Congenital malformations occur frequently in the nervous system and are exceeded only by prematurity as a cause of death in infants. In most cases, no specific cause can be demonstrated, although a number of teratogenic factors are recognized:
maternal infections, eg, rubella, toxoplasmosis, cytomegalic inclusion disease, and syphilis;
drugs ingested by the mother during gestation, eg, thalidomide, LSD, methotrexate;
systemic disease, electrolyte imbalance, and dietary deficiencies.
Although specific genes responsible for some congenital defects have been identified, it is reasonable to assume that the final phenotype results from a complex interplay of genetic predisposition and various intrauterine factors.
Malformations of the Skull and Vertebral Column
A spectrum of anomalies can result from either improper ossification of bony elements or failed segmentation during early development. Children may be asymptomatic or profoundly disabled.
Craniosynostosis is the premature closure or absence of one or more of the sutures that normally separate the individual bones of the skull. The majority of these cases involve a single suture and are isolated. The incidence is 4:10,000 births. Sutural closure prevents radial and symmetric growth of the head and is manifest as a secondary deformation of head shape with compensatory growth occurring parallel to the plane of the fused suture. Single suture synostosis is associated with abnormal neurologic function. When the process involves two or more sutures, growth and development of the brain can be affected, particularly during the first year of life when the brain triples its volume. Primary craniosynostosis must be differentiated from the secondary approximation and fusion of sutures in microcephaly and that which sometimes follows operative procedures on the skull or shunting for hydrocephalus. Premature closure of a specific suture results in a characteristic visual appearance.
Treatment consists of excision of the fused suture and sometimes reconstruction of the skull. Most procedures are performed between 3 months and 6 months of age, when the growing brain is still able to mold the relatively thin cranial vault into a “normal” shape.
Craniofacial syndromes such as Crouzon’s and Apert’s involve closure of several sutures resulting in cranial vault and midface abnormalities such as hyper- or hypotelorism, maxillary hypoplasia, and proptosis. Treatment may require multiple osteoplastic techniques at the skull base to achieve an acceptable result.
Split cord malformations (also known as diastematomyelia) are characterized by a bony spur or fibrous band projecting through the middle of the spinal canal dividing and spinal cord into two compartments. They are usually accompanied by other skeletal anomalies. Surgical removal of the midline abnormality is needed to untether the spinal cord and prevent progression of neurologic deficits.
Incomplete Formation of the Neural Tube
Failure of the neuroectoderm to fuse in the midline during the third and fourth week of fetal life leads to secondary failure of formation of overlying mesoderm and ectoderm. These defects may be small and concealed or they may be exposed and involve large areas of the spinal cord, meninges, vertebral column, overlying muscles, and skin. The most frequently involved anatomic level is the lumbosacral area; the least frequently involved is the thoracic area. (Cervical myelomeningoceles are very rare.)
Spina bifida occulta does not require treatment. Cases that involve multiple levels and those associated with skin abnormalities (eg, hemangioma, patches of hair, dermal sinus, or subcutaneous lipoma) may produce neurologic dysfunction by tethering of the spinal cord. The clinical picture includes progressive weakness, scoliosis, gait instability, urinary or bowel difficulties, and sometimes pain. MRI is the diagnostic procedure of choice. In newborn infants, spinal ultrasound can assist in determining the position of the anomalies.
Myelomeningocele is protrusion of nerve roots or cord elements along with the meninges. It occurs at least ten times more often than simple meningocele and always causes some degree of neurologic deficit. Findings range from mild weakness and slight sphincteric disturbance to complete sensory and motor paralysis below the lesion and incontinence. Hydrocephalus is associated with at least 80% of lumbosacral myelomeningoceles; Chiari malformation type 2 is always present and may cause brain stem dysfunction.
Meningocele consists of herniation of the meninges through a spina bifida without abnormality of the spinal cord or nerve roots. Neurologic function is usually preserved in these patients.
Encephalocele is a much less common midline protrusion of the meninges through the skull. It is usually occipital or at the base of the nose. The extent of brain tissue within the encephalocele often determines the functional outcome.
Treatment of congenital malformations of the nervous system
For all open spina bifida lesions, the fundamental surgical principle is reconstitution of the neural tube by layered closure of the dura, skin, and sometimes fascia usually within a few days of birth. The goals of early repair are prevention of meningitis and preservation of neurologic function. For lesions that are covered by normal skin, repair can be delayed for several months, unless symptoms develop. Early recognition and treatment of hydrocephalus is essential.
A team approach is required to manage a broad range of orthopedic (spasticity, club feet, scoliosis), urologic (urinary reflux, neurogenic bladder, hydronephrosis), and rehabilitation (orthoses, therapy, special schooling) concerns. Entry into a normal school, completion of education, and physical independence are the ultimate goals. Urosepsis and shunt malfunction remain the primary causes of long-term morbidity and mortality. Repair of myelomeningoceles is being performed in utero, but the long-term benefits of this approach are unknown.