Reference values of amniotic fluid neuron-specific enolase

Enolase is a dimeric cytoplasmic enzyme whose double gamma isoenzyme, neuron-specific enolase, is predominantly found in neuronal and neuroendocrine tissues. Cell injury causes its release into the blood and cerebrospinal fluid (CSF). Neuron-specific enolase has been measured in the serum and CSF of adults and full-term asphyxiated neonates as a marker of neurological injury. We recently observed an elevation of neuron-specific enolase in the amniotic fluid of women whose neonates subsequently developed intraventricular hemorrhage or periventricular leukomalacia. The purpose of our study was to establish reference values of neuron-specific enolase in the amniotic fluid as a function of gestational age.

A total of 110 amniotic fluid samples, obtained primarily for genetic studies (16-20 weeks, n = 22), for evaluation of preterm labor (21-35 weeks, n = 66) and for fetal lung maturity studies (36-40 weeks, n = 22), were analyzed for neuron-specific enolase. Samples were from women who subsequently delivered term neonates with normal neurological examinations or who delivered preterm neonates with normal neurosonograms up to the 7th day of life. Descriptive statistics and non-parametric correlations were used for analysis.

There was no correlation between gestational age and concentration of neuron-specific enolase (Spearman's r = 0.059, p = 0.63). The overall mean neuron-specific enolase value was 2.5 +/- 1.39 microg/l. The highest value obtained was 6 microgl. Of the 110 women, 105 (95.5%) had neuron-specific enolase values of less than 5 microg/l, while five (4.5%) had values ranging from 5 to 6 microg/l.

The amniotic fluid level of neuron-specific enolase does not change as a function of gestational age. These stable levels may have utility in the evaluation of cases with fetal neurological injury.