Surgical Management of Spinal Dysraphism

Positioning and Sterile Preparation

The operating room must be kept warm to avoid hypothermia. All prep solutions are warmed to body temperature. The infant is intubated in the lateral position or supine with a doughnut-sponge on the back to accommodate the sac. The pelvis can be propped up with a horizontal roll to render the lumbar spine hyperlordotic and give maximum relaxation of the skin and soft tissues. The membranous portion of the sac, containing the exposed neural placode, is irrigated profusely with antibiotic-reinforced saline (betadine compounds have been shown to be neurotoxic), and the surrounding skin scrubbed with the usual antiseptic agents.

Opening the Sac

Loupe magnification is used from the very beginning. The sac is entered through the diaphanous leptomeningeal membrane halfway between the margin of healthy skin and the edge of the placode ( eFig. 74.4 ). Neural tissue of the placode is recognizable by its pink, felty surface, transverse wrinkles, and a straight, longitudinal median raphe ( Fig. 74.1A ). Since the epithelialized membrane itself is relatively avascular, any substantial bleeding from the cut edges signifies breaching of the neural tissue (see Fig. 74.1B ). Bleeding is controlled with a pair of ultrafine irrigating bipolar cautery forceps. After the initial gush of CSF and collapse of the cyst, the edge of the neural placode is gently flipped up to identify the ventral nerve roots. Several crossing blood vessels may have to be coagulated to free the placode margins. The pearly epithelium must be meticulously trimmed circumferentially from the placode to avoid later occurrence of inclusion dermoid cyst. At the caudal extreme of a terminal placode, the epithelial membrane may remain thin, or one may encounter a bandlike thickening, probably representing a remnant of the medullary cord, which must be divided to free the tip. At the rostral extreme of the placode, careful incision of the epithelium–neural tissue junction on both sides exposes the delicate bevel-shaped transition between the neurulated cylindrical spinal cord and the un-neurulated, flat placode ( eFig. 74.5 ). At the apex of the bevel, the central canal of the normal cord can be seen unfurling into the median raphe of the placode, from which CSF sometimes slowly oozes.

Handling the Neural Placode

The neural placode is always handled gently with jeweler’s microforceps. If the placode is pliable and thin enough, it is rolled on itself and sewn up with 8-0 nylon sutures through the delicate pial edges ( eFig. 74.6 ). There is no evidence that this improves neurologic function, but it reduces the “sticky” surface from a flat plate to a seam and theoretically lessens the chances of later tethering. If the placode is too thick or too stiff, it should not be forced to roll up, for fear of strangulation. Also, tugging too hard on the proximal spinal cord will cause ascension of the neurologic level. In addition, the caudal end of the placode must be checked for the presence of a neurulated cord in the rare case of a segmental placode (see below).

Paradoxically, small and compact neural placodes are likely functional and therefore should be treated with extreme care, whereas large, thin, and “spread-out” placodes commonly found in middle to high thoracic lesions are usually nonfunctional. It is sometimes advisable to cordectomize these large placodes, for while they do not convey volitional movements, the decentralized neuronal pools within them can produce pathologically hyperactive local reflexes underlying high-pressured, spastic bladders and prominent ureteral reflux. Reflux is less prevalent when the bladder is rendered flaccid following elimination of this type of placode.

Dural Closure

The margins of the dural flaps are created by sharply incising the ventral dura from the lumbosacral fascia and periosteum (see Fig. 74.1C and eFig 74.7 ; see also eFig. 74.4A and B ), and a new dural tube is reconstructed in the midline (see Fig. 74.1D ). One aims to obtain as capacious a dural sac as possible commensurate with the size of the placode, the theory being if the placode passively flops freely within a large CSF space, it is less likely to adhere to the dorsal dura. This is almost always achievable with the patient’s own dura, even if some of the periosteum overlying the bifid neural arches must be mobilized with the dura proper to enlarge the sac. In the rare event of insufficient dura, bovine pericardium can be used as a graft because its texture is compatible with newborn dura and because it seldom has suture-hole leakage of CSF. At the end of closure, the suture line is tested with a Valsalva maneuver held at a pressure of 30 to 35 cm of H ₂ O for 10 seconds.

