The Spine

Congenital spinal malformations

 

The vast majority of congenital anomalies of the spine represent manifestations of spinal dysraphism which refers to a failure of incomplete midline fusion of mesenchymal, osseous, and nervous tissue.

Spinal dysraphic abnormalities are described as open (not covered by skin) and closed (covered or occult) lesions and are divided into different groups depending on the presence of a back mass. The anomalies with back masses and uncovered protrusion of all or part of the intraspinal contents include myelocele, myelomeningocele and meningocele. They are called spina bifida aperta and are usually clinical obvious at birth. Occult spinal dysraphism includes skin-covered masses such as subcutaneous lipoma, skin-covered meningoceles or myelocystoceles and the group of spinal dysraphism without an associated mass. This group encompasses dorsal dermal sinus, spinal lipoma, tethered cord (tight filum terminal e syndrome), fibrolipoma of the filum terminale, diastematomyelia, and anterior sacral meningocele.
Spinal dysraphism is typically associated with midline cutaneous lesions such as a hairy tuft, dimple or hemangioma. Moreover, scoliosis, clubfoot and neurological deficits are frequently found in malformations of the spine and spinal cord.

Modalities

Plain films

For the assessment of bony abnormalities of the spine, anteroposterior and lateral films are adequate in most cases. With CT and MRI superior soft tissue contrast is achieved and no superimposition of other anatomical structures is found. Sagittal, coronal and oblique reformats, as well as 3-D surface rendering of CT data sets allow for visualisation of fine anatomical details and may be preferred to conventional tomography.

Computed tomography (CT)

For precise evaluation of bony structures and subtle anatomical details, CT is superior to other imaging modalities. In diastematomyelia, CT with 2-D reformats and 3-D rendering is indispensible. Delayed post-myelography CT is able to demonstrate communications of a syringohydromyelia or cystic structures with the subarachnoid space. MR! has replaced myelography in most instances.

Ultrasound (US)

In the newborn, US can be used to good advantage in order to visualize the spinal canal, the spinal cord and the thecal sac. Spinal dysraphism, especially when associated with cystic components, can be detected or excluded by ultrasound. In children older than l year and adults, however, no appropriate acoustic windows are available for ultrasound examination of the spine.

Magnetic resonance imaging (MRI)

For the evaluation of spine anomalies, MRI is extremely useful. Due to its multiplanar capabilities, the lack of ionizing radiation and its superior soft tissue contrast, MRI allows for the delineation of the spinal cord, the subarachnoid space, the vertebral bodies and the intervertebral discs and can be employed in infants and children without harmful effects.

For MRI of infants and children, however, sedation or even general anesthesia is required. For the assessment of spinal malformations, axial and sagittal T1-weighted images are adequate and the administration of contrast media is not necessary. High-resolution technique and a slice thickness of 2 or 3 mm should be employed.

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Figure 49. Lower thoracic and upper lumbar spine, ap (a) and lateral (b). Left and dorsal hemivertebra of T11, partial block vertebra T11/T12 with fusion of the spinous processes.
	Figure 50. Block vertebra C5/6 without spondylosis at this segment and incomplete development of the disk. Degenerative hypermobility in the two segments above in the dorsifiexion film.

Pathology

Osseous spinal abnormalities

Anomalies of the vertebral body

Disturbance in the formation of the somites results in lateral hemivertebra or anterior agenesis with posterior hemivertebra (Fig. 49). Lateral hemivertebrae are usually accompanied by angulated scoliosis. Butterfly vertebrae are broadened and constricted centrally. Non-segmentation will form block vertebra (Fig. 50). The discs are rudimentary or absent and at the disc level a waist-like constriction indicates the lack of full development of endplates. Block vertebrae can either be congenital or a result of juvenile spondylitis.
Both neural arches and pedicles originate from separate chondrification centres to form the spinal canal. The cartilaginous closure of the dorsal arches starts from the 3rd month of gestation in the mid thoracic spine to progress step-like down- and upwards. Failure of dorsal closure causes spinal dysraphism which is discussed later. Pedicular absence or 
 

Figure 51. Os odontoideum, ap (a) and lateral (b).

hypoplasia may be asymptomatic. Failure of segmentation results in the fusion of adjacent laminae or pedicles forming vertebral bars.

Klippel-Feil syndrome
The Klippel-Feil syndrome describes a segmentation disorder of the cervical and upper thoracic spine with fusion of multiple vertebrae, in coexistence with a short neck and low hair line. Association with Sprengel's deformity (elevated scapula), omovertebral bone, cervical ribs, and kyphoscoliosis is common. The omovertebral bone bridges the scapula and spinous processes and is composed of osseous, cartilaginous or fibrous tissue.

Craniocervical junction abnormalities
Craniocervical junction abnormalities include agenesis of the odontoid process, os odontoideum, ossiculum terminale and assimilation of the atlas. The os odontoideum occurs along with a hypoplasic dens axis and carries a risk of anterior, posterior and lateral atlanto-axial instability (Fig. 51). It is rare for cases of craniocervical junction abnormalities to result in cervical myelopathy.

