The Skull Base

10.1055/b-0034-87895

The Skull Base

The skull base is one of the most complex regions of the human body because of its intricate network of neural, vascular, and lymphatic structures. An additional complexity exists in the pediatric population because of the postnatal maturation of the skull base. This may lead, among other things, to misinterpretations of sutures and synchondroses as fractures. This section will focus not on skull base maturation itself, but on the more important aspects of diagnosis, differential diagnosis, and diagnosis pitfalls in pediatric patients with lesions of the skull base using conventional X-ray, CT, or MRI of the skull and skull base.

Extensive involvement and guidance by the pediatric radiologist is important to both surgeon and oncologist because it provides an exact assessment of the topography, degree of vascularization, and extent of the lesion for the appropriate surgical or radio-oncologic planning.

CT can detect lesions and fractures, using ultrathin slices, 3D reconstructions, and contrast medium administration, even with a recently introduced CT-angiography (CTA) technique. Regarding tumorous lesions, MRI is superior to CT due to its better soft-tissue contrast and the possibility of choosing a variety of parameters emphasizing different types of tissues and substances. Another important advantage of MRI, especially in children, is its lack of radiation burden.

**Table 4.8 Pathology at the skull base**
Diagnosis	Findings
Nonneoplastic tumors
Arachnoidal cyst Cephalocele Fig. 4.18a–e, p. 310	See arachnoid cysts, p. 344 Occurs usually at the midline. The most common basal cephalocele is the sphenopharyngeal type. CT can show the bony defect. CTA can show the relation to vascular structures around the cephalocele. MR is better to depict the extent and content of the cephalocele and accompanying (midline) structure abnormalities.
Benign tumors
Cholesterol granuloma	MRI is the imaging modality of choice. The tumor has increased signal intensity on T1 and T2 sequences with a hypointense rim that represents expanded cortical bone and hemosiderin peripheral deposits.
Epidermoid	Most common at the cerebellopontine angle cisterns and parasellar region. Epidermoids are typically extra-axial lesions, like arachnoid cysts. On CT, epidermoids appear hypoattenuating with possible marginal calcifications. On MRI, the tumor is hypointense on T1 and hyperintense on T2, with no significant contrast enhancement (CE). There is usually some internal heterogeneity, which is best seen in the proton-density and FLAIR images, and this could help distinguish these cysts from arachnoid cysts, which they closely mimic. DWI is the most helpful imaging sequence in diagnosing an epidermoid cyst. Epidermoid tumors demonstrate an ADC that is similar to that of gray matter and lower than that of CSF. In contrast, arachnoid cysts or other cystic intracranial lesions do not show restricted diffusion and follow the CSF signal on DWI and ADC maps. Epidermoids often show encasement of vessels without displacement.
Lipoma	A lipoma is thought to result from a maldifferentiation of the primitive meninx. The majority of lipomas are located around the corpus callosum. On CT, a lipoma appears exactly the same as subcuteous fat: homogeneous hypoattenuation. On MRI, a lipoma has a signal intensity compatible with subcutaneous fat on all sequences.
Schwannoma/neurofibroma	Schwannomas usually arise from the vestibular division of the eighth cranial nerve. Acoustic schwannoma may occur either sporadically or as part of a clinical complex in NF2. In the latter situation, patients usually present at an earlier age and sometimes with bilateral tumors. Sporadic or non-NF2 vestibular schwannomas are very rare in children. On CT, the lesion is hypodense and may show homogeneous enhancement. On MRI, the lesion is hypointense on T1 and hyperintense on T2 and show marked enhancement after Gd. Schwannoma and neurofibroma are indistinguishable by neuroimaging.
Meningioma Fig. 4.19a–c	Pediatric meningiomas are rare, comprising less than 5% of all pediatric brain tumors and less than 2% of all meningiomas. Risk factors for the development of meningiomas include a history of radiation therapy or a diagnosis of NF2. MRI is the imaging modality. The lesion is iso-/hypointense on T1 and hyperintense on T2 with occasional cysts and calcifications. The lesion shows a marked homogeneous enhancement after Gd. On CT, the demarcated exostosis of bone in the area of the lesion is better seen than on MRI.
Glomus tumor	Benign but locally aggressive tumors, destroying the bones of the skull base with a moth-eaten appearance at CT. MRI shows signs of a vascular tumor and a mixture of multiple punctate and serpentine signal voids, due to high-flow intratumoral vessels and intratumoral hemorrhage, producing the characteristic salt-and-pepper appearance. It is a rare tumor in children.
Malignant tumors
Rhabdomyosarcoma Fig. 4.20a–c, p. 312	Rhabdomyosarcomas at the skull base, by virtue of an often parameningeal location, show an invasive behavior. They can extend intracranially and produce neoplastic meningitis. The four anatomic sites showing this potential are the nasopharynx/nasal cavity, the middle ear, the paranasal sinuses, and the infratemporal fossa/pterygopalatine space. Most patients are under 10 y old at diagnosis (72%) and present with skull base erosion, cranial nerve palsy, and intracranial extension.
Chondrosarcoma	Usually more lateral in location than a chordoma. Otherwise, see chordoma, discussed subsequently.
Chordoma	Originate from embryonic remnants of the primitive notochord and are located in the midline, near the clivus. On CT, chordoma typically appears as a centrally located, well-circumscribed, expansile soft-tissue mass with extensive lytic destruction of the clivus. On MRI, the tumor is heterogeneous with an intermediate to low signal on T1 with foci of hyperintense signal due to ossified fragments of the skull base, tumor calcifications, collections of proteinaceous fluid, or hemorrhage. On T2, a chordoma has high signal intensity and septa of low signal intensity. Slight enhancement after Gd.
Metastatic disease	Rare in the pediatric population. May mimic a meningioma or schwannoma. Often surrounded by peritumoral edema.

