(41-4) (Top) Autopsy shows occipital cephalocele , brain with pachy-/polymicrogyria. (E. T. Hedley-Whyte, MD.) (Bottom) Sagittal T1WI shows occipitocervical meningoencephalocele with traction of cervicomedullary junction .

The foramen lacerum (11) is an irregular cartilage-filled aperture that lies between the sphenoid bone and petrous apex. The internal carotid arteries exit the petrous temporal bone at the endocranial carotid canal (12). The dorsum sellae continues posteroinferiorly as the upper part of a smooth concavity, the clivus (13).

Posterior Skull Base

The posterior skull base (PSB) is formed by the temporal bones posterior to the petrous ridges and the occipital bone. The petrooccipital fissure (14) lies between the petrous apex and the occipital bone.

The internal acoustic meatus (15) lies along the posterior aspect of the petrous temporal bone and transmits the facial (CN VII) and vestibulocochlear nerves (CN VIII) as well as the labyrinthine artery, which is a small branch of the anterior inferior cerebellar artery.

The jugular foramen (16) lies below the internal acoustic meatus. The jugular foramen transmits CNs IX-XI, the jugular bulb, and the inferior petrosal sinus. The hypoglossal canal (17) transmits CN XII. The foramen magnum (18) contains the medulla, both vertebral arteries, and the spinal segment of CN XI.

Cephaloceles

"Cephalocele" is a generic term for the protrusion of intracranial contents through a calvarial or skull base defect. Cephaloceles that contain herniations of brain tissue, meninges, and CSF are called meningoencephaloceles. If the

(41-5) (L) T1WI, (R) T2WI show an occipital cephalocele that contains meninges and CSF and dysplastic brain . Note traction and distortion of the cerebellum .

meninges and accompanying CSF are herniated without brain tissue, the lesion is termed a meningocele.

An atretic cephalocele is a small defect that contains just dura, fibrous tissue, and degenerated brain tissue. Atretic cephaloceles most commonly occur at the obelion (along the sagittal suture on a level with the parietal foramina). A gliocele is a glia-lined pouch that contains only CSF.

Cephaloceles can be congenital or acquired lesions. They are generally classified by location and are named according to the roof and floor of the bone(s) through which they herniate. They can be open or skin-covered. Most congenital cephaloceles have coexisting intracranial abnormalities of varying severity. Cephalocele prevalence and type vary significantly with geographic location and ethnicity.

Cephalocele imaging has four goals: (1) depict the osseous defect, (2) delineate the sac and define its contents, (3) map the course of adjacent arteries and determine the integrity of the dural venous sinuses, and (4) identify any coexisting anomalies.

CT should be avoided when investigating frontonasal masses or suspected sphenoethmoidal cephaloceles in the central skull base during the first 18 months of life. This helps reduce both false positive and false negative examinations during a time of active ossification. The imaging modality of choice is MR with a small FOV (10 or 12 cm).

We now discuss four of the most common forms of cephalocele: occipital, frontoethmoidal, parietal, and skull base cephaloceles.

(41-6) Graphic depicts frontonasal cephalocele with brain herniating through a patent fonticulus frontalis between the frontal bone above and nasal bone below.

(41-7) Graphic demonstrates nasoethmoidal cephalocele with brain herniating into the nose through a patent foramen cecum in front of the crista galli .

Occipital Cephaloceles

Terminology and Classification

Three subtypes of occipital cephalocele are recognized and identified according to the involved bone(s). From most to least extensive, they are occipitocervical (involving the occipital bone, foramen magnum, and neural arches of the upper cervical spine) (41-4), low occipital (involving the occipital bone and foramen magnum), and high occipital (involving the occipital bone only).

Clinical Issues

Occipital cephaloceles account for 75% of cephaloceles in European and North American Caucasians. There is a 2.4:1 male predominance.

Occipital cephaloceles are almost always recognized at birth as a variably sized occipital or suboccipital soft tissue mass. The affected infant is often microcephalic with visible craniofacial disproportion. Neurodevelopmental outcome is related to cephalocele size and contents as well as the presence and type of associated abnormalities.

Imaging

Bone CT with 3D reconstruction delineates the osseous defect well, and multiplanar MR best depicts the sac and its contents. The herniated brain—which can derive from both supraand infratentorial structures—is always abnormal, appearing dysmorphic, disorganized, and dysplastic (41-5). Depending on the size of the cephalocele, severe traction and distortion of the brainstem and supratentorial structures can be present.

Dura and CSF-filled structures (including the fourth ventricle and sometimes part of the lateral ventricles) are often contained within the sac. In addition to delineating the sac and its contents, identifying the course and integrity of the dural venous sinuses is essential for preoperative planning.

