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Pathology - Nervous System Malformations
This guide summarizes common nervous system malformations, focusing on understanding their causes, presentations, and associated complications.
I. Neural Tube Defects (NTDs)
This guide summarizes common nervous system malformations, focusing on understanding their causes, presentations, and associated complications.
I. Neural Tube Defects (NTDs)
- Definition: Failure of the neural tube to close properly during embryogenesis (weeks 3-4).
- Etiology: Multifactorial – both genetic predisposition and environmental factors contribute. Maternal folate deficiency during early gestation is a significant risk factor.
- Types:
- Anencephaly: Fatal malformation; absence of brain and cranial vault. Anterior neural tube defect.
- Encephalocele: Protrusion of malformed brain tissue through a skull defect (often occipital). Large encephaloceles are usually fatal.
- Spina Bifida: Group of malformations due to incomplete closure of the caudal neural tube. Lack of fusion of vertebral arches and skin. Severity varies:
- Meningocele: Outpouching of meninges only.
- Myelomeningocele: Outpouching of meninges and spinal cord. Can cause urinary incontinence, constipation, and motor/sensory impairment in legs.
- Definition: Failure of the glial bridge to form between cerebral hemispheres, preventing axon crossing. Can be complete or partial.
- Association: Frequently occurs with other malformations.
- Presentation: Variable degrees of psychomotor retardation.
- Definition: Narrowing of the cerebral aqueduct.
- Etiology: Developmental anomaly; can be inherited (autosomal recessive or X-linked).
- Presentation: Congenital hydrocephalus (build-up of cerebrospinal fluid in the brain).
- Definition: Downward displacement of cerebellar structures into the foramen magnum.
- Type II (Arnold-Chiari): Severe; small posterior fossa, downward displacement of cerebellar vermis and medulla. Usually associated with lumbar myelomeningocele.
- Type I: Milder; cerebellar tonsils extend into the spinal canal. May be asymptomatic.
- Definition: Absent or rudimentary cerebellar vermis; large cyst (dilated fourth ventricle) fills the posterior fossa.
- Association: Often associated with other malformations.
- Definition: Fluid-filled cavity (syrinx) within the spinal cord's central grey matter.
- Location: Usually cervical and upper thoracic segments; extension into the medulla is called syringobulbia.
- Effect: Syrinx expansion causes adjacent spinal cord atrophy.
- Presentation: Typically presents in early adulthood with isolated loss of pain and temperature sensation in the upper limbs (due to spinothalamic tract damage).
- Embryological timing: Understand the developmental windows during which these malformations occur.
- Etiological factors: Recognize the interplay of genetic and environmental influences, particularly folate's role in NTDs.
- Clinical presentations: Learn to differentiate between the various malformations based on their symptoms and associated findings (e.g., hydrocephalus, motor deficits).
- Associated anomalies: Many of these malformations occur together, highlighting the interconnectedness of neural development.
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