19 -Normal Anatomy and Congenital ANOMALIES of THE SPINE and SPINAL CORD .

C H A P T E R Normal Anatomy and Congenital Anomalies of the Spine and Spinal Cord Normal Lumbosacral Spine Thoracic Spine Cervical Spine Meninges and Spinal Cord Congenital Malformations Open Spinal Dysraphism Occult Spinal Dysraphism Anomalies of Abnormal Canalization and Retrogressive Differentiation Split Notochord Syndromes Miscellaneous Malformations Spine and spinal cord examinations comprise a significant and important segment of clinical neuroirnaging Familiarity with normal gross and radiologic anatomy is a prerequisite to understanding the broad spectrum of disorders that affect the spine and spinal cord In this chapter the normal gross and imaging anatomy of the spine, spinal cord, and nerve roots, as well as their congenital anomalies, are delineated Nonneoplastic disorders, including trauma, infection, demyelinating, vascular, and degenerative dis- eases, are covered in Chapter 20 Tumors, cysts, and tumorlike masses are discussed in the concluding chapter, Chapter 21 NORMAL ANATOMY Lumbosacral Spine The lumbosacral spine has many components It can be divided into anterior elements (vertebral bodies and intervertebral disks), posterior elements (pedicles, articular pillars, and facet joints), ligaments, soft tissues (e.g., epidural fat and venous plexuses), and neural tissue Neural tissue in this region includes the conus medullaris and cauda equina, lumbar roots and nerves, and the sacral plexus Anterior elements Vertebral bodies The lumbosacral spine normally has five lumbar segments and the sacrum, which is composed of five fused segments Each lumbar segment has a large, somewhat square-shaped vertebral body The superior and inferior end plates of the vertebral bodies are covered by a fenestrated cartilage to which the intervertebral disks attach (Figs 19-1 and 19-2).1 786 PART FIVE Spine and Spinal Cord Fig 19-1 Anatomy of the lumbosacral spine in the axial plane A to C, Anatomic drawings through the neural foramen (A), intervertebral disk (B), and pedicles (C) Each vertebral body has an outer layer of dense, compact cortical bone that surrounds an inner medullary portion composed of bony trabeculae and marrow The two types of marrow, hematopoietically active (red or cellular) and inactive (yellow or fatty) marrow, are easily distinguished on MR scans In young children, marrow is typically cellular and appears isointense with paraspinous muscle on T1WI (see Fig 19-15, B) In patients less than years of age, bone marrow and cartilage may show marked en- hancement following contrast administration Mild marrow enhancement persists but gradually diminishes and disappears around age years.2 From age to adolescence there is also progressive conversion of red to yellow marrow.3 This replacement of cellular marrow by fatty marrow results in high signal intensity on T1WI and relatively low signal intensity on standard T2-weighted spin-echo sequences Inhomogeneous signal is common, and focal fat deposition is seen as localized zones of high Chapter 19 Normal Anatomy and Congenital Anomalies of the Spine and Spinal Cord 787 Fig 19-1, cont’d D, Axial cryomicrotome section shows gross anatomy at the intervertebral disc level E, Axial T1-weighted MR scans show normal imaging anatomy of the lumbosacral spine 1, Vertebral body 2, Nucleus pulposus 3, Inner anular fibers of disk 4, Outer anular fibers of disk 5, Pedicles 6, Lamina 7, Superior articular facet 8, Inferior articular facet 9, Facet joint 10, Ligamentum flavum 11, Epidural fat (curved arrow indicates neural foramen) 12, Epidural venous plexus 13, Basivertebra venous plexus 14, Thecal sac with roots of cauda equina 15, Exiting roots 16, Dorsal root ganglia 17, Extraforaminal nerve 18, Transverse process 19, Pars interarticularis 20, Spinous process (D, Courtesy V.M Haughton.) signal intensity on T1WI (see Chapter 20).4 Marrow in adolescents and adults normally does not enhance following contrast administration.2 Intervertebral disks The intervertebral disks are composed of a central gelatinous core (the nucleus pulposus) surrounded by dense fibrocartilage and fibrous connective tissue (the anulus fibrosus) A normal lumbar intervertebral disk is slightly concave posteriorly, except at L5-S1, where it appears rounded The intervertebral disks of infants are typically high signal on T2-weighted scan except for a central low signal area that represents the notochord remnants (see Fig 19-19) Sharpey's fibers are seen at the periphery as low signal intensity regions Beginning in the second decade of life, a dark band of compact fibrous tissue develops in the disk centrum.5 Adult intervertebral disks are slightly hyperdense compared to adjacent muscle on NECT scans On MR scans, predominately fibrous compact tissue such as Sharpey's fibers and the outer anulus is low signal on both T1- and T2WI, whereas fibrocartilagenous tissue with mucoid matrix such as the nucleus pulposus, has high signal intensity on T2WI (Figs 19-1, E; and 19-2, G).5 Age-related changes of disk dessication and degeneration begin in the midteens and continue throughout life (see Chapter 20) Posterior elements The pedicles and neural arch form the posterior part of the vertebral column The neural arch is composed of the articular pillars and facet (zygoapophyseal) joints, the laminae, and the