USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 32-year-old woman comes to the clinic for a routine prenatal visit. She is 15 weeks pregnant and during her last visit was informed her child is at an increased risk of spina bifida (mother has a history of obesity and diabetes in addition to a family history of birth defects). She has been taking folic acid supplements as instructed, but wants to determine as soon as possible if her child will be born with spina bifida. She has no other concerns and the rest of her visit is unremarkable. Which of the following most accurately describes how to begin the diagnostic process for spina bifida?
Content Reviewers:Rishi Desai, MD, MPH
The term “spina bifida” comes from the latin “split spine,” which is actually a pretty good description, because spina bifida is a birth defect where the tissue on the left and right side of the back that normally come over the spinal cord to protect it, don’t completely meet up to form a nice seal, leaving behind various degrees of an opening right down the middle of the lower back.
Very early on in fetal development, the ectoderm, which is the outer layer of the fertilized egg, starts to develop a bit of a ridge that eventually becomes the neural tube.
This neural tube goes on to become the spinal cord, the brain, as well as tissues that enclose and protect them, called the meninges, and spina bifida occurs when a portion of this neural tube fails to close properly, typically in the lower back.
This constitutes a defect or absence of the vertebral arches due to failure of mesoderm to organize over the region of the defect, and it may or may not involve the underlying meninges and neural tissue
Alright so there are three main types of spina bifida, the first is myelomeningocele, also called meningomyelocele, and this is the most severe of the three and occurs when the spinal cord and the surrounding meninges protrude out of an opening in the bony vertebrae and are held together by a sack of skin that pouches out from the back.
In really severe cases, there is no skin at all, and the nerves of the spinal cord are therefore exposed, a condition called ‘open spina bifida, and this can cause serious damage to the exposed nerves as well as risk an infection, resulting in symptoms like loss of sensation or paralysis in the areas of the body below the damaged site, bladder or bowel movement problems, seizures, as well as leg and foot deformities.
Now a myelomeningocele is typically associated with a condition called an Arnold Chiari II malformation, which is where cerebellar and brainstem tissue slip down into the foramen magnum—the opening at the base of the skull.
In this condition, the prenatal spinal deformities result in only the meninges (and not the spinal nerves) slipping into the gaps between the deformed vertebrae.
This one’s the least common form of spina bifida, and because the spinal cord itself is not damaged, these individuals often do not experience the severe symptoms that are seen in myelomeningocele.
Alright the third type and final type is spina bifida occulta, and this is the most common and most mild form of spina bifida.
In fact, ‘occulta’ is latin for hidden, which is appropriate since many of the normal prenatal tests used to diagnose spina bifida don’t catch the disorder because the deformities in the tissues of the lower back are tiny.
In spina bifida occulta, the spinal cord and surrounding tissue don’t protrude, nor is any of the tissue forced into the spaces in between the vertebrae.