Classification of Anomalies

A large number of overlapping and imprecise descriptive terms contribute to the confusion regarding NTDs. Spinal dysraphism includes all forms of spina bifida, although the latter term is used by both patients and health care professionals variably to describe either myelomeningocele or other spinal dysraphic anomalies. In this chapter, 'spina bifida' is considered synonymous with spinal dysraphism. One classification scheme for spinal dysraphism divides these anomalies into two general categories: open defects (also known as spina bifida aperta) and closed defects (also known as spina bifida occulta). While this division does not imply a different em-bryological origin for the two groups, it does offer some advantages in understanding general treatment principles for these conditions.

Open forms of spinal dysraphism are those anomalies that have deficiencies in the overlying skin along with defects of the bone, meninges and/or nervous tissue, leading to a direct communication between the lesion and the external environment. The most common form is myelomeningocele, in which there is a focal segment of structurally abnormal spinal cord that is exposed in the midline of the back

Pediatric Neurosurgery, edited by David Frim and Nalin Gupta. ©2006 Landes Bioscience.

Table 1. Clinical features associated with spina bifida occulta

System

Features

Cutaneous

Midline dimple Asymmetric gluteal cleft Capillary hemangioma Hypertrichosis ('hairy patch') Lipoma

Neurological

Lower-extremity weakness Gait instability Sensory deficit

Back pain or lower-extremity pain

Clubfoot High-arched foot Leg-length discrepancy Muscle atrophy Scoliosis

Orthopedic

Urological

Incontinence Delay in toilet training through a defect in the vertebral arches and overlying skin. The birth incidence of myelomeningocele is approximately 0.1% to 0.2%. A meningocele is a sac-like structure composed of meninges without involvement of the underlying spinal cord. Open lesions are easily diagnosed in newborns and need to be repaired shortly after birth to prevent infectious complications.

Closed forms of spinal dysraphism are those anomalies in which the underlying midline defect, either neural or bony tissue, is masked by intact skin. Simple spina bifida occulta is the congenital absence of a spinous process and variable amounts of lamina. Although not strictly 'occult,' certain dysraphic conditions are associated with cutaneous anomalies (Table 1), but are still considered within the broader group of spina bifida occulta. Cutaneous lesions include skin tags, hemangiomas, lipomas, hairy patches and skin dimples. The other malformations included in this category are lipomyelomeningocele, fatty filum terminale, inclusion cysts (dermoid and epi-dermoid), dermal sinus tract and split cord malformations. These lesions are often diagnosed in infancy, but if the cutaneous abnormality is subtle, diagnosis may be missed until symptoms develop many years later. Surgical repair of these lesions is often delayed.

Normal Embryology of the Spine and Spinal Cord

Neurulation is defined as the formation of the neural tube and is the process that leads to the development of the brain and spinal cord. In the embryo neurulation is first visible when the flat neural plate, which is composed of ectoderm, folds along the dorsal midline into the neural groove. This is followed by fusion of the edges of the groove, resulting in a closed tube that is then covered by a layer of cutaneous ectoderm. Closure of the open neural tube occurs first in the upper cervical region, then extends caudally to the level of L-1 or L-2, and rostrally to the region of the nasion.

Neurulation occurs in two overlapping processes designated as primary and secondary neurulation. Primary neurulation forms the spinal cord only to the segments corresponding to the lower lumbar level. Abnormalities at specific points during primary neurulation lead to various forms of dysraphism such as myelomen-ingocele, lipomyelomeningocele, intraspinal dermoid and epidermoid cysts, and split cord malformations. This process occurs during days 18 to 28 of embryonic development. The caudal-most portion of the neural tube is formed by a process called secondary neurulation. This process occurs during days 28 to 48 of embryonic development and is divided into two steps, canalization and regression. During canalization, a mass of undifferentiated cells caudal to the neural tube and notocord assembles into a structure called the caudal cell mass. A canal within the caudal cell mass connects with the rostral neural tube formed during primary neurulation and ultimately forms the distal segments of the spinal cord (below L2). Errors during canalization can lead to a terminal myelocystocele or lipomyelomeningocele. The caudal cell mass also forms associated genitourinary and anorectal structures, accounting for the coexistance of anomalies such as imperforate anus and bladder.

