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INTRODUCTION-
Spinal cord development occurs through three consecutive periods of gastrulation, primary nerulation and secondary neurulation. Aberration in these stages causes abnormalities of the spine and spinal cord, collectively referred as spinal dysraphism. They can be broadly classified as anomalies of gastrulation (disorders of notochord formation and of integration); anomalies of primary neurulation (premature dysjunction and nondysjunction); combined anomalies of gastrulation and primary neurulation and anomalies of secondary neurulation. Correlation with clinical and embryological data and common imaging findings provides an organized approach in their diagnosis.
Spinal dysraphism refers to the congenital abnormalities of the spine and spinal cord. Clinico-radiological classification of spinal dysraphism has been well established and widely followed. The objective of this article is to illustrate the common magnetic resonance imaging (MRI) findings of various spinal dysraphisms based on embryological events
Spinal dysraphism is an umbrella term that describes a number of conditions present at birth that affect the spine, spinal cord, or nerve roots.
- Spine: the bony structure also known as the spinal column. Made up of individual vertebrae (bones), the spine protects the spinal cord.
- Spinal cord: the bundle of nerves and other tissue that connects brain and body. Inside the spine, the spinal cord is also protected by a series of membranes (coverings). The spinal cord relays sensory information from the body to the brain, and movement instructions from the brain to the body.
- Nerve roots: nerves that branch off the spinal cord to reach the rest of the body.
All forms of spinal dysraphism result from an event very early in an embryo’s development. In about the third week of development, a sheet of cells called the neural plate folds up to form a structure called the neural tube. The top of the neural tube develops into the brain, and the rest of the neural tube develops into the spine and spinal cord. Spinal dysraphism results when a section of the neural tube that will become the spine and spinal cord does not close completely.
Types of spinal dysraphism include myelomeningocele (also known as spina bifida aperta or open spina bifida), spina bifida occulta, split cord malformation (diastematomyelia), spinal cord lipoma (lipomyelomeningocele), dermal sinus tract, tight filum terminale, and tethered spinal cord. A person with spinal dysraphism may have more than one type.
- Myelomeningocele, or spina bifida aperta: a condition in which the spinal cord and its membranes are not contained within the spinal column, but protrude into a sac outside the body
- Spina bifida occulta: a condition in which one or more vertebrae (bones of the spinal column) has a slight defect, but the spinal cord and its membranes are not affected
- Split cord malformation (diastematomyelia): a complex type of spinal dysraphism in which the spinal cord splits lengthwise into two distinct cords. Often, the split is caused by a thin segment of bone or cartilage that protrudes from the spinal column into the spinal cord space. Sometimes there is only one protective membrane or sleeve (the dura) around both parts of the spinal cord. Sometimes each part has its own dural sleeve.
- Spinal cord lipoma (lipomyelomeningocele): a condition in which an abnormal growth of fat attaches to the spinal cord, its membranes, and the space outside the spinal canal. Dermal sinus tract : a channel in the skin that may reach all the way to the spinal cord. Dermal sinus tracts are often associated with tumors in the space around the spinal cord called dermoids or epidermoids. Benign (non-cancerous) tumors and cysts can be found along with any type of spinal dysraphism. Tumor types include lipoma (a fatty tumor), dermoid(a tumor containing tissue of hair, bone, or cartilage), and epidermoid(a tumor of skin layers). These tumors are not cancerous and will not spread. However, they may compress the spinal cord or
- tethered spinal cord.Tethered cord: a condition that may occur as a result of any spinal dysraphism, or as a result of other conditions (e.g. tumor, infection, or scar tissue formation). In this condition, the spinal cord is restricted at its base and cannot move freely in the spinal column. The resulting “stretch” on the spinal cord tissue can cause damage to the spinal cord leading to neurological problems (weakness, sensory loss), urological problems (incontinence), orthopedic problems (scoliosis and foot, ankle or leg deformities) and pain (see below). Accumulations of fluid in the spinal cord can also occur.
CLASSIFICATION-
Spinal dysraphism can be broadly divided into two different clinicoradiological entities 8,9:
- open spinal dysraphism (formerly spina bifida aperta or cystica): occurs when the cord and its covering communicate with the outside; no skin or tissues cover the sac
- myelomeningocele (98% of open spinal dysraphism)
- myelocele
- hemimyelomeningocele
- hemimyelocele
- closed spinal dysraphism (formerly spina bifida occulta): occurs when the cord is covered by other normal mesenchymal elements
- with subcutaneous mass
- lipoma with dural defect
- lipomyelomeningocele
- lipomyelocele
- terminal myelocystocele
- meningocele
- limited dorsal myeloschisis
- lipoma with dural defect
- without subcutaneous mass
- posterior spina bifida (isolated defect of the posterior neural arch of vertebra)
- intradural lipoma
- filar lipoma
- tight filum terminale
- persistent terminal ventricle
- disorders of midline notochordal integration
- dorsal dermal sinus
- dorsal enteric fistula
- neurenteric cyst 5,6
- split cord malformations
- diastematomyelia
- diplomyelia
- disorders of notochordal formation
- caudal regression syndrome
- Type 1
- Type 2
- segmental spinal dysgenesis
- caudal regression syndrome
- with subcutaneous mass
CAUSE AND RISK FACTOR-
The causes of spinal dysraphism are not yet completely understood. Genetic and environmental factors both seem to play a role.
