Spina Bifida Occulta

Spina bifida occulta

Spina bifida occulta is a closed spinal dysraphism (neural tube defect) that involves failure of vertebral arch fusion but no cystic lesion is present on the back – the defect is covered in skin.

There is often either a tuft of hair or small dimple present on the back.
Closed spinal defects are often distinguished based on whether or not a lipomatous subcutaneous mass accompanies the defect.
For instance, in lipomyeloschisis, lipomatous subcutaneous tissue invades into the spinal canal through the bony defect and attaches itself to the spinal cord.

On routine radiographic survey, 5-10% of the population has some degree of closed spinal defect, thus the vast majority are asymptomatic. However, spina bifida occulta can accompany pathologies that are quite severe and we shouldn't mis-infer pathologic severity from whether the spinal dysraphism is open or closed until we know if there is an associated underlying disorder that causes pathology via (most often) tethering of the spinal cord.

Tethered cord syndrome

Tethered cord syndrome causes traction damage to the spinal cord and serious neurological deficits.

Multiple forms of closed spinal dysraphism cause tethered cord syndrome, but we often think of this syndrome in its relationship to a tight filum terminale or a terminale lipoma [a thickening of the filum terminale due to residual caudal mass cells (of the conus medullaris)]. These scenarios make it easy to imagine the resultant traction on the cord.

The caudal spinal cord forms during secondary neurulation and so we can imagine that tethered cord syndrome is often a secondary neurulation defect (as opposed to the anterior and posterior neuropore defects that occur during primary neurulation and cause anencephaly and myelomeningocele). Note that tethered cord syndrome is often also a secondary cause of syringomyelia a cavity in the spinal cord because the downward traction on the cord expands the central cavity that runs within the spinal cord.

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Image References

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