**Fig. 4.18a–e Cephalocele.** (a) 3D CT demonstrates midline schisis and bulging of soft tissue in the oronasal pharynx in a 3-month-old boy. (b) Sagittal reconstruction of a CTA. The pericallosal arteries are herniating through the encephalocele. T1 (c) and T2 (**d, e**) MRI shows a frontal nasal encephalocele including herniation of the pituitary gland.

**Fig. 4.19a–c Meningioma.** (a) A 9-year-old girl with a destructive lesion of the calvaria due to an intraosseous meningioma in the midline and paramedian left. Tumor shows invasion of the SSS. Note the swollen cortical veins due to impaired venous drainage of the sinus. (b) 3D map of contrast-enhanced MRA show the involvement of the SSS. (c) Sagittal T2 image shows the involvement of the diploic space. Note the relative low signal intensity of the tumor.

**Fig. 4.20a–c Rhabdomyosarcoma.** (a) A 4-year-old boy with progressive neurologic deterioration due to compression of the brainstem. The tumor is located in the left mastoid with cerebellopontine expansion, histologically an embryonal rhabdomyosarcoma. Note the high signal intensity on this T2-weighted image. (b) On this axial T1-weighted image, the tumor is hypointense. (c) Intense contrast enhancement (CE) on this Gd-enhanced T1 sequence is noted.

**Table 4.9 Sclerosis and hyperostosis of the skull base**
Diagnosis	Findings
Physiologic	In prematures and neonates.
Chronic infection	Mastoiditis, inflammation of the apex of the petrous bone. Mucocele. Sinusitis complicated with surrounding osteomyelitis, known as Pott puffy tumor at the level of the frontal sinus.
Fibrous dysplasia Fig. 4.21a, b Fig. 4.22	See Table 4.4 and skeletal dysplasia in Table 4.9
Hemolytic anemias	Hemolytic anemia may cause hyperplasia of the bone marrow as well as hyperostosis of the entire calvarial bone.
Hyperparathyroidism	Bone changes are primarily due to high bone turnover, often combined with a mineralization defect leading to increased bone fractures and bone deformities. Although rarely considered, the craniofacial skeleton represents one of the peculiar targets of this complex metabolic disease whose more dramatic pattern is a form of leontiasis ossea.
Mucopolysaccharidosis Fig. 4.23a–d Fig. 4.24	MPS may cause sclerosis and enlargement of the skull.
Vitamin D toxicity Fluorosis Idiopathic hypercalcemia Hyperphosphatasia
Tumors
Skeletal dysplasias	These include osteopetrosis, pycnodysostosis, sclerosteosis, craniometaphyseal dysplasia (Pyle disease), FD, hyperostosis corticalis generalisata/endosteal hyperostosis (van Buchem disease), Camurati-Engelmann disease, frontometaphyseal dysplasia, dysosteosclerosis, and hyperostosis cranialis interna. Only FD has a pagetoid pattern with ground-glass appearance on CT. On MRI, the affected bone areas show low to intermediate signal on T1, heterogeneous signal on T2, and heterogeneous enhancement. In all other diseases, the affected bone sites show low signal intensity on T1 and T2 and no enhancement.
Osteopathia striata Fig. 4.25a–d, p. 316	Rare skeletal dysplasia characterized by longitudinal striations of the long bone dia- and metaphyses and sclerosis of the cranial vault and base. Typical physical presentations of this disorder are a squarelike skull, frontal bossing, flat nasal bridge, palate abnormalities, and hearing loss. Mental retardation is present in many patients with osteopathia striata with cranial stenosis.