The spinal cord arises very early in fetal development–in the first several weeks of gestation. Many forms of spinal dysraphism develop during this time. Robust maternal nutrition early in pregnancy, especially adequate levels of a vitamin called folate, seems to protect against some forms of spinal dysraphism.
SPINAL CORD DEVELOPMENT-
Spinal cord development can be summarized in three basic embryologic stages – gastrulation (2–3 weeks), primary neurulation (3-4 weeks) and secondary neurulation (5–6 weeks). The rostral spinal cord (to about the level of S2) is formed by primary neurulation and the caudal spinal cord (distal to S2 level) by secondary neurulation, also referred to as canalization and retrogressive differentiation.
Gastrulation-
Gastrulation is the process of conversion of bilaminar disc into a trilaminar disc initiated by primitive streak. Primitive node, a depression at the cranial end of streak, contains cells that are important for organizing the embryonic axes. Epiblast cells migrate toward and through the streak and node, detach and form two new layers ventral to the remaining epiblast. The first cells through the streak displace the original hypoblast to form endoderm, whereas cells migrating slightly later create a new middle layer, the mesoderm. Nonmigrating cells of epiblast constitute the ectoderm. Some cells migrate cranially in the midline to form the prechordal plate and notochord, which initiate the process of neurulation by inducing the formation of the neural plate from overlying ectoderm cells. Thus, neural plate is derived from ectoderm and forms in the central part of this upper layer. Remainder of the ectoderm surrounding the neural plate forms the epidermis.
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PRIMARY NUTRITION-
Lateral borders of neural plate elevate into neural folds which later fuse in the midline to form the neural tube . The open regions of the neural tube called the anterior (cranial) and posterior (caudal) neuropores close by the zippering process. This neurulation process is called primary neurulation which is responsible for establishing brain and spinal cord regions down to the sacral levels (probably up to S2). On neural tube closure, overlying non-neural epidermal cells form the ectodermal layer of the skin. Normal neural tube closure occurs by day 25 to 27. Meanwhile, the neural tube separates from the overlying ectoderm, a process called dysjunction.
The neuroepithelial cells (neuroblasts) around the inner neural tube form the mantle layer, which produces the spinal cord gray matter. The outermost layer forms the marginal layer, which subsequently myelinates to produce the spinal cord white matter. The central neuroepithelial cells differentiate into ependymal cells along the central canal. Neural crest cells along each side of the neural tube form the dorsal root ganglia, autonomic ganglia, Schwann cells, leptomeninges and adrenal medulla.
SECONDARY NUTRITION-
The cord caudal to S2 level is formed by this process. Totipotent mesodermal cells called caudal cell mass or tail bud coalesce to form neural tube which then epithelialize, reorganize around a lumen and finally become continuous with the cranial part of the tube initially formed by primary neurulation. Part of the caudal cell mass undergoes both regression and differentiation (a process called retrogressive differentiation) to form filum terminale, terminal ventricle, tip of conus medullaris, and most of the sacrum, coccyx and coccygeal medullary vestige. By the third gestational month, spinal cord extends the entire length of the developing spinal column. Rapid elongation of the vertebral column and dura relative to the cord produces the apparent ascent of the cord during the remainder of the gestation and the conus is at the adult level soon after birth.
Clinico-radiologically, spinal dysraphism is classified into two categories. The first category is spinal dysraphism with back mass that is not covered by skin, i.e. open dysraphism. The second is spinal dysraphism with skin-covered back mass, i.e. closed dysraphism, which can be further subcategorized on the basis of the presence or absence of a subcutaneous mass.
Anomalies of Gastrulation-
Failure of notochord formation causes complex dysraphic states such as caudal regression syndrome and segmental spinal dysgenesis. Incorrect notochordal induction leads to the incomplete splitting of the neural plate from the notochord, producing the split notochord syndromes (neurenteric cyst and diastematomyelia).
