Apical Ectodermal Ridge Disruption and Hypoplastic Digits in Amyoplasia

The cause of Amyoplasia remains controversial, with the predominant theory being loss of anterior horn cells secondary to vascular disruption as the primary insult. Disruption of the apical ectodermal ridge (AER) is extremely rare, occurring in 1 in 2 million live births. Amyoplasia has an incidence of 1 in 10,000 births, making the probability of both conditions occurring simultaneously by chance 1 out of 20 billion. Muscle precursor cells do not differentiate until they reach the limb and contact signaling factors that are AER dependent. Without sufficient AER function, muscle cell line development may be limited. We report on an association between disruption of the AER and Amyoplasia, which may offer a clue regarding pathogenesis. We identified 22 patients who presented to our institution during a 2-year time span. Inclusion criteria include diagnosis of Amyoplasia and at least one hypoplastic digit. Five children were excluded due to lack of data or unconfirmed diagnoses. Radiographs, clinical photos, clinic visit notes, and operative reports were reviewed for this project. Hypoplastic fingers were seen in 41% of cases, hypoplastic toes in 47%, hypoplastic fingers and toes in 6%, and loss of a foot in 6%. No patient had fully preserved ectoderm at the transverse amputation or amniotic bands. Loss of AER function with resultant hypodactyly could be a characteristic of Amyoplasia. There may be a causal or downstream association between AER loss and muscle hypo-development.