How is spina bifida diagnosed before birth ?
Antenatally, spina bifida
is diagnosed by the following tests:
- Maternal serum Alfa Fetoprotein (ms AFP)
One of the tests for antenatal diagnosis of NTDs is measuring msAFP at 16th - 18th week of gestation. Fetal liver synthesizes this protein. It is present in amniotic fluid in high amounts. From amniotic fluid, a small amount of AFP leaks into maternal serum. In presence of open NTD much higher amounts of AFP ooze into amniotic fluid and from there to maternal serum. Thus the level of msAFP is significantly higher if the fetus in utero carries an open NTD. At cut - off value of greater than 2.5 x mom (multiple of median) more than 90% cases of anencephaly and 80% of Spina Bifida cystica may be detected by msAFP measurement. The specificity of the test however is not very high since it may be elevated in several fetal disorders as well. The utility of combining serum uE 3 (Low Estriol) measurement along with msAFP has been recently proposed. The diagnosis of NTD can be confirmed by high - resolution fetal ultrasonography and or amniotic fluid acetyl - choline esterase, both of which are capable of making correct diagnosis of NTD in great majority of cases.
Also the maternal fluid AFP can be measured and this can be the informative index in the diagnosis of NTDs. But as this procedure is more invasive and real time ultrasonography is the preferred method.
- Antenatal Ultrasonography
Ultrasound evaluation for spina bifida includes both spinal and cranial imaging. Spinal findings include widening of the posterolateral spinal ossification centres, absence of the continuity of the skin over the spine, and a bulging sac past the dorsal skin line. Cranial findings suggestive of Spina Bifida include the presence of ventriculomegaly, small head size, and elongated cerebellum with obliteration of cisterna magna from a chiari II malformation (Banana sign) and scalloping of the frontal skull region (lemon sign). The lemon sign is best obtained superior to the plane used for biparietal diameter (BPD) measurement at level of lateral ventricles.
Because of increasing density of the fetal skull with increasing gestational age, the scalloping of the frontal bones usually resolves in the third trimester. This is a highly sensitive test for open neutral tube defects only in the second trimester. Its absence between 14-24 weeks makes Spina Bifida unlikely. Posterior fossa changes with open neutral tube defects are caused by the Chiari II malformation with herniation of the cerebellum into the upper spinal canal. The cerebellum takes on a crescent or banana shape and the cisterna magna becomes obliterated. This banana sign in the second trimester is both sensitive and specific for open neutral tube defects without false positive of the lemon sign. In the third trimester the obliterated cisterna magma may be easier to see, and more helpful than the banana sign. The posterior fossa changes in the presence of a spinal defects are called Arnold Chiari Type II malformation. Hydrocephalus develops in up to 90% of these cases and conversely one-third hydrocephalus fetuses have a spinal defect.
SELECTING PATIENTS FOR TREATMENT :
We apply following selection criteria for treatment advice:
- Potential for ambulation: After thorough motor and sensory evaluation, the potential for ambulation is decided.
- Family status and support: The economical, educational status of the family is assessed and the amount of support, which the family is giving to deliver for the case of Spina Bifida, is judged.
- Associated anomalies: Gross hydrocephalus, presence of bowel, bladder incontinence and other congenital anomalies are seen.
Those patients who have very less potential for ambulation in future i.e. patients with total paraplegia
& those who were from very poor families, living remote from specialized centers and child with gross other congenital anomalies are told of a poor prognosis.
Back closures are usually done earlier in the first 48 hrs in all newborns unless there was contraindication like infection.
Hydrocephalus when present is treated with Chabra's medium pressure V-P shunt. But the real task starts after the initial management of the back closure and hydrocephalus.