eMedicine Specialties > Radiology > Pediatrics
Spinal Dysraphism/Myelomeningocele
Updated: Jan 14, 2009
Introduction
Myelograms in a 5-year-old patient show the dorsal region of the spine and an anterior thoracic meningocele. Note the gross dorsal kyphosis.
T2-weighted sagittal MRIs of the sacrum show an anterior sacral meningocele.
Myelograms in a 4-year-old patient show the lumbosacral region; a long, tethered cord; and diastematomyelia.
Background
Spinal dysraphism, or neural tube defect (NTD), is a broad term encompassing a heterogeneous group of congenital spinal anomalies that result from defective closure of the neural tube early in fetal life and anomalous development of the caudal cell mass.1,2
Some forms of spinal dysraphism may cause progressive neurologic deterioration. The anatomic features common to the entire group is an anomaly in the midline structures of the back, especially the absence of some of the neural arches, and defects of the skin, filum terminale, nerves, and spinal cord.
Spinal dysraphism occurs in closed forms and open forms. Open forms include myelocele, meningocele, and myelomeningocele. These open forms are often associated with hydrocephalus and Arnold-Chiari malformation type II and may be classified as spina bifida aperta. In 1886, Von Recklinghausen gave a detailed account of spina bifida cystica (aperta). Spina bifida is described in literature of the Middle Ages, although the condition was recognized even earlier. Indeed, the mythologic figure of the satyr may have its origins in the association of foot deformities with lumbar or lumbosacral hypertrichosis.
The closed form of spina bifida is termed spina bifida occulta. Of the general population, 5-10% may have bony spina bifida occulta with intact overlying skin. Most of these cases are found incidentally.
Open NTD (ONTD) represents a serious congenital anomaly. If the neural tube fails to fuse at the skull, the result may be that of anencephalus or encephalocele. Open NTDs, such as meningomyelocele, occurs when the tube fails to fuse along the spine. Infants with NTDs frequently have additional serious neurologic, musculoskeletal, genitourinary, and bowel anomalies.
Spina bifida occulta is characterized by the variable absence of several neural arches and various cutaneous abnormalities, such as lipoma, hemangioma, cutis aplasia, dermal sinus, and hairy patch; it is often associated with a low-lying conus and other spinal cord anomalies. In infants, whenever the conus lies below the L2-3 interspace, cord tethering should be considered. The term tethered cord implies that the cord may be attached to the vertebral column or subcutaneous tissues by a thickened filum terminale, fibrous band, dermal sinus tract, diastematomyelia, or a lipoma (lipomyelomeningocele). Patients with spina bifida occulta may present with scoliosis in later years.
Approximately 95% of couples that have a fetus affected with ONTD have a negative family history for ONTDs. Most ONTDs are caused by factors relating to multifactorial inheritance, including genetic and environmental factors.
Related eMedicine topics:Myelomeningocele
Spina Bifida
Pathophysiology
The defect of spinal dysraphism occurs in the first 8.5 weeks of fetal life. The neural tube develops from ectodermal cells, whereas mesoderm forms the bony elements, meninges, and muscle. The skin is separated from the neural tube by the mesoderm. Incomplete separation of ectoderm from the neural tube results in cord tethering, diastematomyelia, or a dermal sinus. Premature separation of the cutaneous ectoderm from the neural tube results in incorporation of mesenchymal elements between the neural tube and skin, which may result in the development of lipomas. If the neural tube fails to fuse in the midline, posterior spinal abnormalities, such as myelomeningoceles, occur.3,4
The disease process may be classified in terms of the deformity of the spine or cord. The widely accepted classification is based on the type of spinal defect. Spina bifida occulta consists of a defect in the vertebral arches not associated with an externally visible sac on the back. These lesions differ from spina bifida cystica, in which the vertebral defect is associated with a cystic mass on the back.
Spina bifida cystica (aperta) is further subdivided into the following groups: (1) a meningocele without cord tissue within the sac; (2) myelomeningocele in which spinal neural tissue forms part of the sac; and (3) rachischisis, which is the most severe form and involves a widely patent dorsal opening of the spine, with or without residual cord tissue. Rachischisis is usually associated with anencephaly. Anomalies of the cord may be classified as amyelia, nonfusion of the dorsal cord, diastematomyelia, hydromyelia, tethered cord, overgrowth of the cord, or nonspecific dysplasia. With either type, confusion may arise if emphasis is given to the mesodermal or neuroectodermal lesions without regard to their mutual relationship.5,6,7
Myelomeningocele open neural tube defect (ONTD) is teratogenic process that results in a failed closure and abnormal differentiation of the embryonic neural tube during the first 4 weeks of gestation. Abnormal development of the posterior caudal neural tube produces a myelodysplasia. The defect may occur anywhere from the base of the skull to sacrum. The anatomic level of the spinal cord lesion roughly correlates with the patient's neurologic deficit.
