Spinal Cord Dysraphism and Myelodysplasia

Embryology

A fundamental understanding of embryology can provide tremendous insight when evaluating and treating patients with spinal dysraphism. The extent of spinal dysplasia that develops depends on the embryological period in which failure occurs. Neurulation is the process by which the neural tube forms ( Fig. 18.1 ). Beginning near day 18 of gestation, there is induction of the neural plate over the notochord. This is followed by formation of a longitudinal neural groove in the midline, surrounded by neural folds laterally. In primary neurulation, the neural folds fuse together dorsally, beginning in the middle and extending in both the cranial and caudal directions. This forms the neural tube, with openings at either end called neuropores. ^, The rostral neuropore closes around day 26, and the caudal neuropore closes around day 28. There is separation of the neural tube from the ectoderm at the dorsal surface; this ectoderm will eventually become the dermis and epidermis.

In secondary neurulation, the most caudal aspect of the neural tube develops from caudal cell masses formed from a regressing tail bud. There is regression of the tail structure, leaving the filum terminale, the coccygeal ligament, and the terminal ventricle of the conus behind. Spinal dysraphisms can generally be divided by the stage of embryology in which failure occurs, specifically whether defects occur pre- or postneurulation. ^,

Spina Bifida Aperta––Open Neural Tube Defects

Open neural tube defects arise from a failure of primary neurulation during embryogenesis. The exact timing of the malformation determines its severity. Insults occurring before day 28 can lead to myeloschisis, the most severe form of spina bifida aperta, in which the neural folds fail to fuse and the nervous tissue lies exposed as an open, flattened mass with no overlying covering ( Fig. 18.2A and B ). Failure of primary neurulation after day 28 will lead to meningocele or myelomeningocele, together referred to as spina bifida cystica. A meningocele is a herniation of a cerebrospinal fluid (CSF)–filled sac covered by meninges, but not containing any neural elements (see Fig. 18.2C and D ). Myelomeningocele is the herniation of a CSF-filled sac in addition to protrusion of the spinal cord and/or nerve roots, all enclosed by the meninges (see Fig. 18.2E and F ). ^,

Myeloschisis

As mentioned, myeloschisis is a severe form of spina bifida aperta caused by failure of primary neurulation. There is herniation of the malformed spinal cord, and, in contrast to meningocele or myelomeningocele, the exposed neural tissue has no meningeal covering. In this condition, the skin is attached to the sides of the exposed neural tissue, and when it extends over multiple levels, it is often referred to synonymously as rachischisis. The defect can be located anywhere along the spine.

In complete rachischisis, the entire length of the spine is involved. In this severe condition, the etiology is usually multifactorial and often accompanied by other fatal defects such as acrania. Survival rates are extremely low; most infants with this condition are stillborn or die within a few hours to days after birth. The few that survive have significant disabilities.

A subtype of myeloschisis is limited dorsal myeloschisis (LDM), in which there is no obvious neurulation failure; rather, a small area of the neural tube contains an incomplete fusion of the neural folds. This is a result of aberrant connection between cutaneous and neural ectoderm because of incomplete separation during embryogenesis, and is most common in the lumbar region. LDM appears as a small saccular extrusion lacking a skin covering but with an underlying fibroneural stalk containing neural tissue ( Fig. 18.3 ). This stalk extends from the opening and attaches to the spinal cord in the dorsal midline. It is said to have a “crater” or “pit” appearance. LDM can present alone or with associated pathologies, such as lipomas or SCM.

Myelomeningocele and Meningocele

A theory of simple nonclosure has been proposed regarding the embryogenesis of meningocele and myelomeningocele. It is relatively accepted based on observations of both human embryos and animal models of spinal dysraphism. ^,

Central nervous system (CNS) anomalies, such as hydrocephalus and Chiari II malformation, are associated with myelomeningocele. ^{, , ,} Based on a mouse model of neural tube defects, a unified theory is proposed to describe these associated malformations. The leakage of CSF from the myelomeningocele is thought to decrease distention and expansion of the ventricular system; this lack of mechanical pressure leads to a smaller bony posterior fossa. As the cerebellum and brainstem develop in the smaller posterior fossa, the tentorium is thought to become dysplastic because of upward herniation, whereas downward herniation leads to the development of a Chiari malformation. The abnormal CSF flow pathways can then result in hydrocephalus.

Myelomeningoceles are usually seen in the distal thoracic and lumbosacral region (about 85%), with 5% in the cervical region and 10% in the upper thoracic region. The pedicles of the vertebrae at involved levels are laterally displaced, which creates a wider spinal canal. The unfolded neural tissue, called the neural placode, usually appears in the center and is surrounded by an arachnoid membrane (called the zone epitheliosa) that contains CSF and is fused to the skin, fascia, and dural margins (see Fig. 18.2F ). The ventral surface of the neural placode is covered by pia mater ( Fig. 18.4 ).

Diagnosis of spina bifida cystica is typically made before birth during routine prenatal screenings, with the presence of elevated maternal serum α-fetoprotein at 16 weeks gestation followed by visualization on ultrasound at 18 to 23 weeks gestation. ^, Ultrasound can demonstrate cranial abnormalities such as ventriculomegaly, downward hindbrain displacement causing flattening of the frontal bones, or anterior bowing of the cerebellum. Fetal magnetic resonance imaging (MRI) allows for better visualization of spinal defects and associated cranial abnormalities ( Fig. 18.5 ). T1-weighted images are useful for visualizing overlying soft tissue or associated lipomas, whereas T2-weighted images allow for visualization of neural elements. Upon detection of a meningocele or myelomeningocele, parents are usually referred to a multidisciplinary team (consisting of a social worker, neonatologist, and pediatric neurosurgeon) to discuss treatment options and provide education on neural tube defects.

The mainstay of management of myelomeningocele is early surgical closure of the defect, with most repairs occurring within 24 to 48 hours after birth. Delaying closure to even 72 hours after birth is associated with significant risk of meningitis/ventriculitis, decreased motor function, and worsened neurological deficits. Benefits of early repair include not only infection prevention and cosmesis, but also motor, sensory, and cognitive function preservation, allowing for continued neural development at the defect level. A minority of patients need surgical detethering of the spinal cord, either primarily or secondary to initial repair. ^, About 85% of patients require CSF diversion for hydrocephalus, either with a shunt or endoscopic third ventriculostomy. Some patients also require surgical decompression for symptomatic Chiari malformation.