Summary

Spina bifida represents a spectrum of condi-tions of disordered development of the spineand spinal cord. In its most severe form –open myelomeningocele – the neurologicaldeficits can be profound, with implicationsfor repeated and lifelong management,although, with the concerted effort of a mul-tidisciplinary team, the outlook for quality oflife may be good. The incidence of this con-dition is declining and may be reduced fur-ther with the introduction of periconceptualfolic acid supplements.

Less obvious forms of spina bifida occurand may not be immediately apparent atbirth – occult spinal dysraphism – althoughthere may be cutaneous manifestations of theunderlying condition. The concept of “tether-ing” of the spinal cord by a dysraphic condi-tion has proved useful in the understandingof the generation of symptoms and in therationale for treatment of these disorders.

Introduction

The term “spinal dysraphism” covers a range ofdevelopmental conditions of the spinal cord andits surrounding structures. Often also referredto as either spina bifida or neural tube defects,spinal dysraphism is sufficiently all-encompass-ing to allow for the scope and complexity ofthese conditions, which the other two terms fail

to convey. Spinal dysraphism includes bothconditions obvious at birth or before, such asmyelomeningocele (spina bifida aperta), andconditions of the spine which may or may notbe apparent on closer inspection. This lattergroup includes so-called occult dysraphism(spina bifida occulta), when the only hint of anunderlying spinal abnormality may be a cuta-neous lesion or the development of orthopedicor urological symptoms.

Substantial progress has been made in theunderstanding and management of these conditions in recent years. Effective treatmentof hydrocephalus and the multidisciplinaryapproach to the needs of children with myelo-meningocele have led to improvement in theoutlook for those affected. The realization of theimportance of tethering as a mechanism fordeterioration in dysraphic states has meant an improvement in the management of theseconditions, directed at the underlying neuro-logical problem, rather than the acceptance ofinevitable deterioration. A fall in the incidenceof dysraphic lesions, improvements in diet andthe use of preconception folic acid, as well as abetter understanding of the genesis of theseconditions, will hopefully mean that fewerchildren are affected in the future.

Epidemiology

Estimates of the true incidence of neural tubedefects occurring in pregnancy are difficult toascertain, since a proportion of severely affected

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27Spinal Dysraphism

Simon Stapleton

fetuses are undoubtedly aborted spontaneously,early in pregnancy. Nevertheless, worldwide,the prevalence of neural tube defects occurringat birth appears to be falling. This can be partlyattributed to screening programmes and possi-bly to improvements in nutrition, but the ratealso seems to be declining for unexplainedreasons. In the UK, the prevalence throughout the country is falling from an overall level ofapproximately 4 per 1,000 live births in the1970s to in the region of 0.3 per 1,000 live birthsnow. There remains a significant geographicalvariation in the prevalence of cases in thiscountry, with the highest rates occurring in thewest of the country, namely in western Scotland,Wales and Northern Ireland, where the inci-dence may be more than twice as high as else-where, but the incidence in these areas is alsofalling. Elsewhere in the world, the prevalencerates are generally lower, ranging down to about1 in 10,000 in sub-Saharan Africa.

There is a significant genetic component tothe development of neural tube defects, since, ifeither parent has had an affected child or if either parent is affected by the condition,there is an approximately 10% risk of furtheroffspring having a neural tube defect. If two affected pregnancies occur, the risk to afurther pregnancy is increased about 20-fold.Secondary prevention of further affected preg-nancies requires screening techniques, includ-ing alphafetoprotein sampling and ultrasound,with selective termination of affected pregnan-cies. Primary prevention, however, requires theprevention of neural tube defects occurring inthe embryo in the first place, even in pregnan-cies not at higher risk of such an occurrence.Since the 1960s, it has been recognized thatwomen with an affected pregnancy had signifi-cantly lower red cell folate levels than those withunaffected pregnancies. The Medical ResearchCouncil Vitamin Study of 1991 [1], in whichwomen who had had a previous pregnancyaffected with a neural tube defect were ran-domized to receive folic acid (4 mg daily) orplacebo, with or without other multivitaminsupplements, demonstrated that the rate ofaffected subsequent pregnancy was significantlyreduced in the folic acid group (relative risk0.29). It is now, therefore, recommended that inorder to prevent a recurrence of a neural tubedefect in subsequent pregnancies, 5 mg daily offolic acid should be supplemented to the diet

prior to conception. In order to prevent a firstoccurrence of a neural tube defect, all womenshould be advised to take 400 mg of folic aciddaily prior to conception, as well as increasingtheir dietary intake of foods rich in folic acid,continued until the 12th week of pregnancy(food high in folate includes green vegetables,yeast, beef extract and breakfast cereals fortifiedwith folic acid) [2].

Etiology

There is clearly a familial tendency to the devel-opment of neural tube defects, although this isprobably a polygenic mechanism. Siblings of anaffected individual have an approximately tentimes greater chance of also being affected(2.5% vs approximately 0.2% risk in the generalpopulation – see above). The children of healthysiblings of an affected child are known also tobe at increased risk.

