Myelomeningocele represents the most severe form of dysra- phism, a so-called aperta or open form, involving the vertebral column and spinal cord, which occurs with an incidence of approximately 1/4,000 live births. ETIOLOGY The cause of myelomeningocele is unknown, but as with all neural tube closure defects including anencephaly, a genetic predisposition exists; the risk of recurrence after one affected child is 3-4% and increases to 10% with 2 prior affected children. Folate is intricately involved in the prevention and etiology of NTDs. Maternal periconceptional use of folic acid supplementation reduces the incidence of NTDs in pregnancies at risk by at least 50%. To be effective, folic acid supplementation should be initi- ated before conception and continued until at least the 12th wk of gestation, when neurulation is complete. The mechanisms by which folic acid prevents NTDs remain poorly understood. CLINICAL MANIFESTATIONS Myelomeningocele produces dysfunction of many organs and structures, including the skeleton, skin, and gastrointestinal and genitourinary tracts, in addition to the peripheral nervous system and the CNS. A myelomeningocele may be located anywhere along the neuraxis, but the lumbosacral region accounts for at least 75% of the cases. The extent and degree of the neurologic defcit depend on the location of the myelomeningocele and the associated lesions. A lesion in the low sacral region causes bowel and bladder incontinence associated with anesthesia in the peri- neal area but with no impairment of motor function. Newborns with a defect in the midlumbar or high lumbothoracic region typically have either a saclike cystic structure covered by a thin layer of partially epithelialized tissue or an exposed fat neural placode without overlying tissues. When a cyst or membrane is present, remnants of neural tissue are visible beneath the membrane, which occasionally ruptures and leaks CSF, whereas the placode is composed of neural tissue. Examination of the infant shows a faccid paralysis of the lower extremities, an absence of deep tendon refexes, a lack of response to touch and pain, and a high incidence of lower extrem- ity deformities (clubfeet, ankle and/or knee contractures, and subluxation of the hips). Some children have constant urinary dribbling and a relaxed anal sphincter. Other children do not leak urine and in fact have a high-pressure bladder and sphincter dys- synergy. Thus, a myelomeningocele above the midlumbar region tends to produce lower motor neuron signs due to abnormalities and disruption of the conus medullaris and above spinal cord structures. Infants with myelomeningocele typically have an increasing neurologic defcit as the myelomeningocele extends higher into the thoracic region. These infants sometimes have an associated kyphotic gibbus that requires neonatal orthopedic correction. Patients with a myelomeningocele in the upper thoracic or cervi- cal region usually have a very minimal neurologic defcit and, in most cases, do not have hydrocephalus. They can have neuro- genic bladder and bowel. Hydrocephalus in association with a type II Chiari malforma- tion develops in at least 80% of patients with myelomeningocele. Generally, the lower the deformity is in the neuraxis (sacrum), the less likely is the risk of hydrocephalus. The possibility of hydrocephalus developing should always be considered, no matter what the spinal level. Ventricular enlargement may be indolent and slow growing or may be rapid causing a bulging anterior fontanel, dilated scalp veins, setting-sun appearance of the eyes, irritability, and vomiting associated with an increased head circumference. About 15% of infants with hydrocephalus and Chiari II malformation develop symptoms of hindbrain dys- function, including diffculty feeding, choking, stridor, apnea, vocal cord paralysis, pooling of secretions, and spasticity of the upper extremities, which, if untreated, can lead to death. This Chiari crisis is due to downward herniation of the medulla and cerebellar tonsils through the foramen magnum as well is endog- enous malformations in the cerebellum and brainstem.
Posted on: Mon, 13 Oct 2014 22:33:55 +0000