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Spine 13:896–98 796 Section Spinal Deformities and Malformations 29 Malformations of the Spinal Cord Dilek Könü-Leblebicioglu, Yasuhiro Yonekawa Core Messages ✔ Spinal cord malformations ( =spinal dysraphisms) are usually diagnosed at birth or early infancy (open spinal dysraphism, closed spinal dysraphisms with a back mass) but are some- times not discovered before adulthood ✔ Spinal cord malformations arise from defects occurring in the embryological stages of gastrulation (weeks 2–3), neurulation (weeks 3–6) and caudal regression ✔ The term “spina bifida” merely refers to a defective fusion of posterior spinal bony elements but is still incorrectly used to refer to spinal dysraphism in general ✔ “Tethered spinal cord” is a broadly used umbrella term for numerous spinal cord abnormalities, such as lipomyelomeningocele, previ- ously operated on myelomeningoceles, or thickened filum terminale, which tether (fasten, fix) the spinal cord in the spinal canal ✔ Tethered cord syndrome is a stretch-induced functional disorder of the spinal cord worsened by daily, repeated mechanical stretching, and distortion may even occur in patients who have the conus at normal level ✔ Patients with spinal cord malformation are either diagnosed at birth or present later because of unexplained pain, neurological deficits, unclear recurrent urologic infections, cutaneous markers or orthopedic deformities ✔ MRI is the imaging modality of choice and has increased the number of tethered spinal cord diagnoses ✔ Prenatal treatment encompasses prophylactic folic acid substitution and intrauterine surgery ✔ Open spinal dysraphism is best surgically treated postpartum to untether the spinal cord, prevent infections, repair the dural/cutaneous defect, and restore normal anatomy as far as possible ✔ Closed spinal dysraphism with tethered spinal cord warrants early untethering, when mini- mum or mild symptoms are detected ✔ Surgery after development of the deficits only stops progression, but symptoms may even further progress after detethering ✔ Individuals with spinal malformations need both lifelong surgical and medical management, which should be provided by a multidis- ciplinary team Epidemiology Myelomeningocele is the most common form of open spinal dysraphism Spine and spinal cord malformations are often collectively summarized under the term of spinal dysraphisms [39]. This term was first employed by Lichten- stein (1940) [36]. Open spinal dysraphism is a common congenital midline defect of the nervous system and has been historically reported in 2–4/1000 live births [14]. However, the true incidence of spinal dysraphism is not well studied. Myelo- meningocele accounts for the vast majority of open spinal dysraphisms (98.8%) [32, 39]. The incidence of myelomeningocele is 0.6 per 1 000 live births My elomeningocele occurs in 0.6 patients per 1000 live births, and females are affected slightly more often than males (by a ratio of 1.3 to 3), with the first-born usually affected [5, 39]. Myelocele is a rare malformation and represents only 1.2% of all open spinal dysraphisms [39]. The most common locations for these malformationsare,indecreasingfrequency,lumbosacral,thoracolumbarand Spinal Deformities and Malformations Section 797 a b c Case Introduction A 17-year-old patient presented with progressive tethered cord syndrome with worsening of hand functions and some leg weakness and increasing spasticity. Postnatally he had had a cervical myelomeningocele and had had only “cosmetic” closure after the birth. The MRI showed a widened spinal canal at C6–C1 ( a, c), cord tethering dorsally at C6–7 and dorsal limited myeloschisis. It is possible to see the hypotrophic right hand ( b). This clinical worsening recovered after an intradural exploration and dissec- tion of the stalk placode. cervical spine [5, 39]. The incidence of myelomeningocele varies from country to country and from one geographical region to another [20]. Since the early 1980s, estimation of the prevalence of open spinal dysraphism in many industrialized countries has been decreased by folic acid administration to pregnant women and the availability of prenatal diagnosis and elective termination [20, 29, 48]. Patients with open spinal dysraphism almost always have associated Chiari II malformation. There are also reports in the medical literature of an association between closed spinal dysraphisms and Chiari II [41]. Spina bifida is present in 90 –100 % of patients with tethered cord Spina bifida occulta occurs in approximately 17–30% of the total population and is present in 90–100% of patients with tethered cord [35, 61]. The dermal sinus is a common abnormality and accounts