Monomelic amyotrophy with late progression.

Monomelic amyotrophy is a sporadic juvenile-onset disease that presents with gradual onset of weakness and atrophy in the hand muscles unilaterally. Generally, this disease is considered a 'benign' and non-progressive motor neuron disease, which stabilizes within five years of onset. We discuss a case that illustrates that monomelic amyotrophy may rarely exhibit late clinical progression to the lower extremities after a prolonged period of disease stability.