Skin and Myofascial Closure

The true size of the skin defect is only apparent after completely excising the epithelialized membrane back to full-thickness skin. Defects up to 5 or 6 cm in diameter can usually be closed primarily after the subcutaneous layer is mobilized a short distance centrifugally, just enough to reduce the tension on the skin edges. The subdermal layer is closed with interrupted absorbable sutures, and the skin with fine nylon sutures. Several surgical techniques have been developed to minimize suture-line tension in large defects. Lateral relaxing incisions with bipedicle flap closure in the midline have been effective, and the relaxing incisions themselves can be left to heal via secondary intention, although larger defects may require skin grafting at the same time or later. Complex multiple rotation skin flaps have also been tried, but this necessitates extensive skin undermining and still does not altogether eliminate all tension spots. For impossibly large defects, composite skin–muscle (myocutaneous) flaps can be used.

It is important to avoid large wound seroma or hematoma, which increases skin tension and prevents flap adherence to the underlying tissues from which revascularization for the flap must be derived. If the flaps are large and “wet,” a small drain without negative suction may be left in for 24 hours. Suction drains may encourage CSF leak and are best avoided.

Early Complications (First Postoperative Week)

The operative mortality for children undergoing repair of an ONTD should be close to 0. The most common cause of postoperative death is related to hindbrain dysfunction (73%), ^, but this seldom occurs acutely in the first week of life. Most of the immediate complications pertain to the wound itself.

Wound Dehiscence

A study of the nutritional status of newborn infants who have had myelomeningocele surgery using body weight, nitrogen balance, serum protein, and total lymphocyte count as parameters showed that these neonates undergo an initial period of severe catabolic changes that do not readjust themselves for as long as 1 month after surgery. This nonspecific catabolic response is caused by rises in circulating levels of ACTH, cortisol, thyroxin, growth hormone, and antidiuretic hormone, stimulated by the extreme stress of surgery, general anesthesia, and blood transfusion. During this period, the resistance to infection is lowered, and all anabolic processes, including wound healing, are temporarily slowed. This metabolically unstable time also coincides with feeding difficulties caused by hydrocephalus, postoperative ileus, neurogenic dysphagia (due to brainstem compression), and prematurity. It is no surprise that wound dehiscence is the single most common complication during the first postoperative week.

Local factors, mostly avoidable, also contribute to this problem. A large sac means higher wound tension and precarious blood supply. An untreated kyphus adds stretching to the suture line and aggravates the local ischemia. Any additional external pressure caused by a tight dressing or improper patient positioning also interferes with healing.

It is common to see erythema along an 8- to 10-mm strip on either side of the suture line, particularly in areas of high tension. Sometimes the skin flaps may even look deep red to dusky as a result of venous stasis. These color changes often pass after a few days. When there is necrosis, the intensely dark red skin edges will turn black, but the necrosis may be limited to the epidermis, and the dermis and subdermis may survive, which should make adequate coverage. If full-thickness necrosis occurs, the blackness extends farther laterally. The junction between dead and viable skin demarcates, and the surrounding skin becomes erythematous and edematous. The sloughing skin edge also begins to pull away from the sutures, and serous exudate from subjacent fat necrosis seeps from the exposed subcutaneous tissues. Demarcation and sloughing are usually complete by the 7th or 10th day.

Sloughing of only the epidermis in small areas requires only simple dressing changes because the wound eventually epithelializes over the underlying dermal and subdermal layers. Skin grafting is unnecessary. If the skin necrosis is full thickness but there is healthy muscle underneath, the wound edges should be carefully debrided back to bleeding skin. It may then be dressed for second-intention healing from below, but this will take some time and delay CSF shunting. A faster way would be partial-thickness skin grafting, which should take well over a well-vascularized bed. If full-thickness necrosis exposes the dural tube, some measure of immediate coverage must be instituted to prevent desiccation and meningitis. This usually means a more radical and extensive flap rotation or even pediculated full-thickness skin flap grafting.

Finally, parenteral or enteral hyperalimentation should be set up to ensure adequate nutrition.

Wound Infection

Considering how badly the exposed neural placode is contaminated during and shortly after birth, it is surprising how rarely wound (extradural) infections occur after closure of an ONTD. The wound infection rate is about 1.5% to 2.5%, which is only moderately higher than clean neurosurgical procedures. However, if one counts the intradural infections, the infection rate rises to 7% to 10% even for early closure. ^,

Systemic signs of sepsis due to gram-negative meningitis are usually present 1 to 3 days after closure. In neonates, these early signs tend to be nonspecific, such as poor feeding, lethargy, or an ashen complexion. It is more common to see hypothermia than pyrexia, and the systemic white blood cell count often drops below 4000/cubic mm. If the dural sac is well invested with a myocutaneous coverage, an intradural abscess may eventually form without any external signs. It is important to obtain CSF for culture from a ventricular tap if there is clinical suspicion of sepsis, for the long-term prognosis of gram-negative ventriculitis in the newborn depends almost solely on the promptness of diagnosis and treatment.

