and dissection of the membranes become necessary, and placement of a drain is recommended. Organisms cultured from these infections have included staphylococci, as well as gram-negative organisms presumably acquired during vaginal delivery.

FIGURE 206-10 A-C, Enlargement of infected cephalohematomas, with septations appreciated. Note that in all sections this cephalohematoma is limited by the coronal suture and extends to the lambdoidal suture, but does not pass beyond the suture line, which is a characteristic of cephalohematomas.

Aneurysmal Bone Cysts

These lesions are tumor-like expansions of the diploic space and tend to be cystic lesions filled with blood and lined with connective tissue and giant cells. More than 80% of these lesions will come to clinical attention before the second decade of life. Only 2% of aneurysmal bone cysts are found in the cranium, usually in the skull base. Cranial neuropathies have been reported with these lesions when they are impinging on exiting cranial nerve foramina.

Characteristic fluid-fluid levels within the cyst are seen frequently, and the cortical bone surrounding the lesion is expanded but not destroyed (Fig. 206-11). Despite displacing surrounding central nervous system tissue, these lesions do not penetrate the dura.

FIGURE 206-11 Coronal (A) and axial (B) magnetic resonance images of an aneurysmal bone cyst showing the large vascular channel.

Surgical management of these lesions should include preoperative angiography accompanied by preoperative embolization to minimize intraoperative bleeding of the very vascular lesions. After complete resection of the lesion, primary reconstruction of the cranial defect is ideal. To reduce the recurrence rate of these lesions, some authors have recommended cryosurgery or low-dose irradiation. Recent reports of medical treatment with methylprednisolone and calcitonin have suggested that a new medical option might be used for lesions in areas difficult to excise, such as in the spinal atlas-axis joint.¹²

Cranial Fasciitis

This unusual skull and scalp lesion occurs rarely in the pediatric population and is generally a painless mass.^13,14 Cranial fasciitis tends to grow rapidly and has been characterized histologically by inflammatory cells and focal multinucleated cells separated by bundles of ionized collagen tissue. These lesions must be differentiated from fibrous histiocytoma and fibrous sarcoma, which rarely occur in pediatric patients. The lesions often erode the cranium and may extend to the dura. Symptoms are generally related to a mass effect.

A distinct finding on MRI is hyperintense areas with central nonenhancement on T2-weighted images (Fig. 206-12). The lesions are readily excisable, and reconstruction of the underlying skull may be required, depending on the age of the child (Fig. 206-13). The masses have been described as firm and fleshy but are readily dissectible from the dura by blunt dissection.

FIGURE 206-12 A, Non–contrast-enhanced cranial fasciitis lesion identified in the parietal area of this patient. B, With administration of contrast material, this lesion enhanced very prominently and its destruction of the underlying skull is appreciated.

FIGURE 206-13 This fasciitis lesion not only involves the scalp but also extends through the skull, yet it has very distinct borders from the surrounding skull.

Skull Metastasis

Most malignant scalp or skull lesions in children are secondary to leukemia, lymphoma, Ewing’s sarcoma, neuroblastoma, or rhabdomyosarcoma. These skull lesions usually appear lytic, with soft tissue masses that may be palpable through the skin, or the lesions may also compress the underlying dura. Surgical resection is indicated for either biopsy of the lesion or relief of the underlying brain compression. In all cases, management of the primary neoplasm will determine further management of the local lesion.

Melanotic Neuroectodermal Tumor (Melanotic Progonoma)

These lesions tend to occur in infancy and have been thought to be derived from neural crest cells. Two thirds of the lesions will be found in the midface region, whereas the remainder may be found in either the mandible or the cranial vault. Radiologically, these lesions resemble osteogenic sarcoma (Fig. 206-14). At the time of surgery, patients tend to have discolored bone that is somewhat yellowish and very thick. The lesion must be resected to normal bone margins, and reconstruction of the cranial defect is required. In rare cases, these lesions have been reported as malignant.¹⁵ Histologically, there is an epithelial-like arrangement of melanotic cells interwoven with small cells resembling neuroblasts.

FIGURE 206-14 Computed tomography scan through a melanotic neurectodermal tumor involving all layers of the skull. An eburnated irregularity of this lesion on its inner aspect is demonstrated.

Osteomas

Osteomas are benign, usually solitary, and slow-growing lesions. They tend to be frontal and are well-defined hyperostotic lesions. They may involve either the outer or the inner table, and they account for less than 5% of skull lesions. Although rare in children, they may be seen as either a deformity or a purely incidental finding on a radiograph.

Imaging studies will demonstrate that these lesions are circumscribed exostoses, with preservation of the diplopic space. They are best seen on the bone windows of CT scans.

Surgical management can be as simple as burring down the lesion to normal bone or as complicated, with larger lesions, as excisional craniectomy, possibly requiring split-thickness autologous bone cranioplasty. Gardner’s syndrome is a hereditary form of these lesions and is associated with multiple osteomas, colon polyposis, and skin inclusion cysts.

