Calvarial Ossification Defects

► [Abnormal openings within the calvarial bones]

Congenital ossification defects of the calvaria are rare. Some of them, such as agenesis of entire bones of the cranial vault, are exceptional anomalies (de Heer et al. 2003). Others, such as foramina parietalia and cranium bifidum, are more common and will be discussed in this section. A miscellaneous group of syndromic disorders manifesting skull defects as one feature is also included.

Cranium Bifidum

Fig. 1.13 a, b. Craniolacunia in a male newborn. Note the typical 'soap-bubble' appearance of the skull, with multiple areas of bone rarefaction interspersed with linear streaks of in creased bone density.Additional features in this child included spinal myelomeningocele, and severe gibbus deformity of the lumbar spine

Fig. 1.13 a, b. Craniolacunia in a male newborn. Note the typical 'soap-bubble' appearance of the skull, with multiple areas of bone rarefaction interspersed with linear streaks of in creased bone density.Additional features in this child included spinal myelomeningocele, and severe gibbus deformity of the lumbar spine

Craniolacunia is a peculiar type of calvarial dysplasia, which is characterized by multiple,'soap-bub-ble' areas of bone rarefactions that can give the faulty impression of abnormal calvarial openings (Fig. 1.13a,b). On the other hand, a true midline cranial defect, with or without meningo(encephalo)cele, can be an associated finding, as craniolacunia almost invariably occurs in patients with spinal or cranial dys-raphism and Arnold-Chiari malformation type II. The mechanism underlying the development of craniolacunia is only partially understood, and it is generally related to dysplastic bone formation associated with focal dural defects. Certainly, congenital hydro-cephalus has no role in this. Lacunar skull develops during fetal life, is apparent at birth, and usually disappears by the 5th month of postnatal life. In sharp contrast, the pattern of increased convolutional markings (or 'copper-beaten' appearance) associated with chronically increased intracranial pressure does not usually develop before the end of the 1st year.

Parietal Foramina. Parietal foramina are small (<5 mm), round or oval openings in the fetal calvarium, located symmetrically and straddling the sagittal suture, that transmit emissary veins connecting the occipital veins to the superior sagittal sinus. They usually disappear at about the 5th fetal month, but occasionally persist into infancy as a normal anatomical variation of skull vein drainage. Enlargement (>5 mm) of per sistently open parietal foramina has traditionally been regarded as the abnormal counterpart of 'normal' parietal foramina. However, this view has been challenged by the recent observation in cadaveric adult skull specimens that 'normal' parietal foramina sometimes coexist with enlarged parietal foramina, suggesting the possibility that they are in fact totally distinct entities (Tubbs et al. 2003). Familial parietal foramina (foramina parietalia permagna, OMIM 168500) are sharply marginated, oval areas of radi-olucency without peripheral sclerosis, located in the parietal bones on each side of the sagittal suture and separated by a narrow bridge of bone (Fig. 1.14). This cranial defect is caused by mutations in the MSX2 gene located on chromosome 5q34-q35 (Wilkie et al. 2000). The defect is often asymptomatic, although seizures, scalp defects, and structural and vascular malformations of the brain can occur in association with them (Wilkie et al. 2000; Reddy et al. 2000). Foramina parietalia permagna occur as part of the Potocki-Shaffer syndrome (OMIM 601224), a contiguous gene syndrome caused by deletion in the 11p13-p11 region. Haploinsufficiency of the human home-obox gene ALX4,located in the D11S1785-D11S1385 region, which encodes a paired-related homeodo-main transcription factor, has been identified as the potential cause of foramina parietalia permagna in this disorder (Bartsch et al. 1996; Wu et al. 2000; Mavrogiannis et al. 2001). Another manifestation of

Foramina Parietalia Permagna
Fig. 1.14. Foramina parietalia permagna: cranial radiographs etal foramina, showing symmetrical, oval openings in the pari-of nine affected individuals from a Chinese family with pari- etal bones. (From Chen et al. 2003)

this syndrome, multiple exostoses, is caused by deficiency of the EXT2 gene, which is located in the same proximal chromosomal region as the ALX4 gene (Po-tocki and Shaffer 1996; Hall et al. 2001). Other occasional manifestations, including craniofacial dysos-tosis and mental retardation, are probably secondary to hemizygosity of genes located outside of the D11S1785-D11S1385 region. Foramina parietalia permagna occur as one feature of cleidocranial dysplasia (OMIM 168550) (Eckstein and Hoare 1963; Golabi et al. 1983) and as an occasional manifestation of Rubinstein-Taybi syndrome (OMIM 180849). A third gene locus for foramina parietalia permagna has been assigned to chromosome 4q21-q23 (Chen et al. 2003).

