Spina bifida is a neural tube defect (NTD) due to the incomplete fusion of the vertebral arches in its mildest form, or the incomplete closure of the neural tube, most commonly at the caudal end (the lumber region) in its most severe and commonest form.
🏘️ Epidemiology
Incidence and prevalence trends reveal striking differences in racial and ethnic susceptibility, with highest rates noted among Hispanic women and lowest rates noted among African and Asian women.
Geographical variability is also noted, with higher incidence rates noted in Northern China, England and Wales, and along the eastern seaboard in the US.
It is 1.2 to 1.7 times more common in girls, except for sacral-level defects, which occur with equal frequency among boys and girls.
Spina bifida occulta have a normal life expectancy, whilst some patients with myelomeningocele may live up to their 3rd decade of life.
⚠️ Risk factors
- Maternal Hispanic ethnicity
- Maternal folate and B12 deficiency
- Maternal folic acid deficiency
- Maternal sickle cell disease
- Maternal obesity (BMI >35)
- Maternal diabetes
- Previous pregnancy with NTD
- Parental NTD
- Low SES
- Antenatal exposure to teratogenic medications - such as valproate & carbamazepine (AED's), isotretinoin, and methotrexate.
- Malabsorption
🔢 Classification
- Spina bifida occulta
- Meningocele (meningeal cyst)
Spina bifida occulta is the mildest type of spina bifida. It is sometimes called hidden spina bifida. It is present in 10-15% of the population.
With it, there is a small gap in the spine due to a defect in vertebral fusion only. There is no sac or bulge at the back. The spinal cord and the nerves usually are normal. Skin and other tissue may form on top of the spinal cord, but not bone.
Many times, spina bifida occulta is not discovered until late childhood or adulthood. This type of spina bifida usually does not cause any disabilities and has no clinical significance. It is simply an incidental radiological finding. It may be characterised by a small dimple or tuft of hair.
With a meningocele, there is herniation of the meninges through an opening in the baby’s back, without the involvement of the spinal cord. There is usually little or no nerve damage. This type of spina bifida can cause minor disabilities.
- Myeloschisis
- Myelomeningocele
This is the herniation of the meninges as well as the spinal components but remains flattened and plate-like.
Myelomeningocele is the most serious type of spina bifida. With this condition, there is herniation of both the meninges and the spinal cord through an opening in the baby’s back. Part of the spinal cord and nerves are in this sac and are damaged. This type of spina bifida causes moderate to severe disabilities, such as problems affecting how the person goes to the bathroom, loss of feeling in the person’s legs or feet, and not being able to move the legs.
🥚 Embryology
In the 3rd week of development, neural development begins:
- The notochord appears in the mesoderm. It is a flexible, rod-like structure that runs along the length of the embryo, serving as a temporary axial support. The notochord plays a crucial role in the embryonic development of the vertebrate central nervous system and spine. It secretes growth factors which stimulate the differentiation of the overlying ectoderm into neuroectoderm, forming a thickened structure known as the neural plate.
- Primary neurulation
- Secondary neurulation
The neural plate undergoes a process of invagination, creating a groove known as the neural groove along its midline. The lateral edges of the neural plate then rise to form neural folds. The neural folds move towards each other and meet in the midline, fusing to form the neural tube (which is the precusor to the brain and spinal cord). This process is known as primary neurulation and it occurs in the upper-middle spinal cord.
During fusion of the neural folds, some cells within the folds migrate to form a distinct cell population known as the neural crest cells. They give rise to a diverse cell lineage including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and glia.
Secondary neurulation refers to the formation of the lower spinal cord, which gives rise to the lumbar and sacral elements. Oppositely to primary neurulation which happens from the ectodermal layer, secondary neurulation is derived from the mesodermal layer. Mesenchymal cells aggregate to form a solid structure known as the neural cord in the caudal region of the embryo. The central part of the neural cord undergoes cavitation, forming a fluid-filled cavity that will become the central canal of the spinal cord. The secondary neural tube eventually fuses with the previously formed primary neural tube.