Developmental abnormality of the cephalic anterior tube gives rise to several CNS anomalies, including Arnold-Chiari type II malformation, characterized by cerebellar hypoplasia and varying degrees of caudal displacement of the hindbrain into the upper cervical canal through a widened foramen magnum. Arnold-Chiari type II malformation may impede CSF flow and cause hydrocephalus; it occurs in more than 90% of infants with myelomeningocele (particularly after treatment of the myelomeningocele). Cerebral cortex dysplasia, including cerebral heterotopias, polymicrogyria, abnormal lamination, fused thalami, and corpus callosum abnormalities, are other anomalies often associated with Arnold-Chiari type II malformations.
Myelomeningocele often occurs with multisystemic congenital anomalies. Mesodermal elements surrounding the neural tube, such as the vertebra and ribs, also may be malformed, leading to congenital or early-onset kyphosis and scoliosis (but only if the myelomeningocele is crossing the thoracolumbar junction). Other anomalies associated with myelomeningoceles are facial clefts, congenital heart disease, and genitourinary tract anomalies. Anomalies of the urinary tract, such as a solitary kidney or ureteric abnormalities, may further contribute to increased morbidity in the presence of neurogenic bladder dysfunction.
In a myelocele, a midline plaque of exposed neural tissue is flush with the skin surface. This reflects a failure of dysjunction. Myelomeningoceles (meningomyeloceles) represent a similar herniation of spinal contents through a bone defect in the posterior spinal arches, except that the neural herniation is elevated above the skin surface by a posterior expansion of the subarachnoid space. In both conditions, the cord is tethered low. An associated Chiari II malformation is present.
Posterior meningoceles, lipomyelomeningoceles, and myelocystocele are closed defects; they may occur with a skin-covered back mass. A posterior meningocele is a skin-covered, CSF-filled mass that is continuous with the CSF in the spinal canal; it is associated with a tethered cord or hydromyelia. A myelocystocele is a herniated, skin-covered, cystic dilatation of the central canal of the terminal portion of a low-lying cord. A lipomyelocele/lipomyelomeningocele occurs when a lipoma extends from the subcutaneous tissues to the dorsal aspect of the cord, tethering the cord inferiorly. This process reflects a premature separation of the cutaneous ectoderm during the process of neurulation that allows mesenchyme to enter the unclosed neural tube and differentiate into fat.
A dorsal dermal sinus is a long, thin, epithelium-lined sinus that extends from the skin surface for a variable distance. This sinus may become symptomatic as a result of infection if it has a connection within the theca; symptoms may also occur as a result of compression of neural tissue if the sinus is associated with a dermoid/epidermoid. An infected dermal sinus may cause a localized abscess or meningitis. Microscopic examination of a dermoid sinus tract shows a single channel or multiple channels lined with cornifying squamous epithelium. The lumen may contain desquamated keratin and hair. Often, the surrounding connective tissue shows follicles and skin appendages.
An intradural lipoma has no anatomic connection with subcutaneous fat; in embryologic terms, it lies wholly within the dural space. In this respect, it is similar to a lipomyelomeningocele, except that the neural tube closes after the mesenchyme has entered.
Abnormal splitting of the notochord usually occurs in association with a persistent tract between the gut and the dorsal skin. The intervening tract may become obliterated at any point, creating an enteric cyst, diverticulum, fistula, sinus, neurenteric cyst, or enteric duplication cyst.
Diastematomyelia refers to a complete or partial clefting of the spinal cord in a sagittal plane into 2 symmetric or asymmetric hemicords. The cord usually reunites more distally. Each of the hemicords has its own central canal and dorsal and ventral nerve roots. They are classified into 2 types: In type A (representing 50-60% of cases), the hemicords are contained within a single dural and arachnoid lining without a dividing spur. In type B, each hemicord has its own dural and arachnoid sac; a spur or septum separates the 2. The septum may be bony, cartilaginous, or fibrous. The most common site for this anomaly is the thoracolumbar region. Associated cutaneous stigmata at the site of the anomaly, cord tethering, and syringohydromyelia are common; for patients with diastematomyelia, a search should be made for these conditions.