spinous processes Pedicles The pedicles are thick, bony pillars that 788 PART FIVE Spine and Spinal Cord Fig 19-2 Anatomy of the lumbosacral spine in the sagittal plane is depicted A and B, Anatomic drawings show structures in the midline (A) and in the neural foramen (B) C to E, Cryornicrotome section shows anatomy in the midline (C) and neural foramen (D) Close-up view (E) of the neural foramen (C to E, Courtesy V.M Haughton.) mostly consist of dense cortical bone They project posterolaterally from the vertebral bodies, connecting them with the neural arch and forming the spinal canal (Fig 19-1, C) Articular pillars The articular pillars consist of the pars interarticularis and the superior and inferior articular facets The pars interarticularis is a bony plate that extends posteriorly from the pedicle and gives rise to the superior and inferior articular facets Facet joints The facet joints are diarthrodial synovial-lined joints that connect the posterosuperior articular process of a lower vertebra with the posteroinferior articular process of the vertebra above (Figs 19-1, B and D; and 19-2, B and D).6 A tough, fibrous capsule is present along the posterolateral aspect of each facet joint There is no fibrous capsule on the ventral aspect of the joint; here, the ligamentum flavum and synovial membrane are the only barriers between the facet joint space and the spinal canal.7 The synovial membrane is intimately bound to the Chapter 19 Normal Anatomy and Congenital Anomalies of the Spine and Spinal Cord 789 Fig 19-2, cont'd F to H, Sagittal high resolution MR scans T1- (F) and T2-weighted (G) scans demonstrate normal midline anatomy H, Sagittal T1-weighted scan through the neural foramina shows the relationship of the soft tissues to the surrounding bone and intervertebral disk 1, Vertebral body 2, Intervertebral disk (nucleus pulposus) 3, Anterior longitudinal ligament 4, Posterior longitudinal ligament 5, Basivertebral venous plexus 6, Epidural fat 7, Epidural veins 8, Spinous processes 9, Interspinous ligament 10, Ligamentum flavum 11, Pedicle 12, Neural foramen with epidural fat and veins 13, Dorsal root ganglion 14, Superior articular facet 15, Inferior articular facet 16, Intranuclear cleft 17, Inner anular fibers of disk 18, Outer anular fibers of disk 19, Cauda equina 20, Conus medullaris 21, Pars interarticularis 22, S1 root 23, Sharpey fibers 24, Facet joint fat in the posteromedial and anterior recesses of the joint space.6 Synovium and joint space extend a variable distance along the articular processes and under the capsule The facet joint capsules are richly innervated by sensory fibers that arise from medial branches of the posterior spinal nerve rami.6 In the upper lumber spine the articular pillars and facet joints are oriented nearly in the parasagittal plane, whereas they are positioned more obliquely in the lower lumbar region.1,8 On axial imaging studies the facet joint has a mushroom-shaped appearance; the superior articular facet forms the "cap" and the inferior articular facet and spinal lamina form the "stem" (Fig 19-1, E) On sagittal MR scans the pars interarticularis lies between the more pointed superior articular facet above and the somewhat roundedappearing inferior articular facet below (Fig 19-2, H) Laminae and spinous processes The laminae are comparatively flat bony plates that extend posteriorly from the articular pillars and join together at the midline where they form the root of the spinous process The spinous processes extend posteriorly and inferiorly from the neural arch (Fig 19-2, A) Ligaments and soft tissues In the lumbosacral spine the ligaments, epidural fat, and the epidural venous plexuses form prominent extradural soft tissues that surround the thecal sac and exiting nerve roots Ligaments The anterior (ALL) and posterior (PLL) longitudinal ligaments are thick, dense fibrous bands that extend along the anterior and posterior surface of each vertebral body from the skull base to the sacrum (Fig 19-2).9 They connect the vertebral bodies and are attached to the intervertebral disks The ALL extends from the basiocciput to S1 It is identified on sagittal T1-weighted MR scans as a very 790 PART FIVE Spine and Spinal Cord low signal line that is in direct contact with and follows the ventral surface of the vertebral bodies and disks (Fig 19-2, A) The PLL is a thinner band that extends from C1 to the first sacral vertebra.1 In contrast to the ALL, the PLL does not adhere to the vertebral body.9 The PLL has a more narrow central segment that widens laterally at the intervertebral disks and attaches firmly to the anulus fibrosus, reinforcing the midline and paramedian zones of the disk.1 On midline sagittal MR scans, the PLL is seen as a continuous low signal band that is molded to the posterior disk surface but spans the vertebral body concavities like a bowstring (Fig 19-2, G) Epidural fat and veins are interposed between the PLL and the vertebral body The ligamentum flavum (LF) arises from the anterior aspect of the lower margin of one lamina and inserts on the posterior surface of the lamina below.1 The appearance of the LF on sagittal MR scans varies with its distance from the midline.10 It is thinnest at the midline where it is seen as an oblique, linear band of low signal that attaches to the superior border of one spinous process and the inferior