Open spinal dysraphism: myelocele and myelomeningocele

Myelomeningocele and myelocele are the quintessential open spinal dysraphisms (open neural tube defects). In open dysraphism, neural tissue and meninges are exposed to the amniotic fluid before birth, allowing leakage of CSF and injury to the underlying neural tissue. This leads to intracranial abnormalities (the Chiari II malformation), including hindbrain herniation, hydrocephalus, loss of subarachnoid spaces, and tectal beaking. These patients have a poorer prognosis than those with closed dysraphism, with higher rates of lower extremity motor dysfunction as well as frequent bowel and bladder incontinence. The neurologic function worsens the higher and the larger the defect (8). The vast majority of these lesions are now diagnosed prenatally, with a combination of fetal ultrasound and MRI. In utero repair has recently become the standard of care and has been shown to significantly improve the hindbrain herniation, rate of ventricular shunting, and lower extremity function (9).

In the myelomeningocele, imaging findings include an expanded CSF space with the neural placode extending beyond the confines of the spinal canal through a posterior osseous dysraphic defect, the exposed neural placode and displaced meninges protruding posteriorly (Fig. 16.1).

In the less commonly encountered myelocele (also referred to as myeloschisis), imaging demonstrates a neural placode that extends to but not beyond the posterior dysraphic osseous defect, there is no expansion or prominent extraspinal protrusion of the subarachnoid CSF space, and the placode typically sits within the posterior osseous defect, subjacent to the cutaneous defect (Fig. 16.2).

Closed spinal dysraphism with a fatty subcutaneous mass: lipomyelocele and lipomyelomeningocele

Lipomyelocele and lipomyelomeningocele are closed, or skin-covered, dysraphic defects that typically present with a subcutaneous mass. Premature disjunction of neuroectoderm from cutaneous ectoderm occurs when the still incompletely tubulated neural tube separates from the adjacent cutaneous ectoderm. Mesoderm then has access through the point of disjunction to the internal surface of the unclosed neural tube and prevents tube closure. Eventually, the mesodermal elements form fat. Lipomatous hypertrophy of this subcutaneous fat can form a mass-like lipoma, which is the predominant component of the clinically evident subcutaneous mass. A posterior osseous defect is also present in the vertebrae of the affected level(s). Treatment is surgical, with many patients presenting outside of the neonatal period.

In the lipomyelocele, imaging shows a subcutaneous lipoma contacting the neural placode, which is within the confines of the spinal canal or at the level of the dysraphism (Fig. 16.3).

In the lipomyelomeningocele, the subarachnoid CSF space is expanded, often asymmetrically, and the neural placode and meninges can be identified with ultrasound and/or MRI as protruding through the posterior osseous defect beyond the confines of the spinal canal into the subcutaneous soft tissue, with the placode–lipoma interface lying in this subcutaneous region, often in an oblique plane (Fig. 16.4).

Closed spinal dysraphism with a nonfatty subcutaneous mass: meningocele and myelocystocele

In the closed dysraphic state of a meningocele, there is also a subcutaneous mass, but in this case, the mass is not formed by fat. Rather, the mass is formed by herniation of the dura, arachnoid, and a CSF-filled sac beyond the normal confines of the spinal canal through an osseous defect, most commonly posteriorly. The spinal cord does not herniate into this sac in a meningocele and should be seen with US or MRI as traversing and not entering the sac. There may be associated fatty infiltration of the filum terminale, making imaging of the distal spine important (4–7,10) (Fig. 16.5).

A myelocystocele is also a closed dysraphism that clinically presents with a subcutaneous mass. In this condition, however, the mass is formed by herniation of meninges and expanded CSF space as well as a portion of the spinal cord with a cyst-like dilation of the central canal of the spinal cord, often creating a “cyst within a cyst” appearance. Myelocystoceles can be terminal, involving the caudal end of the spinal cord and canal, or nonterminal, involving a more cranial aspect of the cord (4–7) (Figs. 16.6 and 16.7). Terminal myelocystoceles are frequently associated with the OEIS (omphalocele, exstrophy of the bladder, imperforate anus, spinal anomalies) complex.

Entities Not Necessarily Including a Dysraphism but Often Discussed with Closed Dysraphic Disorders

Caudal regression syndrome

Caudal regression syndrome (CRS) is a rare congenital malformation characterized by varying degrees of developmental failure, with roughly 15% of affected infants having diabetic mothers (11,12). It is believed that this syndrome results from disturbances of the caudal mesoderm, including the caudal cell mass and cloaca, very early in gestation. The syndrome is composed of a spectrum of anomalies, which includes absence of caudal vertebrae and spinal cord, and may include anorectal malformations, malformed external genitalia, bladder exstrophy, renal ectopia or aplasia, neural tube defects, and lower extremity malformations (12–14). Milder forms, such as isolated sacral hypogenesis, with or without anal atresia, are more common than the more severe forms (11).

