What is fetal congenital adrenal hyperplasia (CAH) and how can it be screened for and diagnosed?

Fetal Congenital Adrenal Hyperplasia: Definition, Screening, and Diagnosis

Fetal congenital adrenal hyperplasia (CAH) is an autosomal recessive disorder characterized by impaired cortisol synthesis, most commonly (95% of cases) due to 21-hydroxylase deficiency caused by mutations in the CYP21A2 gene, leading to accumulation of cortisol precursors that are diverted to sex hormone production and resulting in virilization of affected female fetuses. 1

What is Fetal CAH?

CAH represents a group of inherited enzyme deficiencies in adrenal steroid biosynthesis, with an incidence of 1:10,000 to 1:20,000 births 1. The disorder exists on a spectrum:

• Classic salt-wasting form (75% of classic cases): Severe enzyme deficiency causing both cortisol and aldosterone deficiency, leading to potentially fatal hypovolemia and shock 1
• Classic simple virilizing form: Cortisol deficiency with preserved aldosterone production
• Nonclassic form: Mild enzyme deficiency with variable postnatal androgen excess, occurring in 0.1-0.2% of the general Caucasian population 1

The pathophysiology involves a cortisol synthetic block that triggers ACTH stimulation of the adrenal cortex, with accumulation of 17-hydroxyprogesterone (17-OHP) and diversion of precursors to androgen biosynthesis 1. The cardinal prenatal manifestation is genital ambiguity in affected 46,XX fetuses due to excess androgen exposure in utero 1.

Screening Approaches

Newborn Screening (Postnatal)

Universal newborn screening for 21-hydroxylase deficiency using a two-tier protocol (initial immunoassay with confirmatory liquid chromatography/tandem mass spectrometry for positive tests) is strongly recommended 1. This approach has been implemented in all 50 U.S. states and 12 other countries as of 2010 1.

The benefits of newborn screening include:

• Markedly reduced time to diagnosis 1
• Prevention of severe salt-wasting crises and associated mortality (4-10% death rate without screening) 1
• Less severe hyponatremia at diagnosis (mean 134 mM with screening vs. 124 mM without) 1
• Equal detection of affected males and females (males are at higher risk of missed diagnosis and death without screening due to lack of genital ambiguity) 1
• Reduced delay in correct sex assignment for virilized females 1

Prenatal Screening and Diagnosis

Prenatal treatment of CAH should continue to be regarded as experimental 1. However, prenatal diagnosis is available when there is a known family history:

First-trimester prenatal diagnosis can be performed through:

• Direct molecular genetic analysis of chorionic villus sampling (CVS) 2, 3
• DNA analysis of CYP21A2 gene mutations 2

Second-trimester prenatal diagnosis options include:

• Hormonal measurement of amniotic fluid 3
• DNA analysis of amniotic cells 3
• Sonographic evaluation showing enlarged adrenal glands (though this is not a primary screening method) 4

Emerging noninvasive approach: Cell-free fetal DNA detection in maternal plasma using haplotype-based sequencing has demonstrated feasibility for noninvasive prenatal testing, with accuracy rates of 96.41% for maternal alleles and 97.81% for paternal alleles 5, 6. This approach constructs parental haplotypes and recovers fetal haplotypes through maternal plasma DNA sequencing, avoiding the miscarriage risk associated with invasive procedures 6.

Diagnostic Confirmation

Hormonal Diagnosis

The diagnosis of 21-hydroxylase deficiency is based on measuring 17-OHP in symptomatic individuals 1. The diagnostic algorithm follows these thresholds:

Baseline 17-OHP levels:

• 300 nmol/L (>10,000 ng/dL): Likely classic CAH

• 6-300 nmol/L (200-10,000 ng/dL): Likely nonclassic CAH
• <6 nmol/L (<200 ng/dL): Likely unaffected or nonclassic CAH 1

For borderline cases, obtain a complete adrenocortical profile after cosyntropin (ACTH) stimulation test 1:

Post-ACTH stimulation 17-OHP levels:

• 300 nmol/L (>10,000 ng/dL): Classic CAH

• 31-300 nmol/L (1,000-10,000 ng/dL): Likely nonclassic CAH
• <50 nmol/L (<1,666 ng/dL): Unaffected or heterozygote 1

Additional elevated steroids include 21-deoxycortisol, androstenedione, and testosterone. Elevated plasma renin activity (PRA) with reduced aldosterone-to-PRA ratio indicates impaired aldosterone synthesis and differentiates salt-wasters from simple virilizers after the newborn period 1.

Genetic Testing

Genotyping should be reserved for cases where hormonal testing results are equivocal or for genetic counseling purposes 1. This recommendation reflects the complexity of CYP21A2 genetic analysis, with over 100 known mutations, gene duplications, deletions, and rearrangements 1.

Important caveat: Genetic testing cannot detect salt-wasting status; this requires careful clinical evaluation 1. Compound heterozygotes typically manifest a phenotype compatible with the milder mutation 1.

Critical Clinical Pitfalls

1. Do not attempt to completely normalize 17-OHP levels during treatment, as this typically results in glucocorticoid overtreatment and iatrogenic Cushing's syndrome 1

2. Males without genital ambiguity are at highest risk of delayed diagnosis and death from salt-wasting crises without newborn screening 1

3. Prenatal dexamethasone treatment, while effective in reducing genital virilization in affected females, remains experimental despite short-term safety data 1, 2. Only 1 in 8 at-risk pregnancies will be an affected female who would benefit, meaning 7 of 8 fetuses are unnecessarily exposed 2

4. Sonographic findings of enlarged adrenal glands may suggest CAH in the second trimester but should not be relied upon as a primary screening method 4