surface of the next (Fig 19-2, F) On parasagittal scans the LF appears as an inhomogeneous triangle with a narrow base inferiorly and a broader base at its caudal end near the lamina.10 At the neural foramen it is seen as a curvilinear, low signal structure covering the anterior surface of the facet joint (Fig 19-2, H) On axial CT and MR studies the LF is seen as a V-shaped structure that covers the facet joint anteriorly and is sometimes filled with fat posteriorly (Fig 19-1, E) On NECT scans the LF is similar in attenuation to muscle; signal on MR is variable because the LF undergoes age-related degenerative change and can calcify or become infiltrated with fat (see subsequent discussion) Small ligaments, the corporotransverse and transforaminal ligaments, are often found in the neural foramina These fibrous bands originate from the intervertebral disk and attach to the pedicle, superior articular process, or ligamentum flavum They reduce the potential space available for nerve roots that traverse the neural foramen.11 Epidural fat and veins Extradural fat surrounds the lumbosacral thecal sac and root sleeves The epidural fat contains numerous small veins that connect to each other in the midline between the PLL and posterior vertebral body to form the epidural venous plexus.9 Basivertebral veins traverse the lumbar vertebral bodies and emerge near the midline to drain into this plexus (Figs 19-1, C; and 19-2, A).1 The lumbar epidural venous plexus is seen as thin, linear, low signal foci on T1- and T2-weighted MR scans (Fig 19-2, F) Enhancement following contrast administration is variable but can sometimes be intense Nerves and meninges Conus medullaris and cauda equina The distal Spinal cord terminates in a slight, diamond-shaped enlargement: the conus medullaris The conus tip is normally at about the Ll-L2 level The lower spinal nerve roots exit the conus medullaris and pass inferiorly within the thecal sac, forming the cauda equina, or "horse's tail" (Fig 19-3, A) Using heavily T2-weighted spin-echo sequences (Figs 19-2, G; and 19-3, G), MR "myelography" provides detailed definition of the thecal margins, nerve roots, and root sheaths that approaches conventional water-soluble lumbar myelograms and CT-myelography (Fig 19-3, E and F).12 On axial section, the roots of the filum terminale typically he in a symmetric, crescent-shaped pattern with the lower sacral roots positioned dorsally and the lumbar roots positioned more anterolaterally (Fig 19-3, F and G).13 Lumbar nerves and neural foramina Between L1 and L5, the nerve roots exit the spinal canal at about a 45 degree angle The nerve root axillae are lateral outpouchings of dura and arachnoid that surround the exiting roots (Fig 19-3, E) The motor roots lie ventral to the sensory roots from the thecal sac exit to the dorsal root ganglia.14 The dorsal root ganglia normally vary considerably in size, and range from mm at L1 to 15 mm at S2.14 The pedicles form the superior and inferior borders of the neural foramen; the articular facet and ligamentum flavum form its posterior border (see Fig 19-2, B) The anterior border is comprised of the vertebral body superiorly and the intervertebral disk and PLL inferiorly.15,16 The normal lumbar neural foramen is widest in its superior aspect and narrows inferiorly Each lumbar nerve root exits the spinal canal through the superior part of the foramen, above the level of the intervertebral disk In 90% of cases, the dorsal root ganglion is directly inferior to the pedicle.14 On sagittal MR scans the fat-filled foramen looks like the head and beak of a bird, with the dorsal root ganglion forming its eye (see Fig 19-2, H) Sacral plexus The sacral plexus is formed by the ventral rami of the L4-L5 and S1-S4 nerves (Fig 19-3, A) Medial to the psoas muscle, the L4-L5 nerves join to form the lumbosacral trunk After they exit the spine, the S1-S4 nerves converge in front of the piriformis muscle and join with the lumbosacral trunk to form the sacral plexus The sciatic nerve (L4-S3) is the continuation of the sacral plexus The sciatic nerve leaves the pelvis through the greater sciatic foramen to enter the thigh.17 Chapter 19 Normal Anatomy and Congenital Anomalies of the Spine and Spinal Cord Fig 19-3 Anatomy of the conus medullaris, cauda equina, and exiting nerve roots A and B, Anatomic drawings with coronal (A) and axial (B) views C and D, Cryornicrotome sections show gross anatomy of the distal cord and filum terminale in sagittal section (C) Axial section (D) illustrates the cauda equina 1, Thoracic cord with central gray matter 2, Conus medullaris 3, Subarachnoid space 4, Anterior roots 5, Posterior roots 6, Cauda equina 7, Sacral plexus 8, Sciatic nerve 9, Pedicles 10, Basivertebral vein 11, Exiting roots 12, Dorsal- root ganglion 13, Central gray matter 14, Posterior longitudinal ligament (C and D, Courtesy V.M Haughton.) Continued 791 792 PART FIVE Spine and Spinal Cord Fig 19-3, cont'd E to I, Multimodality imaging studies show the conus me dullaris and filum terminale Water soluble myelogram, AP view (E) Axia CT scan (F) with intrathecal contrast Axial T2-weighted MR scan (G) through cauda equina Compare with F H and I Axial T2-weighted MR scans through conus medullaris with “MR myelogram” effect Compare with (J), an axial post myelograrn CT scan of the conus medul laris 1, Thoracic cord with