Genetic testing - spinal muscular atrophy associated with the X chromosome (See spinal muscular atrophy types I, II, III, IV, ...).

See:

Spinal Muscular Atrophy Type I, II, III, IV, X - linked dominant and adult (Spinal muscular atrophy types I, II, III, X-linked, dominant, adult) - Genes SMN1, SMN2, UBA1, Dync1h1 and VAPB