Disorders of Secondary Neurulation : Mainly Focused on Pathoembryogenesis

Fundamental origins of neural tube defects with a basis in genetics and nutrition

Neural tube defects (NTDs) are leading congenital malformations. Its global prevalence is one in 1000 pregnancies and it has high morbidity and mortality. It has multiple risk factors like genetic errors and environmental stressors like maternal malnutrition and in utero exposure to pollutants like chemicals. The genetic program determines neural tube development based on timely expression of many genes involved in developmental signaling pathways like BMP, PCP and SHH. BMP expression defines ectoderm. SOX represses BMP in ectoderm and convertes to the neuroectoderm. Subsequently, PCP molecules define the tissue patterning for convergent-extension, a critical step in neural tube genesis. Further, SHH sets spatial patterning of the neural tube. Nutrients are the essential major environmental input for embryogenesis. But it may also carry risk factors. Malnutrition, especially folate deficiency, during embryogenesis is a major cause for NTDs. Folate is integral in the One Carbon metabolic pathway. Its deficiency and error in the pathway are implicated in NTDs. Folate supplementation alone is insufficient to prevent NTDs. Thus, a comprehensive understanding of the various risk factors is necessary to strategize reduction of NTDs. We review the current knowledge of various risk factors, like genetic, metabolic, nutritional, and drugs causing NTDs and discuss the steps required to identify them in the early embryogenesis to avoid NTDs.

Association of spinal cord abnormalities with vertebral anomalies: an embryological perspective

PURPOSE

To analyze the relationship between spinal cord and vertebral abnormalities from the point of view of embryology.

METHODS

We analyzed the clinical and radiological data of 260 children with different types of spinal cord malformations in combination with vertebral abnormalities.

RESULTS

Among 260 individuals, approximately 109 presented with open neural tube defects (ONTDs), 83 with split cord malformations (SCMs), and 83 with different types of spinal lipomas. Pathological spina bifida emerged as the most frequent vertebral anomaly, affecting 232 patients, with a higher prevalence in ONTD. Vertebral segmentation disorders, including unsegmented bars, butterfly vertebrae, and hemivertebrae, were present in 124 cases, with a higher prevalence in SCM. The third most common spinal anomaly group consisted of various forms of sacral agenesis (58 cases), notably associated with blunt conus medullaris, spinal lipomas, and sacral myelomeningocele. Segmental aplasia of the spinal cord had a typical association with segmental spinal absence (N = 17).

CONCLUSION

The association between SCM and neuroenteric cyst/canal and vertebral segmentation disorders is strong. High ONTDs often coincide with pathological spina bifida posterior. Type 1 spinal lipomas and focal spinal nondisjunction also correlate with pathologic spina bifida. Segmental spinal absence or dysgenesis involves localized spinal and spinal cord aplasia, sometimes with secondary filar lipoma.

Cartilage within lipomyelomeningocele and ulnar longitudinal deficiency syndrome as VACTERL association, alliance in SHH/GLI3, and Wnt pathway: illustrative case

BACKGROUND

Lipomyelomeningocele associated with an ulnar club hand in the spectrum of VACTERL association ([costo-]vertebral abnormalities; anal atresia; cardiac defects; tracheal-esophageal abnomalities, including atresia, stenosis, and fistula; renal and radial abnormalities; limb abnormalities; single umbilical artery) is a very rare and infrequently reported phenomenon. Within the fat mass of the lipoma, it is not common to find a well-defined cartilaginous mass with no attachments to the surrounding tissue.

OBSERVATIONS

The authors present the case of a 3-month-old male with low-back swelling that was off-center to the left, accompanied by a left short forearm displaying outward bowing. Echocardiography showed an atrial septal defect. This rare VACTERL association comprises lipomyelomeningocele, atrial septal defect, and ulnar longitudinal deficiency syndrome. During surgical intervention for the lipoma, a well-defined cartilaginous mass was discovered within the adipose tissue.

LESSONS

The manifestation of VACTERL association can be partially explained by the Shh/Gli and Wnt pathway defects. It is prudent to screen children with neural tube defects to be aware of any associated syndromes. This case is very rare, and the literature has contained no prior report on the VACTERL association of lipomyelomeningocele, atrial septal defect, and ulnar longitudinal deficiency.

