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Abnormalities in the closure of the neural tube

Introduction

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)
    • Myelocele
    • Myelomeningocele - with or without cysts
    • Myeloschisis
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Factors that could hinder neural tube closure:
The reasons for the malformation of the neural tube have yet to be found. One knows, though, that they must be multi-factorial. Genetic, as well as environmental and nutritional factors appear to play an important role. Studies have shown that the consumption of vitamins and folic acid can prevent the occurrence of the abnormality. Certain medications, though, for example valproic acid (an anti-epileptic medication) can increase the susceptibility to develop spina bifida.

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.

Fig. 29 - Spina bifida occulta
(longitudinal section)
media/module7/h6a_occulta_a.gif

  1. Spinous process
  2. Spinal cord
  3. Tuft of hair
  4. Skin
  5. Dura mater

Fig. 29a - Spina bifida occulta
(cross section at the level of A)
media/module7/h6a_occulta_b.gif

  1. Vertebral arch
  2. Spinal cord
  3. Tuft of hair
  4. Skin
  5. Dura mater

Legend
Fig. 29, 29a

Schematic drawing of a spina bifida occulta.
The meninges and the spinal cord are in their right places.
Optically, only a tuft of hair gives its presence away.

Fig. 29a
 

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.

Fig. 30 - Meningocele
(longitudinal section)
media/module7/h6b_meningozele_a.gif

  1. Spinous process
  2. Spinal cord
  3. Mostly intact skin over the meningocele
  4. Meningocele with cerebrospinal fluid
  5. Dura mater

Fig. 30a - Meningocele
(cross section at the level of A)
media/module7/h6b_meningozele_b.gif

  1. Spinous process
  2. Spinal cord
  3. Mostly intact skin over the meningocele
  4. Meningocele with cerebrospinal fluid
  5. Dura mater

Legend
Fig. 30, 30a

Schematic drawing of a meningocele. The spinal cord is in its normal location. Only a sack filled with cerebrospinal fluid, bounded by the dura mater, protrudes from below the skin.

Fig. 30a

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.

Fig. 31 - Myelomeningocele
(longitudinal section)
media/module7/h6c_myelomeningo_a.gif

  1. Spinous process
  2. Central canal
  3. Skin partially covering the neural tube defect
  4. Spinal cord
  5. Dura mater

Fig. 31a - Myelomeningocele
(cross section at the level of A)
media/module7/h6c_myelomeningo_b.gif

  1. Spinous process
  2. Central canal
  3. Skin partially covering the neural tube defect
  4. Spinal cord
  5. Dura mater

Legend
Fig. 31, 31a

Schematic drawing of a myelo-meningocele.
The spinal cord comes to the surface through the osseous defect and is normally covered only by a thin skin.

Fig. 31a
 
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Macroscopic pictures

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.

Fig. 32 - Myeloschisis
(longitudinal section)
media/module7/h6d_myeloschisis_a.gif

  1. Spinous process
  2. Spinal cord
  3. Dura mater
  4. Skin up to the neural tube defect

Fig. 32a - Myeloschisis
(cross section at the level of A)
media/module7/h6d_myeloschisis_b.gif

  1. Spinous process
  2. Spinal cord
  3. Dura mater
  4. Skin up to the neural tube defect

Legend
Fig. 32, 32a

Schematic diagram of a myeloschisis.
The nerve tissue lies open at the surface without a dermal covering.
This leads to the severest disorder of the innervation.

Fig. 32a
 

Anencephaly

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.