At least half of all patients with occipital cephaloceles have associated abnormalities such as callosal dysgenesis, cerebellar malformations (including Chiari 2 and DandyWalker spectrum disorders), and gray matter heterotopias.

Frontoethmoidal Cephaloceles

Terminology and Classification

Frontoethmoidal cephaloceles are also called sincipital cephaloceles. In frontoethmoidal cephaloceles, brain parenchyma herniates through a persisting dural projection into the midface, typically the forehead, dorsum of the nose, or orbit.

There are three subtypes of frontoethmoidal cephaloceles. The frontonasal subtype is most common, representing 4060% of frontoethmoidal cephaloceles.

In the nasoethmoidal subtype (30%), the sac herniates through a midline foramen cecum defect into the prenasal space.

The least common subtype is nasoorbital (10%). Here the cephalocele herniates through the maxilla and lacrimal bone into the inferomedial orbit.

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Etiology

Frontonasal cephaloceles protrude through an unobliterated fonticulus frontalis into the anterior forehead at the glabella/dorsum of nose (41-6). Nasoethmoidal cephaloceles herniate into the nasal cavity through a patent foramen cecum

(41-7).

Developmental defects in the lacrimal bones and frontal processes of the maxillary bones result in a nasoorbital cephalocele, which herniates into the orbit.

Clinical Issues

Epidemiology and Demographics. Frontoethmoidal cephaloceles represent 10-15% of all cephaloceles and are typically present at birth. There is no sex predilection.

Frontoethmoidal cephaloceles are the most common type of cephalocele seen in southeast Asia and among ethnic

(41-8A) 3D CT soft tissue reconstruction in a newborn with a frontonasal encephalocele shows a large mass protruding anteriorly between the eyes. (41-8B) 3D reformatted bone CT shows a well-delineated frontonasal bony defectjust above the bridge of the nose.

(41-8C) Sagittal T1WI in same patient shows the skin-covered soft tissue mass protrudes through a patent fonticulus frontalis . Note absence of corpus callosum with "highriding" third ventricle, azygous anterior cerebral artery . Chiari 1 malformation with tonsillar herniation is also present. (41-8D) T2WI shows cephalocele is mostly dysplastic brain. Note arachnoid cyst, polymicrogyria . (Courtesy M. Michel, MD.)

southeast Asian immigrants to the United States and Europe, where they are now almost as common as the occipital type.

Associated Abnormalities. Associated abnormalities are present in 80% of patients with frontoethmoidal cephaloceles. These include hypertelorism and eye anomalies, corpus callosum dysgenesis, interhemispheric lipomas, hydrocephalus, seizures, neuronal migration anomalies, and microcephaly.

Imaging

NECT scans show a well-demarcated, heterogeneous, mixeddensity mass that extends extracranially through a bony defect.

In a frontonasal cephalocele, brain herniates into the forehead between the frontal bones above and the nasal bones below (41-8). In the nasoethmoidal type, the nasal bone is bowed anteriorly by the cephalocele, and the crista

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(41-12A) Autopsy of sphenoethmoidal cephalocele shows central skull base defect . Basal view of brain shows the cephalocele sac .

(41-12B) Sagittal view shows the cephalocele , pachygyria, and corpus callosum dysplasia. (Courtesy E. T. Hedley-Whyte, MD.)

(41-13) Sagittal T1WI shows sphenoethmoidal cephalocele . Hypothalamus and anterior third ventricle are retracted into the sac.

galli is posterior to the defect. The cribriform plate is deficient or absent; the crista galli may be absent or bifid. A nasoorbital cephalocele protrudes inferomedially into the orbit through a defect in the lacrimal/frontal process of the maxillary bone.

MR shows a soft tissue mass in direct contiguity with the intracranial parenchyma. The mass is usually heterogeneous in signal intensity but mostly appears isointense with cortex. It does not enhance following contrast administration.

Differential Diagnosis

The major differential diagnoses of a frontoethmoidal cephalocele are nasal dermal sinus with or without associated dermoids/epidermoids and nasal cerebral heterotopia (nasal "glioma"). All three lesions present clinically as midline nasal masses. All three have similar embryologic origin (i.e., the dura that normally extends through the embryonic foramen cecum between the developing nasal bone and nasal cartilage fails to regress).

A nasal dermal sinus is seen clinically as a small dimple or pit on the nose. It is the opening of a dermal-lined sinus tract that extends intracranially for a variable distance. A dermoid or epidermoid cyst can develop anywhere along the tract. Nasal dermal sinuses have an epithelial lining and do not contain brain parenchyma.

A nasal glioma is a congenital nonneoplastic heterotopia that consists of dysplastic glial tissue. Most nasal gliomas are extranasal (60%), located along the dorsum of the nose. Approximately one-third are intranasal, lying under the nasal bones. MR scans show no connection between the mass and intracranial contents.