Nutritional factors are thought to play a role and may explain partly the social class differences in incidence. The use of preconcep-tion folic acid stems from the concept that low maternal folate intake is implicated in theetiology of neural tube defects (see above).Numerous teratogens have been identified inanimal models, including anticonvulsants suchas valproate and phenytoin, other folate antag-onists such as aminopterin, hypervitaminosis A, alcohol and mitomycin C. The extent towhich these factors operate in humans remainsuncertain.

Recently, abnormalities in homeobox geneshave been implicated in the genesis of neuraltube defects in mice. In particular, the humananalogue of the mouse Pax3 gene has been iden-tified. It is unknown, however, whether this is asine qua non in humans [3].

Embryology

Under normal circumstances, development ofthe neural tube proceeds as follows. The sup-posed relevant deviations from this related tospecific forms of dysraphism are described inthe appropriate section.

The primitive streak develops at the caudalend of the bilayered embryonic disc by devel-opmental day 14. From the primitive streak,

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cells migrate between the layers of ectodermand endoderm to form the embryonic meso-derm. Hensen’s node is located at the cranialend of the primitive streak and, from here, thenotochordal process develops in a cranial direc-tion between the two embryonic layers by day17. The solid notochord becomes a hollow cylin-drical structure, which transiently fuses with theunderlying layer of endoderm. There exists,therefore, a communication between the amni-otic cavity dorsally and the yolk sac ventrally viathe primitive pit, known as the neurentericcanal. This communication closes as the noto-chord again separates from the endoderm byday 20. At this time, Hensen’s node and theprimitive streak regress with growth of theembryonic disc, in a caudal direction, ultimatelyto lie in the low sacral or coccygeal region.

The notochord induces the overlying ecto-derm to thicken and cells heap up to form theneural plate. The neural groove develops in theneural plate, producing lateral folds which ulti-mately meet in the midline as the neural groovedeepens, to form the neural tube. This is theprocess of primary neurulation. Closure of theneural tube begins in the mid-thoracic regionand extends both cranially and caudally. At thecranial end of the neural tube (the future laminaterminalis), closure of the anterior neuroporeoccurs by day 24, while closure of the caudal orposterior neuropore occurs by day 28. The loca-tion of the posterior neuropore is a matter ofsome debate but probably lies in the region ofL1 or L2. Caudal to this level, development of the spinal cord does not occur by primaryneurulation. In this region, Hensen’s node andthe primitive streak give rise to an undifferenti-ated clump of cells known as the caudal cellmass, destined to form the conus medullarisand the filum terminale. This occurs by a ratherpoorly defined process – secondary neurulation– of vacuolation, condensation and subsequentfusion to the spinal cord formed by primaryneurulation. The significance of this process inthe human remains rather uncertain.

As the process of primary neurulation occurs,the ectoderm lateral to the developing neuralplate fuses in the midline to cover the neuraltube, while embryonic mesoderm from the scle-rotome at each level of the embryo migratestowards the midline to surround the notochordand the neural tube to give rise ultimately to thevertebral bodies and neural arches, as well as the

dural sheath. The process of secondary neuru-lation (vacuolation, condensation and fusion ofthe caudal cell mass) occurs after the overlyingectoderm has fused to form the skin.

Myelomeningocele

This is the single most common form of spinaldysraphism and is synonymous with spinabifida aperta or an open neural tube defect. It is the most severe form of spinal dysraphismand presumably represents a failure of closureof the neural tube at approximately day 21 ofdevelopment (primary neurulation).

Clinical Presentation andAssessment

Unless detected antenatally by ultrasound ormaternal screening, open myelomeningocele isimmediately apparent at birth (Fig. 27.1). Thismanifests usually as a defect on the back, withevidence of a neural placode representing the open spinal cord, often with normal- and

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Fig. 27.1. Myelomeningocele in the lumbar region of a neonate.Note the cystic appearances, with fragile blood vessels visible.

abnormal-appearing nerve roots coursing fromit ventrally and surrounded by arachnoid adhe-sions, an incomplete dura and associated para-vertebral soft tissues. The central canal of thespinal cord may be visible at the rostral end ofthe placode; the defect may be covered by a thinepithelial or arachnoid layer, but this may haveruptured and CSF will be seen to leak from thedefect.

There may be associated developmentalanomalies, usually of the nervous system. Mostinfants will develop hydrocephalus and a ChiariII malformation is very common. Abnormalitiesof cerebral gyration, of the posterior fossa con-tents and agenesis of the corpus callosum, aswell as associated vertebral anomalies, may alsooccur. Unsuspected, more rostral “occult”forms of dysraphism may coexist.

Delivery of an infant with a suspected openmyelomeningocele should be by Cesareansection, in order to avoid the risk of infectionduring passage along the birth canal; the childshould be nursed on its front or side, with asterile moist dressing covering the defect, andkept warm. Having examined the defect itself,clinical assessment aims at determining theneurological deficit, both sensory and motor.Much of this can be done by observation andgentle stimulation of the limbs to ascertain sen-sation and movement. Bladder and bowel func-tion are difficult to assess with any certainty buta good urine stream may suggest an incompletedeficit, although almost all children will go on to have some degree of bladder and boweldisturbance. Further examination is directedtowards possible associated congenital ano-malies and hydrocephalus, as well as generalcardiopulmonary status.