**Fig. 4.21a, b McCune-Albright syndrome** in a 15-year-old girl involving the left side of the vault and skull base.

**Fig. 4.22 McCune-Albright syndrome** in a 16-year-old boy. CT shows a ground-glass aspect of the clivus and sphenoid region on the left. Foramina are smaller and there is aplasia of the left part of the sphenoid sinus.

**Fig. 4.23a–d Mucopolysaccharidosis.** (a) Macrocephaly of 2-year old girl with Hurler syndrome (MPS I). (b) Enlarged perivascular spaces are hypointense on T1 image in the same patient. (c) Enlarged perivascular spaces are hyperintense on T2 image; fluid contains mucopolysaccha-rides. Note the hydrocephalus. (d) Sagittal T1-weighted image of same patient shows a narrow foramen magnum.

**Fig. 4.24 Mucopolysaccharidosis type IV (Morquio syndrome)** in 2-year-old boy with compression on the myelum at the cranio-cervical junction.

**Fig. 4.25a–d Osteopathia striata.** (a) A 16-year-old girl showing macrocephaly and a dense vault and skull base. (b) Lateral view. (**c, d**) CT showing the dense skull base and vault.

**Table 4.10 The enlarged sella**
Diagnosis	Findings	Comments
Chronically elevated ICP Fig. 4.26	Enlarged sella without changes in contour and structure. Sometimes with an empty sella appearance.	Triventricular hydrocephalus due to obstruction at the aquaduct as a result of tumor compression, postinfection, or posthemorrhage. May also occur in (non) syndromal craniosynostosis.
Parasellar mass effect
Glioma of the optic nerve and hypothalamus	Enlarged sella with changes in contour and structure. Hypothalamic and optic chiasm glioma are sometimes indistinguishable.	MRI is the imaging modality of choice.
Suprasellar cysts	Part of a cystic form of a craniopharyngioma or glioma, or arachnoid cyst.	MRI is the imaging modality of choice.
NF/neuroma/schwannoma	Enlargement is rare. The most frequent schwannoma is of the trigeminal nerve.
Chordoma	May extend into the sellar region, destructing the floor and dorsum sellae.
Intrasellar mass effect
Rathke cleft cyst Fig. 4.27a–c	Located between the anterior and intermediate lobes of the pituitary gland. Almost always intrasellar in location. On MRI, the signal intensity depends on its content. If the protein content is less than 15%, the cyst is hypointense on T1 and hyperintense on T2. Between 15%–25%, the cyst may be hyperintense on T1 and T2. In more than 25%, it may be hyperintense on T1 and hypointense on T2. Rim enhancement may be seen.	Rarely symptomatic, unless larger than 1 cm. MRI is the imaging modality of choice.
Craniopharyngioma	Look for density (CT) or signal intensity (MRI) differences in the cystic parts of the mass, due to special “motor oil” content.	Most common intrasellar tumor. CT may be helpful for identifying calcifications.
Hypophyseal tumors Fig. 4.28a–d	Adenoma is most common sellar/parasellar mass. Macroadenoma (> 1 cm) causes visual disturbance or hypopituitarism. Usually erosion of the bottom of the sella. Rarely invasive with extension into the sphenoid sinus. Macroadenoma may also give parasellar extension into the cavernous sinus.	Presentation near puberty. MRI is the imaging modality of choice.
Germinoma Fig. 4.29a–c	Thirty-five percent are intra-/suprasellar. Usually large at presentation. On CT, the mass is well-marginated and isodense to hyperdense to brain parenchyma with homogeneous enhancement. On MRI, the mass is hypointense to isointense to gray matter on T1 and hyperintense on T2 and shows intense enhancement after Gd. CSF spread and systemic metastasis is possible.	Clinical presentation includes diabetes insipidus, visual disturbances, and panhypopituitarism.
Meningioma	Rare in children.
Metastasis	Leukemia and lymphoma do occur.	Most patients die before becoming symptomatic, but diabetes insipidus may occur.
Untreated hypothyroidism	Massive enlargement without destruction.	Enlargement due to reactive response of hypophysis.
Empty sella syndrome (ESS) Fig. 4.26, p. 317	MRI shows a flattened gland with increased CSF within the sella turcica. Enlargement and erosion of the sella may be seen. Normal position differentiates ESS from arachnoid cyst.	The causes of ESS may be high ICP, neglected or improperly treated hydrocephalus, and suprasellar arachnoid cyst. Primary ESS has also been described.