A tethered cord (filum terminal syndrome) was first described by Garceau in 1953. Tethering implies that the anchoring lesion is formed by dense fibrous connective tissue. Normal spinal nerve roots do not anchor the cord, but the dentate ligaments do. Tethered cord is associated with low conus below the mid L2 level and a short, thick filum. Garceau's concept that anchoring of the cord by the filum causes Chiari malformation is open to criticism, but his observations on the clinical manifestations of tethering were confirmed in many subsequent reports.
In filum terminal syndrome, a thickened filum is attached to the dura, or an extradural band may anchor the cord. The filum usually retracts dorsally upon transection. In some patients, the cord is bound down by lipomatous tumors or fibrous tissue, the sacral roots ascending. Lipomas may attach dorsally to the conus and be sessile or pedunculated, whereas other lipomas occupy the distal end of the conus, elongating the latter and terminating in a small lipoma with attached nerve roots. The dorsal and caudal lipomas may occur in combination. Filar lipomas occupy an enlarged filum terminale.
Lipomyelomeningoceles are more extensive lesions; they represent a combination of a splayed cord fused with a lipomatous mass, which in turn fuses with the subcutaneous adipose tissue. The neurologic deficits of cord tethering are probably caused by impaired circulation in the stretched cord, as evidenced by a reduction in oxidative metabolism or by abnormal development resulting in neurologic deficits, as in myelomeningocele.
Hydromyelia refers to an abnormal dilatation of the central canal, which is lined by ependyma; it usually occurs in the lumbosacral region. Myelomeningocele or diastematomyelia is commonly associated with this condition. The central canal dilatation varies from oval to irregular or slit-shaped; it projects either bilaterally or dorsally. A hydromyelia must be differentiated from a syringomyelia, which is a fluid-filled cavity within the spinal cord; syringomyelias are not lined by ependyma. Syringomyelia is rare in children; it is usually related to trauma, inflammation, or tumor.
A myelocystocele is a variant of hydromyelic dilatation of the central canal, the cystic cavity being within the cord and the spinal roots originating at the ventral and dorsal outer surface of the cyst wall. A myelocystocele is often associated with defects of the vertebral bodies or with an intestinal fistula. These lesions are often located in the cervical or upper thoracic cord at the level the underlying bony defect. Heterotopic cerebellar tissue has been described within the myelocystocele.
Multiple factors are implicated in the etiology of a myelomeningocele, including genetic, racial, and environmental factors. Exposure to a risk factor may alter hyaluronate metabolism, which results in the failure of vertebral architecture to develop normally. Most infants with myelomeningocele are born to mothers with no previously affected children. Other offspring in a family with one affected child are at increased risk of NTD, as compared with children who do not have a sibling affected by NTD.
The risks of the occurrence of an open neural tube defect (ONTD) in various familial situations have been determined as follows:
- One previous child with ONTD — 2%
- Two children with ONTD — 5%
- Three children with ONTD — 10%
- Patient and child with ONTD — 6.5%
- Patient's sibling with ONTD — 2%
- Maternal aunt with ONTD — 1%
- Parents who have a first- or second-degree relative with an ONTD — 0.3%
- One child with multiple vertebral or spinal dysraphism — 2%
- Maternal age older than 35 years — 0.33%
- Maternal age younger than 35 years — 0.14%
The risk is 1 in 20-30 for subsequent pregnancies. If 2 children are affected, the risk becomes 1 in 2. As many as 10% of fetuses with an NTD detected during early gestation have an associated chromosome abnormality. Associated chromosome abnormalities include trisomies 13 and 18, triploidy, and single gene mutations (13%).
Maternal risk factors include insulin-dependent diabetes mellitus and hypothermia. Intrauterine drug exposures to valproate, carbamazepine, and drugs to induce ovulation have been identified as risk factors (1.5%).
The type of neural tube defect influences the associated urologic pathology and the urologic prognosis. For patients with caudal regression syndrome, the incidence of renal agenesis and vesico-ureteric reflux is high. The risk of renal damage and, in patients with neuropathic bladder, of urinary incontinence is similar to that for patients with meningomyelocele.8
Studies in the 1980s showed that correction of folic acid deficiency is an effective means of primary and recurrent prevention. In the early 1990s, studies showed that 50% of NTDs are related to a nutritional deficiency of folic acid; thus, they are preventable.
Frequency
United States
Statistics on the frequency of spina bifida occulta vary; as many as 2-24% of the population is affected. This variation appears to be age related. Spina bifida occulta of the first sacral vertebral body is found in 51.6% of children 7-8 years of age and in 26% of adults. The fifth lumbar vertebral body is affected in 16.1% of children 7-8 years of age and in 2.2% of adults.
In 1972, James and Lassman examined routine radiographs of 1172 consecutive autopsies and found the incidence of spina bifida occulta in adults to be 5%.