Fibrous hamartoma of infancy of the spinal cord resembling conus and filum, with a coexisting sacral dimple

Fibrous hamartoma of infancy (FHI) is a rare benign soft tissue lesion of infants and young children. It usually occurs within the first 2 years of life at the superficial layer of the axilla, trunk, upper arm, and external genitalia. FHI in the central nervous system (CNS) is extremely rare. So far, only two spinal cord FHI cases have been reported. We present a case of a 1-month-old girl who presented with a skin dimple in the coccygeal area. Her MRI showed a substantial intramedullary mass in the thoracolumbar area with a sacral soft tissue mass and a track between the skin lesion to the coccygeal tip. Her normal neurological status halted immediate surgical resection. A skin lesion biopsy was first performed, revealing limited information with no malignant cells. A short-term follow-up was performed until the intramedullary mass had enlarged on the 5-month follow-up MRI. Based on the frozen biopsy result of benign to low-grade spindle cell mesenchymal tumor, subtotal resection of the mass was done, minimizing damage to the functioning neural tissue. Both the skin lesion and the intramedullary mass were diagnosed as FHI. Postoperative 5.5-year follow-up MRI revealed minimal size change of the residual mass. Despite being diagnosed with a neurogenic bladder, the patient maintained her ability to void spontaneously, managed infrequent UTIs, and continued toilet training, all while demonstrating good mobility and no motor weakness. This case is unique because the lesion resembled the secondary neurulation structures, such as the conus and the filum, along with a related congenital anomaly of the dimple.

Surgery for spina bifida occulta: spinal lipoma and tethered spinal cord

The technical evolution of the surgery for spina bifida occulta (SBO) over the course of a half-century was reviewed with special foci placed on the spinal lipoma and tethered spinal cord. Looking back through history, SBO had been included in spina bifida (SB). Since the first surgery for spinal lipoma in the mid-nineteenth century, SBO has come to be recognized as an independent pathology in the early twentieth century. A half-century ago, the only option available for SB diagnosis was the plain X-ray, and pioneers of the time persevered in the field of surgery. The classification of spinal lipoma was first described in the early 1970s, and the concept of tethered spinal cord (TSC) was proposed in 1976. Surgical management of spinal lipoma with partial resection was the most widely practiced approach and was indicated only for symptomatic patients. After understanding TSC and tethered cord syndrome (TCS), more aggressive approaches became preferred. A PubMed search suggested that there was a dramatic increase of publications on the topic beginning around 1980. There have been immense academic achievements and technical evolutions since then. From the authors' viewpoint, landmark achievements in this field are listed as follows: (1) establishment of the concept of TSC and the understanding of TCS; (2) unraveling the process of secondary and junctional neurulation; (3) introduction of modern intraoperative neurophysiological mapping and monitoring (IONM) for surgery of spinal lipomas, especially the introduction of bulbocavernosus reflex (BCR) monitoring; (4) introduction of radical resection as a surgical technique; and (5) proposal of a new classification system of spinal lipomas based on embryonic stage. Understanding the embryonic background seems critical because different embryonic stages bring different clinical features and of course different spinal lipomas. Surgical indications and selection of surgical technique should be judged based on the background embryonic stage of the spinal lipoma. As time flows forward, technology continues to advance. Further accumulation of clinical experience and research will open the new horizon in the management of spinal lipomas and other SBO in the next half-century.

Retained medullary cord and caudal lipoma with histopathological presence of terminal myelocystocele in the epidural stalk

Background

The retained medullary cord (RMC), caudal lipoma, and terminal myelocystocele (TMCC) are thought to originate from the failed regression spectrum during the secondary neurulation, and the central histopathological feature is the predominant presence of a central canal-like ependyma-lined lumen (CC-LELL) with surrounding neuroglial tissues (NGT), as a remnant of the medullary cord. However, reports on cases in which RMC, caudal lipoma, and TMCC coexist are very rare.

Case Description

We present two patients with cystic RMC with caudal lipoma and caudal lipoma with an RMC component, respectively, based on their clinical, neuroradiological, intraoperative, and histopathological findings. Although no typical morphological features of TMCC were noted on neuroimaging, histopathological examination revealed that a CC-LELL with NGT was present in the extraspinal stalk, extending from the skin lesion to the intraspinal tethering tract.

Conclusion

This histopathological finding indicates the presence of TMCC that could not be completely regressed and further supports the idea that these pathologies can be considered consequences of a continuum of regression failure during secondary neurulation.

Limited dorsal myeloschisis associated with intramedullary infantile hemangioma in the conus medullaris: illustrative case

BACKGROUND

Limited dorsal myeloschisis (LDM) and intramedullary infantile hemangioma rarely coexist in the spinal cord.