Without closure of the myelomeningocele,meningitis is likely to develop within a few days,often with fatal consequences. In view of thesevere, often devastating neurological deficitsand the poor outlook for a fulfilling, self-caringand dignified existence, Lorber [4] proposed apolicy of selective non-treatment, based on thelevel of the lesion, the severity of associatedhydrocephalus and degree of spinal deformity,as well as the presence of other congenitalabnormalities. This policy has clearly led tomany severely affected infants not surviving.Nevertheless, not all untreated infants suc-cumb, with the effect that they may go on tosurvive, with more severe disabilities than had

they been treated initially. McLone [5–7] haspresented evidence that, with a concerted mul-tidisciplinary team approach whereby all openmyelomeningoceles are closed at birth, theoverall outcome with respect to mortality, intel-lect and mobility is not improved by selectivenon-treatment.

Surgery for open myelomeningocele is aimedat protecting the existing neural structures andpreventing infection. Surgery will not restoreneurological function; nevertheless, it is essen-tial to preserve any functioning nervous tissuethat does exist. Because of the risk of infection,closure of the defect should be carried outwithin 48 hours of birth. Closure of the defectinvolves defining the neural placode and freeingthis from arachnoid adhesions. Some surgeonsreconstitute the neural tube by folding over and suturing the neural placode in an attemptto prevent future cord tethering; however, thisprocedure is not essential and may unnecessar-ily damage the delicate existing nervous tissueif the sutures are inappropriately placed. It should be ensured that there are no skinappendages attached to the placode. Theextradural space is identified, the dura is mobi-lized and this plane developed around the defect to allow closure of the dura in a water-tight fashion. If necessary, a dural graft may berequired to close the dura, without compromis-ing the neural structures and maintaining theclosure free from tension The muscle and fasciaon either side of the defect are mobilized; thismay require lateral releasing incisions if thedefect is large, and then approximated. The skinis then closed in a watertight manner. For verylarge defects, plastic surgical procedures withmyofascial or cutaneous flaps may be requiredto achieve adequate closure. At all stages of the closure, it is essential that the tissue layersare not approximated under tension, otherwisewound breakdown and CSF leakage will occur.

Approximately 80% of children with myelo-meningocele will require a shunt at some stage.For those with obviously severe hydrocephalus,this may need to be carried out within severaldays of closure of the spinal defect. For thosechildren less severely affected, observation, withhead circumference measurements and assess-ment of signs of raised intracranial pressure, willdictate the need for and the timing of shuntinsertion. The majority of children who will needa shunt will do so by the age of 5 months [8].

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Long-term Care and Outcomein Open Myelomeningocele

The long-term care and follow-up of childrenwith myelomeningocele requires the input ofmany disciplines, preferably in a combinedmultidisciplinary clinic devoted to the manage-ment of children with spina bifida and its complications. This will include assessment ofall aspects of physical, as well as cognitive,development.

Overall survival with myelomeningocele afterthe first 4 years appears to stabilize at approxi-mately 85% [6], although there continues to bea significant mortality. This can be related tomany of the associated problems faced by thesechildren, including hind-brain disturbance,causing respiratory distress, apneic spells andgastro–esophageal reflux with tracheal aspira-tion, shunt malfunction and infection, andbladder and renal problems.

Myelomeningocele itself does not appear to have a significant impact on intelligence.Although the average IQ of affected children isbelow the mean, the majority fall within thenormal range, with only 9% having an IQ scorebelow 70 in McLone’s series, followed up forover 15 years. Factors which do seem to affectintelligence include initially severe hydro-cephalus, neonatal ventriculitis and shunt infections, as well as the level of the lesion.Hydrocephalus and Chiari malformations pre-sumably also account for the consistently founddeficits in fine motor function in the upperlimbs and in visuo–spatial processing, as well aschanges in age-related language ability. Two-thirds of children, with appropriate support,can be kept in mainstream education. Ultimateemployment opportunity seems to be relatedmore to intellectual ability than to physical disability [9].

Numerous factors contribute to mobility. Theability to walk will depend not only upon thelevel of motor and proprioceptive deficit, butalso on factors such as spasticity, deformity,ataxia and obesity, as well as intelligence,support and motivation. In McLone’s series,75% of surviving children by school age weremobile without a wheelchair; nevertheless this figure declines with age [10], largely due to increasing difficulty and effort required tomaintain posture, especially with weightincreasing proportionately faster than strength.

Again, bladder and bowel continencedepends chiefly upon the level of the neuro-logical lesion. Most children manage thebladder with clean intermittent catheterizationand many are able to perform this for them-selves. Management of the bowel depends upona combination of bulk laxatives and enemas,avoidance of constipation and the use of aShandling catheter [11].

The long-term care and assessment of child-ren with myelomeningocele requires attentionto many details of their development, with par-ticular scrutiny for the possible complicationswhich may arise in time. From a neurologicalperspective, aside from the ever present risk ofshunt malfunction and Chiari malformationand syringomyelia-induced problems, this willinclude the possibility of cord re-tethering.

Re-tethering of the spinal cord (see below)following closure of a myelomeningocele orafter any other “untethering” operation, such asafter surgery for lipomeningocele, should beconsidered in any child with a clinical deterio-ration. Re-tethering may manifest as a deterio-ration in motor power or gait, pain, alteredsensation or deformity in the legs, or withchanges in bladder function. Increasing scolio-sis has also been considered to result from re-tethering.

