Reduced strength of the distal musculature of the legs.

Abnormally increased curvature (anterior concavity) of the lumbar or cervical spine.

Spinal muscular atrophy (SMA) is characterized by muscle weakness and atrophy resulting from progressive degeneration and irreversible loss of the anterior horn cells in the spinal cord (i.e., lower motor neurons) and the brain stem nuclei. The onset of weakness ranges from before birth to adulthood. The weakness is symmetric, proximal > distal, and progressive. Before the genetic basis of SMA was understood, it was classified into clinical subtypes based on maximum motor function achieved; however, it is now apparent that the phenotype of SMN1-associated SMA spans a continuum without clear delineation of subtypes. With supportive care only, poor weight gain with growth failure, restrictive lung disease, scoliosis, and joint contractures are common complications; however, newly available targeted treatment options are changing the natural history of this disease.

A decrease in the strength of the diaphragm.

Atrophy of the interosseus muscles (including the palmar interossei that lie on the anterior aspect of the metacarpals, the dorsal interosseus muscles of the hand, which lie between the intercarpals, the plantar interosseus muscles, which lie underneath the metatarsal bones, and the dorsal interossei, which are located between the metatarsal bones.

Muscular atrophy affecting muscles in the distal portions of the extremities.

The presence of characteristic findings of denervation on electromyography (fibrillations, positive sharp waves, and giant motor unit potentials).

An abnormal reduction on the vital capacity, which is defined as the total lung capacity (volume of air in the lungs at maximal inflation) less the residual volume (i.e., volume of air in the lungs following maximal exhalation) of the lung.