central gray matter 2, Conus medullaris 3, Sub arachnoid space 4, Anterior roots 5, Posterior roots 6, Cauda equina 7, Sa cral plexus 8, Sciatic nerve 9, Pedicles 10, Basivertebral vein 11, Exiting roots 12, Dorsal root ganglion 13, Centra gray matter 14, Posterior longitudina ligament Chapter 19 Normal Anatomy and Congenital Anomalies of the Spine and Spinal Cord 793 Thoracic Spine Anterior elements Vertebral bodies The dorsally convex thoracic spine consists of twelve vertebrae that gradually increase in size from rostral to caudal (Fig 19-4, A) The weight-bearing vertebral bodies are slightly wedge-shaped from front to back and appear somewhat cone- or triangular-shaped in axial section.18 Interverteral disks The height of the thoracic intervertebral disks is less than either the cervical or lumbar counterparts, but the anulus fibrosus is thicker here Fig 19-3, cont'd J, Axial postmyelogram CT scan of the conus medullaris Posterior elements Pedicles and laminae The pedicles project posteriorly from the superior aspects of each vertebral body The laminae are broad, short, and overlap each other like the tiles on a roof (Fig 19-4, B).18 The laminae fuse in the midline to form the dorsal canal wall and give origin to the spinous processes The thoracic spinous processes are long and gracile, extending posteriorly and inferiorly from the spinal canal (Fig 19-4, D) Articular pillars and joints Articular processes arise from the superior and inferior aspects of the laminae and form the facet joints In the thoracic spine, most facet joints lie in the coronal plane Transverse processes project laterally from the articular pillars between the superior and inferior articu- Fig 19-4 Anatomy of the thoracic spine and spinal cord A to C, Anatomic drawings with sagittal midline view (A), sagittal view through the neural foramen (B), and axial view (C) 1, Spinal cord with central gray matter 2, Conus medullaris 3, Spinous process 4, Ligamentum flavum 5, Dura 6, Cauda equina 7, Subarachnoid space 8, Rib 9, Facet joints 10, Basivertebral venous plexus 11, Superior articular facets 12, Inferior articular facets 13, Lamina 14, Posterior longitudinal ligament 15, Dentate ligaments 16, Epidural fat 17, Epidural veins 18, Nerve root 19, Costovertebral joint 20, Pedicle 21 Neural foramen 794 PART FIVE Spine and Spinal Cord Fig 19-4, cont'd D, Sagittal midline cryornicrotome shows the thoracic spine and intervertebral disks in a young child E to G, Imaging anatomy of the thoracic spinal cord is shown on T2-weighted MR scans Sagittal scans through the midline (E) and neural foramina (F) are shown Axial view (G) shows the rib articulations 1, Spinal cord with central gray matter 2, Conus medullaris 3, Spinous process 4, Ligamentum flavum 5, Dura 6, Cauda equina 7, Subarachnoid space 8, Rib 9, Facet joints 10, Basivertebral venous plexus 11, Superior articular facets 12, Inferior articular facets 13, Lamina 14, Posterior longitudinal ligament 15, Dentate ligaments 16, Epidural fat 17, Epidural veins 18, Nerve root 19, Costovertebral joint 20, Pedicle 21, Neural foramen (D, Courtesy V.M Haughton.) lar facets The tip of each transverse process from T1 to T10 bears an oval costal facet Costotransverse joints are formed by the articulation of the rib tubercles and tips of the transverse processes.19 Ribs Ribs articulate with the thoracic vertebrae at two sites Rib heads articulate with the vertebrae at the disk (Fig 19-4, C and G), and the rib tubercle joins with the transverse process at the costotransverse articulation (see previous discussion).19 At all levels except T1, TH, and T12, demifacets above and below the disk articulate with the rib head to form the costovertebral joint, which is a true synovial joint The rib heads are therefore helpful landmarks in identifying the intervertebral disk during axial imaging.19 Ligaments The anterior longitudinal ligament is thicker in the thoracic region than in the cervical or lumbar spine.18 It is also more prominent opposite the vertebral bodies than the disks The posterior longitudinal ligament is also thicker in the thoracic spine Other ligaments such as the ligamentum flavum and the interspinous ligaments are not significantly different from their configuration at other spinal segments.19 Nerves A number of rootlets emerge from the thoracic spinal cord and merge to form two roots: a large dorsal sensory root and a smaller ventral motor root (see Figs 19-3, H and 19-4, C) These descend a 804 PART FIVE Spine and Spinal Cord within the subcutaneous tissues or pass through the median raphe or bifid laminae toward the dura.38 The tract extends into the spinal canal in one-half to twothirds of all cases.32 Incidence, age, and gender Dorsal dermal sinuses are uncommon lesions Most are identified in early childhood, although some present as late as age 35 There is no gender predilection.33 Clinical presentation Most congenital dermal sinuses become symptomatic because of infection Sometimes symptoms arise from the mass effect caused by an associated dermoid or epidermoid tumor.38 Physical examination discloses a midline dimple Dorsal Dermal Sinus Epithelial-lined sinus tract from skin; >50% in lumbosacral region; occipital area is second most common site May terminate in subcutaneous tissue, dura, subarachnoid space, spinal cord, or nerve root; 50% end in dermoid or epidermoid