Neural tube formation starts at the 17th day of gestation during which the notochord induces the overlying ectoderm to differentiate into neuroectoderm to form the neural groove in the dorsal midline of the embryo. Neural folds on either side of the groove elevate and meet, the cells fuse dorsally, and a primitive spinal cord is formed as a hollow cylinder. This process is completed by days 27–28 of gestation. Interruption of the neural tube formation dorsally at a particular segment(s) prevents normal neural tissue differentiation at that level. While this maldevelopment of the spinal cord structure causes a more or less complete neurological impairment below the affected level, additional abnormalities appear as a result of altered induction of the neural tube to the surrounding tissues [2, 5]. The consequence is a visible spinal cord segment, placode, representing an unclosed primitive neural tube remnant without meningeal, bony, or cutaneous enclosures dorsally. The term “spina bifida,” although denoting only the missing posterior bony elements over the unclosed neural tissue, is confusingly used to describe the whole anomaly. Likewise, myeloschisis, spina bifida cystica, and myelomeningocele are terms attributed to the different morphological appearance of the same pathology contributing no practical purpose in terms of decision making, surgical technique, or outcome other than confusion. The term “myelomeningocele” is currently preferred to represent almost all variations of open spinal dysraphism that result from a common mechanism of maldevelopment [1, 6, 7].

Disordered embryogenesis of the segmental neural tube formation in myelomeningocele is attributed to either primary failure of neural tube closure or secondary opening after appropriate tube formation. The nonclosure theory is probably for the majority of human myelomeningoceles; however, overdistention may contribute to some experimental neural tube defect models [2, 6]. Regardless of the causative mechanism, unclosed neural tube triggers a cascade of events concerning the nonneural tissue that is designated to cover the spinal cord dorsally. The cutaneous ectoderm remains attached to the open neural tube segment and fails to form the future skin over the lesion. Moreover, the normal mechanism in which the cutaneous ectoderm detaches from the dorsal side of the neural tube to allow paraxial mesoderm to move in between to give rise to bone and soft tissue is also distorted. The final pathological anatomy is an exposed lesion at birth, representing the inner surface of the spinal cord on the dorsal midline covered with membranous tissue as the epidermal remnants or exudate, with a groove continuous with the central canal of the unaffected segments. Immature laminar remnants and paraspinal muscles occupy the lateral border of the widened spinal canal and can be palpated under the border of the skin defect (Fig. 15.2).

Current hypotheses suggest that complex interactions between extrinsic and intrinsic variables are responsible for myelomeningoceles. Clinical and epidemiological data in humans imply maternal illnesses, medications, environmental toxins, and dietary factors such as lack of folic acid that play causative or at least contributing roles in the development of myelomeningocele. On the other hand, increased incidence demonstrated in certain families situates neural tube defects into complex genetic disorders in which genes and the environment interact through an unknown relationship [2–4].

The incidence of myelomeningocele is generally accepted as 1/1000 live births regardless the ethnic and geographic variability. Although several studies demonstrate a variation in different parts of the world depending on the geographical region, seasons at conception, gender of the affected infants, ethnicity, and socioeconomic status of the parents, maternal age and parity and population-based surveillance studies fail to confirm a definitive correlation with the incidence [8]. A decrease in frequency of myelomeningocele has been reported recently in some areas, while the incidence has been stable elsewhere [9]. Although this decrease has been attributed to increased prenatal diagnosis, selective terminations, genetic counseling, and mostly folic acid supplementation during pregnancy, there are no hard data to indicate that the decrease is due to a single factor [10, 11].

Myelomeningocele represents one of the most devastating congenital malformations that are compatible with life. This is due to the fact that neurological impairment is inevitable for a patient with myelomeningocele and the severity is proportional to the affected level of the spinal cord. With up to 80 % of the disclosure taking place at the thoracolumbar spine, paraplegia is the result. The level of neurological deficit descends as the lesion level moves caudally; at best, sacral localization avoids a major motor disturbance but does result in a neurogenic bladder [7, 12, 13].

The neurological deficit in myelomeningocele is thought to be due not only to incomplete differentiation of the neural tube but also to exposure of the uncovered neural tissue to amniotic fluid. Furthermore, associated anomalies extending to 63 % as reported in fetal autopsy series contribute to the disability of the myelomeningocele cases [3]. Besides morphological abnormalities of the adjacent vertebral elements, almost all patients with myelomeningocele have associated Chiari II hindbrain malformation. The simplest representation of Chiari II malformation is herniation of the cerebellar tonsils and vermis to the cervical spinal canal through a tight foramen magnum. Additionally, medullary kinking, low-lying tentorium, tectal beaking, brain stem nuclei changes, polymicrogyria, and gray matter heterotopias may be associated with the Chiari malformation. The main contribution of the tonsil herniation to the clinical picture of myelomeningocele is hydrocephalus, which is present in almost 90 % of patients either during delivery or becoming apparent after surgical treatment [1, 12]. Hydrocephalus are one of the major coexisting factors that are responsible for morbidity and overall unfavorable outcome in myelomeningocele cases. Craniolacunae, a mesodermal self-limiting skull abnormality, is also a frequent finding in the newborn.

