Developmental Abnormalities of the Nervous System

Introduction

if a step goes wrongÞ a problem

NEURULATION- days 18-27, 3^rd and 4^th somites close first, anterior neuropore closes day 24-25, posterior day 26-27

CANALIZATION – days 28-48, a coalescence of vacuoles in the neural tube

RETROGRADE DIFFERENTIATION – day 48, the formation of the filium terminale, atrophy of the caudal neural tube, ascent of the conus medullaris

Spinal Dysraphisms – failure of neural tube closure

Spina bifida Cystica (Myelomeningocele) – failure of neural tube closure Þ open spine and spinal cord

Major skin dysplasia (e.g. no skin over the open neural tube)

Major spinal cord dysplasia, no function below level of open neural tube (immediate severe neurological defect)

Low lying conus medullaris

Often progressive hydrocephalus because a Chiari II malformation is almost always present

Generally occurs in the lumbosacral region but can be any part of the spinal cord

No clear sex predilection, US incidence has dropped to 3/10,000 live births (becase of folate and abortion)

Prenatal screening via a -feto-protein levels and ultrasound

Spina bifida Occulta – failure of fusion of the posterior element of the lumbar spine

Affects female infants twice as much as males
NO Chiari, NO hydrocephalus
Low lying conus medullaris (below L1)
"Tethered Cord Syndrome" – progressive neurological deterioration because of longitudinal traction on the conus medullaris resulting in ischemia of the conus. This can be treated by prophylaxis surgery.
Covered with skin, often has a minor cutaneous abnormality over it (e.g. hemangioma, lipoma, or hairy patch)
Anal/rectal malformations have a strong association with Spina bifida occulta.

Chiari Malformations

Chiari was a pathologist in Prague at the end of 19^th century who described these lesions

(1) Chiari I – herniation of the cerebellar tonsils below the foramen magnum. Not associated with other CNS abnormalities. May be congenital or acquired (after too many LP’s or after placement of an over-efficient shunt)

Hydrosyringomyelia – a blockage of CSF leading to an accumulation of CSF in the spinal cord. Leads to "cape-like" distribution of pain sensation. Children chew off fingernails (because they can’t feel ‘em) and get "Griffin-hands"

(2) Chiari II – caudal displacement of the cerebellar vermis, 4^th ventricle, and brainstem below the foramen magnum. Also see herniation of the cerebellum rostrally. Due to a malformation of the brainstem. Associated with other CNS abnormalities (beaked tectum, polymicrogyria, hydrosyringomylia, dysplastic nerve nuclei) Treated by removing bone to decompress.

Pontine Flexure – one of three flexures during development, where the brainstem folds back on itself; this fails to occur in the Chiari II malformation, possibly due to a problem with the thoracic neurocele that induces brainstem folding

(3) Chiari III – rare, caudal displacement of the cerebellum and brainstem into a cervical meningocele

(4) Chiari IV – even rarer, cerebellar hypoplasia

Arachnoid Cysts

benign lesions arising from a splitting of the normal arachnoid that fills with CSF

may occur intracranially or intraspinally, however 50% are found in the Sylvian fissure

these cysts can cause symptoms by slowly enlarging and displacing brain

normal blood vessels may be stretched by the growing cyst and rupture Þ further deficit

not all cysts require treatment; those that do can have a shunt placed or fenestrations of the cyst made via endoscope to allow for drainage of the CSF

Supracellar Cysts – present with the triad of (1) hydrocephalus, (2) endocrinopathy, and (3) visual symptoms.

Encephaloceles

occur as a result of failure of closure or disruption of the neural tube permitting herniation of the brain and/or the meninges through a defect in the skull.

Occipital encephalocele – occurs in the cranial vault, more common in the Western hemisphere, associated with retardation and developmental delay, the goal of surgery is to repair the defect and to save any viable neural tissue

Anterior encephaocele – occurs in the cranial base, may project through the cribiform plate, more common in Southeast Asia, no intellectual retardation or developmental delay, goal of surgery the same + cosmetic repair.

Corpus Callosum Abnormalities

agenesis of the corpus callosum may be complete or incomplete, if incomplete the defect may not affect neurological development. Surgery is not indicated, large ventricles in this setting are usually static and do not represent symptomatic hydrocephalus