Regression refers to the process by which most of the caudal spinal cord arising from the caudal cell mass with the exception of the conus medullaris and nerve roots involutes, leaving a thin and nonfunctional filum terminale. Failed or inaccurate regression can lead to a fatty filum terminale. After formation of the terminal filum, the vertebral canal grows at a faster rate than the neural tube. This differential growth results in the relative 'ascent' of the spinal cord, such that at the time of birth, the conus medullaris lies at the L2-L3 level, and reaches the normal adult level of the L1-L2 interspace by 3 months of age.

Myelomeningocele

Embryological Origin

If the neural plates fail to fold and then fuse into the neural tube, it remains as a flat piece of tissue called the neural placode. The midline placode is interposed between the superficial cutaneous ectoderm, which cannot meet in the midline. Mesenchymal elements, including bone, cartilage and muscle, are unable to migrate between the neural tube and superficial ectoderm and are absent in the mid-line. The exposed dorsal surface of the placode represents what should have been the interior of the spinal cord, while the underlying ventral face represents what should have been the outer surface of the closed spinal cord (Fig. 1). The unfused neural placode thus floats on top of a closed subarachnoid space, and is surrounded by normal skin. The size of the sac varies and may cause the lesion to appear flat or significantly elevated from surrounding skin. The exact cause of the embryological defect that leads to the formation of myelomeningocele is likely to be multifacto-rial. In genetically altered mice, a variety of gene knockouts lead to a phenotype of an open neural tube.

Figure 1. A typical open spina bifida lesion (myelomeningocele). The large sac surrounds a midline neural placode. The muscles of the legs are atrophic with bilateral club feet. The anus is protruding, or 'patulous'.

Management

Antenatal Diagnosis and Counseling

Maternal alpha fetoprotein (AFP) is sampled by a serum blood test during the early part of the second trimester. An elevated AFP level compared to a median value corrected for maternal age is predictive of a higher chance of having a child with a NTD. It should be recognized that a large number of other congenital anomalies also result in elevated maternal serum AFP. If the predicted risk is higher than a specific threshold value (typically 1:500), then a high-resolution ultrasound is recommended in the middle of the second trimester. The antenatal ultrasound often identifies the level of lesion (in approximately two-thirds of patients) and the presence of lower extremity deformities, and characterizes the severity of associated abnormalities such as the Chiari II malformation and hydrocephalus. Amniocentesis is performed if the maternal AFP and ultrasound are equivocal.

Some parents will choose early termination for a fetus with a myelomeningocele. This accounts for some of the reduction in birth incidence of spina bifida in the US in the past 10 to 20 years. For those parents who choose to carry the pregnancy to term, the most important concern is long-term functional outcome (see below). This discussion should focus on the primary disabilities associated with myelomen-ingocele and the treatment of hydrocephalus. The specific disabilities include lower extremity dysfunction, urinary and fecal incontinence, symptoms attributable to a Chiari malformation, and cognitive impairment. The lower the lesion is in the spine the more likely the possibility of a favorable neurologic outcome. Quality of life issues are always discussed with the parents at every antenatal visit in order to prepare them for what will likely be a lifelong commitment to a child with multiple special needs. Finally, the risk of myelomeningocele is increased (to an incident risk of 2.8%) if there is a history of previous children with myelomeningocele.

Perinatal Care

The early involvement of a multidisciplinary team is essential. The initial assessment should include evaluation of the following systems:

1. General. The size of the lesion should be measured. If the placode has ruptured, antibiotics should be started with coverage of both Gram-negative and Gram-positive organisms. If the placode is intact, no antibiotics are necessary. The lesion should be covered with a moist, nonadherent dressing to prevent desiccation. The dressing may also be covered with plastic wrap to avoid evaporation. The patient is usually nursed on his or her stomach, although the side position is also acceptable. Dressings should attempt to exclude urine and stool from the exposed placode.