cyst May terminate several spinal segments from cutaneous ostium Symptoms usually from infection Imaging shows tract; underlying spine often dysraphic or ostium that is often associated with hyperpigmented patch, hairy nevus, or capillary angioma.30 Location Slightly more than half of all mal sinuses occur in the lumbosacral spine The occipital area is the second most common site, followed by the thoracic spine.39 A dermal sinus may extend over a considerable distance and terminate several spinal segments away from its cutaneous ostium The dermatomal level of the cutaneous defect corresponds to the neural ectodermal level of the CNS structure with which it is connected via the tract.38 Imaging NECT scans typically show a relatively hyperdense sinus tract that traverses the subcutaneous fat and passes through a dysraphic or dysplastic lamina into the spinal canal where it penetrates for a variable depth The tract may merge with the dura, terminate in the subarachnoid space, or traverse the subarachnoid space to terminate in the conus medullaris, filum terminale, a nerve root, or a concomitant dermoid or epidermoid cyst.31 The subcutanous portions of dermal sinus tracts and associated intramedullary tumors such as dermoid are easily identified on MR scans (Fig 19-12); the intraspinal segments may be difficult to delineate unless they are lined by fat.31 Associated anomalies Approximately half of all dermal sinuses terminate in deep dermoid or epidermoid cysts; 20% to 30% of dermoid tumors are associated with dermal sinuses.31 Fig 19-12 This 18-month-old infant had repeated episodes of meningitis Sagittal (A) and axial (B) T1-weighted MR scans show a linear focus of low signal (arrowheads) that extends from the skin toward the spine The sinus tract is surrounded by high signal subcutaneous and epidural fat A marker capsule delineates the site of a surface dimple (curved arrow) Dorsal dermal sinus Chapter 19 Normal Anatomy and Congenital Anomalies of the Spine and Spinal Cord Spinal lipomas The malformation most frequently associated with all forms of occult spinal dysraphism is lipoma Spinal lipomas are masses of matire fat and fibrous tissue that are connected with the lieptomeninges or spinal cord.30 Spinal lipomas are divided into the following three principal categories (see box, below)31: Lipomyelomeningocele (84%) Filum terminale fibrolipoma (12%) Intradural lipomas (4%) Lipomyelomeningocele Lipomyelomeningocele is a neural tube closure defect A lipomyelomeningocele is basically analogous to a myelinomeningocele that has superimposed lipomas, fibromuscular capsules, and intact skin (Fig 19-7, C).33 Lipomyelomeningoceles account for 20% of skin-covered lumbosacral masses and up to half of occult spinal dysraphisms.31 There is a moderate female predominance Most patients with lipomyelomeningocele present before months of age but others may remain undetected into adulthood.33 Neurogenic bladder, sensory abnormalities, and orthopedic deformities are common presenting symptoms A large subcutaneous semifluctuant lumbosacral mass is often present on physical examination Imaging findings on plain film radiographs include focal spina bifida and widened spinal canal Segmentation anomalies are common MR scans disclose a low-lying spinal cord that is continuous dorsally with the neural placode The nerve roots arise from the placode (not the lipoma) and cross the subarachnoid space to exit the spinal canal.33 The lipoma itself lies outside the dura and is contiguous with subcutaneous fat (Fig 19-13).31,37 Syringohydromyelia is present in 25% of cases.40 Lipomyelomeningocele, is not associated with Chiari II malformation but has been reported with a Chiari I malformation.33,40a 805 Filum terminale fibrolipoma These lipomas may result from faulty retrogressive differentiation.30 Asymptomatic filum fibrolipomas without tethered spinal cord have been reported in 1% to 6% of random autopsies and are noted incidentally on 0.24% to 5% of lumbosacral MR scans.41,42 Small lipomas typically occur within the filum itself, whereas larger lipomas are usually found at the lower dorsal dural attachment.43 Filum fibrolipomas are seen as thin, linear high signal areas on TI-weighted MR scans The conus medullaris ends at the normal level, i.e., at or above the L1-L2 interspace (Fig 19-14) Intradural lipomas Intradural lipomas are intradural, subpial, juxtamedullary lesions (see Fig 19-7, D).30 The spinal cord is open posteriorly, and the lipoma is situated between the unapposed lips of the placode.30 Most lipomas are located in the cervical and thoracic spine The dorsal cord is the most common site.30 T1-weighted MR scans show a high signal mass interposed between the central canal of the spinal cord and pia (Fig 19-15).30 Anomalies of Abnormal Canalization and Retrogressive Differentiation The distal embryonic neural tube normally elongates through the process of canalization and retrogressive differentiation, ultimately forming the conus medullaris, ventriculus terminalis, and the filum terminale.44 Failure to regress normally results in a spectrum of lesions that includes tethered spinal cord, tight filum terminale syndromes, and caudal spinal anomalies.30 Spinal Lipomas Three types Lipomyelomeningocele (84%) Filum terminale fibrolipoma (12%) Intradural (subpial) lipoma (4%) Incidence Most common lesion with all occult spinal dysraphic disorders; most common cause of tethered cord Key points Lipomyelomeningocele is not part of the Chiari II malformation; myelomeningocele is Filum lipomas often incidental, asymptomatic Intradural lipoma on dorsal cord surface Fig 19-13 Lipomyelomeningocele (large arrows) is shown on this sagittal T1-weighted MR scan Note spinal cord with hydrosyringomyelia (small arrows) tethered into the lipoma (large arrows) The lipoma extends posteriorly through a widely dysraphic spine, where it is continuous with the subcutaneous fat (Courtesy D Baleriaux; reprinted from Neuroradiol 35:375-377, 1993.) 