Most neural placodes or lipomas probablyadhere to the dura soon after the initial opera-tion; however, it is only with continued trac-tion on the cord that symptoms are likely. Re-tethering remains a clinical diagnosis, sincestatic MRI will only show the location of theconus, rather than its mobility.

Re-operation to release the cord should beconsidered in any child with a significant clini-cal deterioration. There are often dense adhe-sions and thick scar, making surgery difficult,and consideration to expanding the dura with apatch should be given. The results of surgery arereported to be gratifying in that most childrencan be stabilized and pain is often improved[12].

“Occult” Spinal Dysraphism

The term “occult dysraphism” implies that the presence of the dysraphic condition is not immediately apparent, as it is in “open”

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myelomeningocele. This may or may not be soat birth, but most dysraphic conditions of thespine become apparent in one way or anotherin childhood or adolescence. The diagnosis may be suspected at birth by the presence of a lumbosacral lipoma or cutaneous lesion with abnormal pigmentation in the midline.Commonly, infants are referred with a sacraldimple with a question as to the presence of adysraphic condition. Shallow dimples, tetheredinferiorly, in the natal cleft are generally inno-cent. Any midline dimple above the natal cleftshould be treated with suspicion, particularly ifthere is any history of discharge. Cutaneouslesions often have abnormal pigmentation andmay have an associated hairy patch.

Other manifestations of “occult” dysraphisminclude orthopedic problems such as scoliosis,pes cavus or inequality of calf girth or foot size.Neurological abnormalities such as leg weak-ness or numbness are common presentations,usually with variable loss of lower-limb reflexes.In older children, adolescents and adults, sen-sory abnormality in the feet may lead to trophicchanges, with smooth, shiny skin and occasion-ally with ulcers which are slow to heal. The childmay be referred as being generally clumsy on hisor her feet. Disturbances of bladder or bowelfunction are also seen frequently. Constipationrequiring regular laxatives and delay in toilettraining may be apparent after the age of 5 years,when the child may still not be dry by day as wellas by night. There may be evidence of a poor uri-nary stream and frequent urinary tract infec-tions. Pain as a feature of spinal dysraphism isuncommon in children but is a frequent occur-rence in adults either with a known dysraphismor who may present for the first time in adult-hood (see below).

Occult dysraphism may be suspected at birthby the presence of a visible abnormality but fre-quently presents during subsequent periods ofrapid spinal growth, at approximately age 6years and in adolescence. Occasionally, presen-tation may occur acutely at times of excessivespinal motion, such as in sporting activities,after being placed in the lithotomy position,after childbirth and as an unsuspected underly-ing condition following surgical correction ofscoliosis. Such modes of presentation supportthe idea that symptoms and signs in dysraphicconditions are the result of “tethering” of thespinal cord by the underlying abnormality, pre-

venting the normal ascent of the spinal cord andconus relative to the bony spine during postna-tal growth and development. Coupled with therepeated stretching of an already “taut” cordduring everyday activity, this leads to neuro-logical dysfunction and the development of theassociated orthopedic and urological features.

Spinal Cord Tethering

Central to an understanding of the developmentof symptoms in occult dysraphism is theconcept of spinal cord tethering. This is adynamic problem with the spinal cord andcannot therefore be identified per se on staticimaging such as MR or CT. Nevertheless, thattethering may exist can be inferred by the pres-ence of an abnormality such as a lipoma, bonyspur or thickened filum terminale, producing anabnormally long spinal cord with a low-lyingconus. The implication is that the cord has beenheld in its original position and prevented fromascending within its thecal sac during growth ofthe individual. There are a number of possibleexplanations of why this should lead to neuro-logical dysfunction:

sustained traction on the cord with growthof the bony spine leads to stretching of the cord itself and disruption of neuronalfunction;ischemia of the cord leads to abnormalitiesof oxidative metabolism [13];repeated hyperflexion (e.g. during sportingactivities) of the spine causes acute injuryto neuronal processes at the level of thetethering lesion.

Attempts have been made to “visualize” teth-ering clinically using MR CSF pulsatility studies[14] or to demonstrate motion of the conususing CT myelography in both prone and supinepositions, but the diagnosis remains a clinicalone, based on the presence of a presumed teth-ering lesion and evidence of neurological dys-function.

If tethering is the underlying cause of neuro-logical deterioration in many dysraphic condi-tions, the principle of surgical treatment is tountether the spinal cord and to prevent it re-tethering. This is generally achieved by remov-ing the tissue responsible for the tethering ordetaching the spinal cord from it, as describedin the specific sections below.

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Tight Filum Terminale

The simplest and perhaps the most easily imagined form of spinal cord tethering is thatproduced by a thickened, fatty, filum terminale.This may present with asymmetric neuro-logical or orthopedic abnormalities in the lower limbs and is well seen on sagittal and axialT1-weighted MRI as high signal within the filum associated with a low-lying conus.Treatment simply involves division of the filum,ensuring that no nerve roots are attached to it.On occasion, at operation, the proximal end ofthe filum may spring out of view superiorly,providing convincing evidence of the tetheringprocess.