2. Neurological. Cranial nerve function should be tested carefully, as lower cranial nerve dysfunction can be seen with a symptomatic Chiari II malformation. Vocal cord paralysis present at birth is often manifested as stridor. A detailed assessment of lower extremity function is mandatory. Particular attention should be paid to the position of the legs at rest, and whether wasting of specific muscle groups is present. It is important to observe and document spontaneous lower extremity function as well as the best lower extremity response to painful stimuli, as this indicates the approximate functional level of the myelomeningocele. Function may be preserved below the anatomic level of the abnormality, but in general function worsens as one examines lower levels. Rectal tone is often reduced or absent, leading to an outward protrusion of the anus (termed a 'patulous' anus). The head circumference should be measured and the status of the fontanelle should be noted (e.g., sunken, flat, full but soft, tense). Hydrocephalus in this age group rarely produces somnolence but can lead to irritability, or apnea and bradycardia. A head ultrasound is useful for evaluation of ventriculomegaly.

3. Orthopedic. Lower extremity range of movement and deformity should be assessed. Clubfeet (equinovarus deformities) are common, as are dislocated hips. Scoliosis may also be present. Anterior-posterior and lateral spine X-ray films are used to evaluate scoliosis, and an orthopedic consultation is obtained for associated spine, hip, or knee deformities.

4. Urological. Since the vast majority of children with spina bifida have a neurogenic bladder of varying severity, a program of clean intermittent catheterization should be begun at once to prevent retention and urinary tract infections. A renal ultrasound should be performed to detect any changes associated with ureteral reflux and bladder-wall thickening. Although not mandatory, our practice is to obtain urodynamic studies shortly after birth.

5. Other. Myelomeningcocele patients have an average of 2 to 2.5 additional anomalies involving other organ systems. These include cardiac defects, renal anomalies, or pulmonary immaturity, which may preclude surgery.

Surgical Treatment of a Myelomeningocele Timing

It is generally accepted that early closure of the myelomeningocele lesion does not lead to improvement in neurological function, but prevents meningitis. The myelomeningocele lesion should be closed within 24 to 48 hours of birth. By 36 hours the lesion begins to be colonized by bacteria, and postoperative infection rates increase. Although the main benefit of early myelomeningocele closure is reduced infection rates, it is crucial to preserve as much neural tissue and vascular supply as possible during the closure procedure.

Operative Technique

The goal of surgery is to identify as many anatomic layers as possible and to perform a watertight closure. In general, the following steps are performed:

1. The neural placode is sharply separated from the surrounding arachnoid membrane and ectodermal elements. Retained fragments of cutaneous epithelium, which can cause a dermoid cyst, should be removed from the placode.

2. The neural tube is reconstituted by gently folding the placode towards the midline and securing the pia with small, nonabsorbable monofilament sutures. A pia-to-pia closure of the placode may prevent retethering of the terminal portion of the spinal cord.

3. Inspection for a thickened filum terminale is performed. This is sectioned if found.

4. Within the sac, the dura extends from the anterior to the spinal canal and flares laterally to merge with the dermis. The most crucial step of the procedure is to identify the dural-dermis boundary. The dura is incised circumferentially at the boundary, separated from the subcutaneous tissues, and mobilized towards the midline where it is closed.

5. Since the mesodermal elements such as muscle and fascia are displaced laterally, it is difficult to bring these layers over the closed dura, but this should be attempted if sufficient tissue is available.

6. Finally, the skin is mobilized by separating it from the underlying normal fascia laterally and then pulling it towards the midline. For large lesions or when the skin edges are under tension, a cutaneous or myocutaneous flap may be required to cover the lesion.

Postoperative Management

After repair, it is important to maintain the patient in the prone position to avoid pressure on the incision. A barrier dressing below the incision is used to avoid contamination from urine or stool. Daily measurements of head circumference are made, and weekly head ultrasounds are obtained to assess for progressive ventriculomegaly, especially in children who do not have shunts. Routine bladder catheterization is also important. Orthopedic and urological consultations continue to be important to plan for future correction of limb, hip, or spine deformities, and for correction of bladder function.

