O'Sullivan-McLeod syndrome is a benign lower motor neuron disorder and a rare variant of monomelic amyotrophy (MA). It is characterized by an initial weakness in the intrinsic hand muscles on one side which eventually spreads in an asymmetrical pattern to the opposite limb. Over a long period of about 20 years, people with this condition experience a very slow progression of muscular atrophy. This description helps in understanding that the condition affects the muscles gradually, with the primary focus on hand muscle weakness leading to eventual atrophy over many years.

People with this condition typically experience weakness in the hand muscles on one side at first. Over time, this weakness spreads to the other hand, and the muscles slowly become smaller and weaker due to gradual atrophy.

Information about causes is currently limited for this condition.