Sinus Pericranii

Sinus pericranii are usually median or paramedian lesions between the extracranial and intracranial veins. They tend to be painless and are characterized as being compressible or deflating in the upright position and engorging either during a Valsalva maneuver or in the recumbent position. Although most of these lesions are congenital, some have been acquired by trauma. There have been cases in which they were noted to enlarge with time, and because of such enlargement, cosmetic issues, and concern about the potential for hemorrhage, surgery has been advocated. These lesions are best visualized on CT or CT angiography. They can also be visualized on magnetic resonance venography (Fig. 206-15).

FIGURE 206-15 A, Sinus pericranii in the parietal area of a child in the prone position. B, On magnetic resonance imaging, one can appreciate the proximity of this lesion to the sagittal sinus. C, The signal within the lesion, however, is isointense with the surrounding brain tissue but separate from the scalp. This lesion had small blood vessels that were originating from emissary veins underlying the area.

Sinus pericranii lesions can usually be excised with careful control of venous bleeding both from the veins draining the lesion in subcutaneous tissue and at the site of entrance of the venous malformation into the skull, where a bone defect can be visualized on scans. Blood vessels that communicate with arachnoid granulations or directly with the sagittal sinus pose a risk for major blood loss, but communication of the venous stalk through the skull to the dural vein is generally easily coagulated with bipolar forceps before excision of the lesion. Endovascular treatment consisting of transvenous embolization or direct sclerosis of the lesion with intralesional alcohol has also been used in the treatment of selected patients.

Fibrous Dysplasia

Fibrous dysplasia, a benign skeletal lesion of immature mesenchymal cells, typically involves the long bones, ribs, and skull.^16,17 It begins as mutated pluripotent embryonic cells from which the skeletal stem cells develop.¹⁸ Lesional bone is formed when cancellous bone is replaced with fibrous, cellular tissue and when collagen is deposited with immature woven bone.¹⁹ Affected bone is rich in versican and osteonectin, which are antiadhesion molecules, and lacks the adhesion-promoting molecules osteopontin and bone sialoprotein.²⁰ The affected bone can be monostotic, or involving one bone, or polyostotic, in which multiple bones are affected.

The genetic mutation for fibrous dysplasia is located on chromosome 20q13 at the GNAS locus. The aberrant gene codes for a constitutively active mutant of the G_sα subunit of the G protein that couples intracellular cyclic adenosine monophosphate production to hormone.^21,22 As a result, the normal guanosine triphosphatase activity required to deactivate the G protein is diminished. The abnormal allele is present in mosaic patterning.²² Interestingly, this mutation is also found in hypersecreting thyroid tumors, in Leydig cell tumors, and in up to 40% of growth hormone–secreting pituitary adenomas.^21,22 Parental imprinting (an epigenetic phenomenon involving post-transcriptional DNA methylamine and histone processing, as well as early embryonic mutation) probably accounts for the phenotypic variance and mosaicism.²¹

McCune-Albright syndrome, also a result of activation of G_sα, consists of polyostotic fibrous dysplasia, endocrinopathy (particularly precocious puberty), and café au lait spots. A clinical diagnosis can be made with two of these three criteria.²³

Radiographically, lesional bone is evident on thin-cut CT and displays a classic ground-glass appearance. Three-dimensional CT and multiplanar views can be helpful in preoperative planning (Fig. 206-16).

FIGURE 206-16 Fibrous dysplasia involving the frontal (A), superior orbital rim (B) and extending on the floor of the anterior fossa (C), with some compression of the orbit. Note the multiple densities within this bone, as well as some porous changes, especially on the coronal image. This illustrates the vascular nature of these hypersclerotic medullary bone-like lesions.

The bone disease usually stabilizes by adolescence and can be managed conservatively, but its sequelae, such as pain and fractures, can last throughout adulthood.²⁴ The bone pain is most often relieved with bisphosphonate therapy, but the effects of other sequelae on bone turnover, bone density, and radiographic changes still remain unclear.²⁵ From a craniofacial standpoint, orbital apical involvement is not uncommon and most often consists of proptosis, dystonia, and distorted facies.²⁶ Although optic nerve encasement is common, blindness is more infrequent.^26,27 Growth hormone excess seems to be a risk factor for both nerve encasement and visual loss.²⁷ One series of patients with craniofacial fibrous dysplasia found that the ethmoid bones (71%) were involved most often, followed by the sphenoid (43%), frontal bones (33%), maxilla (29%), temporal bones (24%), parietal bones (14%), and occipital bone (5%).²⁸ Goals of surgery include decompression of any neurological structures and cosmesis. Malignant transformation of fibrous dysplasia is rare, and the possibility that radiation could induce malignant transformation is controversial. In one large retrospective series, the incidence of sarcomas arising from fibrous dysplasia was 2.5%, with just under half of the patients reporting a history of irradiation.²⁹