Understanding Meningocele In Spina Bifida

Introduction

Spina bifida is a neural tube defect where the spine and spinal cord do not develop correctly in the womb, causing gaps or missing vertebrae in the baby’s spine. Spina bifida symptoms can range from mild to serious impairments. Symptoms are dependent on the location of the spine opening and the size of the opening. The severity of symptoms and characteristics of the opening are used to classify the three main types of spina bifida:

• Spina bifida occulta - It is the most common and mildest form, where 1+ vertebrae do not develop properly, but the gap is minuscule 
• Myelomeningocele - It is the most severe form in which the baby’s spinal canal does not close along many vertebrae in the back
• Meningocele (muh-NIN-guh-seal) - It is also a severe but rare form where protective membranes surrounding the spinal cord (meninges) protrude from the spine

Figure 1. Types of spina bifida - Centres for Disease Control and Prevention.

They can be grouped into two main types: 

• Closed spina bifida - spina bifida occulta 
• Open spina bifida - also known as spina bifida aperta (including myelomeningocele and meningocele) 

The latter category is more dangerous and problematic. 

By Centers for Disease Control and Prevention - Centers for Disease Control and Prevention, Public Domain, Link

Figure 2. Illustration of an infant with open spina bifida - Centres for Disease Control and Prevention.

Neural tube development in meningocele

During early pregnancy, the neural tube forms and eventually develops into the spine and spinal cord. The spine should close by the 4th week after conception.¹

In meningocele, the neural tube fails to close, and a defect in the formation of vertebral arches means meninges protrude through the opening. These protrude through the dura and arachnoid mater of the meninges but not into the pia mater. This creates a sac that is filled with cerebrospinal fluid.² Unlike myelomeningocele, the meningocele sac does not contain the spinal cord or nerve endings within the sac.

Furthermore, the large sac can exert pressure on the surrounding tissues, which leads to various degrees of neurological impairment, depending on the location along the spine. 90% of the abnormalities found are located in the lumbar and sacral regions of the vertebral column (lower back), and fewer in the cervical region (upper neck).

Causes and risk factors 

A main cause of spina bifida is thought to be genetics, responsible for 60-70%.³ There is no specific gene responsible (Mendelian inheritance) for spina bifida; however, it is a complex trait determined by many genes. The exact mode of inheritance is unknown, but researchers are working to determine it.

The other cause is environmental factors such as:

• Insufficient folic acid - folate is the natural form of vitamin B9, which, when deficient, causes spina bifida, so folic acid supplements (the synthetic form) can be given to patients
• Pregnant person assigned female at birth (AFAB) with diabetes/obesity - increases the risk of spina bifida by 2 to 10 times⁴
• Specific medications - e.g., valproic acid to treat seizures
• Family history - the risk is greater if one child has spina bifida, and even greater if a person AFAB has two children with spina bifida

Preventative strategies

Since folic acid deficiency increases the risk of spina bifida, taking 400 micrograms of folic acid supplements when trying to get pregnant and during the first 12 weeks of pregnancy has been found to significantly reduce the likelihood of spina bifida.

If you are at a higher risk, 5 milligrams of folic acid is prescribed. Eating foods naturally high in folate can be useful. These include leafy green vegetables like spinach and chickpeas/kidney beans.

In addition, carefully managing health during pregnancy, especially obesity and diabetes, is important. Furthermore, avoiding hot tubs or saunas, to not overheat the body, is recommended, but the evidence is less well supported.

Diagnosis

There are two categories in which diagnoses fall, first is before birth or prenatal and second is after birth or postnatal.

Prenatal

Spina bifida can be detected prenatally, and over 90% of cases are detected before birth.⁵ This is done primarily through ultrasound because it is non-invasive, cheap, and safe.

