Objectives
On completion of this chapter, you should be able to:
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Describe the sonographic technique to image the neonatal and infant spinal column
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Describe the sonographic appearance of normal anatomy of the spinal cord, the dura, the nerve roots, and the cauda equina
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Describe how to determine the level of the lumbar vertebrae in the sonographic examination
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List the common pathologic conditions of the spinal cord and their sonographic appearances
The spinal canal and its contents can be demonstrated sonographically with great clarity in the neonatal and early infant period. Thus high-resolution sonography of the spine has emerged as the optimal screening modality for the detection of occult dysraphic conditions, such as a tethered spinal cord, with diagnostic sensitivity equal to magnetic resonance imaging (MRI) for certain anomalies. Spinal dysraphism includes a wide range of developmental anomalies of the spinal canal and are broadly divided into open and closed types. Neural tube defects are a type of spinal dysraphism involving the absent or incomplete closure of the neural tube. The severity of the defect ranges from mild spina bifida occulta to severe spina bifida aperta. Open spinal dysraphisms have a skin defect with the neural tissue exposed to the environment, and because spine sonography is contraindicated when the skin is thin or no longer intact, occult or closed conditions are more commonly examined.
Certain lumbosacral stigmata ( Box 29-1 ), or cutaneous back masses and midline deformities, are known to be associated with spinal dysraphism. Clinically, the infant may present with a dimple on the posterior surface of the body along the spinal canal. Although it is not uncommon for the buttocks to contain a shallow dimple near the anus, at times the dimple appears unusually deep or asymmetric. The dimple may also be suspicious if it is more than 2.5 cm above the anus, larger than 5 mm, and most suspicious when associated with other spinal lesions. These findings may suggest the possibility of an underlying maldevelopment of the spinal cord or the adjacent elements, and sonography is used to determine the relationship of these stigmatas to deformities in the spinal canal. If these abnormalities are not recognized early, the patient may have difficulty walking or experience other neurologic problems in infancy or childhood.
Midline or paramedian masses
Hair tufts
Skin tags or discolorations
Hemangiomas
Pinpoint midline dimples
Paramedian deep dimples
Tail-like projection
Advantages of sonography include the ability to perform the procedure easily, dynamically, and at the bedside without ionizing radiation. The availability of high-frequency transducers now leaves operator inexperience as the main reason for unsuccessful neonatal spinal sonography. The sonographer may observe the spinal cord as it pulsates normally within the spinal canal. The vascular supply to the spinal canal may be evaluated with color Doppler ultrasound. The development of fluid collections, cysts, or fatty tumors (lipomas) may be seen. Malformations of the spinal cord may be imaged and will be presented in this chapter, along with other indications for spine sonography, including the detection of sequelae of spinal cord injury, often from a spinal tap.
Embryogenesis
The defects of the spinal canal occur in the first 8.5 weeks of life as the fetal nervous system develops. The neural tube and the subsequent spinal cord arise from ectodermal cells. The surface ectoderm separates from the neural tube, with the mesoderm coming to lie between the neural tube and the ectoderm. The mesoderm forms the bony spine, meninges, and muscle. Incomplete separation of the neural tube from the ectoderm may result in cord tethering, diastematomyelia, or a dermal sinus. Premature separation of the cutaneous ectoderm from the neural tube can result in abnormal mesenchymal elements, such as lipomas forming between the neural tube and skin. If the neural tube fails to fold and fuse in the midline, defects such as myelomeningocele occur. Disorders of the distal cord may lead to fibrolipomas of the filum terminale.
Normal anatomy and sonographic findings
The vertebral column extends from the base of the skull to the tip of the coccyx along the posterior surface of the body. The vertebral column is the central bony stabilizer of the body. Within the vertebral cavity lie the spinal cord, the roots of the spinal nerves, and the covering meninges, which provide protection for the vertebral column.
The vertebral column consists of 33 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral (fused to form the sacrum), and 4 coccygeal fused bones ( Figure 29-1 ). The pads of fibrocartilage, called intervertebral disks, are found between each vertebra and allow flexibility in the spine.
Vertebrae
Each vertebra consists of a rounded body anteriorly and a vertebral arch posteriorly ( Figure 29-2 ). These enclose a space called the vertebral foramen, through which run the spinal cord and its coverings. The vertebral arch consists of a pair of cylindrical pedicles, which form the sides of the arch, and a pair of flattened laminae, which complete the arch posteriorly. The vertebral arch gives rise to seven processes: one spinous, two transverse, and four articular. The two superior articular processes of one vertebral arch articulate with the two inferior articular processes of the arch above, forming two synovial joints.