If the infection is confined to the extradural space, the wound will become red and fluctuant on day 5 to 7. The surrounding skin will also appear edematous but, unlike CSF infection, there are often no systemic signs of sepsis and the infant may continue to feed and move normally. A spinal magnetic resonance imaging (MRI) with contrast may be used to evaluate the presence and extent of an abscess. A red, fluctuant wound may be diagnostically aspirated for purulent material. An abscessed wound must be opened immediately, widely debrided, irrigated with antibiotic solution, and reclosed over suction drains. Associated vascular occlusion and myonecrosis may require refashioning of a new myocutaneous closure. Depending on whether the CSF indices and imaging study indicate intradural infection, the dura may have to be opened to rule out an intradural abscess. The patient is then put on broad-spectrum, CSF-penetrating antibiotics.

Cerebrospinal Fluid Leak

Newborns with ONTD should be vigilantly monitored for the development of hydrocephalus with frequent measurement of the head circumference and use of serial cranial ultrasounds to detect progressive ventriculomegaly. Prompt treatment of hydrocephalus by CSF diversion is the best way to prevent CSF leak from the myelomeningocele repair site.

The dura nearest its lateral margin may be severely attenuated in large myelomeningocele defects. This, plus the often tense and precarious myocutaneous closure, makes large lesions particularly prone to leak CSF. Also, the timing of the slowly climbing CSF pressure happens to coincide with weakening of the tenuous suture line, around the fifth to eighth postoperative day. A small amount of transdural CSF leak probably occurs through the suture holes in most cases, considering the thinness of the newborn dura. The appearance of slight fluctuance under the skin flaps during the first few postoperative days is likely due to a combination of CSF and blood. As long as the skin closure holds, and there is no outward leak of CSF, there is no risk of infection. This small amount of seepage is self-limiting. A large transdural leak causes a tense subcutaneous accumulation that will eventually threaten the viability of the suture line. When CSF breaks through the skin barrier, the risk for gram-negative infection rapidly rises, and treatment must be promptly initiated.

A shunt is effective in preventing CSF leak but is not recommended after the leak has sprung, especially if the leak has already breached the skin closure. Even a low-pressure shunt maintains a constant lumbar CSF pressure of 5 to 6 cm H ₂ O, still considerably higher than that in the subcutaneous pocket, which is near atmospheric. The preferential passage of CSF is still out through the back wound and not through the shunt. If a CSF leak persists in the presence of a shunt, the latter becomes infected sooner or later. The EVD is a much better means of decompression after a substantial leak has already existed. The drainage chamber can be lowered to subzero pressure to siphon CSF away from the back wound. With elimination of outward leak, the probability of infection is mitigated. The skin edges can now be oversewn, and the infant may have to be sedated to minimize the milking action of muscles overlying the thecal sac. Many leaks can be successfully managed by such measures without a reoperation if the leak is not a consequence of progressive hydrocephalus. If the leak persists post-EVD, the wound needs to be explored to close the dural defect or a CSF shunt may need to be implanted.

Open Neural Tube Defect With Segmental Placode

The term segmental placode describes a portion of open neural plate bounded both rostrally and caudally by perfectly neurulated spinal cord ( eFig. 74.8 ). It is found in approximately 4% of all ONTDs. The exact mechanism of its genesis is unknown, but somehow it must involve a “square-pulse” type teratogenic insult to the process of primary neurulation; that is, normal neurulation resumes post facto to an isolated failure of neural plate fusion, both in space and time. Alternatively, it could be a manifestation of “collision site” failure from two adjacent neurulation sites proceeding in opposite directions, although no proof of the multisite closure hypothesis yet exists. The most common site for the segmental placode seems to be midthoracic to thoracolumbar. It is unclear why the teratogenic insult in these cases, unlike in terminal placodes, does not disrupt secondary neurulation, and allows for normal closure of the posterior neuropore and formation of the conus.

It is important to recognize the placode as segmental before surgical closure because the surgeon should be mentally prepared to handle the distal end of the placode delicately. One reliable clinical clue is the preservation of distal lower extremity movements while the open defect is located high up in the thoracic region. A preoperative MR should be obtained, not only to visualize the distal spinal cord beyond the placode but to spot other associated paradysraphic malformations such as a split cord malformation (SCM), ^, a lipoma, or a thickened filum. It is even possible the segmental placode represents a hemimyelomeningocele, in that the other hemicord of the SCM is fully neurulated and stays uninvolved in the open defect itself. ^,

The technique of closure of the segmental placode is the same as for the terminal one. Every bit of neural tissue must be preserved during trimming of the extraneural membrane, and every effort should be made to reconstruct the tube ( eFig. 74.9 ). The critical decision is whether to deal with the other associated malformation at the same time or at a later date. The latter is recommended, since one wants to inflict as little stress on the newborn infant as possible and the immediate goals of infection prevention and neural conservation have been met by the mere closure of the open sac. The definitive procedure of complete untethering usually involves more extensive bone and soft tissue dissection, and should be left till 2 to 3 months later when the infant can better withstand a longer anesthesia and larger blood loss, when hydrocephalus is no longer an issue, and after thorough neuroimaging studies have been obtained.