From the practical viewpoint, a patient with myelomeningocele is born with signs of functional cord transection at the lesion level and neurogenic bladder, and has a very good chance of hydrocephalus. The open lesion carries a substantial risk of getting infected; cerebrospinal fluid exposure to the external environment through the incomplete dural barrier can initiate meningitis and ventriculitis. Meningitis not only complicates a probable treatment for hydrocephalus but also adds the potential risk of seizures and further neurological impairment in terms of intellectual outcome.

The aim of surgical treatment for myelomeningocele is to stabilize the clinical and neurological status of the newborn and prevent the potential risks of deterioration. This is best achieved by reconstructing the open neural tube and its coverings as soon as possible after birth. The initial management aims to stabilize the infant, avoiding contamination of the lesion and excluding the associated malformations. First, the important concern at this point is the decision to treat. This has medical, ethical, and legal ramifications to be discussed among the parents and the physicians. In cases with a prenatal diagnosis, the parents have already acknowledged the consequences of a congenital anomaly and treatment options. Otherwise, the anticipated problems in myelomeningocele with the limited role of surgical repair in the final outcome may result in the refusal of surgical treatment. From the physicians’ side, severe forms with multisegment large lesions complicated with associated vertebral anomalies and hydrocephalus may cause hesitation to treat. Unless there is a life-threatening coexisting malformation, the current ethicolegal opinion is to provide surgical treatment to all cases [6, 7, 12]. Once treatment has been decided, the second concern is the timing of the surgical procedure. Risks of immediate repair in a newborn should be weighed against the risk of contamination at delayed closure. Surgical repair within 48–72 h after birth is universally accepted and does not necessarily carry increased risk of contamination compared to very early treatment (within the first 24 h). Furthermore, the time interval provides sufficient postnatal evaluation and stabilization of the infant. The simplest description of the surgical technique is to mimic the normal embryological pattern of development. This is to isolate the nonfused segment, establish the original tube shape for the placode, and recreate and close the dural envelope followed by approximation and closure of the skin over the lesion (Fig. 15.3a–d). Even very large defects can be closed by relaxation incisions along the axis, avoiding complex muscle and skin flaps once advocated but proved to have major consequences. Almost 10 % of cases exhibit hydrocephalus at birth, necessitating simultaneously shunting with repair of the spinal defect. Operative mortality is nearly zero, but major morbidity includes progressive hydrocephalus, wound infection, breakdown, and leakage of cerebrospinal fluid.

Following myelomeningocele repair, the treatment of the other conditions may range from simple observation to extensive surgical procedures. The vast majority of patients will require shunts for hydrocephalus before being discharged; future treatment might be required for associated kyphosis, Chiari malformation, foot deformities, and secondary tethering of the spinal cord for CM-II, syringomyelia, and/or tethered cord syndrome. While myelomeningocele may be regarded a static and nonprogressive defect, clinical worsening is caused by associated problems. Owing to the fact that myelomeningocele is located at thoracolumbar segments in almost 80 % of the cases, the child faces lifetime complications of paraplegia and neurogenic bladder. Therefore, the initial closure is just the beginning, and outcome and long-term results greatly depend on the management of the associated conditions [7, 12]. At least 75 % of children born with an open spina bifida can be expected to reach their early adult years. Survivors have a high incidence of problems related to pressure sores, obesity, severe renal disease, hypertension, depression, and visual impairment. Mortality is mostly related to shunt dysfunction and infection, urinary complications of neurogenic bladder, or respiratory tract infection [13].

Neurulation is responsible for the formation of the spinal cord until the future second sacral segment and the most distal segment of the spinal cord develop by a process called secondary neurulation from the neural ectoderm cell mass caudal to the neural tube, the caudal eminence. Caudal eminence is formed from pluripotent cells derived from the regressing primitive streak. The mesenchymal neural cord then becomes an epithelial cord, acquires a lumen by canalization and regression process, attaches to the primary neural tube, and forms the remaining sacral and coccygeal segments of the spinal cord including the filum terminale [1, 3, 22]. Developmental errors during secondary neurulation, besides several anomalies, lead to the formation of a fatty and short/thick filum, a classical representative of occult spinal dysraphism. Other major occult forms of dysraphism include split cord malformations (SCMs, diastematomyelia), lipomyelomeningoceles, and dermal sinuses which represent disordered mesodermal differentiation belonging to different stages during primary neurulation, before secondary neurulation begins [23]. The setoff time for the mesodermal maldevelopment and the stage of neurulation at that instance are critical for the neurological consequences. The more the primary neurulation is disrupted, the chances are higher that the child is born with a neurological compromise. This is one of the main reasons why, in different forms of occult dysraphism, resultant neurological status ranges from normal to severe impairment, sometimes compatible with myelomeningocele. Segmental, asymmetrical involvement of neurulation results with lower extremity changes, including leg or buttock asymmetry, hip and knee problems, and foot deformities that typically worsen due to muscle imbalance, weight bearing, and gravity as the child grows [6].