Spina bifida

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 three main layers. The ectoderm on the outside, the mesoderm in the middle, and the endoderm on the inside. Around the third 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 fourth 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 7 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 5 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 “cele” 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.