Skull Midline Defects/Cranium Bifidum. Cranium bifi-dum, or cleft cranium, is the cranial counterpart of spina bifida, the spinal dysraphic state (see Chapter 3). This term designates a single midline skull defect, with or without associated encephalocele, that is to say congenital herniation of intracranial contents. In the absence of encephalocele (atretic encephalo-cele), this defect is referred to as cranium bifidum oc-cultum. In these cases, the scalp overlying the occult cranial dysraphism is often atrophic and alopecic, occasionally displaying associated angioma forma tion, without brain malformations or ventricular deformity, except for the abnormal position of the venous sinuses (Inoue et al. 1983). As discussed later, encephaloceles may occur at the site of a normal cranial opening, such as the fontanels, or protrude through a cranial ossification defect, either on the midline (cranium bifidum) or away from it. If the herniation contains brain it is termed a 'meningoen-cephalocele,' whereas if it contains only meninges it is referred to as a 'cranial meningocele.' In some cases the appearance and structure of the herniated brain are preserved, but in others they can be grossly disorganized. In general, the presence of cerebral tissue in the sac means a poorer prognosis (Lorber and Schofield 1979; Martinez-Lage et al. 1996). Encephaloceles are classified by the site of the cranial defect through which the brain and/or meninges are protruding into the following types: (1) occipital en-cephaloceles (cervico-occipital, low and high occipital) (Fig. 1.15a-c); (2) encephaloceles of the cranial vault (temporal, interparietal, interfrontal, anterior and posterior fontanel) (Fig. 1.16); (3) frontoeth-moidal encephaloceles (nasofrontal, nasoethmoidal, naso-orbital); (4) basal encephaloceles (transeth-moidal, sphenoethmoidal, transsphenoidal, fron-tosphenoidal); and (5) encephaloceles associated with cranioschisis (Suwanwela and Suwanwela 1972;

Frontonasal Meningocele

Fig. 1.15 a-c. Occipital encephalocele. a In a female newborn. Note the small, ovoid midline defect in the occipital region. There was meningeal herniation through the occipital defect (cranial meningocele) and a lobar form of holoprosencephaly in this child. b, c In a 20-day-old female newborn with Meckel syndrome. Observe the wide, round occipital defect, with external migration of meninges and tissue brain (meningoencephalocele). Additional findings included severe microcephaly, characteristic sloping of the forehead, and malar hypoplasia

Nager Syndrome

David 1993; Nager 1987). Encephaloceles located in the anterior part of the skull are often referred to as 'sincipital,' and they manifest clinically as external masses protruding along the nose, orbital margin, or forehead. Encephaloceles are common defects, with a prevalence of 1 per 4,000 live births (Blumenfeld and Skolnik 1965). Occipital encephaloceles are the most common type (65 to 80%), followed by sincipital (15%) and basal (1-2%) encephaloceles (Komolafe et al. 2003; Hoving 2000). Occipital encephaloceles are linked to neural tube defects, such as myelo-meningocele (Anegawa et al. 1993), and are most commonly seen in individuals of European descent. By contrast, sincipital encephaloceles are not associated with neural tube defects and are particularly common among people of Southeast Asian ancestry

(Hoving 2000). In occipital encephaloceles, skull defects are most commonly small (<2 cm), well-mar-ginated, oval midline openings that usually do not require cranioplasty. In the case of frontoethmoidal encephaloceles the cranial defect is located at the junction of the frontal and ethmoid bones, most commonly in the shape of a single central ostium corresponding to the foramen caecum, through which a soft tissue mass of variable size protrudes (David and Proudman 1989). However, in a minority of cases the defect manifests as paired ostia each located at one side of an intact midline bridge of bone. In these cases, a bilobed mass can be detected on clinical inspection (Fig. 1.17). The soft tissue mass can be firm or compressible, pulsatile or nonpul-satile, covered by intact skin or uncovered, with ex

Meningo Hidro Encephalocele

Fig. 1.16 a, b. Encephalocele of the cranial vault in a male new- with the anterior fontanel. Observe dysplastic changes of the born. Note wide midline opening in the frontal-parietal re- frontal and parietal bones adjacent to the bone defect gion,with a large encephalocele. The bony defect is continuous