In the United States, the incidence of neural tube defects (NTDs) declined 50% between 1970 and 1989 (from 1.3 cases per 1000 live births to 0.6 cases per 1000 live births). In the same period, the proportion of spina bifida cases increased relative to anencephaly.
The incidence of spinal dysraphism is 1-2 cases per 1000 live births.
An open neural tube defect (ONTD) is the most common major birth defect. The incidence at birth of ONTD was reported to be 4.4-4.6 cases per 10,000 live births in 1983-1990. The highest rates of ONTD in the United States have been reported in Appalachia. The incidence is higher in the eastern United States than on the West Coast.
International
Neural tube defects (NTDs) are among the most common birth defects; there are substantial racial and geographic variations. The incidence rates of open NTD (ONTD) vary widely, not only among countries but also among regions within countries. Rates are significantly higher in areas of low socioeconomic status. Over the past 50 years, epidemics of ONTDs have been reported in Boston, MA; Rochester, NY; Dublin, Ireland; the Republic of China; and Jamaica.
On the basis of data reported in the 1970s and 1980s, the disease is 2.5 times more frequent in whites than blacks; it is particularly common in Belfast, Liverpool, and Dublin; and it is uncommon in Japan. The incidence is higher in Great Britain than in Japan. The incidence in the United Kingdom and in mainland Europe is decreasing.
Currently, the highest reported incidence is in Northern China (3.7 cases per 1000 live births). The incidence of NTDs is also relatively high in Indian and Eastern Mediterranean populations (with the exception of Israeli Jews). However, unlike the Western white populations, in these populations, anencephaly occurs more frequently than spina bifida.
Mortality/Morbidity
Spina bifida occulta
Developmental spinal defects in the lumbosacral region are a major source of disability among children and adults. In cases of spina bifida occulta, overlying, intact skin masks the underlying abnormality of the lower neural axis, and the cutaneous signs of spinal dysraphism may be subtle. Early recognition is important because symptoms may not be obvious until later in childhood or early adult life, by which time an irreversible neurologic deficit may already have occurred. Because the disease is progressive, prophylactic surgery is indicated in most cases.
In infants with established spinal dysraphism, urodynamic studies in infants with established spinal dysraphism may be normal, unlike urodynamic findings in older children. This underscores the importance of early diagnosis, in order that appropriate surgery may be offered to the patient. In adults, the most common warning sign of deterioration is the appearance and exponential increase of pain, particularly in the lower back and legs, and the further tightening of feet and leg tendons.
Over 90% of patients experience pain relief after surgery, but few patients experience an improvement in bladder control. Although some clinicians advocate that detethering surgery be performed as soon as possible so as to maintain continence, this approach remains controversial. If the dysraphism and associated cord tethering is neglected, the risk of urologic and neurologic damage may be irreversible and permanent.
Myelomeningocele
In Europe and the United States, myelomeningocele is the leading identified cause of infant death from congenital birth defects. Mortality rates of 90-100% have been reported in patients whose myelomeningoceles go untreated. Most affected infants die within the first year of life if untreated. Hydrocephalus or meningitis is a leading cause of death in untreated infants in the first 2 years of life. There is only a 28% likelihood that a 2-month-old infant with untreated myelomeningocele will live to be 7 years of age.
Recent advances in clinical approaches have led to a dramatic improvement in survival rates for infants with myelomeningocele, primarily as a result of effective antibiotics and the development of neurosurgical techniques for the management of hydrocephalus. In Europe and the United States, antibiotics, sac closure, and ventriculoperitoneal shunt placement are instituted in 93-95% of affected infants during the perinatal period. Early death in both treated and untreated patients occurs as a result of advanced hydrocephalus and associated congenital anomalies.
Neurologic consequences of myelodysplasia include paraplegia and functional bladder and bowel problems, which may be present at birth. A variety of chromosomal and nonchromosomal anomalies associated with myelomeningoceles may cause further neurologic deficits that appear acutely or chronically at birth or later. These deficits may be related to hydrocephalus, Arnold-Chiari II malformation, or a variety of other intraspinal anomalies.
Seizures occur in 10-30% of affected children and adolescents. Seizures may be related to brain malformation, shunt malfunction, or infection. Renal compromise is common; it is a leading cause of death after the first year of life. Death occurs as a consequence of a neurogenic bladder. Neurologic deficits may cause musculoskeletal disease, resulting in progressive bony and joint deformities, pathologic fractures, and muscle atrophy.
Difficult childbirth may occur with subsequent traumatic birth, leading to complications such as cerebral palsy from ischemic brain injury; permanent disability, with weakness or paralysis of the legs; loss of bowel or bladder control; frequent urinary tract infections; and meningitis.
