Congenital Spine and Spinal Cord Malformations—Pictorial Review
Objective
Congenital abnormalities of the spine and spinal cord are referred to as spinal dysraphisms. This article reviews normal embryological development of the spine and spinal cord and the imaging findings of congenital abnormalities of the spine and spinal cord with particular focus on MRI.
Conclusion
Knowledge of the normal development of the spine and spinal cord provides a framework for understanding these complex entities.
Spinal Cord Development
Spinal development can be summarized in three basic embryologic stages [1, 2]. The first stage is gastrulation and occurs during the second or third week of embryonic development. Gastrulation involves conversion of the embryonic disk from a bilaminar disk to a trilaminar disk composed of ectoderm, mesoderm, and endoderm. The second stage in spinal development is primary neurulation (weeks 3–4) in which the notochord and overlying ectoderm interact to form the neural plate. The neural plate bends and folds to form the neural tube, which then closes bidirectionally in a zipperlike manner (Fig. 1A, 1B, 1C, 1D). The final stage of spinal development is secondary neurulation (weeks 5–6). During this stage, a secondary neural tube is formed by the caudal cell mass. The secondary neural tube is initially solid and subsequently undergoes cavitation, eventually forming the tip of the conus medullaris and filum terminale by a process called retrogressive differentiation. Abnormalities in any of these steps can lead to spine or spinal cord malformations.
Categorization of Spinal Dysraphisms
Spinal dysraphisms can be broadly categorized into open and closed types [1–3]. In an open spinal dysraphism, there is a defect in the overlying skin, and the neural tissue is exposed to the environment. In a closed spinal dysraphism, the neural tissue is covered by skin. Closed spinal dysraphisms can be further subcategorized on the basis of the presence or absence of a subcutaneous mass [4]. Appendix 1 summarizes the key features of open and closed spinal dysraphisms.
Open Spinal Dysraphisms
Myelomeningocele and myelocele—Myelomeningoceles and myeloceles are caused by defective closure of the primary neural tube and are characterized clinically by exposure of the neural placode through a midline skin defect on the back. Myelomeningoceles account for more than 98% of open spinal dysraphisms [1]. Myeloceles are rare. Open spinal dysraphisms are often diagnosed clinically, so imaging is not always performed. When imaging is performed, the main differentiating feature between a myelomeningocele and myelocele is the position of the neural placode relative to the skin surface [2]. The neural placode protrudes above the skin surface with a myelomeningocele (Fig. 2A, 2B, 2C) and is flush with the skin surface with a myelocele (Fig. 3A, 3B).
Hemimyelomeningocele and hemimyelocele—Hemimyelomeningoceles and hemimyeloceles can also occur but are extremely rare [5]. These conditions occur when a myelomeningocele or myelocele is associated with diastematomyelia (cord splitting) and one hemicord fails to neurulate.
Closed Spinal Dysraphisms With a Subcutaneous Mass
Lipomas with a dural defect—Lipomas with a dural defect include both lipomyeloceles and lipomyelomeningoceles. These abnormalities result from a defect in primary neurulation whereby mesenchymal tissue enters the neural tube and forms lipomatous tissue [6]. Lipomyeloceles and lipomyelomeningoceles are characterized clinically by the presence of a subcutaneous fatty mass above the intergluteal crease. The main differentiating feature between a lipomyelocele and lipomyelomeningocele is the position of the placode–lipoma interface [4]. With a lipomyelocele, the placode–lipoma interface lies within the spinal canal (Fig. 4A, 4B, 4C). With a lipomyelomeningocele, the placode–lipoma interface lies outside of the spinal canal due to expansion of the subarachnoid space (Fig. 5A, 5B).
Meningocele—Herniation of a CSF-filled sac lined by dura and arachnoid mater is referred to as a meningocele. The spinal cord is not located within a meningocele but may be tethered to the neck of the CSF-filled sac. Posterior meningoceles herniate through a posterior spina bifida (osseous defect of posterior spinal elements) and are usually lumbar or sacral in location but also can occur in the occipital and cervical regions (Fig. 6A, 6B, 6C). Anterior meningoceles are usually presacral in location but also can occur elsewhere [7] (Fig. 7A, 7B).
Terminal myelocystocele—Herniation of large terminal syrinx (syringocele) into a posterior meningocele through a posterior spinal defect is referred to as a terminal myelocystocele [2] (Fig. 8A, 8B, 8C). The terminal syrinx component communicates with the central canal, and the meningocele component communicates with the subarachnoid space. The terminal syrinx and meningocele components do not usually communicate with each other [8].
Closed Spinal Dysraphisms Without a Subcutaneous Mass
Closed spinal dysraphisms without a subcutaneous mass can be subcategorized into simple and complex dysraphic states.
Simple dysraphic states—Simple dysraphic states consist of intradural lipoma, filar lipoma, tight filum terminale, persistent terminal ventricle, and dermal sinus.
An intradural lipoma refers to a lipoma located along the dorsal midline that is contained within the dural sac (Fig. 10A, 10B). No open spinal dysraphism is present. Intradural lipomas are most commonly lumbosacral in location and usually present with tethered-cord syndrome, a clinical syndrome of progressive neurologic abnormalities in the setting of traction on a low-lying conus medullaris [2].
Fibrolipomatous thickening of the filum terminale is referred to as a filar lipoma. On imaging, a filar lipoma appears as a hyperintense strip of signal on T1-weighted MR images within a thickened filum terminale (Fig. 11A, 11B). Filar lipomas can be considered a normal variant if there is no clinical evidence of tethered-cord syndrome [10, 11].
