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PubMed This is a summary of 25 peer-reviewed journal articles Updated
Neurosurgery

The Biology of Spina Bifida and Its Subtypes

At a Glance

Spina bifida develops when the early structure that becomes the spine and spinal cord does not close completely. It ranges from hidden spinal gaps to open defects, and testing may include blood screening, ultrasound, and MRI.

Learning about a spina bifida diagnosis can feel overwhelming, but understanding the biology behind it is a powerful first step. Spina bifida is a type of neural tube defect (NTD)—a condition where the brain, spinal cord, or their protective coverings do not develop or close completely during early pregnancy [1][2].

How the Spinal Cord Forms

In the first few weeks after conception, a flat layer of cells called the neural plate begins to fold inward. This process, called neurulation, is like a zipper closing from the middle toward both ends to form a tube [1][3].

For the tube to close correctly, several biological “engines” must work together:

  • Cell Folding: Cells must change shape to bend the plate into a tube [1].
  • Zippering: The edges of the folds must meet and fuse together perfectly [4].
  • Surface Sealing: The skin (surface ectoderm) must grow over the tube to protect it [4].

If this “zipper” fails to close at any point along the spine, the result is spina bifida. Because this happens so early—often before a person even knows they are pregnant—prevention and early diagnosis are key [5][6].

The Spectrum of Spina Bifida

Spina bifida is not a single condition but a spectrum. Doctors generally categorize it into “open” or “closed” defects based on whether the spinal cord is exposed to the environment.

Spina Bifida Occulta (Closed)

Spina bifida occulta is the mildest and most common form. “Occulta” means hidden. In an incidental bony spina bifida occulta, there is a small gap in the bones of the spine (the vertebral arch), but the spinal cord and nerves are usually healthy and stay inside the spinal canal [7][8].

  • Appearance: The skin over the gap is intact. There may be a small birthmark, a dimple, or a tuft of hair (sometimes called a “faun tail”) at the site [9][10].
  • Clinical Note: While often harmless, “occulta” does not always mean “no symptoms.” It can sometimes be part of occult spinal dysraphism, where the spinal cord is abnormally attached or “tethered” which has a much higher neurologic significance [2][11].

Meningocele (Closed)

In a meningocele, the protective membranes around the spinal cord (the meninges) push through the gap in the spine, forming a fluid-filled sac [2].

  • Anatomy: The sac contains spinal fluid but does not contain the spinal cord itself. Because the nerves are often not involved, the risk of severe paralysis is lower than in myelomeningocele, but meningocele can still be associated with neurologic problems [12].

Myelomeningocele (Open)

Myelomeningocele is the most severe form. In this case, the spinal canal remains open along several vertebrae, and both the membranes and the spinal cord (or nerves) protrude through the opening into a sac [12][13].

  • Exposure: If the sac is not covered by skin, it is an “open” defect, which exposes the delicate nerves to amniotic fluid during pregnancy, potentially causing further damage [14].
  • Impact: This type usually causes moderate to severe disabilities, affecting how a person walks and how they control their bladder and bowels [15][11].

Understanding “Tethering”

A major concern in “closed” or “occult” types is a tethered spinal cord. Normally, the spinal cord floats freely within the spinal canal. In a tethered cord, the cord is stuck to the surrounding tissues [11][16]. As a child grows, the “stuck” cord is stretched, which can lead to:

  • Progressive leg weakness or changes in how a child walks [15].
  • New or worsening bladder or bowel accidents [16].
  • Back or leg pain [10].

Why Does It Happen?

Spina bifida is multifactorial, meaning it is usually caused by a combination of genetics and environmental factors [17]. It is important to know that a spina bifida diagnosis is not your fault or your parents’ fault, and it usually cannot be attributed to any single action.

Factor Description
Folic Acid This B-vitamin lowers but does not entirely eliminate the risk. Ask your doctor for individualized dosing advice for future pregnancies [18][19].
Genetics Certain gene variations (such as those in the VANGL2 gene) are areas of active susceptibility research, but they are not routine single-gene causes for most cases [20][3].
Maternal Health High blood sugar levels before and during early pregnancy are linked to a higher risk of NTDs [21].
Medications Certain anti-seizure medications, particularly valproate (valproic acid), can increase the risk if taken during the first weeks of pregnancy [5][13].

How Is It Diagnosed?

Most cases are now found during pregnancy through routine screening, but some “closed” forms may not be detected on prenatal imaging and are discovered at birth or later in life.

  • Maternal Serum Alpha-Fetoprotein (MSAFP): A routine blood test screening for markers of open neural tube defects. A positive result is a screening, not a definitive diagnosis.
  • Prenatal Ultrasound: Doctors look for “cranial signs” (like the lemon sign or banana sign, which describe the shape of the skull and brain) that suggest the spine is open [22][23].
  • Fetal MRI: This provides a highly detailed view of the spinal cord and can help plan for surgery before or after birth [24].
  • Postnatal MRI: After birth, MRI is the gold standard for looking at the spinal cord and checking for tethering or fatty growths (lipomas) [25][8].

Common questions in this guide

What is spina bifida, and when does it develop?
Spina bifida develops very early in pregnancy when the neural tube, which becomes the brain and spinal cord, does not close completely. The condition can affect the bones of the spine, the spinal cord, its nerves, or the protective membranes around it.
How are spina bifida occulta, meningocele, and myelomeningocele different?
Spina bifida occulta is usually a closed gap in the spinal bones, with the spinal cord remaining inside the spinal canal. A meningocele is a fluid-filled sac containing the protective membranes, while a myelomeningocele is an open defect in which the spinal cord or nerves also protrude and may cause more serious disability.
Can spina bifida occulta cause symptoms?
Often, spina bifida occulta causes no symptoms and is found by chance. However, some people have a tethered spinal cord, which may lead to worsening leg weakness, changes in walking, back or leg pain, or new bladder and bowel problems.
How is spina bifida diagnosed during pregnancy?
Prenatal evaluation may include a maternal blood test for alpha-fetoprotein, ultrasound, and sometimes fetal MRI. A positive blood screening result does not prove that a fetus has spina bifida, so clinicians use imaging and follow-up testing to clarify the diagnosis.
What causes spina bifida, and can the risk be reduced?
Spina bifida usually results from a combination of genetic and environmental factors rather than one action or mistake. Folic acid lowers the risk but does not eliminate it, and doctors may also consider maternal blood sugar, medications such as valproate, and individualized folic acid dosing before a future pregnancy.
What does a tethered spinal cord mean in spina bifida?
A tethered spinal cord is attached to surrounding tissue instead of moving freely within the spinal canal. As a child grows, stretching of the cord can contribute to leg weakness, walking changes, pain, or worsening bladder and bowel control.
What imaging is used after birth to evaluate spina bifida?
A postnatal MRI provides a detailed view of the spinal cord and can evaluate tethering, fatty growths called lipomas, and other spinal abnormalities. The findings help the care team understand the anatomy and plan appropriate monitoring or treatment.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Which specific subtype of spina bifida does my child (or do I) have, and is it considered 'open' or 'closed'?
  2. 2.Does the imaging show a 'tethered cord,' and what does that mean for my long-term mobility and bladder function?
  3. 3.Are there other spinal abnormalities present, such as a lipoma, dermal sinus, or split cord malformation?
  4. 4.If we are planning a future pregnancy, what dose of folic acid should I take, and when should I start taking it?
  5. 5.How will we monitor for changes in leg strength, walking, or bathroom habits over time?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

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This page is for informational purposes only and does not constitute medical advice. A qualified clinician should interpret imaging and discuss spina bifida risks, symptoms, and next steps for you or your child.

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