Congenital Disease

Congenital Spinal Stenosis

No measurements are absolute; however, AP dimensions of the spinal canal less than 10 mm in the cervical region ( Fig. 3.14 ) and 12 mm in the lumbar region (see Fig. 3.7) are considered to be stenotic. To some extent canal dimensions are more critical in the cervical spine, due to the presence of the cord, and in this region AP diameters of 10 to 13 mm are considered to represent relative spinal stenosis. It should be noted that the normal dimensions of the cervical spinal canal vary according to anatomic level, sex, age, and height. One easily recognized imaging finding in congenital spinal stenosis of the lumbar region is tapering of the canal dimension from the upper to the lower lumbar levels, with this region normally equal in size or greater in dimensions when compared to the thoracic spine. Patients with congenital cervical spinal stenosis are predisposed to traumatic spinal cord injury. Clinical presentation with congenital cervical spinal stenosis is typically due to myelopathic symptoms. However, radicular symptoms may be present, in either cervical or lumbar congenital stenosis, due to nerve root impingement with narrowing of the lateral recesses or neural foramina. A major element of congenital spinal stenosis is short pedicles. Achondroplasia is well known for symptomatic lumbar stenosis, with the entire spine stenotic in some patients. Down syndrome is known for congenital stenosis of both the cervical and lumbar spine.

**Fig. 3.14** Congenital cervical spinal stenosis. Sagittal images identify a reduced AP dimension to the cervical spine, with little CSF either anterior or posterior to the cord. Axial images are presented at the C4–5 and mid C5 levels. At C4–5 a broad-based disk osteophyte complex leads to moderate spinal canal stenosis with moderate cord flattening, which otherwise in a patient with a normal diameter canal would have had little impact. Note also that even at the mid C5 level there is markedly reduced CSF anterior and posterior to the cord, on axial imaging, with mild to moderate cord flattening.

Scoliosis

Scoliosis is simply lateral curvature of the spine. Ninety percent of cases are idiopathic with no underlying cause. The typical idiopathic scoliosis is an S-shape curve, with the thoracic curvature convex to the right. The remaining 10% are congenital ( Fig. 3.15 ), neuromuscular, and post-traumatic in etiology. In congenital scoliosis, vertebral anomalies can be seen (e.g., a hemivertebra) ( Fig. 3.16 ) and in other instances the scoliosis is associated with a congenital abnormality such as diastematomyelia or a Chiari I malformation (with hydromyelia). Neuromuscular causes include cerebral palsy, with an incidence as high as 50% in patients with severe disability as assessed by the gross motor function classification system. Posttraumatic etiologies include prior fracture, chronic osteomyelitis, prior surgery, and radiation therapy. Plain films are utilized for quantitation of the curvature and monitoring for possible progression. MR is the imaging modality of choice for evaluation of atypical or progressive scoliosis, specifically to exclude an underlying abnormality, with coronal imaging important (in addition to the more routinely acquired sagittal and axial scans).

**Fig. 3.16** Butterfly vertebral body. Coronal and axial images well depict a not uncommon vertebral body anomaly, a “butterfly” vertebral body. In this entity, there is failure of fusion of the two halves of the vertebral body due to persistent notochordal tissue centrally.

Tethered Cord

This congenital anomaly is defined by a low position of the conus, with the conus being tethered (held in that position) due to an additional abnormality. Causes include a tight (often slightly thickened) filum terminale, lipomyelomeningocele (and variants thereof), diastematomyelia, and retethering following meningomyelocele repair ( Fig. 3.17 ). The clinical presentation is typically that of a young child with progressive neurologic dysfunction, specifically gait disturbance, motor and sensory loss in the lower extremities, and bladder dysfunction (all presumably due to cord traction and ischemia). In a tight filum terminale there will be an identifiable conus. The most common appearance of a tethered cord, however, is that of the cord extending without change in caliber to the lumbosacral region, tethering posteriorly with an associated lipoma and dysraphic posterior spinal elements. There may be associated hydromyelia. Treatment is surgical with release of the tether, for prevention of symptom progression. Following release, the level of the conus/distal cord typically will not change.

Syringohydromyelia

Hydromyelia is strictly defined as dilatation of the central canal of the spinal cord, lined by ependyma. Syringomyelia is defined strictly as the presence of a fluid-filled cavity within the spinal cord, lined by gliotic parenchyma, specifically not representing dilatation of the central canal. Unfortunately, these terms are commonly confused, and used interchangeably, by physicians. Thus the term syringohydromyelia has emerged, being less specific and including both hydromyelia and syringomyelia. Syringobulbia refers to the extension of a fluid collection into the brainstem, often accompanied by cranial nerve findings (due to compression). These entities are all difficult to visualize on CT. MR well depicts the cord and any pathology therein, and specifically abnormal fluid collections. The sagittal plane is commonly used to define the extent of syringohydromyelia, with the axial plane providing localization relative to the cross-section of the cord. Axial images also better identify very small cavities, and can determine with greater certainty the true extension of hydromyelia (with minimal dilatation of the central canal difficult to assess on sagittal images due to partial volume effects). Neoplastic disease and trauma are common etiologies for syringomyelia, whereas the Chiari I and II malformations account for the majority of cases of hydromyelia ( Figs. 3.18 and 3.19).