Lipomeningocele

Lipomeningocele is an abnormality of the spinecharacterized by a low-lying conus medullaris,infiltrated with fatty tissue, which extendsthrough a bony dysraphic defect and into thesubcutaneous tissues. It is thought to occur as aresult of an abnormality of secondary neurula-tion of the caudal cell mass, whereby pluripo-tential mesenchymal cells fail to regress and maylead to lipomas, hamartomas and teratomas in the lumbosacral region. The lipoma is invari-ably covered by skin but may have pigmenta-tion, hair or cutaneous dimples on it. They oftenlie asymmetrically across the midline in the lum-bosacral region and can reach a very large size.Intradurally, the lipoma may be attached to thedorsal surface of the cord or it may be insertedinto the terminal end of the conus. The lipomamay enlarge in infancy or may be associated withobesity. Although uncommon, lipomeningoce-les may be associated with other developmentalanomalies, including syringomyelia, Chiari mal-formation and anal and genitourinary malfor-mations. The lesion is considered to lead totethering of the spinal cord, thereby producingsymptoms.

The cutaneous abnormality is frequentlyobvious at birth but, in the past, has not alwaysbeen recognized for what it is. Progressive, oftenasymmetrical neurological deterioration in thelegs is a common mode of presentation, as wellas discrepancies in foot size and leg length inolder children. Bladder and bowel dysfunctionsare also frequent complaints. As with otherforms of dysraphism, pain tends to be a feature

in adulthood. Similarly, acute neurological dete-rioration has been described.

The approach to treatment of lipomeningo-cele depends largely upon the perceived naturalhistory of the condition. Arguments in favor of prophylactic untethering include the fact that many infants appear to be neurologicallynormal, whereas most adolescents and adultspresent with some neurological deficits. In onestudy [15], all children were symptomatic by theage of 4 years. The condition therefore appearsto carry a high risk of progressive deterioration,while the risks of surgery remain relatively low. Once neurological deficits or orthopedicdeformity have occurred, they tend not to be reversible, even with surgery, and there is abackground risk of acute deterioration andparaplegia without surgery. For these reasons,many neurosurgeons consider untethering thecord before the age of 6 months or do so as soonas the lesion is recognized thereafter [16,17].Counter to this argument is the fact that not allchildren with lipomeningoceles do deteriorateas demonstrated by the occasional unsuspectedadult presentation; that surgery does carry somerisk of neurological damage; and that acuteparaplegia in the absence of any precedingsymptoms is rare. Added to this is the frequentproblem of re-tethering after surgery. There hastherefore been a reappraisal of the need for earlyprophylactic untethering and consideration ofsurgery only at the onset of the development of symptoms. This requires close and regularassessment of all patients with lipomeningoce-les, to detect any neurological change [18,19].There is little argument over the place of surgeryin preventing further deterioration in those pre-senting with progressive neurological deficits.Recently, Chumas [20] has summarized thesearguments and emphasized the need for long-term follow-up of all these patients if a consen-sus is to be achieved.

Pre-operative assessment requires carefulneurological examination of the legs and uro-dynamic assessment. MRI in the sagittal planeclearly demonstrates the lesion (Fig. 27.2a) andwill also show an associated hydromyelia. Axialimages reveal the relationship of the lipoma tothe cord itself (Fig. 27.2b).

Surgery is aimed at untethering the spinalcord, reducing the bulk of the lipoma and recon-stituting the dura to enable the conus to liefreely. This may require a dural patch to try to

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prevent re-tethering by local scar tissue. Surgeryis generally carried out through a midline inci-sion, although plastic surgical considerationsmay need to be taken into account in largelesions. The first normal lamina above the lesionis identified and the dura here exposed. Thedura is then opened to expose the attachment ofthe lipoma to the cord and the opening contin-ued around the exiting fatty defect. The line offusion of the lipoma with the cord can then beidentified and, with the exiting nerve roots ante-rior to this line, the lipoma can be detached. Thebulk of the lipoma can then be removed and the dura closed, ensuring that the mobility of the conus is not compromised by the closure.The frequent difficulty in obtaining a satisfac-tory dural closure leads to the two chief com-plications of the procedure: CSF leakage andre-tethering. The former is generally manage-able with CSF drainage and re-suturing, if need be. The latter may become apparent within months or many years of the initialsurgery and is characterized by the onset ofneurological deterioration or pain. Since the

conus, following even successful surgery, may not change position on MRI studies, re-tethering can be difficult to prove radiologicallyand re-exploration may be necessary on clinicalgrounds alone (see above).

Surgery carries a low risk for neurologicaldeterioration (less than 5%) and appears toprevent the subsequent development of neuro-logical deficits. In those with clinical signs oftethering, surgery appears to be effective at preventing further deterioration and, for earlyintervention, may reverse some of the neuro-logical dysfunction. For long established deficitswith orthopedic deformity, surgery will not leadto improvement but can effectively prevent aworsening deficit.