Parietal Cephaloceles

Parietal cephaloceles comprise just 5-10% of all cephaloceles. Most have significant underlying brain and vascular anomalies, such as a persistent falcine sinus, sinus pericranii, and/or partial absence of the straight sinus.

MR without and with contrast enhancement is best to delineate parietal cephalocele contents (41-9). Because of the proximity to the superior sagittal sinus, it is important to delineate the position of all dural sinuses and adjacent cortical draining veins with MRV, CTV, or DSA prior to surgery.

A number of parietal cephaloceles are termed atretic cephaloceles (APCs), small lesions that typically present as midline scalp masses near the posterior vertex (41-10). They have been associated with maternal folate deficiency and valproic acid use. APCs have limited defects in the skull that are best visualized on 3D bone CT. They are often associated with a persistent falcine sinus and frequently split the superior sagittal sinus (41-11).

Skull Base Cephaloceles

Skull base cephaloceles account for 10% of all cephaloceles. They result from developmental failure of proper skull base ossification, which in turn allows migration of neural crest cells and their derivatives through the bony defect.

Skull base cephaloceles can be midline or off-midline (lateral) and are subtyped according to which bony component(s) they involve. There are three types of midline skull base cephaloceles. Sphenopharyngeal cephaloceles involve just the sphenoid body, whereas sphenoethmoidal lesions affect both the sphenoid and ethmoid bones (41-12). Transethmoidal cephaloceles herniate through the cribriform plate.

Lateral basal cephaloceles can be sphenomaxillary (orbital fissure plus maxillary sinus with herniation into the pterygopalatine fossa) or sphenoorbital (through the sphenoid bone into the orbit).

Occasionally, middle cranial fossa arachnoid granulations are seen as multiple focal outpouchings (arachnoid "pits") in the greater sphenoid ala. These skull base defects can be associated with CSF leak or skull base cephalocele.

Imaging of skull base cephaloceles is essential to delineate the sac contents completely. The pituitary gland, optic nerves and chiasm, hypothalamus, and third ventricle can all be displaced inferiorly into the cephalocele (41-13).

Intracranial anomalies are frequent findings in association with skull base cephaloceles. Midline anomalies such as corpus callosum dysgenesis and an azygous anterior cerebral artery are common.

Persistent Craniopharyngeal Canal

Terminology

Persistent craniopharyngeal canal (PCPC) is also known as persistent hypophyseal or basipharyngeal canal.

Etiology and Clinical Issues

PCPC is a rare developmental anomaly with a persistent tract in the intersphenoidal synchondrosis (41-14) that extends from the nasopharynx to the bottom of the pituitary fossa. It is usually small, uncomplicated, and noted incidentally on imaging studies or at autopsy. However, occasionally PCPCs can present as large complex skull base lesions with cysts, cephaloceles, midline craniofacial malformations, or pituitary anomalies. Some cases have been linked to genetic defects in SOX3, an early developmental transcription factor involved in pituitary development.

Imaging

High-resolution bone CT with 3D reformatted images best delineates the skull base abnormality. Most PCPCs are small, typically less than 1.5 mm in diameter. A larger lesion appears as a smoothly marginated cylindrical or ovoid midline bony "canal" extending obliquely downward from the sellar floor to the nasopharynx.

MR findings depend on the contents within the canal. Small, uncomplicated PCPCs may be difficult to identify. Larger lesions show variable signal intensity within the canal itself. Coronal images sometimes show the adenohypophysis sitting on the top of the PCPC, resembling a "golf ball on a tee."

Associated anomalies of the midface (e.g., hypertelorism, cleft palate, and sphenopharyngeal cephaloceles) or pituitary gland/stalk (e.g., duplicated gland, ectopic adenoma, and hypothalamic hamartoma) are common.

Differential Diagnosis

The major differential diagnosis of PCPC is a sphenooccipital synchondrosis, a linear developmental cleft between the basisphenoid and basiocciput. The sphenooccipital synchondrosis gradually decreases in size with increasing age and usually disappears by adulthood. It lies behind the dorsum sellae; a PCPC lies in front of the dorsum (41-16).

A persistent medial basal canal is a developmental variant of the lower clivus and occurs posteroinferior to the sphenooccipital synchondrosis (4115).

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(41-14) Presphenoid (green), postsphenoid with basisphenoid (yellow), basiocciput (red), foramen cecum , and intersphenoid synchondrosis .

(41-15) Intersphenoid and sphenooccipital synchondroses and notochord migration path (green) forming medial basal canal are shown.

(41-16) Sagittal T1WI shows persistent craniopharyngeal canal and sphenooccipital synchondrosis posterior to the PCPC.