Trisomy 18 (Edwards Syndrome)
The clinical triad of micrognathia, overlapping digits, and rocker-bottom feet in a newborn is pathognomonic for Trisomy 18 (Edwards syndrome). 1
Key Diagnostic Features
This constellation of physical findings represents the classic phenotype of Trisomy 18, which is the second most common autosomal trisomy after Down syndrome, occurring in approximately 3 per 10,000 live births 1. The syndrome predominantly affects females (2-3 times more common than males) 1.
Characteristic Physical Findings
The specific features mentioned in your question are hallmark findings:
- Micrognathia (small jaw) - part of the characteristic facial dysmorphism 2, 3, 4
- Overlapping digits - typically clenched hands with overlapping fingers, a pathognomonic hand anomaly 1, 2, 4
- Rocker-bottom feet - distinctive foot deformity that is highly specific for this condition 2, 3, 4
Additional Associated Features to Recognize
When you encounter this triad, expect to find other associated anomalies that strengthen the diagnosis:
- Facial features: Prominent occiput, low-set malformed ears, small palpebral fissures, high arched palate 2, 4
- Cardiac anomalies: Congenital heart disease is extremely common 1, 3
- Neurologic abnormalities: Ranging from tone abnormalities to severe developmental delays and seizures 1
- Other skeletal findings: Talipes equinovarus (clubfoot) may accompany rocker-bottom feet 2
Clinical Context and Prognosis
Approximately 95% of live-born infants with these findings have full trisomy 18 (complete duplication of chromosome 18), though mosaic and partial forms exist with more variable presentations 1. The condition historically carries high mortality in the neonatal period and first year of life, though outcomes vary based on the degree of mosaicism and specific organ involvement 1.
Diagnostic Confirmation
While the clinical phenotype is highly recognizable, chromosomal analysis of peripheral blood is required for definitive diagnosis 4. In current practice, many cases are identified prenatally through noninvasive prenatal screening (NIPS), though diagnostic certainty requires amniocentesis and correlation with physical findings 1.
Important Clinical Pitfall
Do not assume microcephaly is required for diagnosis - while common, some cases of Trisomy 18 can present with other CNS anomalies including hydrocephalus rather than microcephaly 4. The triad you've described (micrognathia, overlapping digits, rocker-bottom feet) remains diagnostic regardless of head size.