Split Cord Malformations

“Split cord malformation” is a term used todescribe a number of congenital abnormalitiesof the development of the spinal cord and its coverings, characterized by a division of the cord into two, not always equal, parts. The

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a b

Fig. 27.2. a Lumbar lipomeningocele. Sagittal T1-weighted MR scan, demonstrating a low-lying conus associated with alipomeningocele. The spinal cord ends in a lipoma, which itself is continuous with the subcutaneous tissues and is coupled with adefect in the lumbar laminae. b Lumbar lipomeningocele. Axial T1-weighted MR image, revealing the relationship between the conusand the lipoma itself.

general term “split cord malformation” ispreferable to the potentially confusing termsused to describe the conditions often referred toas diastematomyelia and diplomyelia. Theabnormality may take the form either of a divi-sion of the spinal cord within a single dural sac(diplomyelia, type II split cord malformation)or of splitting of two hemicords, each with itsown dural covering, by a septum, usually madeup of a bony spur and/or fibrocartilagenousband (type I split cord malformation).

The embryological origin of split cord mal-formations remains unclear. The presence oftwo hemicords within a single dural tube hasbeen considered to occur as a result of a dou-bling of the neural tube without fibrous tissuedividing the cords and therefore without thepotential for causing tethering. On the otherhand, two hemicords, each with its own duralsheath and surrounding bony structures, havebeen thought to be due to splitting of the noto-chord by an interposed adhesion between theprimitive endodermal and ectodermal layers orof incomplete persistence of an accessorycranial neurenteric canal. This leads to a septumsplitting the cord and, with growth, tethering asthe bony spine lengthens relative to the corditself. However, Pang has considered that allsplit cord malformations represent a spectrumof a single abnormal embryological process[21,22]. This is characterized by the formationand persistence to a variable degree of an abnor-mal connection between the endodermal andectodermal layers of the embryo, resulting in an endomesenchymal tract. The endomes-enchymal tract results in the independent development of two heminotochords and twohemineural tubes. The degree to which theinvading mesoderm (future meninges) is asso-ciated with the endomesenchymal tract betweenthe two developing neural tubes affects theextent to which the two future hemicords aresplit. If both hemicords are each surrounded byinvading mesoderm, then two separate duraltubes result, with associated bony and fibrocar-tilaginous tissue producing the dividing spur.Pang has termed this the “split cord malfor-mation type I”. If the mesoderm of the futuremeninges is not associated with the endomes-enchymal tract dividing the hemicords, then asingle dural sac is created, with only thin sagit-tal fibrous bands, representing the remains of the endomesenchymal tract, attaching the

hemicords to the dural sleeve. This Pang termsthe “split cord malformation type II”. Theimplication here is that both types of split cordmalformation represent tethering lesions andtherefore both require surgical exploration in order to prevent subsequent neurologicaldeterioration.

Split cord malformations are characteristi-cally associated with a midline hairy patch or“horse’s tail”. This is much more commonlyfound in split cord malformations than withother forms of “occult” dysraphism (Fig. 27.3).There may occasionally be an associated dermalsinus or pigmented skin lesion. The spur is gen-erally found in the lumbar or lower thoracicregion. Clinically, this may produce one leg withmuscle wasting or sensory loss, with or withoutorthopedic manifestations, such as pes cavus.One or both ankle jerks are often absent. Theremay be involvement of the bladder and bowels,particularly in those children presenting in earlyadolescence. Scoliosis may also occur, as mayother vertebral anomalies. The anatomy of thedefect is demonstrated on MRI scans; particu-larly well seen on axial imaging are the twohemicords. Once the level has been established,

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Fig. 27.3. Lumbar “horse’s tail” in the presence of a split cordmalformation.

and this may not be the level of the cutaneousstigma (an important point preoperatively), CTscanning with intrathecal contrast may demon-strate a bony spur. Nevertheless, with modernMRI, CT myelography is generally no longernecessary.

Neurological deterioration in split cord mal-formations is generally considered to be a resultof tethering, although associated lesions such ashydromyelia may contribute. The aim ofsurgery, therefore, is to remove the bony orfibrocartilaginous spur and to excise the duralsleeve surrounding it. In cases with separatedural tubes, each containing a hemicord (type Isplit cord malformations), the tethering spur isfound at the caudal end of the divided duraltube (Fig. 27.4a). The cutaneous mark may notnecessarily lie over the bony abnormality butthere is usually an abnormal lamina present.Surgery involves identifying the lowermost

normal lamina and then carrying out a laminec-tomy of the abnormal lamina below, it usuallybeing necessary to include the lowermostnormal lamina. The spur is then isolated andremoved down between the two dural tubes.The stalk and surrounding epidural veins canbleed briskly and this should be anticipated.Once the stalk has been removed, the dura is opened around the cleft, revealing thehemicords, which usually unite just below the location of the spur. Pang emphasizes theimportance of dividing any fibrous bands andmedian nerve roots, which are non-functioning,coursing dorsally to tether the hemicords. Thesebands may continue to the subcutaneoustissues, forming the entity known as a meningo-cele manqué [23]. The dural sleeve around thespur should be excised to ensure that the unionof the hemicords is free to “ride up”. It is notnecessary to close the ventral dura but the

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a

b

Fig. 27.4. a Type I split cord malformation at operation. The two dural tubes containing the hemicords unite just below the dividingspur, with associated nerve roots visible. b Type II split cord malformation. Two hemicords visible within a single dural tube on CTmyelography. The lateral nerve roots are well seen.

dorsal dura may need a patch to obtain satis-factory closure without constriction of the intra-dural contents. Since the hemicords usuallyunite below the spur, a single, thickened filumterminale may be present. This should besought and divided through a separate incisioninferiorly, if necessary.

