Illustrated Encyclopedia of Human Anatomic Variation: Opus III: Nervous System
Ronald A. Bergman, PhD
Adel K. Afifi, MD, MS
Ryosuke Miyauchi, MD
Peer Review Status: Internally Peer Reviewed
Hydromyelia or hydrorrhachis interna is a defective closure or arrangement of the divisions of the primary fetal central canal. Its presence may be indicated by subsequent development of syringomyelia or by its association with spina bifida. Hydrorrhachis externa is the abnormal congenital accumulation of fluid between the meninges of the cord causing a diminution of cord volume.
Heterotopias (misplacement of parts) of the spinal cord include gray matter islands located in the white matter; these may be isolated or partially joined to the gray horns by slender strands of gray matter.
Malposition of bundles of white matter or passage of bundles of white matter (nerve fibers) through unusual sites to reach their destination has been described. Additional variations include: (a) doubling of the spinal cord in some of its parts; (b) the spinal cord (in an adult) extending to the level of the third sacral vertebra; and (c) doubling of the central canal. The denticulate ligaments vary in number between 18 and 22 in normal individuals.
In one study of 129 adults, the point of termination of the spinal cord varied from the lower third of the twelfth thoracic vertebra to the middle third of the third lumbar. In 95% of these 129 cases, the cord terminated at the level of the first and second lumbar vertebra.
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