Being born without one or more limbs is a congenital condition. While rare, it represents a significant variation in human development. Precise medical terminology helps describe these presentations.
Amelia: The Term for Complete Limb Absence
The medical term for the complete absence of one or more limbs is amelia. This congenital condition stems from developmental issues during fetal growth. The term “amelia” originates from ancient Greek, combining “a-” (lack of) and “melos” (limb).
Amelia can manifest as the absence of a single limb (unilateral amelia) or all four limbs (tetra-amelia). This condition is uncommon, with prevalence estimates typically ranging from 0.04 to 0.15 per 10,000 births or about 1.41 per 100,000 births. In amelia, no skeletal structure is present beyond the point of complete limb absence.
Understanding the Causes
Amelia is a birth defect arising during the early stages of fetal development, specifically during limb formation. This period occurs between 24 and 36 days after fertilization. Disruptions during this timeframe can prevent development of limb primordia, leading to their complete absence.
Causes of amelia include both genetic and environmental factors. Some cases are sporadic, without a clear identifiable cause. Genetic mutations can play a role, with forms like tetra-amelia syndrome linked to specific genes (e.g., WNT3, RSPO2), often inherited recessively. Amelia may also occur as part of broader genetic syndromes, such as Roberts syndrome or VACTERL association, which involve multiple congenital anomalies.
Environmental exposures during pregnancy also contribute to amelia. A historical example is the drug thalidomide, which caused severe limb deficiencies, including amelia, when taken by pregnant women in the late 1950s and early 1960s. Other potential environmental factors include maternal infections, maternal diabetes, alcohol consumption, and vascular disruptions.
Variations of Limb Deficiency
Amelia differs from other congenital limb deficiencies where a limb is partially formed or malformed.
One variation is meromelia, the partial absence of a limb. In meromelia, some limb portion remains, often with the hand or foot attached, but intervening segments are missing. This includes conditions like terminal transverse hemimelia (limb ends abruptly) or longitudinal hemimelia (deficiencies along the limb’s length).
Another condition is phocomelia, where limbs are severely underdeveloped, resulting in hands or feet attached directly to the trunk, giving a “flipper-like” appearance. While phocomelia can be associated with genetic factors, it is also linked to historical thalidomide exposure.