Two hemicords within a single dural tube(type II split cord malformations) generally donot require surgical exploration unless there isevidence of a thickened filum or of dorsalintradural fibrous bands (Fig. 27.4b). Pang,however, argues that all these malformationsshould be explored since, in all 18 of hisreported cases, intradural tethering bands werefound, even in their apparent absence on pre-operative investigations. This is clearly an areawhich awaits further clarification.

Congenital Dermal Sinus

Infants are frequently referred to the pediatricneurosurgeon with a midline spinal dimple,occasionally with discharge from the tract. This may be the initial presentation of manycongenital dermal sinuses but a significantnumber present with meningitis due to skin orgut organisms. Multiple episodes of meningitismay even occur before the diagnosis is made,since the cutaneous opening of the tract may be minute. Nevertheless, meningitis due toorganisms such as Staphylococcus aureus or Escherichia coli in an infant rather than aneonate should arouse suspicion and, particu-larly after recurrent episodes, a concerted effortto identify a sinus tract should be made. Theopening may be anywhere along the midline ofthe spine and may even occur in the occiput.

Many infants are referred with a dimple in thenatal cleft, fixed inferiorly. These are generallyinnocent and should not be explored surgicallybecause of their benign nature and, if surgery is carried out, they almost always becomeinfected.

The congenital dermal sinus is an epithelial-lined tract that may end in the soft tissues ormay extend deeper, to be attached to or pene-trate the dura; may end in the subarachnoidspace; or, more commonly, be attached to the filum terminale and end at the conusmedullaris. There may be inclusion dermoidmaterial, forming a mass anywhere along thetract.

There is rarely any neurological deficit, unlessa dermoid cyst has compressed local nerve rootsor meningitis has caused a deficit. MRI scanningreveals the extent of the tract and any associateddysraphic abnormality (Fig. 27.5). Contrastmedia or probes should not be inserted alongthe tract.

Surgical excision of the tract is indicated bothbecause this may be a tethering lesion andbecause of the risk of meningitis. This should becarried out without undue delay. If meningitisoccurs, this should be treated as appropriatefirst, and only when the inflammation hasresolved should surgery be undertaken. Oncemeningitis has occurred, the tract and sub-arachnoid spaces may be scarred, making theoperation technically more difficult.

The object of surgery is to excise the tract inits entirety. This involves excising its cutaneousorifice and following this to its termination. Thismay require opening the dura and excision of the attachment to the filum terminale. Anyassociated dermoid cyst, either intradural orextradural, should also be excised.

Complete excision achieves a cure and neuro-logical outcome is generally very good, with few,if any, long-term problems [24].

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Fig. 27.5. Sagittal T1-weighted MR image of a 14-year-old boywith a congenital dermal sinus entering the theca at the L4 level.He had had two episodes of staphylococcal meningitis prior toreferral.

Neurenteric Cyst

Neurenteric cysts are epithelial-lined cystsderived from the neurenteric canal, which tran-siently connects the embryonic yolk sac with theamniotic cavity during the third week of embry-onic development (see “Embryology” section).Due to the endodermal origin of the cyst lining,gastrointestinal or respiratory epithelium maybe found and associated abnormalities of thegut, respiratory tract and vertebrae may occur.Such cysts (also called, for this reason, enteroge-nous cysts) tend to present as intraduralextramedullary lesions situated ventrally in thecervical region. They may also be found in thethoracic region, where a dorsal intradural loca-tion is more often seen. Clinically, these lesionsgenerally present in adolescence or early adult-hood with neck pain and spinal cord compres-sion, causing a cervical myelopathy. MRIscanning reveals the cyst and its location, allow-ing surgical excision, which can usually beachieved from a posterior approach. Even if acomplete excision cannot be achieved safely,due to dense attachments to the cord or nerve roots, partial excision and cyst drainageprovide lasting benefit, since re-accumulation isextremely slow. The prognosis for improvementin neurological function is generally very good.

Anterior Sacral Meningocele

An out-pouching of dura containing CSF mayoccur through a defect in the body of the sacrum(anterior spina bifida). This may be an isolateddefect or may be in association with a moresevere developmental abnormality of the wholecaudal region of the embryo, as in caudal agen-esis, where abnormalities of the genitourinarytract, rectum and anus may also occur in asso-ciation with sacral agenesis. Presumably, thedefect in the bone is the primary abnormalityand, with the pressure of CSF, the meningocelegradually enlarges. The meningocele maycontain sacral nerve roots. As the meningoceleenlarges into the pelvis or retroperitoneal space,it produces symptoms of compression of thepelvic organs, including constipation, urinaryfrequency and abdominal or pelvic pain, as wellas low back pain. Anterior sacral meningocelesare more common in females and may presentas an incidental mass identified on pelvic examination or ultrasound. The diagnosis is

established by CT myelography, which willconfirm the connection with the spinal thecaand reveal the bony anatomy well. MRI scan-ning will demonstrate the presence of the massand its relationship with the pelvic organs, butmay not identify the neck of the sac. It is impor-tant to distinguish these masses from otherpelvic masses such as an ovarian cyst or othertumor, and, similarly, no attempt should bemade to aspirate the cyst through a potentiallyinfected area.

