Spina bifida

Definitions & Key takeaways

Spina bifida is a congenital birth defect of embryonic development where there is incomplete closing of the vertebral column and membranes around the spinal cord. There are three main types: spina bifida occulta, meningocele, and myelomeningocele. In Spina bifida occulta there is only a small gap in the bones of the spine, and the spinal cord and surrounding tissue don't protrude. In meningocele, the meninges protrude through the opening in the spinal column, but the spinal cord itself is not affected. In myelomeningocele, there is a protrusion of the spinal cord and nerves through the opening of the spinal cord. For treatment, prenatal surgery is done to close myelomeningocele, but this surgery can be dangerous to the developing fetus as well as the mother. In cases where postnatal surgery is chosen, it is often done within the first few days of an infant's life to minimize the risk of infection like meningitis.

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With spina bifida, spina refers to the spine, while bifida means split. So spina bifida literally means split spine.
It occurs during embryonal development when the posterior part of the spine doesn't fully close, leaving behind various degrees of defects, usually in the lumbosacral region of the spine.
To understand spina bifida, we need to zoom in on the embryo during its earliest weeks of development. At this stage, the embryo has 3 main layers the ectoderm on the outside, the mesoderm in the middle, and the endoderm on the inside.
Around the 3rd week of development, the ectoderm forms a flat sheet of cells known as the neural plate. Soon after, the plate begins to fold inward along the midline, creating a neural groove with raised folds on each side.
By the 4th week, these folds move closer and fuse, forming the neural tube. The top part of this tube, called the cranial end, will become the brain, while the bottom part, known as the caudal end, will develop into the spinal cord.
Also, the ectoderm will give rise to three protective layers called the meninges, which wrap around the brain and spinal cord, cushioning them with cerebrospinal fluid.
But that's not all. Thanks to the mesoderm, the body organizes to form structures like muscles, bones, and the overall skeletal framework.
This adds another layer of protection because the mesoderm forms the vertebral column that surrounds and protects the spinal cord.
The vertebral column is divided into five regions, each with its own set of vertebrae. The cervical region has seven vertebrae, labeled C1 to C7.
The thoracic region has 12, named T1 to T12, and the lumbar region includes 5, from L1 to L5. Below these, the sacral region consists of five fused vertebrae, S1 to S5.
And the coccygeal region has 4 fused vertebrae, CO1 to CO4. Now, each vertebra has two main parts, the vertebral body at the front and the vertebral arch at the back.
Together, they form a central opening called the vertebral foramen. When the vertebrae stack up, these openings align to create the vertebral canal, which houses the spinal cord.
Now, spina bifida occurs when the caudal end of the neural tube fails to close properly during embryonic development. As a result, the mesoderm above the affected region fails to organize, which results in a defect or absence of the vertebral arches, usually around the L5 to S1 vertebrae.
Depending on the severity of the defect, spina bifida can range from a small harmless gap in the bone to a complete protrusion of the meninges and the spinal cord through the spine.
The exact reason why spina bifida occurs, remains a mystery, but several factors can increase the risk. The most important one is not getting enough folic acid during the first trimester of pregnancy, because folic acid helps the neural tube close properly.
Next, taking certain anti-seizure medications during pregnancy, such as valproic acid, can block folic acid from doing its job, increasing the chance of neural tube defects.
Other factors include obesity, consuming alcohol during pregnancy, and poorly controlled diabetes. Now, depending on the size of the vertebral defect, spina bifida can be classified into several types.
First, there's spina bifida occulta, which is the most common and mildest type. The word occulta means hidden, because this type often goes unnoticed during prenatal ultrasounds, and usually doesn't cause any symptoms.
That's because, even though one or more vertebrae don't fully close, the opening is so small that neither the meninges nor the spinal cord can bulge through.
Still, subtle signs to keep in mind include a small tuft of hair or a dark patch of skin over the lower back. Next up is meningocele, which breaks down into meningo, referring to the meninges, and the suffix seal, meaning hernia, or simply protrusion.
So in this type, the meninges slip through a gap in the vertebrae and form a visible sac on the lower back, filled with cerebrospinal fluid.
Since the spinal cord itself stays in place within the spinal canal, the risk of neurologic complications is usually low.
The last and most severe type of spina bifida is myelomeningocele. Myelo means spinal cord.
Meningo refers to the meninges, and seal means protrusion. So, in this case, both the spinal cord and meninges bulge through the opening, forming a fluid-filled sac, often about the size of an orange.
Because the spinal cord is no longer in its position, there are almost always neurological complications. Damage primarily affects the lower extremities, causing weakness, paralysis, and loss of sensation, which could be associated with bowel and bladder incontinence.
Myelomeningocele usually doesn't come alone, and it's typically associated with Arnold Chiari malformation type 2. In this condition, the cerebellum and the medulla slip down through the foramen magnum, which is the opening at the base of the skull.
The downward herniation can lead to the obstruction of the cerebral aqueduct, which is a narrow channel that allows the cerebrospinal fluid to flow through the brain.
When the cerebrospinal fluid can't flow as it should, it builds up in the brain, leading to hydrocephalus. In more severe cases, the herniation can press on the respiratory centers in the medulla and result in respiratory failure.
Lastly, myelomeningocele can occur together with Dandy-Walker malformation. In this condition, the cerebellar vermis, which normally connects the two halves of cerebellum, is either underdeveloped or missing altogether.
Without the vermis filling its usual space, the fourth ventricle expands abnormally, creating a fluid-filled cyst in the posterior fossa.
This cystic dilation of the 4th ventricle pushes surrounding structures outward, causing the posterior fossa to become larger than normal, a classic finding of this condition.
Moving on to diagnosis. More severe forms of spina bifida, like meningocele and myelomeningocele, are often detected before birth, through routine prenatal screening.
One important test is the 2nd trimester fetal ultrasound, which can detect defects in the spine. Another key clue comes from the maternal serum alpha fetoprotein, or MSAFP, which is elevated if an open spinal defect is present.
During pregnancy, the fetus produces AFP and under normal conditions, only a small amount moves into the amniotic fluid, and then into the maternal bloodstream.
However, when neural tissue is exposed outside of the body, extra AFP leaks out and causes elevated levels in the mother's blood.
Keep in mind though, that spina bifida isn't the only cause of elevated maternal serum alpha fetoprotein, and therefore, ultrasound is still needed to confirm the diagnosis.
On the other hand, more mild cases of spina bifida may not be detected until after the baby is born. In this case, if physical examination findings are suggestive of spina bifida, a spinal ultrasound can evaluate the spinal cord and vertebral structures.
Treatment depends on the severity. Spina bifida occulta is asymptomatic and typically doesn't require treatment.
However, in meningocele and myelomeningocele, surgery is the way to go, to prevent future complications. In some cases, surgeons can repair the defect with in utero interventions, but the procedure carries serious risks for both the baby and the mother.
All right, it's a quick recap. Spina bifida is a neural tube defect where the caudal end of the neural tube doesn't close properly during early embryonic development.
This leads to incomplete formation of the vertebral arches, creating an opening through which the meninges and spinal cord can protrude.
The exact reason why spina bifida occurs remains a mystery, but several factors can increase the risk. The most important one is not getting enough folic acid during the first trimester of pregnancy.