Sacral agenesis
Sacral agenesis frequently occurs in children of diabetic mothers. Concurrent malformation of the genitourinary tract, alimentary tract, agenesis of the lumbar spine and dysplasia of the pelvis and legs is 

Figure 52. Spina bifida occulta at L5. Persisting ossification centres at the tips of the inferior articular processes of the apophyseal joints of L1.

common. Imaging has to evaluate the urinary tract and to rule out other dysraphic conditions.

Spinal dysraphism

Osseous abnormalities associated with spinal dysraphism include incomplete fusion of the posterior elements of the vertebra, termed spina bifida, in occasion with formation of hemivertebra, butterfly vertebra and block vertebra. Spina bifida occulta has come to mean a mild non-union of the laminae, usually at the L5 and S1 level, a common finding without clinical importance (Fig. 52).

Spina bifida aperta

Myelocele and myelomeningocele

In this disorder, the neural tissue fails to form a tube and develops as a flat plate, termed a placode, herniating through a bony defect. Usually the lumbar spine is involved. The placode is exposed to the air and the skin ends at the edge of the placode. The nerve roots arise from the ventral surface of the placode. In myelomeningocele, a CSF filled space is found anteriorly to the placode. The placode and leptomeninges protrude dorsally (Fig. 53). In myelocele, no distension of the subarachnoid space is found. Hemimyelocele is a special type of myelomeningocele with diastematomyelia and it is present in about 10% of myelomeningoceles.

Myeloceles and myelomeningoceles are usually operated upon within the first 24 hours of life. Postoperative complications after repair of myelomeningocele may be a tethering of the cord and placode by scarring or cord compression by a lipoma, a dermoid or a epidermoid tumor. Following surgery of myelomengingocele syringohydromyelia and scoliosis can develop in childhood.

In all patients with myelomeningoceles, Chiari II malformation is found. Chiari II malformation is characterized by downward herniation of the hindbrain through the foramen magnum. The posterior fossa is too small. The brainstem together with the fourth ventricle, the tonsils and vermis is stretched inferiorly and is located behind the cervical cord. This inferiorly displaced cervico-medullary junction forms a characteristic kink. Frequently, dysplasia of the corpus callosum, hydrocephalus and hydromyelia are associated findings in Chiari II malformation.

Occult spinal dysraphism

Dorsal dermal sinus
A dorsal dermal sinus is a midline epithelium-lined tube which extends from the surface of the skin to a varying degree inferiorly. The ectoderm of the skin can be connected with the ectoderm of the spinal cord. Frequently, a dimple or small ostium is seen on the skin associated with hairs, hemangioma or hyperpigmentation. Complications are meningitis and subcutaneous or intraspinal abscess. Approximately 50% of dorsal dermal sinuses end within a dermoid or epidermoid cyst.

Spinal lipoma
Three types of spinal lipomas can be differentiated: intradural lipomas, lipomyelomeningoceles and fibrolipoma of the filum terminale. In intradural lipomas, the dura is intact, and they can lie in any part of the spine. These subpial-juxtamedullary lesions fill the dorsal deft between the central canal and the pia. When located in a low position spinal lipomas may tether the conus medullaris.
Lipomyelomeningoceles present as a skin-covered fatty, slightly firm back mass. They account for 50% of cases of occult spinal dysraphism. The cord is usually tethered to a large fatty mass extending from the subcutaneous region through a bony spina bifida into the spinal canal. Lipomas exhibit high signal intensity on T1-weighted MR

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Figure 54. Lipoma of the filum terminale with tethered cord, low conus medullaris and syringiform widening of the central canal at the conus medullaris (T1-weighted sagittal (a) and transverse MR images (b).

images and negative density values on CT.

Tethered cord
In tethered cord syndrome, a short, more than 2 mm thick filum terminale holds the conus medullaris in a low position. Patients may be asymptomatic or become symptomatic at any age, usually when the spine grows too long for the fixed cord. Frequently, tethered cord syndrome is associated with mild forms of osseous spina bifida without spina bifida aperta. In about one third of cases a lipoma or fibrolipoma of the filum terminalae is found (Fig. 54). Lipomyelomeningoceles are typically associated with a tight filum terminale syndrome.
Myelocystocele is an occult spinal dysraphism with herniation of an ependyma-lined cyst from a syringohydromyelia through a spina bifida defect into the subcutaneous tissue. Anterior sacral meningoceles are CSF filled meningoceles extending through an anterior sacral defect into the pelvis.

Diastematomyelia
Diastematomyelia is a form of split notochord syndrome with sagittal division of the spinal cord into two pia-lined hemicords. In complete clefts with formation of two dural sacs, a fibrous, cartilaginous or osseous spur is present in half of the cases. The doubling of the cord can occur in the thoracic or lumbar spine. The spinal column is nearly always grossly 

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b

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Figure 55. Cervical AVM. a) T1-weighted image shows tortuous vessels in the cervical cord and marked atrophy of the cord. b) The T2-weighted image shows enlarged draining veins. c) Angiography shows fine details of the arterial supply and venous drainage of the AVM.

abnormal. Hemivertebrae, butterfly vertebrae, block vertebrae and in almost all cases, spina bifida, are present. Cutaneous naevi overlying the site of diastematomyelia, tethering of the cord, hydromyelia, and club foot deformity are frequently found in children with diastematomyelia.

 

Stig Holtås, Maximilian F. Reiser and Axel Stäbler