for 23.7%of all closed spinal dysraphisms. Overall, caudal regression syndrome is not uncommon, accounting for 798 Section Spinal Deformities and Malformations 16.3% of all closed spinal dysraphisms. Sacral agenesis occurs in approximately one per 7500 births without a gender predisposition. The conus normally terminates at L2 In the normal adult population the conus terminates at L2 in 95% of cases [19, 48]. In its classical form, tethered cord implies a low-lying conus, but tethered cord syndrome may occur in the presence of a conus in normal position [19, 37, 40, 46, 48, 54, 56]. Up to 15% of patients with repaired myelomeningoceles will experience a secondary tethered cord syndrome later in life [36]. Pathogenesis Embr yological Aspects Knowledge of normal embryology is essential for the understanding of the pathogenesis and a wide spectrum of pathoanatomy of spine and spinal cord anomalies as well as tethered cord. The most comprehensive embryonic staging system is that of O’Rahilly [23] and most of the information on early human development has been obtained through study of the Carnegie collection [23]. Early neural development has been reviewed in various basic science articles [21]. O’Rahilly provides a timetable for each important event in early neural morpho- genesis: the embryonic p eriod begins at conception with stage 1 and ends at stage 23. Beyond this time, the developing human enters the fetal period [6, 23] ( Table 1). Table 1. Human embryogenesis Weeks Days Carnegie stage Process Size (mm) Somite number Events Embryonal period Week 1 1 1 fertilization 0.1–0.15 fertilized oocyte, pronuclei 2– 3 2 cleavage 0.1–0.2 cell division with reduction in cytoplasmic volume, formation of inner and outer cell mass 4– 5 3 blastula 0.1– 0.2 loss of zona pellucida, free blastocyst 5– 6 4 0.1– 0.2 attaching blastocyst Week 2 7–12 5 0.1 – 0.2 implantation 13–15 6 0.2 extraembryonic mesoderm, primitive streak Week 3 15–17 7 gastrulation 0.4 gastrulation, notochordal process 17–19 8 neurulation 1.0–1.5 primitive pit, notochordal canal 19–21 9 somatization 1.5 – 2.5 1– 3 neural folds, cardiac primordium, head fold Week 4 22–23 10 2– 3.5 4–12 neural fold fuses 23–26 11 2.5 – 4.5 13–20 rostral neuropore closes 26–30 12 3– 5 21 – 29 caudal neuropore closes Week 5 28–32 13 organogenesis 4–6 30 leg buds, lens placode, pharyngeal arches 31–35 14 5–7 lens pit, optic cup 35–38 15 7– 9 lens vesicle, nasal pit, hand plate Week 6 37–42 16 8– 11 nasal pits moved ventrally, auricular hillocks, foot plate 42–44 17 11 – 14 finger rays Week 7 44–48 18 13 – 17 ossification commences 48–51 19 16 – 18 straightening of trunk Week 8 51–53 20 18– 22 upper limbs longer and bent at elbow 53–54 21 22 – 24 hands and feet turned inward 54–56 22 23 – 28 eyelids, external ears Fetal period Week 9 56–60 23 phenogenesis 27 – 31 rounded head, body and limbs longer Malformations of the Spinal Cord Chapter 29 799 Relevant Embryogenetic Steps Spinal cord embryological development occurs through three consecutive peri- ods [11, 19, 26, 39, 48, 58]: Gastrulation The trilaminar embryo develops by day 18 of gestation. At this point, the embryo is composed of endoderm, mesoderm and ectoderm. Shortly thereafter, the mesoderm releases factors which induce the differentiation of the overlying neu- roectoderm, thereby forming the neural tube. Neurulation After gastrulation the ectoderm above the notochord folds to form a tube, the neural tube; this gives rise to the brain and the spinal cord, a process known as neurulation. Primary neurulation (weeks 3–4): The process of fusion begins in the region of the lower medulla and proceeds rostrally and caudally. The anterior neuropore closes at about 24 days and the posterior neuropore at 26–28 days. The brain and the spinal cord are formed by primary neurulation, which involves the shaping, folding, and midline fusion of the neural plate. It is completed about the 25–26th day of conception. The central canal is formed and is lined by ependyma. The caudal cell mass, a group of undifferentiated cells at the caudal end of the neural tube, develops vacuoles. These vacuoles merge together and expand, ultimately meeting the central canal of the rostral cord and causing elongation of theneuraltubeinaprocesscalledcanalization.Secondar y neurulation and retrogressive differentiation (weeks 5–6) results in formation of the conus tip and Filum terminale and conus medullaris are formed during the process of neurulation filum terminale. The formation of the lower lumbar, sacral, and coccygeal por- tions of the neural tube are by canalization and retrogressive differentiation. Overlapping with canalization, the process of retrogressive differentiation of the