Symptomatic cysts are best treated by sacrallaminectomy, aspiration and ligation of the cystneck. If access is not possible posteriorly, anabdominal approach may be indicated, but it isnot necessary to excise the meningocele wall.Asymptomatic cysts may require no treatment,although progressive enlargement with timemay be expected.

Spinal Dysraphism in Adults

Adult patients with spinal dysraphism includethose with new symptomatic onset of a previ-ously unsuspected occult dysraphic conditionand those with a known dysraphic lesion in childhood but with symptom onset only inadulthood. In both groups, unlike in childhood,pain is the most frequent presenting symptom.This may be poorly localized and bilateral, andcoupled with weakness in the legs as well assensory disturbance. Problems with bladdercontrol, as well as erectile dysfunction, alsooccur frequently. Not infrequently, the problemonly comes to light as a result of excessivestretching of the conus, as may occur in child-birth or trauma [25]. In those with a known dys-raphic lesion, presentation in adulthood may bewith a progressive scoliosis or foot deformity,although these features are generally not seen in an adult with a previously unsuspected dysraphism.

There is often no cutaneous clue to an under-lying dysraphic lesion, which may take the formof a thickened or tight filum, or an intradurallipoma, as well as containing dermoid material.

As for childhood dysraphism, surgicaluntethering is recommended for symptomaticadults. This usually involves division of thefilum or release of adhesion and debulking of a lipoma.

Pang and Wilberger [25] have reported verysatisfying improvements in pain following

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surgery in adults, with reasonably good resultsfor motor and sensory improvement but, as inchildhood, bladder and bowel dysfunction tendsnot to improve significantly with surgery.Surgery for fixed orthopedic deformities doesnot improve, although it may prevent progres-sive deterioration

Key Points

● The incidence of open myelomeningocele is indecline.

● Periconceptual folic acid supplements reducethe risks of neural tube defects.

● A combined multidisciplinary team approachcan lead to a good long-term outlook forpatients with open myelomeningocele.

● Midline cutaneous lesions should raise thesuspicion of an underlying occult spinal dys-raphism.

● “Tethering” of the spinal cord is considered tobe the underlying mechanism for the devel-opment of clinical problems in most occultdysraphism and the surgical approach totreatment, therefore, has been aimed at“untethering” the spinal cord.

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2. Folic acid and the prevention of neural tube defects.Report from an expert advisory group. Department ofHealth, 1992.

3. Goulding M, Paquette A. Pax genes and neural tubedefects in the mouse. In: Boch G, Marsh J, editors.Neural tube defects. CIBA Foundation Symposium.Number 181. Chichester, UK: John Wiley, 1994; 103–17.

4. Lorber J. Results of treatment of myelomeningocele: ananalysis of 524 unselected cases, with special referenceto possible selection for treatment. Dev Med ChildNeurol 1971;18:279–303.

5. McLone DG, Dias L, Kaplan WE, Sommers MW.Concepts in the management of spina bifida. In:Humphreys RP, editor. Concepts. Pediatr Neurosurg1985;5:97–106.

6. McLone DG. Continuing concepts in the management ofspina bifida. Pediatr Neurosurg 1992;18:254–6.

7. McLone DG. Treatment of myelomeningocele and arguments against selection. Clin Neurosurg 1986;33:359–70.

8. Rekate HL. To shunt or not to shunt? Hydrocephalusand dysraphism. Clin Neurosurg 1984;32:593–607.

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10. McLone DG. Disorders of the pediatric spine. Pang D,editor. New York: Raven Press Ltd, 1995; Chapter 9,137–57.

11. Shandling B, Gilmour RF. The enema continencecatheter in spina bifida: successful bowel management.J Pediatr Surg 1987;22:271–3.

12. Herman JM, McLone DG, Storrs BB, Dauser RC.Analysis of 153 patients with myelomeningocele orspinal lipoma reoperated upon for a tethered cord.Pediatr Neurosurg 1993;19:243–9.

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15. Hoffman HJ, Taecholarn C, Hendrick EB, HumphreysRP. Management of lipomyelomeningoceles. J Neuro-surgery 1985;62:1–8.

16. LaMarca F, Grant JA, Tomita T, McLone D. Spinal lipo-mas in children: outcome of 270 procedures. PediatrNeurosurgery 1997;26:8–16.

17. McLone D. Occult dysraphism and the tethered spinalcord. In: Pediatric Neurosurgery, 1999; 61–78.

18. Zerah M, Pierre-Kahn A, Catala M. Lumbosacral lipo-mas. In: Pediatric Neurosurgery, 1999; 79–100.

19. Pierre-Kahn A, Zerah M, Renier D et al. Congenital lum-bosacral lipomas. Childs Nerv Syst 1997;13:298–335.

20. Chumas PD. The role of surgery in asymptomatic lum-bosacral spinal lipomas. Brit J Neurosurg 2000;14:301–4.

21. Pang D, Dias MS, Ahab-Barmada M. Split cord malfor-mation: Part I: A unified theory of embryogenesis fordouble spinal cord malformations. Neurosurgery. 1992Sep;31(3):451–80.

22. Pang D. Split cord malformation. Part II: clinical syn-drome. Neurosurgery 1992;31:481–500.

23. James CCM, Lassman LP. Spinal dysraphism: spinabifida occulta. London: Butterworths, 1972.

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