**Fig. 3.18** Chiari I and II. Although sharing a common name, these two entities are distinct. A Chiari I is defined by herniation (*small black arrow*) of the wedge-shaped cerebellar tonsils > 5 mm below the foramen magnum (first patient, left image). Note in this instance the tonsils extend well below the level of C1 (*white arrows*). In symptomatic Chiari I patients there may be accompanying dilatation of the central spinal canal (hydromyelia), also present in this patient. A Chiari II is however a hindbrain dysgenesis, with many distinctive features involving the brain (second patient, right image). Note the fused, beaked (*) tectum (colliculi), the slitlike fourth ventricle, the foreshortening of the pons in the AP dimension, and the low tentorial insertion. The tonsillar herniation is more peglike than wedged, often extending a much greater distance than seen with a Chiari I malformation.

**Fig. 3.19** Chiari I with extensive, marked hydromyelia. Wedge-shaped cerebellar tonsils are noted extending well below the C1 level. There is little if any CSF surrounding the brainstem at the level of the foramen magnum. There is marked dilatation of the central canal of the spinal cord, with a characteristic haustral-like appearance and the cord substance itself compressed peripherally. Complete evaluation of the cord from the foramen magnum to the conus is important in such cases, to establish the caudal extent of the central canal dilation, which may extend to the conus.

Intravenous contrast administration is strongly recommended in patients with syringomyelia, improving the sensitivity for detection of neoplastic disease. In the patient population at large, hydromyelia is much more commonly encountered, congenital in etiology, than syringomyelia due to either neoplasia or trauma. Both syringomyelia and hydromyelia can be treated by direct shunting of the cavity, amongst other options. An enlarging syrinx in a post-traumatic patient, for example following cervical cord injury, can cause neurologic deterioration unless so treated. With MR imaging at 3T now commonly available clinically, the normal central canal of the spinal cord is routinely visualized on high-resolution images, with the canal slightly smaller or larger (specifically with a range of normal diameters) depending on the individual ( Fig. 3.20 ).

**Fig. 3.20** Normal central canal of the spinal cord. The central canal can be visualized on high-resolution MR images in some individuals, as seen on sagittal and axial images, appearing slightly prominent size wise but within the spectrum of normal.

Meningomyeloceles and Lipomyelomeningoceles

In a meningomyelocele and a meningocele, there is incomplete closure of the posterior bony elements, with the contents of the spinal canal extending through the defect (open spinal dysraphism) ( Fig. 3.21 ). A meningocele, by definition, contains only dura and arachnoid, with neurologic deficits uncommon. A meningomyelocele, by definition, contains neural tissue within the expanded posterior subarachnoid space, with cord tethering. Imaging studies are rarely acquired at presentation in the newborn, with the exposed neural placode readily evident (in a meningomyelocele) and surgery typically performed within 48 hours ( Fig. 3.22 ). MR and CT are often obtained many years following surgery, and demonstrate a wide dysraphic defect with an accompanying CSF-filled sac covered by skin. Retethering is a common long-term complication. It is important to note that Chiari II malformations are virtually always associated with a meningomyelocele ( Fig. 3.23 ).

**Fig. 3.21** Tethered cord with a meningomyelocele. In distinction to spina bifida, a meningomyelocele consists of not only a posterior arch defect but also herniation of the meninges and neural structures through this defect. Here, the midline sagittal image reveals a CSF-filled sac posteriorly in the lower lumbar region communicating with the normal thecal sac. The spinal cord extends to at least the level of the lumbosacral junction and dysraphic posterior osseous elements are present from L4 to S1. Abundant fatty tissue inferior to the defect is also seen, with high SI. Note the hypointensity of the vertebral bodies relative to the intervertebral disks, characteristic for an infant.

**Fig. 3.22** Meningomyelocele, at birth. The midline sagittal T2-weighted image from the MR exam of this newborn, on the first day of life, is presented. A large spinal defect is noted posteriorly in the low lumbar region, with a CSF-filled sac protruding dorsally. It is very rare to ever see such a case on MR, given that surgery (repair with closure) is performed almost immediately. By definition, this defect lacks a skin covering, with neural tissue (and specifically the CSF sac) exposed to air.

**Fig. 3.23** Tethered cord in a patient with a Chiari II malformation. The typical tethered cord patient presents with neurologic dysfunction early in life, with many cases repaired at birth. Retethering may occur, as illustrated. The cord gradually tapers until reaching the end of the thecal sac, with no distinct conus—a typical appearance for either simple tethering or retethering. Nearly all Chiari II patients present at birth with a meningomyelocele, as was the case with this child.

Lipomyelomeningoceles (and lipomyeloceles) differ from the two entities just described by the presence of a lipoma attached to the dorsal surface of the cord termination and intact skin overlying the defect (closed spinal dysraphism) ( Fig. 3.24 ). The lipoma extends through the dysraphic spinal canal merging with and becoming indistinguishable from, subcutaneous fat. The distal cord is tethered by the lipoma. When a mass is present posteriorly, it presents clinically under the age of 6 months ( Fig. 3.25 ). If the mass is subtle, presentation may not be until 5 to 10 years of age when neurologic or urologic deficits are noticed. Occasionally this entity goes undetected until adulthood, since the lesion is skin-covered.