806 PART FIVE Spine and Spinal Cord Fig 19-14 Axial (A) and sagittal (B) T1-weighted scans in a 52-year-old man with low back pain and no neurologic abnormalities show a "fatty filum" (arrows) The spinal cord is not tethered, and the conus medullaris ended at the normal level This is probably an incidental finding Fig 19-15 Intradural lipoma in a 2-year-old child with a prominent fatty soft tissue mass at the iliac crest Axial (A) and sagittal (B) T1-weighted MR scans show a high signal mass (arrows) in the dorsolateral distal thoracic cord Note conus medullaris terminates at the normal level Also note low signal in vertebral body marrow, normal at this age because of active hematopoiesis Chapter 19 Normal Anatomy and Congenital Anomalies of the Spine and Spinal Cord 807 Tethered spinal cord and thick filum terminale syndrome Etiology and pathology Failure of terminal cord involution or normal nerve fiber lengthening may use the so-called tight filum terminale syndrome 33 syndrome is a complex of neurologic and orthopedic deformities associated with a short, thick filum ale and low-lying conus medullaris (see box, p.808).33 Incidence, age, and gender Tethered cord is a common feature of many, if not most, spinal malformations, and a thickened filum terminale is usually as sociated with clinical tethered spinal cord syndrome.41,45 Reported causes of spinal cord tethering spinal lipoma (72%), tight filum terminale (12%), diastematomyelia (8%), and 44 myelomeningocele (8%) Tethered spinal cord and tight filum terminale syndrome can be seen at any age44; symptom onset typpically begins between and 35 years of age There is gender predilection.33 Clinical presentation Symptoms of tethered cord and tight filum terminale vary Pain, dysesthesias, neurogenic bladder, and spasticity are common.44 Congenital or developmental kyphoscoliosis is seen in 25%.31 Imaging Imaging findings in tethered cord and and syndromes vary Plain films may show a spine (Fig 19-16, A) Myelography typically shows a low-lying conus without or with a lipoma (Fig 19-16, B) The exiting nerve roots have a Fig 19-16 A 12-year-old boy had repeated urinary tract infections and uremia No cutaneous stigmata were present AP plain film radiograph (A) and myelogram (B) show classic findings in tethered cord syndrome A, Plain film shows the entire distal lumbosacral spine is widely dysraphic (small arrows) Spina bifida (large arrow) is present above the enlarged canal B, Myelogram shows the thinned, elongated spinal cord (small arrows) is tethered inferiorly into a prominent mass (large arrows) The nerve roots (open arrows) are tautly stretched and course superolaterally instead of inferolaterally Fig 19-17 CT-myelogram with axial scans (A) and reformatted coronal scan (B) shows a stretched spinal cord (curved arrow) tethered into a large lipoma (straight arrows) (Courtesy R Jahnke.) 808 PART FIVE Spine and Spinal Cord Fig 19-18 Two cases of tethered spinal cord Sagittal T1- (A) and axial T2-weighted (B and C) MR scans show tethered spinal cord (arrows) Note the spinal cord at L2-L3 Normally, only roots of the cauda equina would be present at this level (compare with Fig 19-3, F) The small intramedullary high signal focus at the distal cord (C, double arrows) represents a small syrinx or possibly a ventriculis terminalis Axial T1-weighted scan (D) in another case shows a tethered spinal cord with lipoma, seen here as a high signal mass (arrow) Tethered Cord, Thick Filum Syndromes Terminal embryonic neural tube fails to involute Back pain, scoliosis common; may have bowel, bladder incontinence Occult tether may have symptom onset delayed into adulthood Often associated with other abnormalities (e.g., lipoma, diastematomyelia, myelomeningocele) Imaging key: axial views because sagittal view can be misleading (lumbar nerve roots usually layer dorsally scan mimic tethered cord) lateral or even "uphill" course in severe cases (Fig 19-16, B) CT-myelography shows a low-lying conus medullaris (below L2), a thickened filum terminale that is greater than 1.5 mm in diameter, and, sometimes, fibrous adhesion bands The tethered cord may terminate in a lipoma (Fig 19-17) On sagittal MR scans, the conus medullaris often appears elongated with no sharp transition between conus and filum Because roots of the cauda equina are normally layered posteriorly in the thecal sac (see Fig 19-3, F and G), this can sometimes mimic a tethered cord if only sagittal MR scans are obtained; axial T2-weighted "MR myelograms" are diagnostic (Fig 19-18) Chapter 19 Normal Anatomy and Congenital Anomalies of the Spine and Spinal Cord Fig 19-19 Sagittal T2-weighted MR scan in a newborn with a mild caudal regression syndrome shows the sacrum (arrow) ends below S1 