caudalcellmasstakesplace.Inthisprocess,thefilumterminale,conusmedulla- ris, and ventriculus terminalis are formed. Caudal Regression The conus medullaris ascends during spinal growth At the time when the neurulation process is complete (weeks 6–7), the terminal filum and cauda equina are formed from the caudal portion of the neural tube by regression. The conus medullaris initially rests in the coccygeal region and appearstoascendasthespinegrowsmorerapidlythanthecord.Atbirththe conusisusuallyatthecaudallevelofL2–L3andby3monthsofageitisatL1–L2, where it remains (relative ascent of the spinal cord). The spinal cord terminates at or above the inferior aspect of the L2 vertebral body in 95% of the population andatorabovetheL1–L2discspacein57%ofthepopulation.Theconusmedul- laris has reached its mature adult level at term in most infants and 100% of cases at approximately 3 months after full-term gestation [39, 48, 58]. The conus medullaris initially rests in the coccygeal region and appears to ascend as the spine grows more rapidly than the cord. At birth the conus is usually at the caudal level of L2–L3 and by 3 months of age it is at L1–L2, where it remains. Interference with normal development at any stage is responsible for the various abnormalities seen in the cases of spinal malformations [19, 26, 38, 39, 58] ( Table 2). 800 Section Spinal Deformities and Malformations Table 2. Embryological classification of spinal dysraphisms Embryological stage Dysraphism Gastrulation Notochordal integration neuroenteric cysts and fistula split cord malformations (diastematomyelia, diplomyelia) dermal sinus, fistula dermoid/epidermoid tumors Notochordal formation caudal regression syndrome segmental spinal dysgenesis Primary neurulation myelomeningocele myelocele lipomyelomeningocele lipomyeloschisis intradural spinal lipoma Secondary neurulation tight filum terminale, filum terminale lipoma Canalization Retrogressive differentiation intrasacral meningocele, sacral cysts Risk Factors Most spinal cord anomalies result from a complex interaction between several genes and poorly understood environmental factors. A list of variables have been implicated as risk factors for spinal dysraphisms but only a few have been estab- lished. Genetic Factors Family history is an important risk factor Spinal cord anomalies occur in many syndromes and chromosome disorders. However, a spinal dysraphism may be the only anomaly in a member of a family, in which case the relatives have an increased risk for all types of tethered cord.A family history is one of the strongest risk factors [20, 26]. Environmental Factors Periconceptual folic acid substitution reduces the incidence of neural tube defects Periconceptual multiple vitamin supplements containing folic acid reduce the incidence of neural tube defects. In England and the United States, it is recom- mended that women planning pregnancy take 0.4 mg folic acid daily before conception and during the first 12 weeks of pregnancy [14, 44]. Up to 70% of spina bifida cases can be prevented by periconceptional folic acid supplementation [20, 26]. Maternal Diabetes Pre-gestational diabetes is a risk faktor for spinal malformation In women with pre-gestational diabetes, the risk of having a child with a central nervous system malformation (including spinal malformations) is twofold higher than the risk in the general population [20]. Medication Valproic acid or carbama- zepine increases the risk of spinal malformation Some drugs taken during pregnancy may increase the risk. These include sodium valproate and folic acid antagonists such as trimethoprim, triamterene, carb- amazepine, phenytoin, phenobarbital and primidone [20]. Malformations of the Spinal Cord Chapter 29 801 Pathophysiology of Tethered Cord Syndrome Tethering of the spinal cord results in progressive neurological deficits Tethered cord is a spinal cord malformation in which the spinal cord is fixed in an abnormally low position and in a relatively immobile state [2, 19, 39, 46, 58]. In this context, the term “tether” refers to “fasten” or “restrain”.Tethered cord exists in open and occult forms of spinal dysraphisms [15, 48]. The normal spinal cord is free, i.e. it is not attached to any surrounding structures in the spinal canal except for denticulate ligaments and nerve roots. A tethered cord is tightly fixed so that there is not a normal movement of the spinal cord. During the formation of the embryonic spinal cord, it fills the entire length of the spinal canal. As the fetus grows, the vertebral column grows faster than the spinal cord. Thus, the dis- tal end of the spinal cord is located at the level of the first or second lumbar vertebral body (L1–L2).If there is an abnormality affecting this “ascension” of the