X-linked infantile spinal muscular atrophy is a condition that affects only boys and is characterized by severe muscle weakness and absent reflexes (areflexia). Affected children often have multiple joint deformities (contractures) from birth that cause joint stiffness (arthrogryposis) and impair movement. In severe cases, affected infants are born with broken bones. The muscle weakness worsens over time; affected children reach some early motor developmental milestones, such as sitting unassisted, but these skills are often lost (developmental regression).
Weakness of the chest muscles used for breathing often leads to life-threatening breathing problems. Children with X-linked infantile spinal muscular atrophy usually do not survive past early childhood due to respiratory failure, although, in rare cases, affected individuals can survive into adolescence.