Caudal Spinal Anomalies Caudal regression syndromes Terminal myelocystocele Anterior sacral meningocele Occult intrasacral meningocele Sacrococcygeal teratoma Caudal spinal anomalies Caudal spinal anomalies are lesions in which malformations of the distal spine, spinal cord, and meninges are associated with hindgut, kidney, urinary bladder, and genitalia anomalies These include the caudal regression syndromes, terminal myelocystocele, anterior sacral and occult intrasacral meningoceles, and sacrococcygeal teratomas (see box).32,33 Caudal regression syndromes The caudal regression syndromes include varying degrees of lumbosacral agenesis combined with other anomalies such as imperforate anus, malformed genitalia, renal dysplasia or aplasia, and sirenomelia (fused lower extremities) They range from absent coccyx usually seen as an isolated incidental finding without neurologic sequelae, to sacral or lumbosacral agenesis (Fig 19- 809 Fig 19-20 Sagittal T1-weighted MR scan in an infant with a terminal myelocystocele 1, Tethered spinal cord (arrowheads) with distal hydromyelia 2, Meningocele (meninges with subarachnoid space are herniated through a large dorsal defect) 3, Large terminal cyst, the myelocystocele, bulges below the meningocele and communicates with the dilated central canal of the tethered cord 4, Anterior lumbosacral subarachnoid space 19).33a, In extreme cases, the last intact vertebra is T11 or T12.32,33 Terminal myelocystocele Terminal myelocystocele, also called syringocele, is a localized cystic dilatation of the distal spinal cord Myelocystocele constitutes between 1% and 5% of skin-covered lumbosacral masses Terminal myelocystocele consists of posterior spina bifida or partial sacral agenesis and tethered spinal cord with hydromyelia The cord, meninges, and subarachnoid space protrude into the dorsal subcutaneous plane The terminal portion of the tethered cord appears ballooned and flared under the subcutaneous fat and enlarged subarachnoid space (Fig 19-20).32 Anterior sacral meningocele Anterior sacral meningoceles are herniations of meninges through defects in the sacrum, coccyx, or adjacent disc spaces to form a CSF-filled hernia sac within the pelvis.32 Anterior sacral meningoceles may occur as an isolated defect, as part of the caudal regression syndromes, or as a 810 PART FIVE Spine and Spinal Cord Fig 19-21 Marfan syndrome patient with extreme dural ectasia and sacral meningoceles A, Axial CT scan with bone windows shows grossly expanded sacral foramine (arrows) B, CT scan obtained following installation of intrathecal contrast shows most of the meningoceles communicate freely with the subarachnoid space, whereas one remains unopacified (open arrows) C, Axial T2-weighted MR scan shows the meningoceles (compare with A) Fig 19-22 A 24-year-old woman with low back pain had this MR scan Sagittal T1- (A) and T2-weighted (B) scans show an intraosseous sacral meningocele (straight arrows) Note the dura (arrowheads) interposed between the subarachnoid space and the meningocele A small potential communication site is indicated (B, curved arrow) Note that the meningocele contents are slightly hyperintense compared to CSF Chapter 19 Normal Anatomy and Congenital Anomalies of the Spine and Spinal Cord Split Notochord Syndromes Dorsal enteric fistula (most severe) Dorsal enteric sinus Dorsal enteric diverticulae Diastematomyelia Spinal enterogenous (neurenteric) cyst chymal dysplasia such as neurofibromatosis type (NF-1) or Marfan syndrome (Fig 19-21).32,46 Widened sacral foramina with smoothly scalloped margins and protruding CSF-filled sacs are present Occasionally the sacral dura is completely absent.46a Occult intrasacral meningoceles Occult intrasacral meningoceles are mild dural developmental anomalies in which the arachnoid herniates through a dural defect, expanding and scalloping the bony sacrum.47,48 These are seen on imaging studies as expansile smoothly marginated, well-delineated cysts that are often slightly higher in signal intensity than CSF within the thecal sac (Fig 19-22) Symptoms are unrelated to cyst size; cysts that communicate freely with the subarachnoid space are typically asymptomatic whereas noncommunicating cysts are often symptomatic.48a Occult intrasacral meningoceles may occur in isolation but are also commonly associated with lipoma, hyperkeratosis, sacral pigmentation, sacoccygeal dimples, and neurofibromatosis.47 Sacrococcygeal teratomas Sacrococcygeal teratomas are developmental in origin and are the most frequently encountered presacral masses in children.49 Pathologically, they range from mature teratomas to anplastic carcinomas.30 Most are mature teratomas and seen as large, well-encapsulated, heterogeneous pre- or postsacral masses Split Notochord Syndromes This group of anomalies results from splitting of the notochord with a persistent connection between the gut and dorsal ectoderm.30 The most severe form f split notochord syndrome is the rare dorsal enteric fistula Other anomalies include diastematomyelia and enterogenous (neurenteric) cysts (see box, above) Diastematomyelia Etiology and pathology Diastematomyelia is characterized pathologically by sagittal clefting of the spinal cord or filum terminale (see box, above right) Recent theories postulate that all variants of split spinal cord malformations arise from adhesions between the