Although the spine closes by the 4th week of pregnancy, spine defects will only become apparent in ultrasounds by around weeks 20-22.¹ 

Abnormalities that can be detected include:

• Changes to the skull shape, such as a flattened skull at the temporal bones (lower front sides of the skull) and changes to the posterior fossa (lower back region of the skull)
• Abnormal cerebral ventricles
• Ventriculomegaly (ventricle enlargement)
• Cystic lesions and abnormal pouches at the base of the spine
• Incomplete closure at the posterior vertebral arches
• Lower limb immobility
• Abnormal positioning of the feet⁵

Spina bifida occulta does not present with intracranial abnormalities, unlike meningocele and myelomeningocele.⁶ Often, it is difficult to distinguish meningocele from myelomeningocele in utero. The later in pregnancy screening is undertaken, the greater the likelihood of being able to differentiate variations in spina bifida. 

Although ultrasound is the best diagnostic tool for detecting spina bifida, sometimes blood tests and amniocentesis are used to confirm the diagnosis or first indicate abnormalities. A maternal serum α-fetoprotein (MASFP) test detects α-fetoprotein, which at high levels indicates a neural tube defect.

However, high α-fetoprotein can also indicate multiple foetuses or being further along in pregnancy than expected, so it can be a less reliable technique. If spina bifida has been confirmed by an ultrasound, amniocentesis will be carried out. It involves using a needle to extract amniotic fluid to test for other genetic diseases. 

Prenatal diagnosis gives parents a prognosis of the baby’s condition. This means parents can make an informed choice whether to continue the pregnancy or terminate if they wish.

Postnatal 

After birth, a doctor carries out a physical examination. If the baby has myelomeningocele or meningocele, then the fluid-filled sac will be positioned outside of their lower back. A CT or MRI can be used to assess the extent of the defect and any potential damage to the spinal cord and nerves. This can show whether the spinal cord and nerves are within the sac or not. To differentiate between myelomeningocele and meningocele. Since spina bifida occulta does not have a sac outside of the body and often has mild symptoms, it is more difficult to diagnose.

Treatment for meningocele

Usually, the only treatment required for meningocele is surgical repair. This will happen soon after birth to prevent the chance of infection. During surgery, the surgeon will make an incision in the sac to drain cerebrospinal fluid, place the meninges back into the spinal canal and close the opening. The surgeon will also explore the spinal canal to assess if the patient has tethered cord syndrome. Tethered cord syndrome is a condition where tissue attaches to the spinal cord to limit mobility. If the condition is present, the spinal cord can usually be released in the same surgery.⁷ Afterwards, patients will be closely monitored in the neonatal unit and electrolytes and red blood cells are assessed.

Prognosis for meningocele 

Meningocele has a favourable prognosis because it has no neural involvement. The prognosis is also good since it can easily be surgically repaired. Problems can sometimes persist after repair, like weakness in bowel and bladder control, if the sac is positioned at the lumbar region of the spine.

Very occasionally, hydrocephalus (the accumulation of cerebrospinal fluid in the brain) occurs in patients with meningocele, where fluid increases pressure in the brain, potentially causing brain damage. It requires a ventriculoperitoneal shunt to alleviate pressure.⁸ This can be a lifelong problem as the shunt needs to be continuously replaced.

Additionally, babies with meningocele have a higher predisposition to allergy to latex than the general population; therefore, latex gloves, nipple dummies, etc, should be avoided for these babies.⁹ 

Therefore, children with meningocele sometimes require ongoing care to manage any residual symptoms or complications. This can involve physical therapy to improve their mobility and strength, which may require seeing an occupational therapist. Despite this, individuals with meningocele can live normal lives, often unaffected by this form of spina bifida.

Summary

Spina bifida is a neural tube defect causing missing vertebrae in the spine. The defect can be closed in spina bifida occulta, or open as in meningocele and myelomeningocele. Open defects tend to have the greatest impact on an individual's quality of life. In meningocele, the meninges protrude outwards without affecting the spinal cord, leading to fewer neurological issues than myelomeningocele. The causes include genetic factors and environmental influences like insufficient folic acid and maternal health conditions. Diagnosis is primarily via prenatal ultrasound, with treatment involving surgical repair post-birth. Meningocele has a favourable prognosis with early intervention, though some complications like bladder issues and hydrocephalus occasionally persist.