At least 75% of children born with a myelomeningocele may be expected to reach early adult life. Late deterioration is common. One of the greatest challenges in medicine today is establishing a network of care for adults with spina bifida.9
Complications may follow surgery for NTDs; they include wound infection, CNS infection, delayed wound healing, CSF leakage, additional damage to the cauda equina, and acute hydrocephalus. Long-term complications include cord tethering and progressive hydrocephalus.
Approximately 25-35% or more of all children with myelomeningocele have hydrocephalus at birth. Furthermore, an additional 60-70% of patients with myelomeningocele develop hydrocephalus after the spinal defect is closed. Hydrocephalus may cause a reduction in cerebral cortical mantle; such a development is associated with a considerable decline in intellect.
Complications from shunt surgery for hydrocephalus include intracerebral hemorrhage, intraventricular hemorrhage, or both; bowel perforation; and infection. Long-term complications include infection, overdrainage or underdrainage, and obstruction of the shunt system. A significant incidence of complications related to the use of CSF diversionary shunts has been reported; these complications include mechanical failure, occlusion, fracture, and the development of loculated CSF collections around the ventricular or distal limbs of the shunt tube. A fibrous sheath may encase the catheter tip, or the catheter may migrate out of the peritoneal cavity.
Secondary tethering of the spinal cord is seen almost universally in patients with myelomeningocele, although the incidence of secondary tethered cord syndrome is relatively low.10
Derotation osteotomy with plate fixation is performed for internal or external tibial torsion that is commonly associated with myelomeningoceles. However, postoperative complications, such as nonunion, delayed union, and infection, are common. Patients are usually immobilized for 6-8 weeks after surgery.
Deformities of the foot and ankle may cause skin ulceration; such deformities may prevent the patient from wearing shoes and compromise ambulation.
Regarding open spina bifida, 13% of cases are associated with aneuploid fetuses. An associated anatomic abnormality has been found in 40%, though in 1 study, only 22% of these abnormalities were detectable on sonograms. If the spina bifida is an isolated abnormality on sonograms, a 4% risk of aneuploidy persists. Prenatal sonography may help in identifying karyotypically abnormal fetuses with spina bifida. However, 20% of cases are missed if sonography is used alone in the setting of a prenatally detected spina bifida. Some authors believe that the use of cytogenetic analysis is justified in these cases.
Race
In studies that predate the introduction of prenatal screening and prophylactic use of folic acid, the incidences at birth of both spina bifida and anencephaly were reported to be higher in the European white population than in the black population.
- In North America, the risk of neural tube defects (NTDs) is 3-fold higher in Hispanics than in non-Hispanics. Migration patterns in the white population showed a prevalence of NTDs that corresponded more closely to the risk of the place to which they had migrated, as opposed to the place of their origin. Similar studies in offspring of the black and Asian migrants in Europe and North America showed prevalences not substantially higher than those of their parent countries.
- In the United States, data from state and national surveillance systems from 1983-1990 showed rates to be highest for Hispanics and lowest for Asians/Pacific Islanders. The incidence is 0.15% in the white population and 0.04% in the black population; a higher proportion of whites than blacks have thoracic-level malformations.
- These variations are consistent with the theory that NTDs are a phenotypically heterogeneous group of malformations with multifactorial inheritance in some cases and single gene defects in others.
Sex
Anencephaly has a female preponderance; this is especially the case in premature births, in which the female-to-male ratio of anencephaly is 3:1. The birth prevalence rate of myelomeningocele is slightly higher in girls than boys (with a ratio of 1.2:1), as reflected in data from both state and national surveillance systems for the period 1983-1990.
- A higher proportion of girls than boys have malformations at the thoracic level. The predilection for girls increases with the severity of the lesion.
- Female excess is greater with stillbirths than with live births. Female excess occurs entirely with myelomeningoceles but not with meningoceles.
- Other neural tube defects (NTDs) above the thoracolumbar junction show a mild female preponderance. No such sex difference has been noted in more distal forms of spina bifida.
Age
Spinal dysraphism is a congenital anomaly. The most severe form of the disease, myelomeningocele, is present at birth. Spina bifida occulta may not be obvious until later in childhood or early adult life.
Anatomy
An understanding of the normal developmental anatomy is essential in the interpretation of imaging of congenital spinal anomalies. Although the exact embryology of spinal dysraphic lesions is not known, dysraphism most likely originates early in embryogenesis, beginning during the third and fourth weeks of gestation.