embryonic ecto- and endoderm This leads to formation of an accessory neurenteric canal An endomesenchymal tract condenses around this accessory canal, bisecting the developing notochord d causing two hemineural plates to form.50 The 811 Diastematomyelia Split spinal cord (not duplicated cord) Hemicords in separate (50%) or common (50%) dural tube Usually between T9-S1 (85%) Spine nearly always abnormal Osseous spur in only 50% Associated with Chiari II malformation, tethered spinal cord, hydromyelia of one or both hemicords result is a split spinal cord, i.e., diastematomyelia (Fig 19-23, A) The spinal cord is typically split into two halves by a fibrous, bony, or osteocartilagenous septum.33,37 The cleft typically extends completely through the cord, although partial clefting occasionally occurs The cord is usually split locally, with a single cord above and below the cleft The two hemicords are typically somewhat asymmetric Each hemicord contains a central canal and one set of dorsal and ventral horns and nerve roots In 50% of all cases, the two hemicords share a single dural tube; in the remaining half, the hemicords are enclosed in separate dural sacs (Fig 19-23, B).33 Some authors have proposed a new classification, dividing so-called double spinal cord malformations into Type I and Type II split cord malformations (SCMs) Type I SCMs consist of two hemicords, each contained within its own dural tube and separated by a dura-sheathed rigid osseocartilaginous median septum A Type II SCM consists of two hemicords contained in a single dural tube with the hemicords separated by a nonrigid fibrous median septum.50 Incidence, age, and gender Diastematomyelia is an uncommon form of occult spinal dysraphism There is a distinct female predominance.33 Age at symptom onset varies Clinical presentation and natural history Cutaneous stigmata overlie the spine in 50% to 75% of patients with diastematomyelia Hair patches, nevi, and lipomas are common, whereas skin dimples and dermal sinuses are less frequently observed.33 Orthopedic abnormalities such as clubfoot occur in nearly half of all patients, and nonspecific neurologic symptoms are present in 85% to 90% of children with diaste-matomyelia.33 Pain is the predominant symptom in adults with split cord malformations Symptom onset may be insidious or abrupt, following a fall on the buttocks or low back Leg weakness and neurogenic bladder are common.51 Location The cleft is located between T9 and S1 in 85% of cases.52 The lumbar spine is the site of nearly half of these anomalies, a thoracic location is 812 PART FIVE Spine and Spinal Cord Fig 19-23 Diastematomyelia is depicted A, Axial gross autopsy specimen shows the two hemicords (large arrows) separated by a fibrous septum (small arrows) B, Axial CT scan with intrathecal contrast shows a grossly abnormal vertebral body with an osseous spur that traverses the entire canal Two hemicords are clearly seen, each contained within its own dural tube C, Axial T1-weighted MR scan in another case shows two unequal hemicords (arrows) D, Water-soluble myelogram, AP view, shows two equal hemicords (straight arrows) Note scoliosis and butterfly vertebral body (curved arrow) (A, Courtesy E Ross.) Chapter 19 Normal Anatomy and Congenital Anomalies of the Spine and Spinal Cord seen in 20%, and combined thoracolumbar lesions occur in 15% to 20%.52 A cervical or basicranial diastematomyelia is very rare.53 The conus medullaris lies below L2 in three quarters of all cases Imaging findings Plain films and NECT scans show a grossly abnormal osseous spine in nearly all cases of diastematomyelia (Fig 19-23, B) (see subsequent discussion) The hemicords and subarachnoid spaces are well delineated at myelography (Fig 1923, D), CT-myelography, or on MR scans (Fig 19-23, C) Associated anomalies Osseous anomalies are typically striking At least 85% of patients with diastematomyelia have vertebral body anomalies, including hemivertebrae, block or butterfly vertebrae, and narrow intervertebral disk spaces Intersegmental laminar fusion with spina bifida is present in 60% of cases.52 An osseous spur is seen in only half of pants with diastematomyelia The spur may traverse or all of the canal and be on or off the midline The spinal canal itself is abnormally wide with increased interpediculate distance.33 Soft tissue anomalies include low-lying or tethered Conus with thickened filum terminale (40%) and hydromyelia of one or both hemicords.31 Between 15% and 20% of patients 813 Enterogenous cyst Etiology and pathology Enterogenous cysts probably result from failure of notochord and foregut separation during formation of the alimentary canal (Figs 19-24 and 19-25).54,55 Grossly, enterogenous cysts are well-delineated, thin-walled, fluid-containing masses (Fig 19-26, A) (see box) Their walls are composed of fibrous connective tissue lined by a single layer of columnar or cuboidal epithelial cells Mucin-secreting goblet cells are often present.53 Incidence, age, and gender Most enterogenous cysts are seen in patients under 40 years of age; peak incidence is during the first or second decade There is a slight male predominance.53,54 Location The most common location is the tho- Enterogenous Cyst Fluid-containing cyst lined by epithelial, goblet cells Thoracic spine most common site; most cysts anterior to spinal cord Bony anomalies seen in