Spinal Dysraphism Overview

Overview

This lecture covers the classification, embryology, and key developmental stages of spinal dysraphism, with a focus on open defects, especially myelomeningocele.

Classification of Spinal Dysraphism

• Spinal dysraphism is categorized as either open (spina bifida aperta) or closed (spina bifida occulta).
• Thinking in terms of open vs. closed spinal dysraphism simplifies diagnosis and surgical considerations.
• The most common open neural tube defect is myelomeningocele, accounting for 98% of open cases.
• Other rare open defects include myelocystocele, hemimyelomeningocele, and hemimyelocystocele.

Embryology of the Spine & Neuralation

• Neuralation refers to the development of the brain and spinal cord.
• Neural tube closure starts at the upper cervical region and extends to L1-L2, then returns rostrally.
• The neural plate folds and is surrounded by ectoderm and neural crest cells.
• Primary neurallation forms the spinal cord to the lower lumbar level (days 18–28 of development).
• Abnormalities in primary neurallation cause myelomeningocele, lipomyelomeninx, intraspinal dermoid/epidermoid cysts, and split cord malformations.
• Secondary neurallation (days 28–48) affects the area below the lower lumbar spine and includes canalization and regression.
• Canalization forms distal spinal segments and associated nerves.
• Regression leads to the formation of the filum terminale (distal, nonfunctional spinal cord segment).

Ascension of Conus Medullaris

• After birth, the vertebral canal grows faster than the neural tube.
• At birth, the conus medullaris is at L2–L3 (can be as low as L3–L4).
• By 3 months of age, the conus ascends to L1–L2, its adult position.
• The location of the conus is critical in assessing neural tube defects and spinal cord tethering.

Key Terms & Definitions

• Spinal Dysraphism — Congenital malformations resulting from improper closure of the neural tube.
• Neuralation — Developmental process of forming the brain and spinal cord.
• Myelomeningocele — Most common open spinal dysraphism, where spinal cord and meninges protrude.
• Primary Neurallation — Formation of the spinal cord to the lumbar level.
• Secondary Neurallation — Development of the lower spinal segments and filum terminale.
• Filum Terminale — Distal cord remnant lacking function.
• Conus Medullaris — Terminal end of the spinal cord.

Action Items / Next Steps

• Review embryology slides illustrating neural tube closure.
• Understand and memorize the timeline and process of neuralation.
• Be able to identify the position of the conus medullaris at different developmental stages.