Two processes form the CNS. The first is primary neurulation; in primary neurulation, the neural structures form a tube, from which is formed the brain and spinal cord. Secondary neurulation refers to the formation of the lower spinal cord; secondary neurulation gives rise to the lumbar and sacral elements. During this process, the notochord induces formation of the neural plate from the surface ectoderm of the embryonic disk. The neural plate forms in the midline on the dorsal aspect of the embryonic disk.
At about the fourth gestational week, the neural plate invaginates along a midline longitudinal axis to form the neural groove. The thicker neural folds form on either side of the neural groove. As invagination progresses, the neural folds come together in the midline and fuse, forming the neural tube.
Any disruption occurring during gestational days 17-23, when the neural plate begins its first fold and fuses to form the neural tube, may cause craniorachischisis, the most severe form of neural tube defect (NTD). Closure of the rostral neuropore occurs during gestational days 23-26. Failure at this point results in anencephaly. Closure of the caudal neuropore occurs during days 26-30; disruption during this period causes myelomeningocele. Beyond day 26, a disruption is unlikely to cause an NTD such as myelomeningocele.
Studies of mice embryos have shed light on the pathogenesis of associated anomalies seen with NTDs, such as hydrocephalus and the hindbrain Chiari II malformation. In 1992, McLone and Naidich postulated that both the hindbrain and the spinal cord anomalies are initially related to the failure of the neural folds to completely close, leaving a dorsal defect or myeloschisis. This permits the CSF to leak from the ventricles through the central canal into the amniotic fluid and causes a collapse of the primitive ventricular system.
Leakage of CSF from the primitive ventricular system produces both downward and upward herniation of the small cerebellum; this in turn prevents the ventricles from increasing in size and volume. In addition, the posterior fossa does not develop to its full size, and the neuroblasts do not migrate outward at a normal rate from the ventricles into the cortex. thus, all of the anomalies of the posterior fossa are related to a single initial inciting event. The process of disjunction involves the separation of surface ectoderm from the lateral margins of the neuroectoderm. This process occurs simultaneously with neurulation. After separation, the surface ectoderm grows across the midline, fuses, and eventually forms skin, which covers the dorsal spine.The filum terminale, conus medullaris, and distal central canal develop through the process of retrogressive differentiation. This involves the formation of the distal neural tube through canalization; much of the neural tube thus formed subsequently involutes and forms the filum terminale. The remaining portion forms the conus medullaris and distal central canal. Skin-covered spinal defects, spinal lipomas, and a dorsal dermal sinus originate in a failure of neurulation, as well as in disjunction.
Premature disjunction before tube closure allows primitive mesenchyme to migrate into the lumen of the developing neural tube, giving rise to lipomatous tissue that prevents tube closure. Failure of disjunction is thought to be the cause of the formation of dorsal dermal sinus. The genesis of diastematomyelia, myelocystocele, thickened filum, and a simple meningocele is not understood. Thickened filum, a simple meningocele, terminal myelocystoceles, and lipomas of the filum most probably result from abnormalities of differentiation of the caudal cell mass.
Presentation
Occult spinal dysraphism and tethered cord
Bony spina bifida occulta at the L5-S1 level is a common incidental finding on radiographs, both in children and adults; it is usually not associated with symptoms or signs. If no signs are present, no further imaging workup is required.
The signs and symptoms of occult spinal dysraphism with a tethered cord are not usually obvious at birth, except in cases of lipomyelomeningocele. Newborns rarely exhibit neurologic deficits, but as the child grows, the consequences of the dysraphism may become apparent.
For babies, whether tethered cords should be released before they become symptomatic is controversial; babies and younger children are at a greater risk of recurrent tethering. Therefore, surgery is tailored to individual circumstances. The overall management of dysraphism may become an ongoing, lifelong process; treatment is personalized, depending on the type, severity, and progression of the disease.
Several cutaneous abnormalities indicative of underlying spinal dysraphism are situated in or near the midline, usually in the lumbosacral region:
- Dermal dimple
- Hairy patch of skin (hypertrichosis, faun's tail)
- Smaller patch of silky hair if the dysraphism is in the cervical or upper thoracic region
- Midline visible or palpable mass overlying the spine (lipoma)
- Dermal sinus
- Capillary hemangioma
- Rudimentary tail (caudal appendage)
- Atretic meningocele, which is seen as a central area of thin, pearly skin surrounded by a halo of red, pink or brown (This has been likened to cigarette-burn type of skin stain.)
In cases of spinal dysraphism associated with a tethered cord, symptoms may first manifest themselves or worsen as a result of trauma, disk herniation, or exaggerated movement of the spinal column. The last-named mechanism may stretch the cord or nerve roots emanating from the cord.