Tight filum terminale is characterized by hypertrophy and shortening of the filum terminale (Fig. 12). This condition causes tethering of the spinal cord and impaired ascent of the conus medullaris. The conus medullaris is low lying relative to its normal position, which is usually above the L2–L3 disk level [2].
Persistence of a small, ependymal lined cavity within the conus medullaris is referred to as a persistent terminal ventricle (Fig. 13A, 13B). Key imaging features include location immediately above the filum terminale and lack of contrast enhancement, which differentiate this entity from other cystic lesions of the conus medullaris [12].
A dermal sinus is an epithelial lined fistula that connects neural tissue or meninges to the skin surface. It occurs most frequently in the lumbosacral region and is often associated with a spinal dermoid at the level of the cauda equina or conus medullaris (Fig. 14A, 14B, 14C). Clinically, patients present with a midline dimple and may also have an associated hairy nevus, hyperpigmented patch, or capillary hemangioma [13]. Surgical repair is of great importance because the fistulous connection between neural tissue and the skin surface can result in infectious complications such as meningitis and abscess.
Complex dysraphic states—Complex dysraphic states can be divided into two categories: disorders of midline notochordal integration, which include dorsal enteric fistula, neurenteric cyst, and diastematomyelia, and disorders of notochordal formation, which include caudal agenesis and segmental spinal dysgenesis.
Disorders of midline notochordal integration: Dorsal enteric fistula and neurenteric cyst—A dorsal enteric fistula occurs when there is an abnormal connection between the skin surface and bowel. Neurenteric cysts represent a more localized form of dorsal enteric fistula (Fig. 15A, 15B, 15C). These cysts are lined with mucin-secreting epithelium similar to the gastrointestinal tract and are typically located in the cervicothoracic spine anterior to the spinal cord [14].
Diastematomyelia—Separation of the spinal cord into two hemicords is referred to as diastematomyelia. The two hemicords are usually symmetric, although the length of separation is variable. There are two types of diastematomyelia. In type 1, the two hemicords are located within individual dural tubes separated by an osseous or cartilaginous septum (Fig. 16A, 16B, 16C). In type 2, there is a single dural tube containing two hemicords, sometimes with an intervening fibrous septum [15] (Fig. 17A, 17B, 17C). Diastematomyelia can present clinically with scoliosis and tethered-cord syndrome. A hairy tuft on the patient's back can be a distinctive finding on physical examination [16].
Disorders of notochordal formation: Caudal agenesis—Caudal agenesis refers to total or partial agenesis of the spinal column (Fig. 18A, 18B) and may be associated with the following: anal imperforation, genital anomalies, renal dysplasia or aplasia, pulmonary hypoplasia, or limb abnormalities. Caudal agenesis can be categorized into two types. In type 1, there is a high position and abrupt termination of the conus medullaris. In type 2, there is a low position and tethering of the conus medullaris [17].
Segmental spinal dysgenesis—The clinical–radiologic definition of segmental spinal dysgenesis includes several entities: segmental agenesis or dysgenesis of the thoracic or lumbar spine, segmental abnormality of the spinal cord or nerve roots, congenital paraparesis or paraplegia, and congenital lower limb deformities. Three-dimensional CT reconstructions can be helpful in showing various vertebral segmentation anomalies [18] (Fig. 19A, 19B).
Conclusion
Congenital malformations of the spine and spinal cord can be complex and variable in imaging appearance. An organized approach to imaging findings with consideration of clinical and developmental factors allows greater ease in diagnosis.
Open Spinal Dysraphisms: not covered by intact skin | |
| Myelocele | Neural placode flush with skin surface |
| Myelomeningocele | Neural placode protrudes above skin surface |
| Hemimyelocele | Myelocele associated with diastematomyelia |
| Hemimyelomeningocele | Myelomeningocele associated with diastematomyelia |
| Closed Spinal Dysraphisms: covered by intact skin | |
| With a subcutaneous mass | |
| Lipomyelocele | Placode—lipoma interface within the spinal canal |
| Lipomyelomeningocele | Placode—lipoma interface outside of the spinal canal |
| Meningocele | Herniation of CSF-filled sac lined by dura |
| Terminal myelocystocele | Terminal syrinx herniating into posterior meningocele |
| Myelocystocele | Dilated central canal herniating through posterior spina bifida |
| Without a subcutaneous mass | |
| Simple dysraphic states | |
| Intradural lipoma | Lipoma within the dural sac |
| Filar lipoma | Fibrolipomatous thickening of filum |
| Tight filum terminale | Hypertrophy and shortening of filum |
| Persistent terminal ventricle | Persistent cavity within conus medullaris |
| Dermal sinus | Epithelial lined fistula between neural tissue and skin surface |
| Complex dysraphic states | |
| Dorsal enteric fistula | Connection between bowel and skin surface |
| Neurenteric cyst | More localized form of dorsal enteric fistula |
| Diastematomyelia | Separation of cord into two hemicords |
| Caudal agenesis | Total or partial agenesis of spinal column |
| Segmental spinal dysgenesis | Various segmentation anomalies |
Acknowledgments
The authors thank Anne Philips, former medical illustrator from the Department of Radiology at the University of Michigan, for providing various illustrations used in this article.
Footnotes
Address correspondence to S. L. Rufener ([email protected]).
Presented at the 2008 annual meeting of the American Roentgen Ray Society, Washington, DC.
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Submitted: November 20, 2008
Accepted: March 14, 2009
First published: November 23, 2012
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