Chapter 6 of 7 · Fundamentals
Neural migration
Neural Migration Disorders
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What the experts said
In gastrulation at the 3rd week, the two-layer embryo becomes three layers by adding mesoderm
Defects in the gastrulation stage can lead to neuroenteric cyst and split cord malformation
A neuroenteric cyst is a CNS cyst lined by epithelium mimicking the GIT
Neuroenteric cysts usually persist and cause recurrent infections and need excision of the whole tract
In split cord malformation type 1, there are two separate spinal cords, each in its own dural sac
In split cord malformation type 2, there is a single dural sac and two hemicords
Type 1 split cord malformation has a bony cartilage spur while type 2 has no such structure
In type 1 split cord malformation, patients more frequently present with weakness and back pain, while type 2 can be only incidentally discovered
Surgery for split cord malformation involves removing the bony spur to avoid more traction to the cord
Dorsal induction and primary neurulation occur between the 3rd and 4th week
The neural crest forms the peripheral nervous system
A small notochord will persist and form the nucleus pulposus of the intervertebral disc
Cranial neuropore closure occurs on the 24th day and caudal neuropore closure occurs on the 26th day
Failure of cranial neuropore closure causes anencephaly or encephaloceles
Failure of caudal neuropore closure causes spina bifida occulta or aperta
Failure of disjunction leads to dermal sinus tract
Anencephaly is failure of the neural tube to form in the neuropore, leading to failure of the skull and part of the brain to be formed
Anencephaly is usually fatal and typically detected before birth
In encephalocele, brain tissue, dura, and CSF protrude through a defect in the back of the head usually
Encephalocele repair is possible and outcomes vary according to the size and location of the protruded brain
Spina bifida aperta (myelomeningocele) is an open spinal defect with a protruding sac containing spinal cord, meninges, and CSF
Myelomeningocele requires urgent surgery to close the defect to prevent infection, especially when ruptured
Spina bifida aperta (myelomeningocele) is usually associated with hydrocephalus that may need VP shunt
Spina bifida occulta is a small vertebral defect only, maybe with skin stigmata, but no symptoms, can be incidentally discovered or develop tethered cord syndrome in the future
Spina bifida aperta requires urgent surgery after birth or prenatal surgery and is usually associated with hydrocephalus
Spina bifida aperta requires lifelong follow-up for shunt, weakness in legs, and sometimes urinary incontinence
Dermal sinus tract is a tube tract from the skin to the deeper spine, intradural, usually lined by epithelium, based on failure of disjunction between cutaneous ectoderm and neuroectoderm
Dermal sinus tract can present with recurrent infection leading to meningitis and potential weakness of the legs
Surgical removal of the whole dermal sinus tract is important
Secondary neurulation occurs between the 20th and 48th days in embryology
Secondary neurulation forms the caudal portion of the neural tube, especially structures lower in the spinal cord including sacral and coccygeal segments
Abnormalities from secondary neurulation include persistent terminal ventricle (fifth ventricle), tight filum terminale, and terminal myelocystocele
Ventral induction including vesiculation occurs between the 5th and 10th week
Three primary brain vesicles (forebrain, midbrain, hindbrain) differentiate into five secondary vesicles
The forebrain divides into telencephalon (which forms the cerebrum) and diencephalon (which forms thalamus, hypothalamus, and epithalamus)
The midbrain (mesencephalon) forms the midbrain
The hindbrain divides into metencephalon (forming pons and cerebellum) and myelencephalon (forming medulla oblongata)
Holoprosencephaly is a forebrain abnormality representing failure of the forebrain to divide, resulting in a single lobed brain structure
Holoprosencephaly results in facial abnormalities and severe neurological deficits
Septo-optic-pituitary dysplasia is incomplete development of midline brain structures affecting vision, hormonal pituitary function, and causing developmental delay
Dandy-Walker malformation is a hindbrain abnormality with vermian agenesis or enlargement of the fourth ventricle, leading to issues with motor coordination, balance, and sometimes hydrocephalus
Neuroproliferation occurs between the 6th and 12th week and involves rapid increase of neural precursor cells in the developing brain
Hypoproliferation causes microcephaly and hyperproliferation causes macrocephaly
Neural migration occurs between the 8th and 20th week (2nd and 5th month) when neuroblasts migrate to specific locations in the brain to form neurons and establish brain layer structure
Polymicrogyria consists of multiple small gyri separated by small sulci, generating an irregularly bumpy cortical surface with excessive small folds, causing intellectual disability and sometimes epilepsy
Schizencephaly is an abnormal cleft or split lined by gray matter, resulting in developmental delay and motor impairment
Schizencephaly can be closed lip or open lip
Gray matter heterotopia is gray matter in abnormal locations in the brain, such as intraventricular or periventricular areas
Lissencephaly means smooth brain in Latin, with lack of normal folds and gyri, leading to severe developmental and motor issues
Porencephaly is a rare congenital disease that occurs from encephalomalacia during early life of the baby
Porencephaly is usually seen on CT scan or MRI and usually communicates with the ventricles
Porencephaly results from ischemic insults
Porencephaly consists of cystic cavities within the cerebral hemisphere that often communicate with the ventricular system or subarachnoid space, usually caused by vascular insults like stroke during fetal or neonatal period
Arachnoid cysts are cerebrospinal fluid sacs located in the arachnoid membrane, resulting from arachnoid membrane splitting abnormality or CSF abnormalities
Arachnoid cysts are extra-axial CSF-filled cysts that typically don't enhance and typically do not communicate with the ventricles
Schizencephaly consists of gray matter-lined clefts splitting the cerebral cortex, usually extending from the ventricular surface to the pial surface, leading to developmental malformation
Clinical presentation of porencephaly is based on the size and location of the porencephalic cyst
Arachnoid cysts are usually asymptomatic and incidentally discovered
Schizencephaly usually presents with developmental delay, seizures, and/or motor deficits
Agenesis of corpus callosum occurs between the 8th and 20th week due to defective tissue and issues in neural migration