Imaging findings with CRS will vary depending upon the severity of anomalies. Patients in the more severe group have a spine that ends at S2 or above and in which there is a blunted, “wedge-shaped,” or “hatchet-shaped” spinal cord seen on sagittal imaging, with a characteristic “double bundle” shape on axial imaging. Patients in the less severely affected group, where the spine ends at S3 or below, frequently have associated stretching of the conus medullaris and a tethered filum terminale (Fig. 16.8).

Segmental spinal dysgenesis

Segmental spinal dysgenesis (SSD) and CRS have been postulated to represent two facets of a spectrum of segmental malformations of the spine and spinal cord. A derangement of the caudal most mesodermal segment results in CRS, while a derangement of an intermediate segment results in SSD (15,16). Although in utero detection is possible, affected children typically are identified after birth as a result of a sharply angulated kyphotic spinal deformity, deformities of the lower limbs, hyperreflexia of the lower extremities, and/or bladder dysfunction (17–19).

Radiographs will characteristically show a moderate to severe kyphosis at the level of vertebral malformation, typically at the thoracolumbar junction with associated vertebral abnormalities (16). Three-dimensional CT reconstructions may be useful in characterizing underlying vertebral formation and segmentation anomalies (20). MRI typically demonstrates tapering of the cord to a point of marked narrowing or complete absence at the level of vertebral abnormalities. Below this point, the spinal cord may resume a normal appearance or may appear thickened (Fig. 16.9). There may be associated lipomas, dermal sinuses, or hydromyelia (16).

Dorsal dermal sinus

The dorsal dermal sinus is an epithelial-lined tract running from the skin surface toward, and sometimes into, the spinal canal. The etiology is proposed to be a focal area of nondisjunction of cutaneous and neural ectoderm. They are most common in the lumbar region and are typically seen as tiny skin dimples or defects with associated cutaneous stigmata. A dorsal dermal sinus that communicates with the central nervous system greatly increases the risk of meningitis and/or abscess. Approximately 50% of dorsal dermal sinuses are associated with intraspinal dermoid or epidermoid inclusions.

With ultrasound or MRI, the dorsal dermal sinus will be seen as a linear, obliquely oriented tract from the skin surface through the posterior subcutaneous fat toward or frankly into the spinal canal (Fig. 16.10). Contrast-enhanced MRI is valuable in the setting of secondary infection and may help to demonstrate the dermal sinus itself, particularly when using fat suppression techniques.

Split cord malformation

The term split cord malformation (SCM) encompasses diplomyelia and diastematomyelia, two entities along a spectrum of spinal cord duplication anomalies. Diplomyelia commonly refers to complete duplication of the spinal cord, in which two dorsal and two ventral nerve roots originate from each duplicated cord segment (21–25). With diastematomyelia, there is a region of sagittal division within the spinal cord with two resulting hemicord segments. Each hemicord contains a central canal, and each gives rise to one dorsal and ventral nerve root, although they may be quite asymmetrical. The division may only affect the anterior or posterior half of the cord (partial diastematomyelia or “horseshoe cord”).

SCM most likely results from splitting of the notochord during embryonic development. The notochord also influences development of the vertebral bodies, which may also explain the spinal column abnormalities, which can be present in SCM. The clinical presentation of SCM depends upon the configuration and level of the underlying malformation, but can include scoliosis, foot deformity, unilateral lower extremity muscle wasting, and low back pain (26–29). Overlying cutaneous stigmata such as hairy patches, dimples, and hemangiomas may also be present (28,29). Additional symptomatic spinal anomalies are present in the majority of patients, including spinal cord tethering, syringohydromyelia, and even open spinal dysraphism (25,27–29).

Radiographs usually demonstrate widening of the interpediculate distance and will sometimes allow visualization of a midline osseous septum in patients with SCM (28). CT is particularly helpful in detailing the anatomy of the osseous anomalies and septum. MRI is the most sensitive examination for the anatomy of the spinal cord and canal, optimally showing the two hemicords, either in a single- or dual-dural tube.

Tethered Spinal Cord

The term tethered cord syndrome is used to describe a spectrum of congenital anomalies that may result in an abnormally low termination of the conus medullaris and potentially associated neurologic, musculoskeletal, urologic, and gastrointestinal abnormalities (30). Although the term is most commonly used in reference to patients with a low-lying conus medullaris and a thickened filum terminale, many of the previously described spinal anomalies and cord abnormalities may result in a “tethered” spinal cord. These abnormalities include open spinal dysraphism, lipoma with dorsal defect, filar lipoma, dorsal dermal sinus, and SCM (30,31) (Fig. 16.11). The true incidence of tethered cord syndrome is not known, but the number of patients diagnosed with this condition is increasing due to incidental detection with more routine use of MRI and greater awareness of these entities (32).

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