What causes a baby to be born with little or no bone in the fifth finger, also known as ectrodactyly or split-hand deformity?

Causes of Little or No Bone in the Fifth Finger (Ectrodactyly)

The absence of bone in the fifth finger, known as ectrodactyly or split hand deformity, is primarily caused by vascular disruption during embryonic limb formation or in already-formed fetal limbs, which can occur spontaneously or be associated with genetic syndromes. 1

Genetic Causes

• Ectrodactyly can occur as an autosomal dominant trait with variable expression, affecting hands and feet to different degrees 2
• It may be part of syndromic conditions such as:
  • Ectrodactyly-ectodermal dysplasia-cleft palate (EEC) syndrome, which is characterized by the triad of ectrodactyly, ectodermal dysplasia, and facial clefts 3
  • Split hand/foot malformation (SHFM), which involves various chromosome regions 4
• Chromosomal abnormalities can be associated with ectrodactyly:
  • Trisomy 10 mosaicism has been linked to ectrodactyly with bilateral radial ray defects 4
  • Other chromosomal disorders may include this feature as part of a broader syndrome 4

Vascular Disruption Mechanism

• Researchers hypothesize that limb deficiencies like ectrodactyly are caused by vascular disruption during critical periods of embryonic development 1
• This disruption can occur during:
  • Initial formation of embryonic limbs
  • Development of already-formed fetal limbs 1
• The timing of the vascular disruption correlates with the severity of the defect:
  • Earlier disruptions (before 68 days' gestation) tend to cause more severe and proximal defects 1
  • Later disruptions (after 70 days' gestation) typically result in more limited digital deficiencies 1

Iatrogenic Causes

• Chorionic villus sampling (CVS) has been associated with limb deficiencies including digital anomalies 1
• The risk appears to be timing-dependent:
  • CVS performed before 68 days' gestation is associated with more severe limb deficiencies 1
  • CVS performed at or after 70 days' gestation is associated with more limited digital defects 1
• Approximately 74% of infants exposed to CVS at or after 70 days' gestation who developed limb deficiencies had digital deficiencies without proximal involvement 1

Osteogenesis Imperfecta Consideration

• While not typically causing complete absence of bone in digits, some forms of osteogenesis imperfecta (OI) can present with bone abnormalities that might be confused with ectrodactyly 1
• OI type I, IV, V, and VI can present with subtle findings that may include bone abnormalities 1
• These conditions should be considered in the differential diagnosis, especially if there are other features such as fractures or blue sclerae 1

Diagnosis

• Prenatal diagnosis of ectrodactyly is possible:
  • Can be detected as early as the first trimester (13 weeks) using three-dimensional ultrasonography 5
  • Second-trimester ultrasound examination can reliably identify hand and foot malformations 6
• Postnatal evaluation should include:
  • Complete physical examination to identify isolated versus syndromic forms 1
  • Genetic testing when appropriate, especially if there are multiple anomalies or family history 1
  • Referral to hand specialists or pediatric plastic surgeons for management of complex deformities 7

Clinical Implications

• The condition ranges from single digit absence to complete split hand deformity 6
• Morbidity varies based on:
  • Whether the condition is isolated or part of a syndrome 6
  • The extent of involvement (single digit vs. multiple digits) 6
  • Associated anomalies that may affect other organ systems 4
• Early identification allows for:
  • Appropriate genetic counseling for families 1
  • Planning for potential surgical intervention 7
  • Screening for associated anomalies in syndromic cases 3

Understanding the specific cause in each case is crucial for providing accurate genetic counseling and determining the risk of recurrence in future pregnancies 1, 6.