OBSERVATIONS

The authors describe the case of a 3-month-old girl who, despite lacking neurological symptoms or signs, had a cigarette burn-like mark at the lumbosacral area and skin dimpling in the gluteal area. Magnetic resonance imaging showed a low-set conus due to a thickened filum and an abnormal subcutaneous stalk connected to the conus medullaris. In combination with the skin lesions, these findings strongly implied nonsaccular-type LDM. An intramedullary mass in the conus medullaris was also shown on magnetic resonance imaging and was homogenously enhanced with isointensity on T1- and T2-weighted images. We prophylactically untethered the spinal cord and partially removed the intramedullary mass, which had no clear borders, for a safe surgical dissection. Histologically, the intramedullary mass was an infantile hemangioma, and the subcutaneous stalk was a lesion associated with LDM. The patient remained neurologically intact after surgery, and then 2 years later, there was spontaneous regression of the residual tumor.

LESSONS

Although rare, nonsaccular type LDM may appear concurrently with intramedullary infantile hemangioma at the conus medullaris. The authors present a possible mechanism behind this concurrent presentation in the same area.

Secondary Neurulation Defects: Retained Medullary Cord

Retained medullary cord (RMC) is a defect resulting from impaired secondary neurulation. Intraoperatively, RMC is recognizable as an elongated cord-like structure caudal to the conus, that contains histologically confirmed neuroglial components and a lumen with an ependymal lining. It characteristically does not possess neurological function. This chapter aims to summarize (1) the mechanisms that lead to the occurrence of RMC; (2) the various forms of RMC, such as cystic RMC and 'possible RMC', and (3) the treatment strategies, especially untethering through limited exposure.

Ectopic dorsal root ganglion in cauda equina mimicking schwannoma in a child

Background

A heterotopic dorsal root ganglion (DRG) is sometimes observed in the vicinity of dysplastic neural structures during surgery for open spinal dysraphism; however, it is rarely associated with closed spinal dysraphism. Distinguish from neoplasms by preoperative imaging study is difficult. Although the embryopathogenesis of a heterotopic DRG has been speculated to be migration disorder of neural crest cells from primary neural tube, its details remain unelucidated.

Case Description

We report a pediatric case with an ectopic DRG in cauda equina associated with a fatty terminal filum and bifid sacrum. The DRG mimicked a schwannoma in the cauda equina on preoperative magnetic resonance imaging. Laminotomy at L3 revealed that the tumor was entangled in the nerve roots, and small parts of the tumor were resected for biopsy. Histopathologically, the tumor consisted of ganglion cells and peripheral nerve fibers. Ki-67 immunopositive cells were observed at the periphery of the ganglion cells. These findings indicate the tumor comprised DRG tissue.

Conclusion

We report detailed neuroradiological, intraoperative and histological findings and discuss the embryopathogenesis of the ectopic DRG. One should be aware of the possibility of ectopic or heterotopic DRGs when cauda equina tumors are observed in pediatric patients with neurulation disorders.

Myelomeningocele as an anomaly of secondary neurulation

PURPOSE

Myelomeningocele (MMC) is the representative entity of open neural tube defects resulting from an error during primary neurulation. However, cases of MMC in the region of the secondary neural tube (below the junction of S1 and S2 vertebrae) are sometimes encountered. We aimed to analyze the clinical features of atypical "low-lying" MMC in comparison to the typical MMC and suggest possible pathoembryogenesis.

METHODS

From 1986 to 2020, 95 MMC patients were treated in our institute. A retrospective review of the radiological and clinical information was performed. We defined "low-lying" MMCs as those with fascia or lamina defects below the S1-2 interspinous ligament.

RESULTS

Thirty-one out of the 95 MMC patients were identified as having low-lying MMC. The percentage of low-lying MMC within the entire MMC group increased dramatically (19% from 1990 to 1999 and 48% from 2000 to 2020). Thirty-nine percent of the low-lying MMCs were associated with hydrocephalus, and 36% showed the Chiari malformation. Clean intermittent catheterization was being performed by 52% of the patients and 46% had a motor weakness. The proportions of hydrocephalus, neurological symptoms, and the number of related procedures in the low-lying MMC were substantially lower than the typical MMC in our cohort and the literature.

CONCLUSIONS

We present cases of atypical MMC occurring in the region of secondary neurulation. These cases provide clues that secondary neurulation may lead to open neural defects. Future experiments with animal models supporting what we have seen in the clinics will greatly enhance the understanding of the developmental process of neurulation and the corresponding anomalies.

