Rachischisis
Rachischisis is failure of the neural tube (the embryonic tube that becomes the brain and spinal cord) to close, leaving neural tissue exposed along the spine.
Remember
Rachischisis is failure of the neural tube to close, leaving the spinal cord exposed and at high risk. It lies at the extreme end of the spina bifida spectrum (spinal defects from mild to severe), opposite spina bifida occulta, a covered gap that usually causes no problems.
Normally the neural tube closes during neurulation (the folding that builds the tube), and bone and skin then form around it. In rachischisis the tube fails to close over a stretch of the spine, so neural tissue lies open along the affected region, with no membranes, bone or skin to protect it.
The cord is exposed and unprotected, so it is damaged and cannot work normally. The result is severe neurological impairment, and rachischisis is typically incompatible with life.
Rachischisis sits at the extreme end of the spina bifida spectrum. From mildest to worst, the forms run like this:
- Spina bifida occulta is a covered gap in the bony arches of the spine, where nothing protrudes.
- A meningocele is a sac of meninges and cerebrospinal fluid (the fluid that cushions the nervous system) that bulges out, with the cord in place.
- A myelomeningocele has a sac that also contains the spinal cord.
- Rachischisis is the open spine: the tube never closed, so there is no sac and no covering.
Anencephaly is a different closure failure of the neural tube. The cranial neuropore (the opening of the tube at the head end) fails to close, so the brain and skull vault are affected rather than the spine.
Key facts
- Definition
- failure of the neural tube to close, so neural tissue is exposed along the affected region of the spine.
- Position on the spectrum
- the extreme end of the spina bifida spectrum, beyond spina bifida occulta, a meningocele and a myelomeningocele.
- Development
- the defect begins in the third to fourth week of development, when the neural tube should be closing.
- Outlook
- severe neurological impairment, and typically incompatible with life.
- Cause
- congenital, because the neural tube fails to close in the embryo.
- Exam clue
- exposed neural tissue with severe impairment points to rachischisis, while no protrusion with usually no problems points to spina bifida occulta.
Exam traps
- A gap in the bony arches of the spine with no protrusion of neural tissue is spina bifida occulta, the mildest form, not rachischisis, because in rachischisis the tube has not closed and neural tissue is exposed.
- Failure of the cranial neuropore to close is anencephaly, which affects the brain and skull vault, while rachischisis is a defect along the spine.
- A sac holding both the spinal cord and meninges is a myelomeningocele, not rachischisis, because a sac forms around that tissue, while in rachischisis the tissue lies open along the spine with no sac.
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