► [Increased sagittal and/or transverse diameter of the spinal canal]
Widening of the spinal canal may occur in response to intraspinal expanding processes, or in association with a congenital disorder. Depending on the underlying causative factor, widening may be confined to a given spinal level, or may extend towards the whole length of the vertebral spine. Benign or malignant intraspinal neoplasms (such as ependymoma, astro-cytoma, neurofibroma, neuroblastoma, ganglioneu-roma, meningioma, metastasis, lipoma, dermoid, teratoma), and cysts (arachnoid, epidermoid, neuro-ectodermal) can cause focal widening of the spinal canal. Slow-growing intraspinal masses have the highest potential to cause widening of the spinal canal and/or neural foramina, while more aggressive neoplasms cause bone tissue infiltration and erosion. On rare occasions, a wide spinal canal is seen in association with intraspinal arteriovenous malforma-tions.'Hydrosyringomyelia,' a term denoting congenital or acquired dilatation of the spinal cord central canal, is another potential cause of spinal canal widening (Schlesinger et al. 1981).
Widening of the spinal canal also occurs with spina bifida (OMIM 182940), or rachischisis, a designation used to indicate a congenital cleft in the neural arches, with (spina bifida aperta or cystica) or without (spina bifida occulta) external protrusion of spinal contents through the dorsal bony defect. Nonunion of the neural arches, which normally fuse in the midline by the age of 2 years, is common at the
L-5 or S-1 level (and even at T-11 or T-12), and is of no clinical significance (Fig. 3.66). Therefore, the use of the designation 'spina bifida' for these minor anomalies should be strongly discouraged. In true spina bifida the bony defect is appreciably wider (Fig. 3.67), and the interpediculate distance is abnormally great (Fig. 3.68 a,b). The vertebral laminae may be thinned and everted. The commonest site for spina bifida is the lumbar spine, and the second most common is the cervical spine. Anterior or posterior spina bifida at the thoracic or cervical level is frequent in patients with cervical fusions. Spinal dys-raphism is a broad term encompassing a variety of disorders and defined as incomplete midline closure of mesenchymal, osseous, and neural tissues. Spinal dysraphism can be classified into three categories, based on the presence or absence of a back mass (Silverman et al. 1993). The first category is spinal dys-raphism associated with a back mass not covered with skin, or spina bifida aperta, and includes such anomalies as myelocele (posterior exposure of a neural placode) and myelomeningocele (dorsal protrusion of meninges, cerebrospinal fluid, and neural contents). Anomalies found in nonrandom association with myelomeningocele include Chiari type II malformation (99 %), callosal dysgenesis, craniolacu-nia, syringohydromyelia (40-80%), hydrocephalus, spinal arachnoid cysts (20%), diastematomyelia (30-40%) and, in anatomical regions other than the
central nervous system, kyphoscoliosis and hip deformities. The second category is spinal dysraphism associated with a skin-covered back mass, or spina bifida cystica, and includes such anomalies as lipo-myelomeningocele (skin-covered back mass containing fat, spinal cord, CSF, and meninges), myelocysto-cele (cystic dilatation of the distal end of the spinal cord herniated through a posterior bony defect), and posterior meningocele (dorsal protrusion of meninges and CSF, without neural contents, through a posterior bony defect). The third category is that of occult spinal dysraphisms, or spina bifida occulta, which encompasses a heterogeneous group of skin-covered lesions without exposed neural tissue or a visible cystic mass. Examples of occult dysraphic lesions include anterior and lateral meningocele (an-
Fig. 3.68 a, b. Myelomeningocele. a Marked increase in the in-terpediculate distance in the lumbar spine. b Severe gibbus deformity of the lumbar spine. There was an association of myelomeningocele, craniolacunia, and spinal deformities in this child terior and lateral protrusion of meninges and CSF, without neural contents), dorsal dermal sinus, spinal lipomas, diastematomyelia, tethered conus, tight filum terminale, hydromyelia, split notochord syndrome, and caudal regression syndrome. With occult spinal dysraphisms, a cutaneous marker consisting of a tuft of hair, skin tag, dimple, and aplasia cutis congenita is found over the defect (Gardner 1973).
Diastematomyelia (OMIM 222500) is a dysraphic malformation in which a bony, cartilaginous, or fibrous septum transfixes the spinal cord longitudinally in the anteroposterior plane. This septum is attached anteriorly to the posterior surface of the vertebral body and posteriorly to the dura, and can be seen on frontal radiograms as a midline vertically oriented ossicle (Fig. 3.69). At the level of the defect,
which sometimes extends for the length of several vertebral bodies above and below, the interpediculate space is increased (Winter et al. 1974). The tethering effect exerted by the septum on the spinal cord prevents its normal cephalic migration as the vertebral column lengthens during growth. The defect is usually located at the thoracic and lumbar level. Vertebral anomalies, including small and fused vertebrae and hemivertebrae, are often associated. The condition is usually isolated, but familial cases, always affecting females, have been reported (Kapsalakis 1964). An X-linked dominant inheritance with lethality in hemizygous males and a multifactorial trait with female sex preference have been suggested (Balci et al. 1999). There is a close association with hydromyelia, lipoma of the filum terminale, and intradural lipoma. Symptoms include motor weakness and sensory deficit in the lower extremities, bladder dysfunction, gait disturbance, and scoliosis. Fifty percent of patients are asymptomatic, however. The split notochord syndrome, also termed 'combined anterior and posterior spina bifida,' is an association of vertebral anomalies (sagittal clefts, nonfusion of laminae and spinous processes, segmentation defects), neural anomalies (split notochord, myelomeningo-cele, meningocele, developmental anomalies of the hindbrain and cervical spinal cord), and visceral anomalies (neuroenteric cysts, enteric duplication, diaphragmatic hernia, imperforate anus, colon agenesis, malrotation of the bowel). These anomalies are believed to be the result of abnormal splitting of the notochord, with a persistent connection that can extend from any area of the gut to the skin of the back. Frequently the anomaly consists in an enteric cyst located in the posterior mediastinum, presenting in infancy or early childhood with dyspnea, cyanosis, and pneumonia. The lymphedema-distichiasis syndrome (OMIM 153400), an autosomal dominant disorder caused by mutation in the forkhead family transcription factor gene MFH1, consists in late-onset bilateral lymphedema in both legs, distichiasis (a double row of eyelashes), corneal ulceration, congenital heart disease, spinal extradural cyst (not obligatory), and spina bifida (Falls and Kertesz 1964; Fang et al. 2000). Spinal extradural cysts (OMIM 271100), which are in fact arachnoid diverticula similar to those occurring in Marfan syndrome, can cause focal enlargement of the spinal canal (Chynn 1967). As already mentioned, focal widening of the spinal canal and neural foramina, with or without meningocele, are common findings in the lumbar and sacral spine in patients with Marfan syndrome (OMIM 154700). There is often associated thinning of the pedicles and laminae. Dural ectasia, possibly due to the generalized defect of connective tissue in this syndrome, is thought to be the mechanism for enlargement of the spinal canal and posterior scalloping of the vertebral bodies (Pyeritz et al. 1988; Villeirs et al. 1999). The presence of meningocele is another potential cause of spinal canal widening (Harkens and el-Khoury 1990).
Increased interpediculate distance and schisis of neural arches are commonly seen in oto-palato-digi-tal syndrome type I (OMIM 311300) and type II (OMIM 304120), in both the lumbar and the cervical spine.
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