**Fig. 4.26 Empty sella** on a sagittal T2-weighted image. Note also the retrocerebellar arachnoid cyst.

**Fig. 4.27a–c Rathke’s cleft cyst.** (a) A 4-year-old girl with familial growth hormone deficiency. MRI shows a Rathke cleft cyst. (b) Sagittal T1-weighted image after contrast shows no enhancement consistent with a Rathke cleft cyst. (c) Coronal T2-weighted image of same patient.

**Fig. 4.28a–d Hypophyseal tumors.** (a) A 16-year-old boy with visual impairment due to a prolactin-producing macroadenoma with suprasellar extension. Note the enlargement of the sella and disruption of the sellar diaphragm. (b) Note the compression on the optic chiasm on this T2 image. (c) Note the lack of enhancement of the macroadenoma on this T1 image. (d) Note the thin wall enhancement of the macroadenoma and the compression on the optic chiasm.

**Fig. 4.29a–c Germinoma.** (a) A 9-year-old boy with diabetes insipidus and thyroid-stimulating hormone and growth hormone deficiency. The space occupying the process in the suprasellar region is most consistent with a germ cell tumor on this T1-weighted image. (b) T2-weighted image shows a solid and cystic component. (c) Sagittal T1 image shows minimal CE.

**Table 4.11 The small sella**
Diagnosis	Findings	Comments
Normal variant, decreased hypophyseal function	There are several syndromes with genetic mutations involving the development of the pituitary gland that may give a hypoplastic sella. There are, for example, bone morphogenetic proteins like fibroblastic growth factor that influence the sella development, but also transcription factors, and signaling proteins like sonic hedgehog are important for the development of the pituitary gland.
After relief of elevated ICP, growth hormone deficiency, Prader–Willi syndrome
Chiari malformation
Fibrous dysplasia Fig. 4.30
Myotonic dystrophy
Primary dysplasia

**Fig. 4.30 Camurati-Engelmann disease** in a 15-year-old boy with polyostotic FD; note the small sella.

**Table 4.12 Changes in contour and shape of the sella**
Diagnosis	Findings	Comments
Rickets	Size of sella is normal. Sella less dense on X-ray and CT as a result of demineralization.	Density returns to normal after treatment.
Chronically elevated ICP	Sella enlarges, and dorsum sellae becomes shorter and angulated.
Leukemia	Demineralization detectable on X-ray and CT.
Nasopharyngeal tumors	Rarely seen, but tumor may cause destruction of the sella.
LCH	Demineralization, may progress to destruction.
Transsphenoidal encephalocele	Defect in the sellar floor as a result of persistence of the craniopharyngeal canal or developmental failure of multiple ossification centers. MRI is necessary to show the extension and herniation of the encephalocele. Bony defect best shown on CT.	Associated anomalies are midline defects like agenesis of the corpus callosum, cleft palate, and hypertelorism. Nasal obstruction.
Chordoma	See Table 4.8, Pathology of the skull base.