Developmental Cns Anomalies

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Developmental Abnormalities of the Central Nervous SystemPediatric NeurosurgeryMahmod Saad

Develop. Anomalies of CNS

Spinal Dysraphism and Related ConditionsContents Definitions Classification of Spina Bifida Epidemiology of Spina Bifida Etiology of Spina Bifida Pathology Principles of Treatment of Congenital Malformations Spina Bifida Occulta Neural Tube Defects (MC, MMC) Dermal Sinus and Associated Dermoid and Epidermoid Tumors Spinal Lipomas Combined Spina Bifida, Split Cord, Neurenteric Cysts Thickened Filum, Myelocystocoele Caudal Agenesis Encephaloceles Chiari II Malformation Tethered Spinal Cord

Definitions Spinal dysraphism. A generic term describing pathologic conditions related to improper closure of the caudal neuropore. Encompasses all conditions associated with spina bifida. Craniorachischisis. Total failure of neurulation. It is the most severe form of spinal dysraphism. There is no dorsal axial skeleton or dermal covering. Myeloschisis. Midline clefting of the spinal cord, partial or complete. Spina bifida cystica. Refers to a meningocele or meningomyelocele. Spina bifida aperta. A defect open to the environment. Neural plaque (or placode). The dorsal neural tissue contained within the meningomyelocele.


Develop. Anomalies of CNS

Classification of Developmental Abnormalities of the CNS A number of schemes have been described. The most straightforward is one that divides anomalies into open and closed. This has practical utility as most closed defects can be observed in the early period. It does not, however, provide a logical etiological framework to link the wide range of dysraphic states. Embryological Classification Anomalies due to defects of neural folding (neural tube defects such as MC and MMC). Anomalies due to incomplete dysjunction (dermal sinus and associated dermoid and epidermoid tumors). Anomalies due to premature dysjunction. (spinal lipomas). Anomalies due to disorders of gastrulation (combined spina bifida, split cord, neurenteric cysts). Anomalies due to disordered secondary neurulation (thickened filum, myelocystocoele). Anomalies due to failure of caudal neuraxial development (caudal agenesis). Anomalies due to disordered postneurulation development (encephaloceles). Secondarily acquired CNS anomalies due to myelodysplasia (Chiari II malformation).

Anatomical Classification Open Neural Tube Defects Meningocele Meningomyelocele Spina bifida aperta Encephalocele

Closed Neural Tube Defects Spina bifida occulta Lipoma Lipomyelomeningocele Dermal sinus tract Thickened filum Myelocystocele Caudal agenesis Diastematomyelia Diplomyelia Neurenteric cyst


Develop. Anomalies of CNS

Epidemiology Spina bifida occulta occurs in 20 to 30% of the population. Incidence for open spina bifida has been declining. Before early 80's, about 1-2/1000 live births in US and 4-5/1000 in Ireland. Now down to 3-6/10000 in US. Declining incidence is probably due to numerous factors. Most are unknown except possibly vitamin supplementation and the impact of antenatal screening and termination. F>M; whites > blacks and Orientals. Increased incidence in those belonging to poorer socioeconomic classes.

Genetics There is familial tendency, but not Mendelian inheritance. Probably multiple genes involved with variable penetrance. Risk of one child with spina bifida is 0.05%. If one sibling affected, risk increases to 5%. If 2 affected, risk increases to 12-15%. Genetic counseling should be offered to all parents.

Pathology Etiology

Unknown. Probably multifactorial. o o o o Genetic (see above) Nutrition (Folate, Zinc) Drugs (valproate, carbamazepine) Environmental (maternal hyperthermia)

Numerous teratogens have been suggested. Folate supplementation of 0.4 mg/day before conception and through early pregnancy reduces occurrence of spina bifida by 72% in highrisk and first pregnancies. Folic acid antagonists (aminopterin) produce neural tube defects in lab animals. Gross

Frequently, much of the CNS is affected in addition to the obvious defects. In addition, there may be GI, GU and CV abnormalities. See path. Under individual conditions. A large variety of brain abnormalities are seen including polymicrogyria, lobar agenessis, holoproencephaly, cerebellar dysplasia, heterotopias, schizencephaly, gyral abnormalities, agenesis of corpus callosum, septum pellucidum cysts, lipomas, arachnoid cysts, and high frequency of hydrocephalus. Spinal cord 85% of lesions are in distal thoracic, lumbar, and sacral areas (10% thoracic, 5% cervical). 4

Develop. Anomalies of CNS

Principles of Treatment of Congenital Malformations1. Counselling of patient and parent. 2. Care of neurogenic bladder. 3. Care of associated anomalies (GI, CVS, etc.). 4. Medical management of spasticity, neurogenic pain, and associated medical conditions. 5. Appropriate surgical procedure. 6. Post-operative care. 7. Rehabilitation and the use of rehabilitation aids. 8. Orthopaedic referral. 9. Regular and close follow up.