Failure of notochord formation causes complex dysraphic states such as caudal regression syndrome and segmental spinal dysgenesis. Incorrect notochordal induction leads to the incomplete splitting of the neural plate from the notochord, producing the split notochord syndromes (neurenteric cyst and diastematomyelia).
Anomalies of notochord formation
Caudal regression syndrome
Caudal regression syndrome (CRS) is a complex dysraphic state with aberrations in gastrulation, secondary as well as primary neurulation. Most cases are sporadic, although a dominantly inherited defect in the HLXB9 gene has been described. Mothers of 15–20% of these infants are diabetic, and the offspring of 1% diabetic mothers are afflicted. Associations with other caudal spinal segment anomalies such as vertebral segmentation and formation anomalies and split cord malformations are noted.
Two types are described. Type 1 features a foreshortened terminal vertebral column, high-lying wedge-shaped conus termination and more severe associated visceral and orthopedic anomalies. Type 2 is less severe and has a low-lying tethered spinal cord with milder associated malformations In general, the higher the cord termination, the more severe is the sacral anomalies. The most severe CRS presentations are lumbosacral agenesis in which the spine terminates at the lower thoracic level and there is severe sacral dysgenesis with fused lower extremities in a “mermaid” configuration (sirenomyelia)
Segmental spinal dysgenesis (SSD) is a very rare dysraphic anomaly characterized by segmental thoracolumbar or lumbar vertebral and spinal cord dysgenesis or agenesis. Congenital thoracic or lumbar kyphosis is characteristic, with a palpable dorsal bone spur located at the gibbous apex. The upper spinal cord is normal, however, the cord segment below the dysgenetic segment is bulky, thickened and low-lying. The spinal canal proximal and distal to the dysgenetic level is of normal caliberSegmental spinal dysgenesis (SSD) is a very rare dysraphic anomaly characterized by segmental thoracolumbar or lumbar vertebral and spinal cord dysgenesis or agenesis. Congenital thoracic or lumbar kyphosis is characteristic, with a palpable dorsal bone spur located at the gibbous apex. The upper spinal cord is normal, however, the cord segment below the dysgenetic segment is bulky, thickened and low-lying. The spinal canal proximal and distal to the dysgenetic level is of normal caliberSegmental spinal dysgenesis (SSD) is a very rare dysraphic anomaly characterized by segmental thoracolumbar or lumbar vertebral and spinal cord dysgenesis or agenesis. Congenital thoracic or lumbar kyphosis is characteristic, with a palpable dorsal bone spur located at the gibbous apex.
Neurenteric cyst and dorsal-enteric spinal anomalies
Neurenteric cyst (NEC) is a complex dysraphic state and consists of an intraspinal cyst lined by enteric mucosa. It is most common in thoracic spine followed by cervical spine. They arise from an abnormal connection between primitive endoderm and ectoderm that persists beyond the third embryonic week. Normally, the notochord separates ventral endoderm (foregut) and dorsal ectoderm (skin, spinal cord) during embryogenesis, in an NEC, a separation failure “splits” the notochord and hinders the development of mesoderm, which traps a small piece of primitive gut within the developing spinal canal. This gut remnant may become isolated forming a cyst or it may maintain connections with gut or skin (or both); this produces the spectrum of fistulas and sinuses that constitute the spectrum of dorsal-enteric spinal anomalies.
The upper spinal cord is normal, however, the cord segment below the dysgenetic segment is bulky, thickened and low-lying. The spinal canal proximal and distal to the dysgenetic level is of normal caliber.
Abnormalities of Primary Neurulation
Premature dysjunction
If dysjunction occurs prematurely, perineural mesenchyme is interposed between neural tube and ectoderm, which may differentiate into fat and prevent complete neural tube closure. It leads to the lipomatous malformation spectrum of lipomyelocele, lipomyelomeningocele and spinal lipomas.
Lipomyelocele (LMC), b) lipomyelomeningocele (LMMC)
The main differentiating feature between a LMC and LMMC is the position of the placode–lipoma interface.With an LMC, the placode–lipoma interface lies within the spinal canal. With an LMMC, the placode–lipoma interface lies outside of the spinal canal due to expansion of the sub-arachnoid space and In both cases, syringomyelia is a commonly associated finding. LMC and LMMC account for 20–56% of occult spinal dysraphism and 20% of skin-covered lumbosacral masses. An important imaging point is that the neural placode is frequently rotated; this foreshortens the roots on one side, predisposing them to stretch injury, and lengthens the roots on the other side, rotating them into the surgeon’s field of view and making them more prone to injury.
The spinal lipoma
The spinal lipoma is a simple dysraphic state and is subdivided into intradural and terminal (filar) lipomas. An intradural lipoma refers to a lipoma located along the dorsal midline that is contained within the dural sac. No open spinal dysraphism is present. They are most commonly lumbosacral in location. Fibrolipomatous thickening of the filum terminale is referred to as a filar lipoma. Filar lipomas can be considered a normal variant if there is no clinical evidence of tethered-cord syndrom.