Fig. 1.16 a, b. Encephalocele of the cranial vault in a male new- with the anterior fontanel. Observe dysplastic changes of the born. Note wide midline opening in the frontal-parietal re- frontal and parietal bones adjacent to the bone defect gion,with a large encephalocele. The bony defect is continuous ternal exposure of the brain (Hunt and Hobar 2003). Individuals with frontoethmoidal encephalocele sometimes have additional anomalies, including microcephaly (24%), microphthalmos (16%), and hydrocephalus (12 %). Mental retardation occurs in half the cases, whereas seizures are rare. Unlike sincipital encephaloceles, basal encephaloceles are not visible externally, as they are intranasal or intrasinusal lesions. The herniated brain (frontal lobes, olfactory bulbs, pituitary gland and hypothalamus, etc.) usually extends inferiorly through a defect in the cribi-form plate or floor of the sella turcica, reaching the nasal cavity or the sinuses. Symptoms are often misinterpreted as those of aspecific nasal obstruction, leading to frequent delay in diagnosis. Occasionally, a basal encephalocele is not detected until late adulthood, when visual disturbance, pituitary-hypothala-mus dysfunction, or CSF rhinorrhea suggests the correct diagnosis (Buchfelder et al. 1987). The presence of other signs of craniofacial dysraphism, such as cleft upper lip, optic nerve dysplasia, and callosal dysgenesis, facilitates the diagnosis.

Anencephaly, a defect of neuronal development associated with severely distorted cortical architecture, can be regarded as the most severe form of cranium bifidum, or cranioschisis, analogous to myelo-schisis in spina bifida (Oi and Matsumoto 1990). There is an open neural tube in the cephalic region, with absent cranial vault and scalp, and an exposed mass of degenerated neural tissue on the skull floor. Depending on whether or not the defect extends through the foramen magnum, anencephaly is classified as holoacrania or meroacrania, respectively. 'Holoacrania with rachischisis' is the designation for a defect extending through the foramen magnum with associated spina bifida (Fig. 1.18). The etio-pathogenesis of cranium bifidum, like that of other forms of the dysraphic state, remains elusive. Several pieces of evidence point to genetic factors. Disruption of the TGF beta signaling in a mouse model has been found to impair cell proliferation in the dura mater, resulting in calvarial agenesis (Ito et al. 2003). The association of anencephaly and spina bifida (OMIM 301410), encompassing various types of neu-

Johanson Blizzard Syndrome
Fig. 1.17. Nasoethmoidal form of frontoethmoidal cephalo-cele. Note the large, bilobed skin-covered mass protruding down along both sides of the nose, and displacing the nasal cartilage and nostrils downward. (From Hoving 2000)

ral tube defects, is inherited as an X-linked recessive trait (Toriello et al. 1980). However, environmental factors have also been implicated, specifically a high concentration of lead in the water,which might eventually compete with intestinal absorption of folic acid (Bound et al. 1997). Cranium bifidum can be an isolated defect, or it can occur as one feature of aminopterin/methotrexate embryopathy, frontonasal dysplasia, fronto-facio-nasal dysplasia, and Knobloch syndrome (Terrafranca and Zellis 1953). Hereditary cranium bifidum and foramina parietalia per-magna may be different expressions, possibly age related, of the same defect (Little et al. 1990). Indeed, progression has been demonstrated from cranium bifidum during early childhood to symmetrical parietal foramina during the middle years of childhood and adulthood (Murphy and Gooding 1970). Frontonasal dysplasia (median facial cleft syndrome, OMIM 136760) includes a constellation of features, such as cranium bifidum occultum frontalis (Fig. 1.19), widow's peak hairline, ocular hypertelorism, and variable nasal defects, ranging in severity from a broad nasal root with lacking nasal tip to wide separation of the nares with absent prolabium and median cleft lip (Sedano et al. 1970). Nasal skin tags are occasionally present. Nearly all reported cases have been sporadic, with no evidence for a genetic basis (Sedano et al. 1970). Only a few familial cases have been reported (Naidich et al. 1988; Cohen et al. 1971; Warkany et al. 1973). Occasional associated abnormalities include congenital heart defects, notably tetralogy of Fallot (de Moor et al. 1987), various skeletal defects, mental deficiency (Prescott et al. 1989), agenesis of corpus callosum with or without lipoma (Nevin et al. 1999), and anterior basal en-cephalocele (Grubben et al. 1990). Fronto-facio-nasal dysostosis (fronto-facio-nasal dysplasia, OMIM 229400) includes brachycephaly, cranium bifidum with or without encephalocele, ocular anomalies (blepharophimosis, ptosis, lagophthalmos, dermoid of the eye, colobomata of the iris), bilateral cleft lip and palate, deformed nostrils, hypertelorism, and severe midface hypoplasia (Gollop 1981; White et al. 1991). Inheritance is probably autosomal recessive (Gollop et al. 1984). Knobloch syndrome (OMIM 267750) is characterized by severe myopia,vitreoreti-nal degeneration with retinal detachment, and occipital cephalocele (Knobloch and Layer 1971). Occasionally, the cranial defect involves the frontal rather than the occipital bones (Snider et al. 2000). Intelligence is not affected, suggesting that a meningocele rather than an encephalocele is involved in most cases (Cohen and Lemire 1982). Considerable intra- and interfamilial variability has been reported (Klie-mann et al. 2003). This syndrome is caused by a mutation in the COL18A1 gene on chromosome 21q22.3 (Sertie et al. 2000) and is inherited as an autosomal recessive trait (Passos-Bueno et al. 1994). Meckel syndrome (Meckel-Gruber syndrome, OMIM 249000) is an autosomal recessive disorder characterized by renal cysts, posterior encephalocele, and polydactyly (Fraser and Lytwyn 1981). Hepatic fibrosis and various developmental anomalies of the central nervous system, notably Dandy-Walker malformation, are additional features (Al-Gazali et al. 1996). The disorder is clinically and genetically heterogeneous, and minimal diagnostic criteria are difficult to define (Seller 1975). Similarities with trisomy 13 are striking, although occipital encephalocele is not a feature of chromosome 13 aneuploidy (Hsia et al. 1971).One locus for Meckel syndrome, MKS1 , has been mapped to 17q21-q24 (Paavola et al. 1995). Additional loci, MKS2 (OMIM 603194) and MKS3 (OMIM 607361), have been assigned to 11q13 (Roume et al. 1998) and 8q24 (Morgan et al. 2002).