Hydrocephalus and Timing of Shunt Placement

Hydrocephalus develops in approximately 60% to 85% of myelomeningocele patients, with 5% to 10% of patients having clinical evidence of hydrocephalus at birth. In general, the vast majority of patients who develop hydrocephalus do so before the age of 6 months. Hydrocephalus, if not present at birth, can also develop after myelomeningocele closure, as surgery eliminates a route for CSF egress. Definitive criteria for shunt placement vary between institutions, but some generally accepted indications are: (1) symptoms such as apnea and bradycardia (unexplained by other causes), (2) rapid and progressive ventriculomegaly, as measured by increasing head circumference or by serial ultrasound exams, (3) CSF leak from the myelomeningocele closure site, (4) significant hydrocephalus in the setting of a symptomatic Chiari II malformation, or rarely (5) a spinal cord syrinx.

In patients with clinically evident hydrocephalus, a ventriculoperitoneal (VP) shunt may be placed at the same time as the myelomeningocele closure is performed. This can be done without increased risk of infection, and may reduce the risk of postoperative CSF leak and wound breakdown. In those patients without clinically overt hydrocephalus, it is reasonable to delay the shunt procedure until signs of hydrocephalus become apparent. It should be noted that 10% to 20% of patients will not require a shunt procedure.

Outcome

Survival

Without treatment, historical data suggest that only 15% to 30% of myelomeningocele patients survive infancy. Current standards of care have improved the survival rate to approximately 85%, although approximately 10% will die before 6 years of age, primarily due to complications from hindbrain dysfunction related to the presence of a symptomatic Chiari II malformation. Late mortality is usually due to shunt malfunction or sepsis. Improvements in voiding schedules and prompt treatment of urinary tract infections have significantly reduced urosepsis as a cause of significant morbidity and mortality.

Neurological Function

Most children with myelomeningocele will have a neurological deficit. Approximately 50% of myelomeningocele patients are ambulatory with bracing; however, most use wheelchairs for ease. Although some authors have found that up to 40% of patients will have improvement in motor function after surgery, it is generally believed that early closure does not improve neurological outcome.

Urinary Control

In the past, myelomeningocele patients underwent extensive urinary diversion procedures to minimize loss of renal function from bladder dysfunction and urine reflux. Currently 75% to 85% of patients are able to achieve satisfactory dryness with intermittent catheterization. Artificial urinary sphincters can also be used to treat urinary incontinence. Approximately 3% to 10% of patients will have normal urinary continence.

Figure 2. A T2-weighted sagittal MR image demonstrating a Chiari II malformation with descent of the cerebellar tonsils to C3, a small posterior fossa and beaking of the tectum.

Figure 2. A T2-weighted sagittal MR image demonstrating a Chiari II malformation with descent of the cerebellar tonsils to C3, a small posterior fossa and beaking of the tectum.

Late Complications Chiari Type II Malformation

Although infrequently symptomatic at birth, hindbrain dysfunction from the Chiari II malformation (Fig. 2) can cause neurological symptoms into adulthood. These include neck and skull-base pain, cranial nerve dysfunction and dystonia. When severe, this condition can also lead to sudden death. Posterior cervical and suboccipital decompression is useful in alleviating symptoms in adult myelomenin-gocele patients with the Chiari II malformation. This involves a decompression of the bone, and in some cases a dural decompression. The dural decompression should be performed with extreme care, as these patients often have dural sinuses that can reach the foramen magnum.

Shunt Malfunction

Hydrocephalus due to shunt malfunction can mimic almost any of the other conditions associated with late complications. In patients with a progressive decline in neurological function, it is important to rule out shunt malfunction. Shunt malfunction can cause hydromyelia or hindbrain compression. Shunt revisions are required in approximately 50% of patients by age 6.

Spinal Cord Tethering

The tethered spinal cord syndrome is diagnosed in a variety of settings, including that of myelomeningocele. By radiographic criteria, e.g., a low-lying conus and/ or attachment of the spinal cord to overlying tissue, almost all myelomeningocele patients will have a tethered cord. However, only a minority develop new symptoms attributable to the tethered cord. These symptoms include progressive scoliosis, changes in muscle tone, weakness and increasing spasticity. In some patients, release

of the tethered cord will result in symptomatic improvement, although it is best to caution the patient that stabilization of symptoms is the most realistic goal. Some authors have suggested that a loose dural closure and a pia-to-pia neural placode closure during the original surgery allow for increased spinal cord mobility and prevent the formation of adhesions. This view is not proven.

