Spinal muscular atrophy with lower extremity predominance (SMA-LED) is characterized by muscle weakness and wasting (atrophy) in the lower limbs, most severely affecting the thigh muscles (quadriceps). (In SMA-LED, the "D" stands for dominant, which refers to the inheritance pattern of this condition.) The loss of nerve cells that control muscle movement (motor neurons) leads to atrophy of the muscles in the lower limbs. Affected individuals often have a waddling or unsteady walk and walk on the balls of their feet. They may have difficulty rising from a seated position and climbing stairs. Some people with SMA-LED also have weakness in upper limb muscles.
Joint deformities (contractures) in the hips, knees, feet, and ankles can occur in SMA-LED, and in severe cases are present from birth and can impair walking. Some individuals with this disorder have rigidity of joints (arthrogryposis) in their shoulders, elbows, and hands.
In most people with SMA-LED, the muscle problems are apparent in infancy or early childhood; however, about one-quarter of affected individuals do not develop muscle weakness until adulthood. The muscle weakness and related health problems typically do not worsen over time. [from
MedlinePlus Genetics]