Abnormalities in the closure of the neural tube
The largest portion of spinal cord abnormalities can be traced back to an unsuccessful closure of the neural tube, caudal to the 4th somite pair during the fourth week. This defect is termed spinal dysraphia and it influences not only the development of the central nervous system but also that of the vertebral arches that lie above it. This is evidenced by a more or less strongly pronounced opening of the vertebral canal (spina bifida). Transplantation experiments have actually shown that the neural tube exercises an inductive effect on the development of the vertebral arches.
According to the number of non-fused neural arches and of the other structures thereby affected, one distinguishes among various types of spina bifida.The clinical consequences can be insignificant, serious or even fatal.
Common to all types of spina bifida is an absent closure of the vertebral arch.
Basically one distinguishes between:
- Spina bifida occulta (= hidden)
- Spina bifida aperta (= visibly present)
- Myelomeningocele - with or without cysts
Spina bifida occulta
Spina bifida occulta occurs very frequently and is usually found accidentally in x-rays or in an examination of the back. They seldom have clinical relevance because only a missing closure of the osseous structures exists in the formation of the vertebral arch, without the spinal cord with its membranes (meninges) being involved. The skin covering is intact. Sometimes a tuft of hair tells where the osseous structures are missing.
Spina bifida in combination with a dermoid cyst
It can happen that in the median, sacral region a fistula stays connected with a dermoid cyst, which indicates the location of the closure of the posterior neuropore in the 4th week. The cyst represents the last place of separation between the superficial ectoderm and the neuroectoderm. Spinal cord and meninges find themselves in their normal locations below the skin.
Spina bifida aperta
In cases of spina bifida aperta, besides fissures of the osseous structures, one also finds abnormalities of the meninges and/or of the spinal cord itself.
The membranous covering can be present or absent. The mildest form is the meningocele where merely the meninges protrude under the skin through the cleft in the malformed vertebral arch.
In a myelomeningocele the meninges as well as the spinal cord (myelon) are found outside of the vertebral arch. They are visible as a protrusion under the skin; sometimes the skin can also be absent or only very thin.
In addition, the central canal is bloated and forms a cyst. In this case one speaks of a myelocystomeningocele.
The myeloschisis (rachischisis) is the severest form of the spina bifida aperta. The nerve tissue is here fully bare and a dermal or meningeal covering is absent. With this abnormality, the closure of the neural folds fails to occur.
Anencephaly represents a special kind of spina bifida. It arises when the neural folds do not fuse in the cranial region of the neural tube. When the fusion fails for the entire neural tube, one speaks of craniorachischisis totalis. Both anomalies are always fatal.