Signs and symptoms of occult spinal dysraphism may appear as the child grows, as more tension is put on the tethered spinal cord. Initially, there is no pain, although neurologic and musculoskeletal deficits may be evident; these include pes cavus and scoliosis. Eventually, however, neurologic symptoms, such as pain and paresthesia, may affect the legs. Subsequently, the patient may even develop neuropathic symptoms, such as numbness of the toes and feet. The pain is sciatica-like and is often unilateral, affecting the buttocks and the back of the thigh muscles.
Depending on the site and type of tethering, neurologic deficits may also affect bladder and bowel function. In a baby, bowel and bladder dysfunction may first manifest itself as an increase in the period of toilet training and as difficulty in keeping the baby dry. Incomplete bladder voiding may lead to recurrent urinary tract infections.
Orthopedic problems include new or progressive scoliosis or kyphosis; unequal size of feet and legs; numbness and paresthesia of the legs and feet; unilateral or bilateral cavovarus deformity; claw toes; rigidity of the ankles, in association with an absence of reflexes; pes cavus; and gait disturbances. Muscle weakness and gait disturbance usually become obvious at approximately 2 years of age, which is the age a child begins to walk.
Myelomeningocele
With routine antenatal ultrasonography, most myelomeningoceles are diagnosed in utero. At birth, a midline defect of the posterior elements of the vertebrae, in association with a protrusion of the meninges and cord elements through an external dural sac, is noted.
Paraplegia from spinal cord malformation is the most common manifestation of a myelomeningocele. The neurosegmental anatomic level of the lesion is tested to assess for neurologic deficit. However, the functional motor level does not always correspond to the anatomic level of the lesion. Serial neurologic assessment of the level of the lesion allows for the early detection of a progressive neurologic deficit.
Thoracic myelomeningocele (25%) results in variable innervation of the musculature of the neck and upper trunk; in most cases, no voluntary movements in the lower limbs are present. In patients with thoracic myelomeningocele, CNS involvement and associated cognitive deficits tend to be more severe.
Lumbosacral myelomeningoceles (75%) are classified as high lumbar, low lumbar, and sacral. In patients with high-lumbar myelomeningocele, hip flexor and hip adductor movement is retained, but movements of hip extensor and abductor muscles and all knee and ankle movements are absent. In patients with low-lumbar myelomeningocele, movement of hip flexor, adductor, medial hamstring, and quadriceps muscles is retained; variable movement is present in the lateral hamstrings, hip abductors, and ankle dorsiflexors. Plantar flexion is usually absent. In patients with sacral myelomeningocele, movement of the hips and knees is retained, but ankle plantar flexor movement is variable. Anesthesia is present in the perineal area.
The muscle tone in any of these groups is usually flaccid; however, as many as two thirds of patients have upper motor neuronal signs, and only 9% have a true spastic paraparesis. Impaired upper limb coordination is not uncommon, particularly in patients with associated hydrocephalus. Arnold-Chiari II malformation may contribute to impaired coordination of the upper limbs, motor-learning difficulty, delayed development of hand dominance, or a combination of these. Spinal and lower-limb deformities and joint contractures are commonly associated with myelomeningocele. In patients with Arnold-Chiari II malformation, signs of cranial nerve dysfunction, such as ocular muscle palsies, swallowing and eating difficulties, may be present; in addition, these patients may have problems with phonation.
Scoliosis associated with a myelomeningocele may be congenital or acquired. Approximately 50% of the children with a myelomeningocele have an acquired form of scoliosis related to muscle imbalances. Increased lumbar lordosis and kyphosis involving the entire spine or the lumbar spine alone is frequently seen. Spinal deformities are seen more frequently in patients with higher spinal myelomeningoceles.
During the first trimester or early second trimester, women may undergo triple screening for spina bifida, Down syndrome, and other congenital diseases in the fetus. Assessment of maternal serum alpha-fetoprotein (AFP) levels has been used since the late 1970s. AFP levels are elevated in 85% of cases of fetuses with spina bifida. Because false-positive results have many causes, a presumptive diagnosis made on the basis of maternal serum AFP levels should be confirmed with amniocentesis and assay of the amniotic fluid for AFP and for acetylcholinesterase (AChE), a neurospecific enzyme. Targeted prenatal sonography (level II sonography) is a reliable method of diagnosing spina bifida. A myelomeningocele may be detected in 99% of affected fetuses with the combined use of these tests.
Results of urinalysis, urine culture, and serum urea nitrogen and creatinine testing at birth provide baseline values for the evaluation of renal function. A baseline renal sonogram may also be taken at this stage. Regular urine cultures may be required for children who have vesicoureteral reflux or signs and symptoms of urinary tract infection. Urodynamic studies are helpful in the evaluation of urologic function because they help in characterizing abnormalities of detrusor control and sphincter function.