The pedicles are notched on their upper and lower borders, forming the superior and inferior vertebral notches. On each side, the superior notch of one vertebra and the inferior notch of an adjacent vertebra together form the intervertebral foramen. These foramina transmit the spinal nerves and blood vessels.
In the neonate, problems typically occur in the lower back near the area of the lumbar vertebrae and the sacrum. Characteristics of the lumbar vertebrae include a large and oval body, strong pedicles directed posterior, and thick laminae with a triangular vertebral foramina. Additionally, the transverse processes are short, flat, and project backward.
Although sonography is performed from the back, the anterior vertebral bodies appear echogenic and lie posterior to the hypoechoic spinal canal. The cartilaginous posterior spinous processes appear hypoechoic, allowing for visualization of the canal when scanning directly over them. Laminae are seen when scanning slightly off midline and appear similar to overlapping roof tiles ( Figure 29-3 ).
Sacrum
The sacrum consists of five bones fused together. The upper border articulates with the fifth lumbar vertebra. The narrow inferior border articulates with the coccyx. The coccyx is mostly or completely unossified and is therefore hypoechoic ( Figure 29-4 ). Laterally the sacrum articulates with the two iliac bones to form the sacroiliac joints. The anterior and upper margin of the first sacral vertebra bulges forward as the posterior margin of the pelvic inlet and is known as the sacral promontory.
The vertebral foramina are present and form the sacral canal. The laminae of the fifth sacral vertebra, and sometimes those of the fourth also, fail to meet in the midline and form the sacral hiatus ( Figure 29-5 ). The sacral canal contains the anterior and posterior roots of the sacral and coccygeal spinal nerves, the filum terminale, and fibrofatty material. It also contains the lower part of the subarachnoid space, or dural or thecal sac, down as far as the lower border of the second sacral vertebra ( Figure 29-6 ).
Intervertebral disks
The intervertebral disks are responsible for one fourth of the length of the vertebral column. They are thickest in the cervical and lumbar regions where the movements of the vertebral column are greatest. Each disk consists of a peripheral part, the annulus fibrosus, and a central part, the nucleus pulposus ( Figure 29-7 ). The annulus fibrosus consists of fibrocartilage. The nucleus pulposus is an ovoid mass of gelatinous material containing a large amount of water, a small number of collagen fibers, and a few cartilage cells.
Ligaments and nerves
The anterior and posterior longitudinal ligaments run as continuous bands down the anterior and posterior surfaces of the vertebral column from the skull to the sacrum. Transverse dentate ligaments of the cord are sometimes visible; seen in a transverse view they appear as thin echogenic lines extending laterally from the spinal cord. The small meningeal branches of each spinal nerve innervate the joints between the vertebral bodies.
Spinal cord
The spinal cord is a cylindrical, grayish white structure that begins above at the foramen magnum, where it is continuous with the medulla oblongata of the brain. In the adult, it terminates below the level of the lower border of the first lumbar vertebra. In the younger child and term neonates, it is relatively longer and should not extend beyond the second lumbar vertebra ( Figure 29-8 ). The conus may end between L2 and L4 in the preterm infant. The cord also has a deep longitudinal fissure in the midline anteriorly, which may be seen on a transverse image.
The spinal cord is hypoechoic with slightly echogenic borders and an echogenic line extending longitudinally along its midline. It is surrounded by cerebrospinal fluid. This central echo complex represents or is close to the cord’s central canal ( Figure 29-9 , A ). One should be aware that slight prominence or widening of the central canal at the caudal end of the cord (not extending cranially into the thoracic spine) is a common finding in neonates. This normal variant is often referred to as ventriculus terminalis, and typically disappears within the first few months of life ( Figure 29-9 , B ).
The size and shape of the spinal cord vary along its length. Its diameter is narrowest in the midthoracic and thoracolumbar junctions. Inferiorly the cord tapers off into the conus medullaris, from the apex of which a prolongation of the pia mater, the filum terminale, descends to be attached to the back of the coccyx. The filum terminale appears echogenic and should measure 2 mm or less in thickness. A normal variant to be aware of is the filar cyst. These small cysts in the filum terminale might be remnants of a terminal ventricle or an arachnoid pseudocyst and have no clinical significance ( Figure 29-10 ).