Meroacrania
Fig. 1.18. Anencephaly in a 32-week fetus. The neural tube defect involves both the cephalic and the spinal region (holoacra-nia with rachischisis). Complete duplication of the spine in its cervical and thoracic segments can be regarded as the extreme end of the spinal dysraphic state
Anencephaly Fetus Image
Fig. 1.19. Frontonasal dysplasia in a newborn male. Note the wide defect in the frontal bones, which is consistent with cranium bifidum occultum frontalis

Syndromic Disorders with Skull Defects. Aplasia cutis congenita (congenital defect of skull and scalp, OMIM 107600) is an autosomal dominant disorder characterized by a localized area of defective skin in the scalp (sometimes in the trunk or extremities) and in the underlying calvaria (Cutlip et al. 1967; Deeken and Caplan 1970; Fullana et al. 1995). An autosomal recessive form of aplasia cutis congenita (OMIM 207700) has been suggested (Gedda et al. 1963; Dubosson and Schneider 1978). All skin layers and subcutaneous tissue are either absent, thus exposing the dura mater and deeper meninges, or extremely hypoplastic, appearing as thin transparent membranes. Congenital heart disease, notably ventricular septal defect and valvular pulmonary steno-sis,have been reported (David et al. 1991; Fryns et al. 1992). Occasional associated malformations include macrocephaly and wide, high forehead (Fryns et al. 1992). A defect in the scalp occasionally occurs in trisomy 13, Wolf-Hirschhorn syndrome (OMIM 194190), and Johanson-Blizzard syndrome (OMIM 243800). Adams-Oliver syndrome (congenital scalp defects with distal limb reduction anomalies, OMIM 100300) encompasses terminal transverse defects of the limbs, and scalp/skull defects similar to those of aplasia cutis congenita (Adams and Oliver 1945). This is a genetically heterogeneous disorder, in which instances of autosomal dominant (Bonafede and Beighton 1979) and recessive (Koiffmann et al. 1988) inheritance, and also sporadic cases (Kuster et al. 1988), have been reported. The limb defects range in severity from shortening of phalanges to absence of entire portions of the limbs. Skull defects occur in about 21 % of cases and may be so large as to imply complete absence of the calvarial bones, or even acra-nia (Farrell et al. 1993; Chitayat et al. 1992). Based on the evidence that vascular skin anomalies such as cutis marmorata telangiectatica and dilated scalp veins are common in Adams-Oliver syndrome (To-riello et al. 1988; Pereira-da-Silva et al. 2000), it has been suggested that this disorder results from a vascular disruption sequence (der Kaloustian et al. 1991; Keymolen et al. 1999).

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