spinal cord (e.g. myelomeningocele, tight filum terminale, diastematomyelia, secondary scar formations, tumors), the spinal cord is tethered [50]. This results in stretching of the spinal cord and causes neurological damage even during the fetal period. By the time a child is born, the spinal cord is normally located between the first or second lumbar vertebral body. After birth, continuing growth puts further stretch on the tethered spinal cord; this damages the spinal cord both by directly stretching it, and by interfering with the blood supply and oxidative metabolism [51]. Atetheredcord canoccurevenwith a normal level conus If neurological findings are already present the further clinical deterioration can be anticipated. Since an adult spine is no longer growing, children are obvi- ouslymoreatriskthanadults.However,evenadultswithtetheredcordcanshow deterioration. This is due to daily repetitive-cumulative stretching on the tethered cord. A sudden flexion movement of the spine can also produce symptom- atic onset of the tethered cord syndrome [9, 51]. Irreversible neuronal damage can occur when there is sudden stretching of the already chronically tethered conus [51]. Yamada and coworkers have nicely demonstrated changes in spinal cord blood flow and oxidative metabolism following tethering of the spinal cord Atetheredcordcanoccur with the conus at a normal level both in experimental animals and humans [9, 51, 52, 55, 58]. Usually a tethered cord results in a low conus position. However, there are many cases of tethered cord syndrome reported with the conus at a normal level [37, 40, 46]. Terminology and Classification Spinal cord malformations can be categorized as: open spinal dysraphisms closed (occult) spinal dysraphism Open spinal dysraphism is characterized by exposure of the abnormal spinal nervous tissue and/or meninges to the environment through a bony and skin defect. Open spinal dysraphism basically includes myelocele and myelomeningocele. In closed spinal dysraphism, there is no exposure of neural tissue (covered by skin). However, some kind of cutaneous stigmata, such as hairy patch, dim- ples,orsubcutaneousmasses,canberecognizedinupto50%ofclosedforms [15, 32, 47]. Spina bifida results from a defective fusion of posterior spinal bony elements and leads to a bony cleft in the spinous process and lamina (L5 and S1). The term has incorrectly been used to refer to spinal dysraphism in general [32, 39]. The terms spina bifida aperta or cystica and spina bifida occulta were used to refer to open spinal dysraphism and closed spinal dysraphism, respectively. These terms have been progressively discarded [32]. 802 Section Spinal Deformities and Malformations Table 3. Chiari malformations Type 1 caudal displacement of the cerebellum cerebellar tonsils below the plane of the foramen magnum no involvement of the brainstem associated with occult spinal dysraphism (e.g. spinal lipomas) note – cerebellar ectopia can be a normal finding (up to 5 mm) Type II small and crowded posterior fossa caudal displacement of the fourth ventricle and medulla into the upper cervical canal tonsils can be at or below the level of the foramen magnum usually association with a variety of cerebral anomalies frequently associated with myelomeningoceles Type III displacement of the posterior fossa structures into the cervical canal (seldom compatible with life) Type IV cerebellar hypoplasia without herniation Placode (neural placode) is a segment of non-neurulated embryonic neural tissue. It is in contact with air in open spinal dysraphism and covered by the integu- ment in closed spinal dysraphism. A terminal placode lies at the caudal end of the spinal cord and may be apical or parietal depending on whether it involves the apex or a longer segment of the cord. A segmental placode may lie at any level along the spinal cord [32, 39]. Differentiate hydromyelia from syringomyelia Hydromyelia is the simple dilatation of the central canal and is lined by the ependyma. An extension into cord parenchyma constitutes a true syringomyelia. Two forms of syringomyelia can be differentiated: communicating syringomyelia non-communicating syringomyelia Communicating syringomyelia is related to a primary dilatation of the central canal and is usually associated with abnormalities of the craniocervical junction (e.g. Chiari malformations). Non-communicating syringomyelia may result from trauma, tumors or inflammation and does not communicate with the central canal or the subarachnoidal space. Chiari malformations are hind brain abnormalities and are observed in con- junction with spinal cord malformations. They are categorized into four types, with Types I and II accounting for 99% of the clinical cases ( Table 3). Classification of Spinal Malformation From a clinical perspective, a practicable classification system of spinal cord anomalies is needed. However, the large variety of features associated with these anomalies makes such classification difficult. Classical classifications rely on the embryological development cascade [11, 19, 22, 39, 58] ( Table 4). We find the mixed clinical-neuroradiological classification system presented by Donati et al. [5, 32, 39] useful. From the clinical perspective, a question framework to approach the spectrum of spinal cord malformation is useful: Is there a back mass? Is it covered with skin? Are there cutaneous markers? Is there a tethered cord syndrome? Malformations of the Spinal Cord Chapter 29 803 Table 4. Classification of spinal malformations Spinal malformations with back mass Open spinal dysraphism With a non-skin-covered back mass (spina bifida aperta) myelomeningocele Almost always associated with Chiari II malformation myelocele (myeloschisis) Closed (occult) spinal dysraphism Withaskin-coveredbackmass(spinabifidacystica) meningocele (posterior) myelocytocele lipomyelomeningocele/lipomyeloschisis Spinal malformations without back mass spinal lipoma (intradural and/or intramedullary) anterior sacral/lateral thoracic meningocele tight filum terminale/filum terminale lipoma dermal sinus, fistula, dermoid/epidermoid tumors neuroenteric/bronchogenic cysts and fistula (split notochord syndrome) split cord malformations (diastematomyelia, diplomyelia) caudal regression/agenesis intrasacral meningocele/sacral cysts neuroectodermal appendages Myelomeningocele and Myelocele Myelomeningoceles and myeloceles are characterized by exposure of spinal intradural elements through a midline defect to the air. The basic defect of myelomeningocele is caused by an abnormality, which occurs at the stage of neurulation that prevents the neural tube from closing dorsally [5, 19, 22, 27, 39]. A myelomeningocele consists of a sac of exposed neural tissue-placode, which is clef- ting dorsally, splayed open and herniates through a large dysraphic defect through the bone and dura beyond the surface of the back. The cord is tethered posteriorly at this level. In myelocele (synonym: myeloschisis), however, the neural placode is flush with the plane of the back and identifiable on the surface. All children with myelomeningocele have tethered cord from the time of birth. One can easily visualize how tethering of the spinal cord might occur ( Case Study 1). Patients with myelomeningocele and myelocele almost always (75–100%) have associated Chiari II malformation ( Table 3) [5, 14, 20, 32, 39]. Distortion and maldevelopment of the medulla and midbrain can cause lower cranial nerve palsies and central apnea (which may be misdiagnosed as epilepsy) [44]. Patients with myelomeningocele and myelocele almost always have associated Chiari II malformation Hydrocephalusmaybepresentatbirth,butusuallyappearswithin2–3days after surgery [14, 32, 45]. The rate of hydrocephalus in patients with occult spinal dysraphism has been reported to be over 80% [14, 43]. Hydromyelia may occurinasmanyas80%ofthesepatients,andmaybelocalizedorextend throughthewholecord.Itmaycauserapiddevelopmentofscoliosisifleft untreated [18, 29, 32]. Meningocele The posterior meningocele consists of a herniated sac of meninges with CSF protruding from the back and covered with skin. It is commonly lumbar or sacral in location, but thoracic and even cervical meningoceles may be found. The spinal cord and conus are seen in the normal position [5, 32, 39], although both nerve roots and, more rarely, a hypertrophic filum terminale may course within the meningocele. No part of the spinal cord is contained within the sac by defini- tion [5]. The spinal cord itself is completely normal structurally, although it is usually tethered to the neck of sacral meningoceles [39]. A Chiari II malformation is found only exceptionally. Anterior meningoceles are typically presacral, 804 Section Spinal Deformities and Malformations . understanding of the pathogenesis and a wide spectrum of pathoanatomy of spine and spinal cord anomalies as well as tethered cord. The most comprehensive embryonic staging system is that of O’Rahilly. and appearstoascendasthespinegrowsmorerapidlythanthecord.Atbirththe conusisusuallyatthecaudallevelofL2–L3andby3monthsofageitisatL1–L2, where it remains (relative ascent of the spinal cord). The spinal cord terminates at or above the inferior aspect of the L2 vertebral body in 95% of the. modality of choice and has increased the number of tethered spinal cord diagnoses ✔ Prenatal treatment encompasses prophylactic folic acid substitution and intrauterine surgery ✔ Open spinal dysraphism