Open Neural Tube Defect and Kyphectomy

A prominent kyphus is almost exclusively found with large, thoracic myelomeningoceles when the neurologic deficits are profound and at a high level. Presumably, the lack of lumbosacral paraspinous muscle action allows overpull by the anterior abdominal and intercostal muscles (innervated from the thoracic cord proximal to the lesion), which causes dorsal buckling of the thoracolumbar spine and secondary wedging of the vertebral bodies at the apex of the kyphosis. A sharp and prominent kyphus exerts enormous tension on the skin flaps and compromises their vascularity. Resection of a bad kyphus not only relieves this perpendicular tension but also in effect shortens the spine and helps in relaxing the surrounding soft tissues. However, kyphectomy should only be attempted if there is no other way to achieve adequate soft tissue closure. When kyphectomy is unavoidable at the time of sac closure, it should be approached with painstaking regard to details because the procedure is fraught with potential mishaps.

Before kyphectomy, the thin, nonfunctional placode is resected, and the dural tube is sewn up as a blind stump rostral to the designated upper cut of the kyphectomy. The vertebral bodies intended for resection are now cleared of their surrounding musculotendinous attachment with careful subperiosteal dissection using the monopolar cautery. Considerable bleeding from the epidural veins may be expected because their thin walls are adherent to the relatively unyielding posterior longitudinal ligament, which prevents the veins from collapsing with the bipolar cautery. The monopolar cautery needle must stay close to bone, particularly while separating the ventral muscles off the bodies. The inferior vena cava, aorta, iliac arteries, and kidneys are all retroperitoneal structures that could be injured by the heat of the cautery or by injudicious action of the periosteal elevator.

The apex of the kyphus is resected through the intervertebral discs with the monopolar cautery. The extent of resection must consider the feasibility of apposing the remaining ends of the spine to fill the gap. The two ends of the stump are then cleared of cartilaginous endplates and brought together using two parallel wires forced through the bony part of the vertebral bodies with sharp cutting needles. A certain amount of downward pressure must be exerted on the bodies during the apposition and twisting of the wires. During twisting of the wires, the fusion surfaces and the wire loops are subjected to tremendous distracting and persistent dorsal-pointing stresses. The wires should never pass through any cartilaginous part of the body or the intervertebral disc. The infant is immediately immobilized in a fitted thermoplastic body brace for a minimum of 3 months and up to 2 years. Nonunion is a serious problem because discarding one or two crumbled and defunct vertebral bodies essentially means widening the gap even more and an even greater stress for the new construct. ^,

Dorsal Lipoma

The lipoma–cord interface is entirely on the dorsal surface of the lumbar spinal cord, sparing the distal conus ( Fig. 74.2A ). The junctional demarcation between lipoma, cord, and pia, the fusion line , can always be traced neatly along a roughly oval track, separating fat from the dorsal root entry zone (DREZ) and dorsal nerve roots laterally (see Fig. 74.2B and eFig 74.10 ). The lipoma therefore never contains nerve roots. The lipomatous stalk runs through an equally discrete dorsal dural defect to blend with extradural fat. The uninvolved conus often ends in a thickened filum terminale.

Transitional Lipoma

The rostral portion of this type is identical to that of a dorsal lipoma, with a discrete fusion line and easily identifiable DREZ and dorsal roots. Unlike the dorsal type, however, which always spares the conus, the transitional lipoma then plunges caudally to involve the conus as the plane of the fusion line cuts ventrally and obliquely toward the tip of the conus, likened to making a slanting, beveled cut on a stick (see Fig. 74.2C ). The lipoma–cord interface thus created may be undulating and tilted so that the neural placode is rotated to one side or even spun into a parasagittal edge-on orientation, but the neural tissue is always ventral to it and the DREZ and the nerve roots are predictably localizable lateral and ventral to the fusion line and therefore do not course through the fat (see Fig. 74.2D ). There may or may not be a discrete filum. The dorsal dural defect extends to the caudal end of the thecal sac and may be much larger on the biased side.