Other problems to be considered
Some reviews of spina bifida include lateral thoracic meningocele occurring in patients with neurofibromatosis or kyphoscoliosis. These are saccular meningeal herniations through the intervertebral foramina, extending anteriorly between the ribs into the thoracic cavity. Most of these lesions are located in the thorax; in exceptional cases, they occur in relation to the lumbar spine. These lateral intrathoracic meningoceles are more closely related to arachnoid cysts and bear little relationship to spinal dysraphic lesions.
Cranial dysraphism includes incomplete raphe closure resulting in cranial bifidum. The bony midline cranial defect allows formation of a cephalocele, which is the congenital herniation of intracranial contents in which the meninges or brain protrudes via a scalp defect. A cranial meningocele is a herniation of meninges and CSF. An encephalocele is a herniation of brain through a skull defect. In most cases, the calvarium and brain are complete, and so anencephaly is complete; in some cases, the calvarium is absent, and anencaphaly is incomplete.
Preferred Examination
Choosing the most appropriate modality for imaging congenital malformation of the spine involves considering many factors. Imaging of the bony spine requires methods different from those used to image the spinal canal and its contents. The age of the patient and the required plane of imaging influence the choice of modality. The best way to image skeletal anomalies is by means of plain radiography, possibly combined with conventional tomography, though this modality has now been more or less replaced by CT.
Plain images may suffice from the orthopedic point of view, but they provide little information of the associated malformations of the spinal cord and its coverings. When spinal malformations are suspected, investigation of the spinal canal and its contents are best performed by MRI.
Skeletal scintigraphy with technetium-99m diphosphonates has high sensitivity but low specificity. Bone scintigraphy is a useful procedure in children with backache of unknown origin. Minor vertebral anomalies may show increased radionuclide uptake because of abnormal stresses and reactive changes. Further imaging may be restricted when an abnormality is localized.
In cases of spinal dysraphism, MRI provides more information than myelography or CT in defining spinal cord anatomy. Sagittal MRIs are superior to CT myelograms in demonstrating the lipoma-cord interface. Individual nerve roots are less well seen with MRI.
In the evaluation of the spinal canal, ultrasonography is limited to the neonatal period, though a spinal defect covered with soft tissue may be imaged well into adult life. Fetal ultrasonography is increasingly used as a primary screening tool for NTDs, usually at about 18 weeks' gestational age. This trend reflects the increasing confidence in fetal ultrasonography. Ultrasonography helps in avoiding the calculated 1% risk of miscarriage associated with diagnostic amniocentesis.
In myelocele or myelomeningocele/meningomyelocele, detailed imaging before closure is usually not required, but after repair or spontaneous reepithelialization, imaging may be performed to depict associated pathology, such as diastematomyelia or lipoma, at other levels and to evaluate intracranial abnormalities.
Deterioration in neurologic function later in the disease course or after surgery suggests a complication; such deterioration is an indication for imaging to look for a surgically remediable cause of the deterioration, such as retethering of the cord as a result of adhesions or an increase in size of an inclusion lipoma, epidermoid, or hydromyelia. Normally, after repair, the cord is low; it should not be tethered unless scars extend into the closure site.
Limitations of Techniques
The amount of radiation involved in plain radiography and CT of the spine is particularly important in the examination of infants, children, and young, fertile women. Plain radiography, as well as CT of the lower spine, delivers a high dose to the gonads, particularly in female patients.
Ultrasonography remains operator dependent; accurate diagnosis depends on the skill and experience of the operator and on the quality of the equipment.
Transaxial CT images may be difficult to interpret because of the complex anatomy of the vertebral bodies, the presence of segmentation anomalies, and the presence of spinal curvature abnormalities. With modern CT scanners, this limitation may not be such a disadvantage, inasmuch as sagittal and coronal reconstruction now provide exquisite images of the spine.
In parts of the developing world, MRI is not readily available. In addition, use of MRI may not be possible in patients with claustrophobia, and it is contraindicated for some patients with implanted devices. Children may require sedation.
Differential Diagnoses
Other Problems to Be Considered
Lateral thoracic meningocele
Cephalocele
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Further Reading
Keywords
spinal dysraphism, myelomeningocele, neural tube defects, NTD, open neural tube defects, ONTD, myelocele, meningocele, myelomeningocele, spina bifida cystica, closed neural tube defects, spina bifida occulta, tethered cord, filum terminal syndrome, cord traction syndrome, diastematomyelia, diplomyelia