Retained medullary cord and terminal myelocystocele as a spectrum: case report

The caudal portion of the spinal cord, the medullary cord, is formed by secondary neurulation. One of the distinctive features of secondary neurulation compared to primary neurulation is that the medullary cord normally degenerates into a filum in humans. Various anomalies have been known to originate from degenerating process errors. One anomaly is terminal myelocystocele (TMCC), which is a closed spinal dysraphism with an elongated caudal spinal cord. The terminal part is filled with cerebrospinal fluid (CSF) and protrudes into the dorsal extradural space. Another anomaly is the retained medullary cord (RMC), which is a nonfunctioning cord-like structure extending to the cul-de-sac. In a 1-month-old boy, we identified an RMC with cystic dilatation of the caudal end extending to the epidural space at the very bottom of the cul-de-sac, resembling a degenerating terminal balloon, which is an essential feature of TMCC. Hence, this case may be considered an intermediate form between TMCC and RMC. This case provides clinical evidence that TMCC and RMC share the same pathoembryogenic origin, namely, failure of the regression phase of secondary neurulation.

Intramedullary abscess at thoracolumbar region transmitted from infected dermal sinus and dermoid through retained medullary cord

Background:

A retained medullary cord (RMC) is a relatively newly defined entity of closed spinal dysraphism that is thought to originate from regression failure of the medullary cord during secondary neurulation. A congenital dermal sinus (CDS) may provide a pathway for intraspinal infections such as repeated meningitis. Intramedullary abscesses are the rarest but most serious complication of a CDS.

Case Description:

We treated a female infant with an intramedullary abscess in the thoracolumbar region, which was caused by infection of the CDS. Surgery revealed that the cord-like structure (C-LS) started from the cord with the intramedullary abscess, extended to the dural cul-de-sac, and further continued to the CDS tract and skin dimple. The boundary between the functional cord and the non-functional CL-S was electrophysiologically identified, and the entire length of the C-LS (the RMC) with an infected dermoid cyst was resected. As a result, the abscess cavity was opened and thorough irrigation and drainage of the pus could be performed. Histopathological examination of the C-LS revealed an infected dermoid cyst and abscess cavity with keratin debris in the fibrocollagenous tissue. The abscess cavity had a central canal-like ependymal lined lumen (CCLELL), with surrounding glial fibrillary acidic protein (GFAP)-immunopositive neuroglial tissues.

Conclusion:

We demonstrated that the transmission of an infection through the RMC was involved in the development of the intramedullary abscess. A good postoperative outcome was obtained because a terminal ventriculostomy for pus drainage could be achieved by excising the nonfunctional RMC.

Disorders of Secondary Neurulation: Suggestion of a New Classification According to Pathoembryogenesis

Recently, advanced knowledge on secondary neurulation and its application to the clinical experience have led to the deeper insight into the pathoembryogenesis of secondary neurulation with new classifications of the caudal spinal dysraphic entities. Here, we summarize the dynamic changes in the concepts of disordered secondary neurulation over the last two decades. In addition, we suggest our new pathoembryogenetic explanations for a few entities based on the literature and the data from our previous animal research. Disordered secondary neurulation at each phase may cause various corresponding lesions, such as (1) failed junction with the primary neural tube (junctional neural tube defect and segmental spinal dysgenesis), (2) dysgenesis or duplication of the caudal cell mass associated with disturbed activity of caudal mesenchymal tissue (caudal agenesis and caudal duplication syndrome), (3) abnormal continuity of medullary cord to the surrounding layers, namely, failed ingression of the primitive streak to the caudal cell mass (myelomeningocele), focal limited dorsal neurocutaneous nondisjunction (limited dorsal myeloschisis and congenital dermal sinus), and neuro-mesenchymal adhesion (lumbosacral lipomatous malformation), and (4) regression failure spectrum of the medullary cord (thickened filum and filar cyst, retained medullary cord and low-lying conus, terminal myelocele, and terminal myelocystocele). It seems that almost every anomalous entity of the primary neural tube may occur in the area of secondary neurulation. Furthermore, the close association of the caudal cell mass with the activity of caudal mesenchymal tissue involves a wider range of surrounding structures in secondary neurulation than in primary neurulation. Although the majority of the data are from animals and many theories are still conjectural, these changing concepts of normal and disordered secondary neurulation will provoke further advancements in our management strategies as well as in the pathoembryogenetic understanding of anomalous lesions in this area.