Confirming the Diagnosis of Edwards Syndrome
Edwards syndrome (trisomy 18) is definitively confirmed by karyotype analysis showing trisomy of chromosome 18, obtained either prenatally through amniocentesis or chorionic villus sampling (CVS), or postnatally through peripheral blood chromosome analysis.
Prenatal Diagnostic Confirmation
Following Positive Noninvasive Prenatal Screening (NIPS)
When NIPS suggests trisomy 18, diagnostic testing with chromosome analysis on amniocentesis or CVS is mandatory 1. NIPS is only a screening test and cannot confirm the diagnosis 1.
Key testing approach:
- Amniocentesis is preferred over CVS for trisomy 18 confirmation, as CVS may show confined placental mosaicism (CPM) that does not reflect the true fetal status 1
- Chromosome analysis demonstrating nonmosaic trisomy 18 consistent with NIPS results confirms an affected fetus 1
- If CVS shows mosaic trisomy 18, it cannot be considered confirmatory and follow-up amniocentesis is required 1
Technical Methods
The definitive prenatal diagnosis utilizes:
- Chromosomal analysis with G-banding on amniotic fluid cells obtained via amniocentesis 2, 3
- Fluorescence in situ hybridization (FISH) can supplement chromosome analysis 2
- Chromosomal microarray (CMA) may be used as confirmatory testing or reflex testing after normal initial chromosome analysis 1
Critical pitfall: CVS-derived cells are placental in origin and may not represent the fetus due to CPM. Trisomy 18 specifically is more likely to be found in mosaic form on CVS 1.
Postnatal Diagnostic Confirmation
When Prenatal Testing Was Declined
If parents declined prenatal testing, postnatal confirmation depends on clinical findings 1:
- Normal physical examination by a clinical geneticist may be sufficient to preclude testing in suspected trisomy 18 cases 1
- Any suspicion of abnormal phenotype related to trisomy 18 warrants cytogenetics evaluation 1
- Extended chromosome analysis to rule out low-level mosaicism or FISH may be performed when warranted 1
Rapid Postnatal Confirmation
For newborns requiring urgent management decisions, karyotyping of bone marrow aspirate provides rapid diagnosis 4. This approach can:
- Shorten the period of diagnostic uncertainty from days to hours 4
- Allow clinicians to avoid invasive surgical procedures in confirmed cases 4
- Facilitate timely discussions with parents about treatment options 4
Standard postnatal confirmation uses peripheral blood chromosome analysis 1, 5.
Clinical Suspicion Triggers
Diagnosis should be suspected with characteristic features including 6, 5:
- Craniofacial dysmorphism
- Arms lifted in supination with permanent finger flexion (index overlapping 3rd finger, little finger overlapping 4th finger)
- Congenital heart disease (most common malformation)
- Polymalformative syndrome on prenatal ultrasound 7
Important consideration: The median age at postnatal diagnosis is typically within the first few days of life (mean 37 days in one series) 6, though clinical suspicion often begins prenatally with abnormal ultrasound findings 7.
Genetic Counseling Context
Once confirmed by karyotype showing homogeneous/free trisomy 18 (most common form) 6, 5, genetic counseling should address:
- Recurrence risk (related to maternal age) 7
- Associated malformations and prognosis 3
- Management options including palliative versus interventional approaches 8
The karyotype remains the gold standard for definitive diagnosis, whether obtained prenatally or postnatally 1, 2, 3, 5.