Occult Spinal Dysraphism

Clinical Features

Spina bifida occulta refers to a number of disparate abnormalities of varying severity that share the common feature of being covered by an intact epithelial layer. Although most often localized to the lumbosacral spine, they can be encountered at any spinal level, from the skull base to the tip of the coccyx. The prevalence of simple spina bifida occulta, characterized by a defect in the spinous process and lamina of L5 or S1, is 10% to 20% and is usually an incidental finding (Fig. 3). In isolation, this condition is of no clinical significance and does not require further investigation. Occult spinal dysraphism also includes anomalies such as dermoid and epidermoid cysts, split cord malformations, fatty filum terminale, lipomyelomeningocele, dermal sinus tract and terminal myelocystocele.

The clinical symptoms and signs that are associated with occult spinal dysraphism presumably occur as a result of the effect of the structural anomaly upon the spinal cord. For this reason, the clinical syndrome of a tethered spinal cord is probably best viewed as arising from a variety of underlying causes and conditions (Table 2).

Figure 3. An AP plain X-ray image of the lumbosacral spine demonstrating a bifid posterior arch at S1 (arrow). No central nervous system anomalies are present.

Table 2. Causes of tethered spinal cord syndrome

Congenital

Traumatic

Neoplastic

Infectious

Iatrogenic

Myelomeningocele Lipomyelomeningocele Spinal cord lipoma Fatty filum terminale Split cord malformation

Spinal-cord injury

Metastatic tumors (PNET, ependymoma) Bacterial meningitis Tuberculosis Fungal meningitis

Ionizing radiation

Postoperative adhesions following surgery

Cutaneous Stigmata

Cutaneous features of spina bifida occulta invariably occur in the midline of the spine although they can be asymmetric, with more of the anomaly being present on one side or another. These include subcutaneous lipoma, dimples, hypertrichosis, skin tags and cutaneous hemangiomas (naevus flammeus) (Fig. 4). Isolated simple dimples located directly over the coccyx are not considered pathological. Other midline dimples should be investigated with either ultrasound (less than 6 months of age), or more definitively with magnetic resonance imaging (MRI). Two or more cutaneous abnormalities can coexist. Hypertrichosis, especially in the lumbar region, is more commonly associated with a split cord malformation.

Figure 4. A) A cutaneous hemangioma in the lumbar area associated with an underlying spinal cord lipoma. B) A smaller cutaneous hemangioma adjacent to a midline dimple that represented the superficial end of a dermal sinus tract extending into the spinal canal.

Neurological Symptoms

Clinical features of the tethered cord syndrome vary in type and severity depending upon the age of presentation. Symptoms include gait difficulty with lower extremity weakness, radicular pain, sensory deficits, asymmetric hyporeflexia, spasticity and bowel/bladder dysfunction. The lower motor neuron symptoms are not directly related to tethering, but are caused by local compression of the spinal cord or nerve root injury. Upper motor neuron changes, however, are thought to arise directly from ischemic cord damage caused by tethering. A tethered spinal cord can also occur with noncongenital conditions such as spinal-cord injury, spinal-cord tumors and meningitis.

Up to 10% of patients with occult dysraphic states will also have a Chiari type I malformation. Thus, symptoms generally related to a Chiari malformation can also be the first presentation of the tethered spinal cord syndrome. Presumably the traction to the spinal cord plays a role in the descent of the cerebellar tonsils into the cervical spinal canal, although this is speculative.

Orthopedic and Vertebral Anomalies

Anomalies in the bone found in patients with spina bifida occulta include bifid vertebrae, laminar defects, hemivertebrae, sacral aplasia and sacral agenesis. The 'neuro-orthopedic syndrome' refers to the combination of foot deformities, limb-length abnormalities, lower extremity muscle atrophy, limb pain and scoliosis.

Anorectal Abnormalities

Occult spinal dysraphism is associated with various anorectal and urogenital disorders, including cloacal extrophy, imperforate anus and bladder extrophy. It is also associated with the VATER syndrome (vertebral defects, anal atresia, tracheoesophageal fistula, radial limb and renal dysplasia). Ten to 15 percent of all patients with anorectal anomalies will have some element of occult spinal dysraphism.

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