Develop. Anomalies of CNS

I. Spina Bifida OccultaDefinition Absence of the spinous process along with minor amounts of the neural arch. Usually, the CNS, cauda equina, and peripheral nervous system are not involved. It is a radiological diagnosis. Occurs in 20-30% of the general population, most common at L5 or S1. Etiology represents failure of fusion of the posterior arch. Clinically there are no obvious external markings. When associated with a skin dimple, hair patch, telangiectasia or other cutaneous marking, there should be increased suspicion of an underlying anomaly (occult spinal disraphysm). No obvious clinical signs or symptoms are associated with this diagnosis. Imaging Plain films show absence of the spinous process along with minor amounts of the neural arch. Most common at L5 or S1.

This AP view of the lumbar spine demonstrates a bifid posterior arch at the S1 level. S1 itself is partially lumbralized. No neurological abnormality was present with this incidental finding.

Treatment and Results Requires no treatment. Questionable association with chronic back pain and enuresis.


Develop. Anomalies of CNS

II. Abnormalities Due to Defective Neural Tube FoldingMeningocele and Myelomenigoceleo Definition A meningocele is a defect consisting of a herniation of meningeal tissue through a defect in the skull and/or spine. A myelomeningocele (MMC) is a defect consisting of a herniation of meningeal tissue and nervous tissue through a defect in the skull and/or spine. Myeloschisis is an old term which represents a completely open caudal neural tube with no evidence of a central canal. Placode is not displaced by underlying CSF. The placode is the open part of the neural tube which remains attached to the surface ectoderm. Meningocele "manque" refers to an elongation of the conus nerve roots or neuroglial tissue, with attachment to the posterior dura with tethering, with or without a lipoma, but without a disruption of the posterior neural arch and overlying tissues. (rare). o Epidemiology For MMC see above. Meningoceles are 10% of all patients with spina bifida. o Etiology Two theories. 1. Primary failure of neural tube closure (originally proposed by von Recklinghausen in 1886). 2. Rupture of a previously closed neural tube due to overdistension (Gardner; unpopular theory). o Embryology Closure of the rostral and caudal neuropores occurs by 4 weeks. Therefore open neural tube defects are felt to result from a primary neural tube closure defect vs. rupture of already closed neural tube. Closed neural tube defects are felt to arise as a complication of secondary neurulation, that is involved in formation of the caudal part of the spinal cord from the neural cell mass caudally. Normally the intervening somatic and epithelial tissue between the main (rostral) neural tube and the caudal mass regresses as these areas unite to complete neurulation, leaving behind a long thin filum terminale, as a remnant. Maldevelopment at this stage (which occurs between the 4th and 6th week of gestation) results in incorporation of mesodermal (lipomyelomingocoele) or epithelial elements (dermal sinus) or incompleted fusion of some


Develop. Anomalies of CNS elements (e.g. meningocele), all of these defects are covered with skin as in this time of development the caudal mass is already skin covered.

o Pathology of Meningocele Pathogenesis is a post-neurulation disorder. Neurulation and dysjunction of the cutaneous ectoderm have occured normally, but subsequent development of overlying mesenchymal and cutaneous ectoderm components is aberrent. Gross. Dorsal half of the vertebra(e) is gone, and the contents of the bulge are limited to CSF, meninges and skin. Microscopic. Meningocele consists of loose firovascular tissue. May contain microscopic islands of aberrent nervous tissue. The cyst is ependyma-lined, under which is a thin glial layer. Hallmark is that glial tissue is superimposed on collagenous tissue without intervening meninges.

o Pathology of Myleomeningocele Pathogenesis is a problem of primary neurulation. Caudal neuropore closes day 26-28. Rostral neuroprore at day 22-24. Failure to close will produce MMC and encephalocele, respectively (other authors argue that because the encephalocele contains brain tissue, the embrylogical defect must occur after rostral neural tube closure and differentiation). The open neural placode represents the embryologic form of caudal end of spinal cord. The narrow groove passing down placode in midline represents primitive neural groove which in normal development forms the spinal canal. The placode is displaced dorsally by underlying CSF. Normal skin surrounds the neural tissue, with an inter