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The Science of the Spine: How OSD Is Diagnosed

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Open spinal dysraphism (OSD) is diagnosed before birth using prenatal ultrasound and fetal MRI. The two main types are myelomeningocele (has a fluid-filled sac) and myeloschisis (flat, exposed nerve tissue). Imaging helps doctors locate the exact spinal defect and plan for surgery.

Key Takeaways

  • • Open spinal dysraphism occurs in the early weeks of pregnancy when the fetal spine fails to fold and close completely.
  • • Myelomeningocele involves a fluid-filled sac containing spinal tissue, while myeloschisis leaves the spinal cord completely exposed without a sac.
  • • Prenatal ultrasound is the gold standard for early screening, often checking for brain markers like missing intracranial translucency.
  • • A fetal MRI provides highly detailed images of the spinal defect to help doctors accurately plan for your baby's surgery.
  • • Babies with myeloschisis have a significantly higher risk of developing severe fluid buildup in the brain (hydrocephalus).

Understanding how open spinal dysraphism (OSD) develops and is diagnosed can help you make sense of the complex medical terms and images you are seeing. While the biology happens early in pregnancy, modern imaging allows doctors to see the details of the defect and its effects on the baby’s body [1][2].

The Biology: How OSD Happens

The development of the spinal cord begins with a process called primary neurulation. This happens very early, typically occurring in the third and fourth weeks of embryonic development [3][4].

During this stage, a flat layer of cells is supposed to fold inward and fuse together at the top, much like a “zipper,” to form a hollow tube [3][4]. If this “fold-and-fuse” process fails at any point along the spine, the bony vertebral arches fail to close completely, resulting in an open defect [3][4][5].

Myelomeningocele vs. Myeloschisis

There are two main types of open defects you may see on a report:

  • Myelomeningocele (MMC): This is the most common form. It is characterized by the presence of a fluid-filled sac (a cyst) containing the spinal cord tissue and cerebrospinal fluid [6][5][3].
  • Myeloschisis (MS): In this form, there is no fluid-filled sac. The spinal cord tissue, called the neural placode, sits flat and is not contained within a sac, leaving it completely exposed [6]. Notably, the absence of a hernia sac in myeloschisis is associated with a statistically significant increased risk of developing shunt-dependent hydrocephalus (severe fluid buildup in the brain) compared to cases with a sac [7][6].

How OSD is Diagnosed

Doctors use a combination of tools to diagnose OSD and plan for the baby’s care:

1. Prenatal Ultrasound

Ultrasound is considered the “gold standard” for early detection [1]. Beyond looking at the spine itself, doctors look for indirect signs in the brain that often occur with open spinal defects:

  • Intracranial Translucency (IT): In the first trimester, doctors may look for the absence of IT, a small fluid-filled space in the brain. The lack of this space can serve as a useful early screening marker for spina bifida aperta [8][9].

2. Fetal MRI

While ultrasound is excellent for screening, a fetal MRI offers increased sensitivity for providing a detailed characterization of the defect [1][5]. It provides a clearer picture of the exact spot on the spine where the opening is, and rules out other potential diagnoses, which is essential for planning surgery [1][10].

Common Terms on Your Report

  • Neural Placode: The exposed, unclosed spinal cord tissue that is not contained within a sac in cases of myeloschisis [6].
  • Hindbrain Herniation (Chiari II Malformation): A condition where parts of the brain (the cerebellum) are pulled downward toward the spinal canal [2][11].
  • Hydrocephalus: Enlargement of the fluid-filled spaces in the brain, often requiring surgical drainage [12][13].

Frequently Asked Questions

What is the difference between myelomeningocele and myeloschisis?
Both are types of open spinal dysraphism. In myelomeningocele, the spinal cord and nerves are contained within a fluid-filled sac on the baby's back. In myeloschisis, there is no sac, and the spinal cord tissue sits flat and is completely exposed.
Why do doctors need to do a fetal MRI if I already had an ultrasound?
While an ultrasound is the standard for early detection, a fetal MRI provides a much clearer, more detailed picture of the exact location of the spinal opening. This detailed view is essential for ruling out other conditions and planning your baby's surgery.
What does 'intracranial translucency' mean on my ultrasound report?
Intracranial translucency refers to a small fluid-filled space in the brain that doctors look for during a first-trimester ultrasound. If this space is missing, it serves as an early warning sign that the baby may have an open spinal defect.
What is a 'neural placode'?
The neural placode is the exposed, unclosed spinal cord tissue seen in cases of myeloschisis. It means the spinal tissue failed to fold into a tube during early development and is sitting flat without a protective sac covering it.
Does myeloschisis increase the risk of fluid buildup in the brain?
Yes. Babies diagnosed with myeloschisis, which lacks a protective fluid-filled sac over the spinal defect, have a higher risk of developing hydrocephalus. This is a severe fluid buildup in the brain that often requires surgical drainage.

Questions for Your Doctor

  • • Is this a 'myelomeningocele' or 'myeloschisis,' and what does that mean for my baby's brain health and long-term function?
  • • What is the 'anatomical level' of the lesion on the report, and how accurately can we determine that before birth?
  • • Did the ultrasound show the 'lemon' or 'banana' signs, and do these indicate any changes in the brain like Chiari II malformation?
  • • Is the 'intracranial translucency' visible on the scans, and what does that tell us about the severity of the defect?
  • • Can we schedule a fetal MRI to get a more detailed look at the 'neural placode' and any potential brain anomalies?

Questions for You

  • • What technical terms on my baby's report am I most confused about or would like my doctor to explain in more detail?
  • • Am I noticing any specific patterns in how I feel when I read or hear these medical terms?
  • • What information from this diagnostic process feels the most 'solid' or helpful to me right now?
  • • What are the top three questions I want to make sure get answered at our next specialist appointment?

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References

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    Cranial magnetic resonance imaging findings in 167 patients with spina bifida aperta: a retrospective study.

    Sezen GB, Alataş İ, Saygı HM, et al.

    Child's nervous system : ChNS : official journal of the International Society for Pediatric Neurosurgery 2025; (41(1)):374 doi:10.1007/s00381-025-07054-2.

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    Concepts in the neurosurgical care of patients with spinal neural tube defects: An embryologic approach.

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    Absence of a Hernia Sack in Patients Undergoing Prenatal Repair of Spina Bifida Increases the Risk of Developing Shunt-Dependent Hydrocephalus.

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    Intracranial Translucency, Its Use as a Potential First Trimester Ultrasound Marker for Screening of Neural Tube Defects.

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  9. 9

    Dynamic acetylation profile during mammalian neurulation.

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  10. 10

    Prenatal ultrasound diagnosis of neural tube defects in the era of intrauterine repair - Eleven years' experiences.

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  11. 11

    Myelomeningocele - a single institute analysis of the years 2007 to 2015.

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    Early Outcome of Endoscopic Third Ventriculostomy With Choroid Plexus Cauterization Versus Ventriculoperitoneal Shunt as Primary Treatment of Hydrocephalus in Children With Myelomeningocele: A Prospective Cohort Study.

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    PMID: 34662909

This page explains the diagnosis and imaging terms for open spinal dysraphism (OSD) for educational purposes only. Your maternal-fetal medicine specialist and pediatric neurosurgeon are the best sources for interpreting your baby's specific scans.

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