Weakness in the movement of the hands is one of the symptoms of spinal muscular atrophy. It has an insidious onset and is most common in middle-aged men. It is characterized by weakness of both hands and atrophy of the intrinsic muscles of the hands, which may result in “claw-shaped hand” or “ape hand” deformity. Types I to III are autosomal recessive genetic diseases and are the most common fatal genetic diseases in infancy. Type IV is autosomal recessive, dominant and X-linked recessive. Pathogenesis The etiology and pathogenesis of SMA has been a difficult problem in neurological research. In recent years, great progress has been made in the study of SMA gene localization, and in 1995, three SMA candidate genes were reported by different research groups. In France, Lefebvre et al. identified the survival motor neuron (survivalmotorneuron, SMN) gene in the region of 5q13.1, with a total length of about 20 kb, containing eight exons, and its transcription product of about 1.7 kb, encoding 294 amino acids, with unknown function. The gene has two copies on one chromosome, with a 5-base difference between the two, and is referred to as SMNt on the telomere side and SMNc on the mitotic side.Studies have shown that exons 7 and 8 of SMNt are purely deleted or truncated in 98.6% of SMA patients, and that the other 1.4% of patients have a small deletion or a point mutation, which strongly supports the idea that SMN is an important determinant gene for SMA. Subsequently, Roy et al. cloned another neuronal apoptosis inhibitory protein (NAIP) gene in the 5q13 region, which has 16 exons, is 70 kb in length, and encodes 1,232 amino acids. 45% of patients with SMA-I and 18% of patients with SMA-III and III have the NAIP gene. 45% of SMA-I and 18% of SMA-III and III patients have deletion of exons 5 and 6, while 2% of normal controls also have deletion of exons 5 and 6, suggesting that the NAIP gene is also associated with the pathogenesis of SMA. As for adult SMA, only some patients were found to have deletion of the SMN gene, suggesting similar genetic alterations to those in childhood SMA, but the gene localization of most patients has not been determined, and the pathogenesis of the disease has not been clarified. Pathologic changes are mainly located in the anterior horn of the spinal cord, where motor cells are markedly reduced and degenerative, residual neuronal cells show sequestration and nucleolysis, axons of the anterior roots of the spinal cord are thinned, and peripheral cells of the axons are swollen. Nuclear degeneration of brainstem motor nerves is common in the facial, vagus, and hypoglossal nerves. For muscle pathology, see the Ancillary Examinations section below.