Pathophysiology
NTDs occur when there is disruptions to the embryological development of the neural tube.
During the early stages of neurulation, there is a temporary opening at the most cranial and caudal ends of the neural tube. The opening at the cranial end is referred to as the cranial neuropore, and the one at the caudal end is called the caudal neuropore. These openings allow communication between the neural tube and the surrounding amniotic fluid. It needs to interact with the amniotic fluid for provision of nutrients, removal of waste products, and for proper developmental signalling. While communication with the amniotic fluid is essential for various developmental processes, it's important to note that the neural pores eventually close as the neural tube completes its formation. The closure of the neural pores is crucial for the protection and proper development of the neural tube.
Improper closure of these neuropores lead to the spectrum of NTDs we see:
- Anencephaly - this occurs due to improper closure of the cranial neuropore. It leads to absence of a major portion of the brain.
- Craniorachischisis - this occurs due to improper closure of both the cranial and caudal neurpores. This leads to an extensive opening along the entire length of the neural tube. The failure of closure at the cranial end leads to a lack of formation of the skull and brain structures. The failure of closure at the caudal end affects the development of the spinal cord and associated vertebrae.
- Spina bifida - this occurs due to improper closure at the caudal neuropore, leading to malformations of the spinal cord and vertebrae.
😷 Presentation
Spina bífida may manifest neurologically or in the musculoskeletal systems:
- Neurological manifestations
- Motor deficits and developmental delay
- Sensory deficits
- Neurogenic bladder/bowel dysfunction
- Hydrocephalus
- Seizures
- Musculoskeletal manifestations
- Increased risk of hip subluxation - primarily attributed to muscle imbalance, reduced weight-bearing, joint laxity, and associated orthopaedic issues (such as scoliosis and contractures).
- Scoliosis
- Contractures
🔍 Investigations
💡 Spina bifida is usually diagnosed on antenatal ultrasound scans.
However, some other routinely done tests in pregnancy that may indicate NTD's are:
- Triple screen - this includes:
- ⭐️ Alpha-fetoprotein (AFP) - elevated. There can be an increased leakage of AFP from the developing spinal cord into the amniotic fluid, leading to higher levels of AFP in maternal blood. .
- Human chorionic gonadotrophin (hCG)
- Unconjugated estriol (uE3)
- Quadruple screen - includes the same 3 mentioned above, as well as inhibin A (although this also typically is not deranged in spina bifida).
These tests are routinely offered between 15 and 20 weeks gestation.
🧰 Management
🔮 Prevention
- 🏆 The single most clinically significant protective factor known to reduce the incidence of spina bifida is maternal folate consumption.
- For women at low risk of NTDs - folic acid (400 micrograms daily), to be taken before conception - until week 12 of pregnancy.
- For women at high risk of NTDs - folic acid (5 mg daily), to be taken before conception - until week 12 of pregnancy.
🔍 Antenatal diagnosis
Options regarding termination of the pregnancy, fetal surgery, and delivery should be discussed between the parents/carers and the obstetrician.
- Fetal surgery - performed between the 19th - 25th week of pregnancy and should be performed by an experienced maternal-fetal surgical team only after careful consideration. The procedure involves opening the uterus and accessing the developing fetus. This is often performed using minimally invasive techniques, such as fetoscopic surgery or using a small incision in open fetal surgery.
👶 Postnatal care
A multidisciplinary approach should be followed based on the individual needs of each patient. This can include:
- Orthopaedic surgeons
- Neurosurgeons
- Physiotherapists
- Occupational therapists
🚨 Complications
- Constipation
- Faecal incontinence
- Obesity
- Hydronephrosis and vesicoureteric reflux
- UTI
- Syringomyelia
- Sexual dysfunction
- Learning disability
- Psychosocial impact