Nondysjunction
Nondysjunction results from failure of dissociation of neural tube from adjacent cutaneous tissue. If dysjunction fails to occur, an ectodermal–neuroectodermal tract forms that prevents mesenchymal migration. Nondysjunction results in open neural tube defect spectrum of dorsal dermal sinus, myelomeningocele, and myeloceles.
Dorsal dermal sinus
The simplest of these is the dorsal dermal sinus connecting skin dimple to the dural sac, conus, or central spinal cord canal. The most common dermal sinus tract (DST) location is in the lumbosacral spine, followed by the occiput. In all dermal sinus cases, there is some degree of focal dysraphism, which may be as subtle as a bifid spinous process. The true congenital dorsal DST usually has an atypical dimple at the ostium that is large (>5 mm), often asymmetric, and remote (>2.5 cm) from the anus A. These features help distinguish the dermal sinus from its clinically asymptomatic mimic, simple coccygeal dimple. The sinus tract/cord is epithelial-cell lined and may or may not be canalized. When patent, it exposes the patient to an elevated risk of meningitis. It is critical to look for this anomaly in all patients with atypical skin dimples, cutaneous back lesions or lipomas. Moreover, 30–50% of DSTs may have an associated dermoid or epidermoid cyst.
Myelomeningocele and myelocele
Myelomeningoceles and myeloceles are caused by defective closure of the primary neural tube and are clinically characterized by exposure of the neural placode through a midline skin defect on the back, and hence, classified under open dysraphic states. Myelomeningoceles account for more than 98% of open spinal dysraphisms. Myeloceles are rare. It is important to note that preoperative imaging of myelomeningocele is usually not done because of the risk of infection. Nevertheless, the main differentiating imaging feature between a myelomeningocele and myelocele is the position of the neural placode relative to the skin surface. The neural placode protrudes above the skin surface with a myelomeningocele and is flush with the skin surface with a myelocele. Myelomeningocele is almost always seen in the context of a Chiari 2 malformation.
Combined Anomalies of Gastrulation and Primary Neurulation
Hemimyelomeningocele and hemimyelocele
Hemimyelomeningoceles and hemimyeloceles can also occur but are extremely rare. These conditions occur when a myelomeningocele or myelocele is associated with diastematomyelia (cord splitting) and one hemicord fails to neurulate.
Anomalies of Secondary Neurulation/anomalies of the Caudal Cell Mass
Failure of expected secondary neurulation leads to conditions such as abnormally long spinal cord, tethered cord syndrome, persisting terminal ventricle, terminal myelocystocele, lipoma of filum terminale and intrasacral – anterior sacral meningocele. It is also implicated in pathogenesis of caudal regression syndrome and segmental spinal dysgenesis.
Persistent terminal ventricle/fifth ventricle
By day 48, a transient ventriculus terminalis appears in the future conus. According to Coleman et al., evidence of a fifth ventricle not accompanied by other pathologies is a frequent finding that does not have pathological significance during the first 5 years of life.
Key imaging features include location immediately above filum terminale and lack of contrast enhancement, which differentiates this entity from other cystic lesions of the conus medullaris.
Tethered cord syndrome
Tethered cord syndrome (TCS) patients most likely present during periods of rapid somatic growth. It manifests clinically as gait spasticity, low back and leg pain that is worse in the morning, lower extremity sensory abnormalities, and/or bladder difficulties. On imaging, TCS strictly refers to patients with a low-lying cord and thickened filum [>1.5 mm] .
Intrasacral – anterior sacral meningocele
The term “intrasacral meningocele” is used to denote a sac lined by arachnoid which lies within an enlarged sacral spinal canal and is attached to the caudal termination of the dural sac by a pedicle that usually permits cerebrospinal fluid (CSF) flow from the tip of the subarachnoid space into the meningocele. Consistent with the possible congenital origin, intrasacral meningocele may occur in association with other anomalies such as sacral vertebral anomalies, diastematomyelia or TCS
Anterior meningoceles are usually presacral in location. It has a large anterior meningocele outpouching that traverses an enlarged sacral foramen and produces a presacral cystic mass. Most ASMs are sporadic but a minority show an inherited predisposition within the Currarino triad or in syndromes that feature dural dysplasia, such as neurofibromatosis type 1 (NF1) and Marfan syndrome.
Terminal myelocystocele
Herniation of a large terminal syrinx (syringocele) into a posterior meningocele through a posterior spinal defect is referred to as a terminal myelocystocele. 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.
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